Category Archives: Lesson

October 19, 2007 · 11:58 pm

How much more light? How much less?

At this point, we know what’s bigger and smaller, faster and slower. We know that f/4 is a bigger opening than f/8, and therefore lets more light in. We know that a shutter speed of 1/200 is faster than 1/100, and therefore lets less light in. We know that ISO 400 is faster than ISO 200, and therefore captures more light.

You’re with me, right? Good.

I’m about to throw a little bit of math at you, but I promise it’s only a tiny amount. No, really. All we really have to do is multiply by 2 or divide by 2. That’s easy, right? Take a deep breath. Ready? Let’s go.

How much more light does a shutter speed of 1/100 let in compared to a shutter speed of 1/200? The obvious guess is twice as much, and it’s precisely right. Any shutter speed that’s half as fast will let in twice as much light. Any shutter speed that’s twice as fast will let in half as much light. So 1/200 is half as much light as 1/100. 1/400 is half as much light as 1/200, and is therefore 1/4 as much light as 1/100. Double the bottom number means half as much light. Easy, huh?

Let’s look at ISO next. Do you think that ISO 200 is twice as much light as ISO 100? Bingo! Just like shutter speed, changing the setting by 2 means twice as much light, or half as much light. ISO 200 is half as fast as ISO 400. ISO 100 is half as fast as ISO 200, so it’s 1/4 as fast as ISO 400. Again, all you have to do is multiply or divide by 2, and you’re good.

Now aperture. Do you think f/4 is twice as fast as f/8?

Sorry, but this time it’s a tiny bit more complicated. When it comes to aperture, making the opening half as big means you’re only going to get 1/4 as much light. Making it twice as big means four times as much light. If you don’t want to know why, skip the rest of this paragraph. Our aperture is basically a circular opening. Remember back in high school, the area of a circle is pi-r-squared? Changing from f/4 to f/8 means that the radius of the opening is divided by 2. Because the area is changed by the square of the radius, the area of the opening is divided by 4. The amount of light coming in is directly related to the area of the opening, so the amount of light coming in is also divided by 4.

As it turns out, the way to cut the light in half is to change the aperture by a factor of about 1.4. In this case, f/5.6 is half as much light as f/4, and f/8 is half as much light as f/5.6.

Let me put it all into a nice tidy table, in case that’s easier to follow:

setting	Starting setting	1/2 as much light	1/4 as much light
ISO	400	200	100
Shutter Speed	1/100	1/200	1/400
Aperture	f/4	f/5.6	f/8

Now, a tiny bit of jargon. Whenever you increase or decrease the amount of light by a factor of 2, that’s called a stop. A shutter speed of 1/400 is one stop faster than 1/200. An aperture of f/4 is one stop wider than f/5.6. ISO 100 is one stop slower than ISO 200.

In summary: for shutter speed and ISO, changing the number by a factor of 2 means twice as fast or twice as slow. For aperture, changing the number by a factor of 2 means four times as fast or four times as slow. Half as much/twice as much is really all of the math you need to know in order to be a competent photographer.

A bonus note for those of you who are vaguely mathematically inclined. Remember that I said that changing the aperture setting by a factor of 1.4 meant twice as much or half as much light? The astute reader will note that 1.4 is (approximately) the square root of 2. That’s not a coincidence.

Next lesson: How the settings play together

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October 15, 2007 · 11:46 pm

ISO: Why You Care

In the last lesson, you learned that ISO was sort of like turning up the volume on your stereo, in that it magnified the light as it came into your camera. That sounds like a pretty good way to get a brighter picture in low light, right? It is, but unfortunately there’s a cost associated with it.

Here are two utterly unremarkable photos that I took of the corner of my office building:

The first one was taken at ISO 100, the lowest my Canon Digital Rebel XT will go, and the second one was taken at ISO 1600, which is coincidentally the highest ISO that camera can do. You probably don’t see much difference in them at this size, but I promise you that they’re very different images.

Let’s go back to music for a bit. If you have a very quiet recording on your stereo and you turn it up, the sound gets louder. Something else happens, too– any background noises in the recording get magnified. If you’re listening to a tape (remember those?) the hiss of the tape gets louder. If there’s a small bit of noise in the background, it gets louder.

Just like sound recordings, our digital images exhibit something called noise, and that noise makes the images look grittier and grainier at high ISOs. For the following images, I’ve magnified everything to 2x the original size, and cropped down to a small section of the image for illustration purposes. First, let’s look at the upper left corner of the sky at ISO 100, 400, and 1600:

As you can see, the sky in the ISO 100 image is very smooth. At ISO 400 you can start to see it getting gritty, and at ISO 1600 the image looks pretty bad. That’s noise.

Here are a few more examples, at ISO 100 and ISO 1600:

In all three cases, the ISO 100 image looks pretty smooth, while the ISO 1600 image is very grainy.

It’s important to know that different cameras can have very different performance characteristics at higher ISOs. Inexpensive point & shoot cameras tend toward really horrible noise as the ISO increases, though I’m sure there are exceptions. Consumer-grade Canon digital SLRs are known for performing reasonably well at high ISOs, and their higher-end cameras often do exceptionally well. I have gotten some excellent low-noise shots out of my 5D at ISO 1600. Of course, I didn’t use that camera to take these test images, since the difference in quality would have been much more difficult to show.

If you’d like to see the full resolution versions of the originals:

ISO 100
ISO 400
ISO 1600

That’s great, but how should use use the ISO setting on your camera? My strategy is very simple. For typical shooting I always leave my camera set on ISO 100 until I find myself unable to get the aperture and shutter speed I need to get the shot. If there’s not enough light to shoot at ISO 100, I turn the ISO up until I get to an acceptable shutter speed and aperture. If I know I’m going to be shooting in a relatively dark environment like a nightclub, I just set the ISO to the highest setting I can and cope with whatever noise I get, since I already know that lower ISOs probably won’t be useful.

It can be useful to consider what you want to do with the images you shoot. If you’re just going to use them on the web, then the extra noise at higher ISOs won’t be much of a consideration, since the noise won’t be all that visible. Likewise, if you’re printing small images, noise won’t affect the output too much. However, if you’re planning to print enlargements, or you crop the image down significantly before printing it or displaying it on the web, noise may be a factor.

I hope that ISO has been demystified for you. Has it?

A quick update from mid-2010: This article was written a couple of years ago. Since then, dSLR technology has improved considerably. Many newer dSLRs can shoot very good images at ISO 800 or even 1600, and some produce usable shots at 3200 or even higher. All of the concepts remain the same, but the top end has moved a bit. I expect that will keep happening in the future.

Next lesson: How much more light? How much less?

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October 8, 2007 · 11:25 pm

ISO: What is it?

We’ve learned that the three things that control the exposure of a digital photo are shutter speed, aperture, and ISO. We know that shutter speed is how long the shutter is open, and that the longer it’s open the more light gets in. We know that aperture is how wide the lens is open, that wider openings let more light in, and that the size of the opening also controls how much of the image is in focus.

So what’s this ISO stuff? Way back when, we learned that there are magic gnomes that live on your camera’s sensor, and that when the shutter opens they record whatever light they see.

But first, a diversion. Imagine you go to a club to see a show– it’s a weird kind of show, but bear with me. There’s someone playing ballads on the guitar and singing along. There’s a guy playing punk rock on the accordion. There’s a trio singing political songs. And then someone gets up and whispers poetry. (Hey, I live in San Francisco. This kind of thing actually happens.)

All of a sudden, you can’t hear the show. The poem whisperer is much quieter than everybody who came before him, and the microphone isn’t sensitive enough to pick up his voice. Eventually someone kicks the sound guy and wakes him up, he turns up the sensitivity on the microphone, and unfortunately you get to hear the end of Ode to a Medium Rare Hamburger With Onions. Oh well.

Now our accordion player comes back for his Sex Pistols encore, and he blasts your eardrums. Whoops! Cover your ears until the sound guy turns the microphone’s sensitivity back down.

If you’d brought your camera you might have gotten a great picture of a room full of people covering their ears while staring at an accordionist, but other than that what does this have to do with photography?

Let’s go back to our imaginary gnomes who sit on your camera’s sensor waiting for light, and then record what they see. They do a great job on a bright sunny day, but what happens when you shoot inside the dark club? All of a sudden there’s not enough light for the gnomes, and you get a very dark image. You can open the aperture, but it only goes so far. You can use a slower shutter speed, but at some point you just get a blur of motion. There’s hope, though. Just like you have to turn up the sensitivity on the microphone to pick up more of a quiet sound, you can turn up the sensitivity on your sensor to pick up more light when the scene is dark. It’s like magnifying the light so that the gnomes can see it better.

That sensitivity is called ISO. Of course, there’s a technical description with lots of math stuff for exactly what it is and how it works, but the truth is that you don’t care. It doesn’t matter. Here’s what you do need to know about ISO– it’s pretty simple:

ISO 100 is generally considered the baseline, and it’s where your camera is set by default. ISO 200 is twice as sensitive as ISO 100. ISO 400 is twice as sensitive as ISO 200, and therefore four times as sensitive as ISO 100. ISO 800 is twice as sensitive as ISO 400, and ISO 1600 is… well, I’m not telling you. You’re smart enough to figure that one out on your own.

In the olden days, you bought film with a particular ISO, and you set your up camera so that it knew what film it had in it. These days, you just tell your camera what ISO that you want it to use, and it factors the ISO into its metering. Most dSLRs have a range of ISO from 100 to 1600, often 100, 200, 400, 800, and 1600. A few will have settings in between. Some will go as low as 50 and as high as 3200, and at least one model will go as high as ISO 6400. I’m pretty sure ISO 6400 will let you take pictures without taking the lens cap off.

See? That wasn’t so hard. Turning up the ISO is basically just magnifying the light that comes into your camera.

Next up: how to use ISO, and what side effects it has.

Next lesson: ISO: Why You Care

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September 26, 2007 · 8:39 am

Depth of Field: Another View

We already know that depth of field is how much of your picture is in focus. In particular, it’s what distances from your lens are in focus. Is it a narrow range? A wide one?

Here are a few sample images that show you how depth of field actually works. I took these very carefully– I placed the camera on a tripod, and focused at exactly the 12-inch mark on the ruler. I made sure to keep everything lined up as carefully as possible, and then took shots at f/4, f/8, f/16, and f/32. Most lenses don’t go down to f/32, but this one did so it was good for shooting these sample photos:

At f/4, pretty much the only thing that’s in focus is the 12-inch mark itself– you can’t even read any of the other numbers. At f/8, you can start to see the numbers 11 and 13 come into focus, though they’re still a little soft. At f/16 the numbers 10-14 are pretty clear, and you can actually make out other numbers. At f/32 you can read the entire visible length of the tape measure, and the majority of it is fairly sharp.

I recommend clicking through on the above images and studying the larger images, since they give you a pretty good idea of how the depth of field breaks down as a percentage of the image. If you have a Flickr account, you can click the “all sizes” link above the photos to see them in their original eight-megapixel glory.

The astute reader will notice that these are the same images that are at the top of every page on this blog. I originally took them to use as examples when I was teaching Stop Shooting Auto! as a class for my coworkers, and I instantly loved them. They seemed to produce an aha! moment for everyone who saw them.

Let’s go back to our six-year-old’s landcscape for a bit. I’ve added something to the illustration this time– an indicator of what percentage of the image would probably be in focus at various apertures. The places where there’s a straight yellow line are the areas that will probably be in focus, and the angled lines indicate what will be out of focus– it’ll be just a little bit soft close to the solid yellow line, and stuff far away will be very blurry. Note that I guessed at these based on my experience rather than using any super-magical scientific math wizardry to figure it out. Treat them as rough guidelines rather than hard scientific fact.

Here’s how it might look at f/4:

and at f/8:

and finally, at f/16:

Clearly, if we want our image to be in focus across pretty much the whole range, we should use f/16 or smaller. If we want to blur most of the image, we should use a large aperture like f/4.

How am I doing? Is aperture less scary now? Does it sort of make sense?

Next lesson: Exercise: Depth of Field

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September 24, 2007 · 10:04 pm

Aperture: Why You Care

At this point, we know that aperture is how big the opening in the lens is when you take a picture. We know that big numbers mean a small opening (and therefore less light getting in), and small numbers mean a large opening and more light. f/4 is a large aperture, which means lots of light. f/16 is a small one, and only a little bit of light.

Aperture does way more than control how much light gets into the camera. Those “f-stop things” are one of the most useful tools in photography. Let’s look at why.

I stopped at a park on the way to work this morning so that I could take pictures of flowers to use as examples in this lesson. The flowers weren’t particularly remarkable, but they were useful patient subjects. Here’s one example. You’ll probably look at it and think, “Yeah, a yellow flower”, and see it as fairly messy and uninteresting. That’s because it is messy and uninteresting and an entirely unremarkable picture. Its purpose in life is to serve as a bad example, and it does a rather fine job of it.

Here’s the exact same flower from the exact same angle, but it’s a much more compelling photo. When you look at it, your eye is drawn to the flower, and it isn’t distracted by all the stems and leaves and junk and crap and ick in the background. The only difference between the two images is that in the first one the background is in focus, and in the second it’s soft and blurred.

“Hey, that’s cool! How can I do that?”

It’s easy! All you have to do is pay a little bit of attention to what’s going on, and change your aperture to get the effect you want.

“Huh?”

It’s time for me to demystify another technical term– this time, it’s “depth of field”. Depth of field is really just how much of your image is in focus. There’s a lot of complicated stuff underlying it, but the basic premise is really simple.

When you take a picture, it’s almost always true that some things in the image will be close to the lens, and others will be farther away. The way depth of field works is very simple– if your lens is very wide open (i.e. small f-stop numbers) only a narrow range of distances will be in focus. If your lens has a very small aperture (a big f-stop number), then a very large range of distances will be in focus.

Confused? I don’t blame you. Let’s have our six-year-old draw us another picture. Imagine that you’re walking through a meadow. In front of you is a yellow flower. A little bit farther back from the flower is the goofiest-looking tree you ever did see, and in the distance is a snow-capped mountain range.

Clearly, the flower is very close to your lens, the mountains are far away, and the tree is somewhere in the middle. Let’s pretend that you’re going to take a picture of this idyllic scene, and you decide to focus on the tree.

If you keep your lens wide open, the tree will be in focus but the flower and the mountains will be blurry. If you use an aperture that’s in the middle of the range then you can get the tree and the mountains in focus, and if you use a very small aperture then you can get everything in focus. The former might be useful if you wanted to focus on one thing (say a person standing in front of the tree) and the latter is great for landscape photography.

Think of your depth of field as a stripe that is centered at whatever you focused on. Whatever is inside the stripe will be in focus. If your lens is wide open, then the stripe will be very narrow. If it has a very small aperture, then the stripe is very wide.

I’ll close today with two excellent illustrations from the nature photography of Joe Decker. One shows the power of a shallow depth of field, and the other illustrates what a wide depth of field can do. I highly recommend clicking through to see larger images, since the thumbnails don’t do them justice. Oh, and Joe’s flowers are much better looking than mine.

You’re probably still confused by this, and I don’t blame you. In the next lesson, I’ll leave the fluffy flowery examples behind and show you depth of field in a completely different way. If you don’t want to wait, stick your camera in aperture priority mode and take a bunch of pictures at different f-stops. The wonderful thing about digital photography is that you can play around a lot. The feedback is cheap and very fast.

Next lesson: Quick Exercise: Focusing

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September 14, 2007 · 10:38 pm

What does f/8 really look like?

Yesterday I talked about apertures (a.k.a. f-stops) and how they work. As the old saying goes, a picture paints a thousand words, and this is after all a photography blog. Without further ado, I present f/8:

Pretty cool, huh? That’s what it would look like if you could look inside your lens while you were taking a picture. As you can see, the blades of the diaphram (that’s the thing in the lens that opens and closes to change the aperture) have closed down to make an opening that’s much smaller than the full opening of the lens. What this means is that a lot less light will get into the camera than if the lens was wide open.For reference, here’s what the lens looks like wide open:

Notice that you don’t see the blades of the diaphram at all. If you were to look into your camera’s lens right now, you’d see something just like this because the diaphram only closes while you’re actually taking a picture.

The lens in the picture is a Canon 50mm f/1.8 lens. (Aside: if you have a Canon dSLR, I highly recommend buying this lens. At a retail price of less than $80, it’s a screaming deal.) Since it’s an f/1.8 lens, it has a maximum aperture of… that’s right, f/1.8. The minimum aperture is f/22, which is pretty typical for non-specialized lenses.

Here’s what the full range of the lens looks like. I didn’t take a picture of every possible setting, but you can probably figure out for yourself what the ones in-between look like:


f/1.8	f/2	f/3.2

f/4	f/5.6	f/8

f/11	f/16	f/22

A lot of what I’ve been saying about aperture probably makes more sense to you now that you’ve seen those pictures. All other things being equal, a photo taken at f/2 is going to be about a bezillion times brighter than one taken at f/22. That’s because the amount light that comes in through a great big window is way higher than the amount that comes in through a pinhole.

By the way, I apologize for the reflections on the lens. Taking pictures of glass is notoriously difficult, and I was feeling a little bit too lazy to set up enough of a lighting rig to do a better job. Mea maxima culpa. (If you want to learn sophisticated lighting techniques, I recommend the excellent Strobist blog. Stick around here for a bit and master the basics first, though.)

Now you’ve seen my f-stops. Want to see your own? Here’s a little mini-exercise to do just that.

Most modern cameras have a button called depth-of-field preview. On my Canons, it’s if you’re looking at the camera from the front, the DOF preview button will be next to the lens, on the right side, toward the bottom of the camera. It’s just below the button that you use to take the lens off. If you can’t find yours, check your camera’s manual.

Now, set your camera for manual mode, and an aperture of f/8. We don’t care about any other settings right now, because we’re not going to take a picture. Look into your lens, and press the depth-of-field preview button. Did the diaphragm close? If so, you now know what f/8 looks like on the lens you’re using. Try different apertures to see how it changes.

If your camera doesn’t have a DOF preview button, there’s another trick you can use. Set the camera for manual mode, f/8, and about a 20-second exposure. Press the shutter button (yes, take a picture) and look into the lens. You’ll have 20 seconds to look into the lens and see the diaphragm. Like before, you can try out different apertures and see what they look like.

When you started reading this blog, you probably kept your camera in automatic mode all the time. And now, a short time later, you actually know your f-stop from a hole in the ground. If I’ve done a good job, your brain isn’t even bleeding out your ears.

As it turns out, aperture is one of the most powerful tools in photography. In the next couple of lessons I’ll show you why.

Next lesson: Aperture: Why You Care

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September 14, 2007 · 4:20 pm

What’s that f-stop stuff anyway?

Newcomers to photography are often intimidated by technical jargon. Walk into a room full of photographers, or read camera reviews, and you’ll quickly be overwhelmed by seeming-gibberish about megapixels, spot metering, chroma noise, low-pass filters, A/D converters, sync speeds, and a myriad of other not-quite-English terms. If only you hadn’t cut Swahili class in high school, maybe it would make more sense to you and your head wouldn’t be swimming.

Of all the technical terms, nothing is scarier for the beginner than f-stops. I’ll do my best to demystify them for you.

First off, a simple English lesson. You’ve almost certainly heard the word aperture before… if you’ve been reading this blog, you’ve eve heard me use it. What exactly is it? An aperture is an opening, and specifically an opening that lets light in. In photographic terms, the aperture is the hole in your lens that lets light into the camera.

That wasn’t so scary, was it? No? Good. let’s keep going.

So what’s an f-stop? In a nutshell, it’s how big that opening in the lens is. That’s it. Really. It’s not some magical, mystical thing that’s going to bite you if you get it wrong. F-stops don’t hurt. They’re not contagious. They won’t kick your dog, drink your beer, or max out your credit cards. OK, they might max out your credit cards someday, but only if you start chasing really big ones.

How do the numbers work? Basically, a small number means that the lens has a very wide opening, and big numbers mean that the lens has a small near-pinhole opening. Of course, we remember that big openings let lots of light in, and small ones only let a little bit of light in. The most common f-stops you will encounter will be in the range of about f/2 to f/22. f/2 is a low number, so it’s a really wide opening. f/22 is tiny. Sometimes you’ll see numbers as low as f/1.4 (and if you have deep pockets, f/1.2), or as high as f/32 or occasionally f/45.

Something important to know is that when you aren’t actually taking a picture, your lens is always open to its widest possible opening even if you have the camera set to use a much smaller f-stop. This lets you get as much light as possible for focusing and composing the image. When you press the shutter button to take a picture, one of the things that happens is that little metal blades in the lens come out to make the opening smaller while the shutter is open. This happens automatically, and you don’t need to think about much about the details.

By the way, photographers tend to use the terms aperture and f-stop interchangeably. That’s OK.

Every lens has a minimum and maximum aperture, but they’re usually only rated in terms of the maximum– that is, the widest opening. For example, Canon sells two 50mm lenses. One is the 50mm f/1.8, and the other is the 50mm f/1.4. The first one can open as wide as f/1.8, which is really rather wide. The other goes as wide as f/1.4, which is even wider. (But you knew that!) Other than the maximum aperture, what’s the difference between the lenses? The f/1.8 lens is about $80, while the f/1.4 is about four times that price. Lenses with with very large maximum apertures are called fast lenses; I think it’s because of how quickly they drain your wallet.

Did that all make sense? Was it not too scary?

Tomorrow I’ll show you some pictures of what it looks like inside a lens, and show you a neat trick for checking it out yourself.

Oh, yeah. If you’re confused, feel free to ask questions in the comments. I’ll do my best to answer them.

Next lesson: What does f/8 look like?

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September 10, 2007 · 3:51 pm

How your camera works– exposure modes

OK, this is what you’ve been waiting for… we’re about to take our camera out of automatic mode. How do we do that? It’s really very easy.

Somewhere on your camera there’s a dial that looks a little bit like this one on my 5D. It will have several settings to choose from, and may have many more than what’s shown in this photo. In order to set the exposure mode, you turn the dial so that the appropriate setting is lined up with the indicator line on the side. The line is a little bit hard to see in this photo, but the camera is in M mode. M is for manual, Magic, Master of the universe, and My favorite mode.

What are all those other modes?

Green box: the camera just does everything for me
P: program mode, which is much like the green box but a little less neurotic
Tv: shutter priority mode
Av: aperture priority mode
M: manual mode
B: bulb mode
C: custom function mode (which I’m going to ignore)

I shoot with Canon gear, and therefore I’m most familiar with Canon’s controls. Those are also the cameras that I have sitting around to use as examples. (If anybody wants to send me a Nikon so that they can have equal representation, I’d be happy to go the extra mile.) I believe that Nikon calls their controls Auto, P, S, A, and M for Automatic, Program, Shutter Priority, Aperture Priority, and Manual. Other brands will have very similar functions– if you can’t figure out what they are, consult your manual.

In Auto and P modes, the camera basically just does everything for you. Typically, Auto mode is extremely obsessive about it, and won’t let you change any of the settings on your own. P mode is kind of a laid back version of Auto, one that will set the shutter speed and aperture for you, but not ignore you if you try to change something else.

Aperture priority and shutter priority are the real workhorses, and many people find that they use aperture priority more than anything else. You might too, once you figure out how they work. In aperture priority mode, you pick your own aperture, and then the camera’s meter figures out what shutter speed to use. As you can probably guess, in shutter priority it’s just the opposite– you pick the shutter speed that you want, and then the camera picks an aperture. Pretty easy, huh?

Manual mode is the final mode that you might want to use on a regular basis. In manual mode, the camera keeps its grubby little paws off the settings, and you get to choose both the shutter speed and the aperture. That sounds like a scary amount of power, right? Well, it’s not so bad. The camera’s not going to do any work, but it will still give you information. Even in manual mode, the camera’s meter works just fine, and you can use the display to help you find the right exposure.

Just for completeness, bulb mode is a special manual mode. In bulb mode, you press the shutter button once to open the shutter, and press it again to stop. This seems like a weird thing to do, and you’re right, but it’s occasionally useful for things like taking pictures of stars. You’ll probably never need to use bulb mode, but tuck the information into a corner of your brain in case it ever comes in handy.l

You’re probably wondering how you set the aperture and shutter speed on your camera. Frankly, I have no idea but if you bring it over here I’ll figure it out and tell you. Or, you can grab the manual and look it up. Here’s how mine work:

Canon 5D: There’s a dial right behind the shutter button– it controls the shutter speed. There’s a thumb wheel on the back of the camera that controls the aperture.

Canon Digital Rebel XT: There’s a dial right behind the shutter button. In aperture priority mode, this button controls the aperture. In shutter priority button, it controls the shutter speed. In manual mode, that dial controls the shutter speed. In order to set the aperture in manual mode, you press the top button (labeled Av +/-) on the back of the camera and hold it down while turning the dial. Yes, that’s a little confusing to describe. It’s actually pretty easy to do, and once you get the hang of it you’ll find it quite easy.

Next up, our first exercise!

Next lesson: Exposure modes exercise

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September 9, 2007 · 4:30 pm

How your camera works– the meter

This is part two of how your camera works, and it’s a little bit more complicated than the first round. I’ll try to keep it as simple as possible, but some of this is pretty close to magic.

As I explained earlier, when you take a picture the aperture closes down part of the way, the shutter opens, and then the sensor records the light it saw. There’s a step that happens before that, and it’s a very important one.

Your camera is surprisingly smart for such a small piece of gear, and it does a lot of sneaky stuff when you aren’t looking. You’ve probably noticed that if you leave the camera in automatic mode, it pretty much just figures out how to take a picture that looks pretty good. Sometimes it guesses wrong, but most of the time it gets pretty close to a good picture. How does that happen?

Your camera has a built-in light meter that measures how much light is coming into the camera. Just before you take a picture, when the shutter button is halfway down, the camera very quickly measures the light coming into the camera and then picks out the settings it thinks will be best for a good picture. If you’re in a dark bar, the camera will want the lens to be wide open and the shutter to be open for a long time. In bright daylight, the camera will want a smaller opening and a quick shutter speed so that the photo doesn’t get overexposed.

I’m sure you’ve seen the weird scale thing somewhere on your camera. There’s almost certainly one in your viewfinder, and quite possibly also one on a screen on the top or back of your camera. It usually has numbers that go from -2 to +2, with a bunch of tick marks in between and a line that moves around seemingly of its own volition. That thing is a display of your camera’s meter, and it’s really very simple to understand. The camera moves the line around to let you know if it thinks the shot is going to be underexposed, overexposed, or just right. When the line is right smack dab in the center, as it is in this photo, the camera is very happy about the exposure. That doesn’t mean the exposure will be perfect– cameras are far from perfect at guessing this stuff. They’re pretty good, though, and the camera’s meter will usually be a very good starting point.

If the line is to the right of center, that means the camera thinks that the shot will be over exposed– the photo will have too much light. A much more common situation is that the line will be way over on the far left, which means that there’s not enough light and the photo will be underexposed. In automatic modes, the camera will do its very best to get that line in the middle of the display, but sometimes it just won’t be able to even at the slowest shutter speed and the widest aperture. Later we’ll talk about ways of handling that. For now, just try to get more light if you can.

Modern cameras generally have very sophisticated meters with all sorts of different modes. You’ll often hear terms like spot, zone, center-weighted, evaluative, partial, super duper, whizbang, and confusing. Camera review sites will list the various modes as badges of honor, and gear geeks will discuss them ad nauseum.

Don’t worry about those for now, and just leave yours set to whatever the camera’s default is. Right now, all you really need to know about metering is that your camera knows how to measure light and pick out shutter speeds and apertures based on the light it sees.

OK, I’ll tell you just a little bit more. Your camera’s default mode is probably one where the middle of the picture matters a lot, the edges barely matter at all, and the stuff in-between matters a little bit. That’s because most people put the important stuff in the middle of the picture. Somewhere down the line we’ll talk about different metering modes and why you might care, but that’s pretty far away. It’s entirely possible that you’ll never have a reason to use anything but the camera’s default mode.

Next lesson: How your camera works– exposure modes

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September 9, 2007 · 3:32 pm

Shutter speed, why you care

OK, those spinning light pictures are kinda cute, but why do you really care about shutter speed if you aren’t trying to capture fire dancers? In short, the shutter speed gives you artistic control of the way your camera captures motion.

One day at work, I heard jets flying very low over my San Francisco office. I happened to have the camera on my desk, so I grabbed it and ran up to the roof to see what was causing the ruckus. I’d completely forgotten that it was Fleet Week in San Francisco, and the Blue Angels were putting on a show for us. I grabbed my longest telephoto lens and started snapping. Since the planes were going by at several hundred miles per hour, I needed a very fast shutter speed to avoid blurriness. This photo was taken at 1/1600 sec, which is pretty speedy.

What if you want to take a picture of your son’s little league game. He’s the pitcher, so of course you want to capture him for posterity. What shutter speed should you use? Well, it depends. Do you want to freeze him in motion just as the ball flies away from him? If so, you should probably use a shutter speed of around 1/500 in order to freeze the motion. Do you want to capture blur from the motion of his arm and see the ball traveling through the air? To achive this result, use something slower, maybe 1/80 or 1/60.

Here’s a neat picture of my drummer friend John. The shutter speed on this was 1/80 sec, which froze almost everything the picture but captured the motion of his drumsticks and his hands. This produces a much more interesting and dynamic shot than if the drumsticks had been completely unblurred– you can almost hear the crash of the cymbal as the fast-moving drumstick comes down on it.

There’s also an interesting remedial component to shutter speed that you should be aware of. Have you ever taken what you thought was a wonderful photo, only to find out later that it was shaky and blurry? It turns out that humans aren’t all that great at holding perfectly still. When we try to take a picture with a slow shutter speed, it doesn’t really matter what our subject is doing. The photographer (that’s us!) wiggles and jiggles around so much that even a picture of a rock will be blurry.

If you’re holding the camera in your hand rather than supporting it on a tripod or some other solid surface, you need to use a fast enough shutter speed that your own motion won’t blur the photo. Some photographers are better at holding still than others, but everybody gets camera shake if their shutter speed is too slow. A general rule of thumb is that your shutter speed should be no slower than the focal length of your lens. For example, if you’re using a 50mm lens, use 1/50 or faster. If you’re using a 200mm lens, use 1/200 or faster. Unless you have nerves of steel, that’s the absolute slowest that you should try to hand-hold a shot. Even you might not be quite that steady; it pays to experiment and figure out how steady your own hands are.

Newer image stabilizing lenses will let you exceed that guideline, but they’re relatively expensive, and they aren’t the cure-all that some people make them out to be.

Thus far, we’ve pretty much just focused on reasons that you might want a fast shutter speed. What about a really slow one? Fireworks are one example. You need a tripod or some other way to stabilize the camera, but fireworks are surprisingly easy to shoot. For this shot, I set the shutter speed to 4 seconds and then did my best to time the shutter so that it opened just before an explosion.

Here’s another great example. Noted nature photographer Joe Decker uses a slow shutter speed to capture the movement of water in this stunning image. Incidentally, Joe has graciously allowed me to use some of his photographs as illustrations on this site. You should check out his work.

Next lesson: How your camera works– the meter

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