Exposure = ISO + Aperture + Shutter Speed. Learn it, know it, live it.
Let me start by saying I don’t buy the top of the line camera body Nikon makes, especially since I’ve gone digital. Digital camera bodies are like computers; in a couple of years, newer, better technology will render today’s bodies almost useless. (Almost. But that’s a discussion for another day.) Lenses, on the other hand, hold their use and value. Lenses contribute more toward making a great photo than a camera body does.
I’d rather have a $1500 lens on a $500 body than a $5000 body with a $500 lens.
One of the most important considerations in buying and/or using a lens is aperture.
This is how I think of aperture: imagine a sheet of glass between you and your subject. That sheet of glass represents aperture when shooting wide open. There’s only that tiny stretch of depth across your frame that will be in sharp focus. This is known as shallow depth of field. I did my best to photograph an example of shallow depth of field using critical aperture (keep reading) where you can literally see the plane of focus:
Nikon D700, 70-200mm f/2.8 lens at 200mm, f5.6 (critical aperture) @ 1/1600 second, ISO 400
Aperture, by definition, is the variable opening by which light enters a camera through a lens. Aperture is the lens opening or f/stop in a photographic equation. Aperture is the amount of light made available to the sensor to record an image.
So let’s review focal length just for a moment and return to the numbers printed on a lens: for example, my Nikon 105mm 1:2.8 lens. We know that the 105mm means focal length, but what does the ratio-looking 1:2.8 mean?
This number designates the widest aperture available for that lens and it’s written as a ratio because aperture is in fact a fraction (stay with me.) For example, my 105mm 1:2.8 lens can be shot wide open at f/2.8. Some zoom lenses will have numbers that look like this: 1:4-5.6 This number means that f/4 is the largest aperture available at the widest (wide angle) end but only f/5.6 at the longest (telephoto) end (say, for example, it’s a 70-200mm zoom, f/4 at 70mm and f/5.6 at 200mm is the widest aperture available for that lens.) My 70-200mm telephoto zoom lens has a 1:2.8 ratio; I can shoot wide open at f/2.8 no matter what focal length I’m at with this lens.
This number is the crux of the issue of lens choice. All of my lenses are f/2.8 or f/1.2 — all of them. Lenses slower than f/2.8 are deal-breakers for me. It’s just the way I shoot. Perhaps you shoot differently; say you shoot landscapes at mostly f/22 and don’t have much use for a lens wide open. Perhaps you never shoot pictures indoors, so a lens that can perform in low-light situations isn’t of much use to you. That’s exactly my point, why I’m going to all of this effort to show you how to use your camera; it’s why I can’t tell you one way or another what camera-lens combo would be the best gear for you to buy. Remember Rilke?
Look at this aperture scale to help you visualize aperture as the amount of light splashing in through your lens toward your sensor:
f/1.2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32
Each stop on the aperture scale = one full stop of light.
Take our exposure equation that we learned last post:
Exposure = ISO (sensitivity to light) + Aperture (amount of light made available to record an image) + Shutter Speed (the length of time the light is made available to record an image)
How sensitive you’re making your camera to light + how much light you’re letting in and for how long. That’s it! That’s your exposure equation. How much light you’re letting in is controlled by aperture. Think of aperture as pouring light onto your sensor through your lens; at f/2.8, you’re just dumping it in wide open but by the time you stop down to f/22, you’re pouring it quite detailed through a small funnel.
Photography is, by definition, painting with light. Not enough light and you might as well take your ball and go home. Light gives a photographer options and options are the paint in a photographer’s hands.
Moving from f/2.8 to f/5.6 on the aperture scale is two full stops of light. In ISO terms, which you might be a little more familiar with if you’re using a point-and-shoot and ISO is really the only adjustment you make, that’s the equivalent of moving from ISO 400 to ISO 1600. Think of the difference in the quality of your images when you shoot on ISO 1600 as compared to ISO 400. (If you don’t already know this offhand, don’t worry; we’ll cover it.) If you’re starting at f/5.6, you’re going to have to make up that loss of light somewhere. Looking back at our exposure equation, there are only two other options: ISO or shutter speed. If you’re working with a long telephoto lens, you might not have as much wiggle-room to slow your shutter speed. That leaves you bumping up your ISO which might give you noisy or not so crisp and clear images.
There’s also the issue of performance. A “critical aperture” is the aperture at which a lens can be expected to best perform, and is generally two stops from the maximum (or largest) lens aperture. So if your largest aperture is f/1.2, then you can expect very good, clean, sharp images from shooting that lens at f/4; but if f/5.6 is your maximum aperture, you’ll have to stop that lens down to f/11 to achieve “critical aperture.” Scroll back up to the aperture chart and look at the circles, particularly at f/11 — visually compare the amount of light available at f/11 with the amount of light available at f/4 or even f/1.2. See my concern?
Generally speaking, the larger the f/stop (think of the size of the circle, not the number!) the better the lens. The larger the f/stop, the “faster” the lens.
Q: Why is f/22 smaller than f/2.8 when 2.8 is a smaller number than 22? It’s very confusing!
A: It is confusing. It’s math. “f/stop” is literally (speaking in mathematical terms) a fraction that indicates the diameter of the aperture (or lens opening). “f” stands for the “focal length” of the lens, “/” is “divided by” and the number indicates the stop. So a 28mm lens set at f/2.8 would be [focal length] 28 divided by 2.8 = 10. The diameter of that lens opening would be 10. A 28mm lens set at f/22 would yield a diameter of 1.27. So in this case, a circle with a diameter of 10 would be larger than a circle with a diameter of 1.27 — whew!
Or, you can simply commit to memory the size of the circles in the chart I drew, f/2.8 being much larger than f/22, each numbered stop along the way halving the light reaching the sensor. That’s what I did.
Q: Why does it go f/1.2 and then f/2.8? Shouldn’t it be f/1.4?
A: That’s a good question. Sometimes it does, some lenses are f/1.4. The numbers don’t quite line up perfectly with halving and doubling. Sometimes cameras factor in half-stops or thirds, producing far more numbers than I included on the scale above. You’ll see the same things happen next post when we get to shutter speed, moving from 1/60 to 1/125. Don’t get too hung up on it.
The numbers on the charts I’m giving you are considered the “whole stops.” It’s a good frame of reference for a beginner. Once you memorize those numbers, you’ll find it becomes second nature to move from whole to whole in calculating exposure. This is useful because if you are shooting f/5.6 at [shutter speed] 1/250 but you’d like to be shooting at f/2.8 to blur your background more, all you have to do is adjust your shutter speed two stops to the right to make up for shifting your aperture two stops to the left. So the exposure of f/2.8 at 1/1000 is equal to the exposure of f/5.6 at 1/250 for this particular photo. Both photos will look the same as far as brightness and contrast, but the image shot at f/2.8 will have less depth of field.
Here’s an example of changing shutter speed and aperture combinations to produce basically the same exposure. In this case, I wanted to illustrate the difference in depth of field (notice the backgrounds) with different apertures. I moved my aperture roughly 5 steps to the right on the scale above, so I had to move my shutter speed roughly 5 steps to the left to compensate for the loss of light in order to achieve the same basic exposure. I was using my 28-70mm lens and probably should have shot 4 photos, two at 28mm and two at 70mm but my model was in a hurry to get to her meet so I shot two frames at roughly “normal” focal length, the first to illustrate shallow depth of field and the second to illustrate great depth of field:
Nikon D700, 28-70mm lens at 48mm, f2.8 @ 1/800 second, ISO 400
See those pretty little circles in the background above? That’s bokeh. Wide open apertures produce blurry backgrounds and beautiful bokeh and shallow depth of field (blurry background.)
Nikon D700, 28-70mm lens at 48mm, f18 @ 1/20 second, ISO 400
F/18 renders great depth of field (focus plane extends much farther throughout the background.) However, depth of field does not always insure sharp focus. A corresponding shutter speed of 1/20 to make up for the loss of light when I moved to f/18 was clearly against the rules we learned in focal length — it’s not a “first learn the rules, then learn to break them” example, it’s an example to illustrate why paying attention to focal length and its relationship to shutter speed is important. Remember, I was handholding my lens (the body-lens combo weighs about 10 pounds) at 48mm with a shutter speed of 1/20 and result is screaming camera shake or blur. Look at these close-up crops of the same images from above:
Nikon D700, 28-70mm lens at 48mm, f2.8 @ 1/800 second, ISO 400
Nikon D700, 28-70mm lens at 48mm, f18 @ 1/20 second, ISO 400
Extra Credit: Let’s say that second image was shot at (I’ll put it into whole stops to make it easier for you) [aperture] f/16 and [shutter speed] 1/30. iTunes treat goes to the first person who can tell me (before I post the answer in the next topic) what my exposure on that second image SHOULD have been, assuming a shutter speed of 1/60.
Bueller? Bueller? Anyone? Anyone?
[return to the PHOTO 101 Table of Contents]
show hide 7 comments