Tag Archives: photography

How the settings play together

After trudging through all those scary numbers, we now know that ISO 200 is twice as fast as ISO 100, that 1/60 second is half the time of 1/30 second, and that f/4 is twice as fast as f/5.6. If I did my job well, we even learned that without anyone’s brain dripping out of their ears and onto their keyboards.

Note: the word “fast” in photography generally means “captures more light” when it’s talking about ISO or aperture.  When talking about shutter speed, though, “faster” means just the opposite– 1/60 is faster than 1/30, but 1/30 lets in twice as much light.  It’s confusing.

What might or might not be obvious now is that we can change two different things and keep the exact same exposure. For example, each of these settings will produce the exact same exposure:

ISO 100 f/4 1/60 sec baseline
ISO 100 f/2.8 1/120 sec faster (wider, more light) aperture, faster (less light) shutter speed
ISO 200 f/4 1/120 sec faster ISO, faster (less light) shutter speed
ISO 200 f/5.6 1/60 sec faster ISO, faster (wider, more light) aperture
ISO 400 f/5.6 1/120 sec ISO four times as fast, shutter speed twice as fast (less light), aperture half as fast (narrower, less light)

The first line is our baseline measurement. On the second one, we opened up the aperture wider to let more light in. In order to keep the exposure constant, we need to capture less of the light, so we keep the shutter open for half as long.

On the third and fourth lines, we increase the ISO, which records more light To balance it out, we either keep the shutter open for less time, or we make the aperture smaller– either of those will let less light in.

The fifth line is tricky! We’ve quadrupled the ISO, which means we’ve increased it by two stops. In order to compensate for that, we made the aperture smaller by one stop and we also made the shutter speed faster by one stop.

The fancy technical term for this is reciprocity– it really just means that if you change one setting, you can balance it out by changing another one, and get the same result.

So how would you use this? Here’s an example. Let’s say you’re taking a picture of a group of people, some of them standing behind the others. You put the camera in aperture priority mode and pick f/4 and ISO 100. When you try to take the shot, you find out that the best shutter speed is 1/30, and that’s too slow for you to hold the camera steady– you need at least 1/50 sec. What do you do?

Well, you can try using f/2.8 and 1/60. That will get you the same exposure, but at a faster shutter speed. But wait… now you’re at f/2.8, and when you focus on the front row of people the back row is out of focus, or vice versa. You really need f/4 to get everyone in focus.

Finally, you set the ISO to 200, which lets you use an aperture of f/4 with a shutter speed of 1/60. Voila! You get your shot.

The way exposure settings play together is a little bit like having a squishy ball in your hand. When you squeeze one part to make it smaller, another part gets bigger. Squish two parts, and the third one gets a lot bigger. Squish squish squish.

Easy, huh?

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Filed under Aperture, ISO, Lesson, shutter speed

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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Filed under Aperture, ISO, Lesson, shutter speed

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:

ISO100FullISO1600Full

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:

ISO100Sky2ISO400Sky2ISO1600Sky2

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:

ISO100BuildingCornerISO1600BuildingCorner
ISO100SkyISO1600Sky
ISO100DarkWindowISO1600DarkWindow

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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Filed under ISO, Lesson

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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