Tag Archives: lens

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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Exercise: Depth of Field

(This shall forever be known as the too-many-yellow-rulers lesson. Sorry!)

f32Up ’til now, you’ve been just reading the exercises and maybe doing them in your head, but not getting the camera out and trying them yourself. It’s OK– I’m lazy too. This one is different though. You’ll want to get the camera and do the exercise. If you don’t, I’ll hunt you down and scowl at you.

In this exercise, you’ll get firsthand experience with how aperture affects depth of field. In order to do the exercise, you’ll need a large flat surface with some sort of measurements, or sharp regular pattern. Some things that would work well:

  • a tile floor with high-contrast lines
  • a striped or checkered blanket with high-contrast lines
  • a long tape measure

You’ll need about five feet of this patterned surface, and you’ll need to be able to get fairly close to it, so that you’re looking down the length of it. See the tape measure picture at the top for an example of what I mean. You’ll also need enough light to take a steady picture at f/16.

Set your camera to aperture priority mode, f/16, and manual focus. If you’re not in a very bright area, set your ISO to 1600, or as high as your camera will go.

Stand near your patterned surface so that you’re looking down the length of it. Pick a spot in the middle, and manually focus on it. Remember that spot, because you’ll be coming back to it. Once you have the image in focus, take a picture.

Without changing your position, change the aperture to f/8. Point your camera at the exact same spot, and take another picture. Do the same thing at f/4, and as wide as your lens will go.

f4f8f16

Pull the photos into your computer where you can examine them more closely, and you should have a set that looks something like the photos above. The photo taken at f/4 will have a narrow band in the center that’s in focus, and the rest will be blurry. The f/8 photo will have a wider focus area, and f/16 will be much wider.

For extra credit: repeat the exercise, but this time don’t take pictures. Instead, use the depth-of-field preview button on your camera to see what the shots would look like. Feel free to try out the apertures in between the ones I suggested too. You should see a much smoother transition of focus than at the widely-spaced apertures, of course.

The astute… OK, the barely-conscious reader will notice that the example photos in this exercise are also in the page header. I originally shot these as examples when I was teaching SSA! as a live class, and I wanted samples to hand out. I was happy when I saw the results. The students in the class seemed to grok depth of field as soon as they saw the images, and they had a field day duplicating them with their own cameras.

Now it’s time for you to go out and play. Put your camera in aperture priority mode, and wander around taking pictures. Create some blurry backgrounds and some crisp ones, just like you’ve always done, but this time do it on purpose. You have the power– use it.

Next lesson:  ISO: What is it?

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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:

f/4f/8
f/16f/32

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:

MountainFlower f/4

and at f/8:

MountainFlower f/8

and finally, at f/16:

MountainFlower 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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Quick Exercise: Focusing

This is a really simple exercise that will help you think about focusing and depth of field. It’ll only take you a minute. Grab your camera.

Find a place where there are things at various distances from you– pretty much any place will do. Put your camera into manual focus mode, and turn the focusing ring all the way either direction.

Look through the camera and see what, if anything, is in focus. We’re not trying to focus on any particular object, but rather just look to see what’s already in focus. Keep looking as you slowly turn the focusing ring all the way to the other extreme. If you want to, repeat this a few times.

As you turned the dial, you probably noticed that things close to you came into focus first and then the farther-away ones did. (Maybe it was the opposite direction. It all depends upon which direction you were turning the ring.) You can imagine that your focus point is a person who walks closer to you or farther away from you as you turn the ring. The person will always be in focus, as will some area around them.

Now look at the top of your lens. You’ll probably see a bunch of numbers with an infinity sign (a sideways figure-8) at one end. Those numbers indicate how far away the focus point is at any given time. If you turn the focusing ring so that the focus is at infinity, things far away from you will be in focus and stuff that’s close will be blurry. You’re smart– I bet you can guess what happens to far away things when you turn the focusing ring the other way.

Next up: another way to look at depth of field. (It’s important. I’m going to repeat myself.)

Next lesson:  Depth of Field: Another View

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Aperture: Why You Care

flower f/22At 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.flower f/4

“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.

While walking through a meadow, you come upon this scene

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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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:

f8

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:

f1_8

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:

f1_8 f2 f3_2
f/1.8 f/2 f/3.2
f4 f5_6 f8
f/4 f/5.6 f/8
f11 f16 f22
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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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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