Get with the program: Exposure 1

Or Part 3 of Getting the Most Out of Automation.

You might remember from the ISO article (Part 2 of this series, which, if you have not already, you should read first) that the art and science of “correct exposure” has everything to do with fitting the full range of tones and brightness in any given scene into the limed range of tones and brightness that the photo sensitive surface (PSS) we are using is capable of capturing. Our PSS might be film coated with silver halide crystals or it might be the silicon wafer containing millions of individual photo receptors that we find in today’s modern digital cameras.

It’s all about Exposure

We accomplish this best-fit trick by changing and balancing three factors: the ISO setting, the aperture, and the shutter speed. Change, because we can change any one of them or all three to achieve correct exposure…but also balance because it is the interaction of the three, working together, that allows us to match the brightness of the scene to the capture range of the PSS.

ISO: To review from Part 2: ISO is a number that represents the sensitivity of the PSS to light. High ISO numbers mean the Photo Sensitive Material takes very little light to produce a image. Low ISO numbers mean it takes a lot.

Shutter speed

Clearly, the amount of light energy that reaches the PSS depends on how long the light is allowed to shine on it. In a camera we control this with, traditionally, a shutter…some device connected to a timer that controls when the light hits the PSS and when it does not (it’s a switch in a digital camera, but it does the same thing). You open the shutter (or throw on the switch) for just long enough to admit (or catch) the amount of light you want. High shutter speeds, or a fast shutter, mean less light falls on the PSS. Low shutter speeds means more light falls on the PSS. It is a direct relationship. Doubling the length of the shutter speed doubles the amount of light falling on the PSS. We are talking small times here, so shutter speeds are generally fractions of a second. 1/125th of a second is twice the light of 1/250th of a second. 1/64th is roughly half the light of 1/125th. Film cameras, which relied on mechanical shutters, had a limited range of settings, which became traditional over time. Digital cameras actually pick whatever fraction works, but it gets displayed or recorded as some fraction that is a rough approximation of one of the traditional shutter speed values.

So, aside from its effect on the exposure, why would you pick one shutter speed over another?

Shutter speed is important to the final image because it determines how still the world has to be to capture an image of it. The world is rarely still and, even if the world were, your camera isn’t. Any camera motion blurs the whole image, and subject motion blurs the subject. To offset camera motion a shutter speed of above 1/125th of a second is often recommended. Go much slower and you need a really steady hand, a camera support, or some kind of image stabilization.

Most people, when needs-must, can hand-hold an occasional shot down to 1/40th of a second, but that’s tricky. And, of course, the more you zoom in (the more you raise the magnification of the zoom) the harder it is to hand-hold the camera. Higher magnifications magnify camera motion along with the size of the subject in the frame. At high zoom settings you had better stick to the 1/125 rule, and, when possible, try for even higher shutter speeds.

Slow shutter speed blurs telephoto shot, faster shutter speed yields sharper image.

Many digital P&Ss have automated exposure warnings…a light that blinks…a little steady hand that pops up on the LCD display…to tell you when you are in danger of getting motion blur because of camera  motion and too low a shutter speed.

In addition, a few (though an increasing number) of high-end P&Ss also have some kind of built in image stabilization…some device in the camera or lens that steadies the shot, allowing you to hand hold much longer exposures. If your camera has image stabilization (about which there will be a future article), you will have to experiment with it to see just how slow you can go at different settings on the zoom.

(True image stabilization should not be confused with the many different flavors of “steady shot” featured on inexpensive P&S from Kodak, Fuji, etc. All these cameras do is bias the auto exposure to select the highest shutter speed and the lowest aperture (read on) possible for the given lighting conditions. True image stabilization moves the sensor or a lens element to counter camera motion and is very sophisticated. To confuse matters Sony also calls their true image stabilization “steady shot” and adds a “super steady shot” with combines the exposure bias with image stabilization.)

To stop moving objects, the shutter speed is related directly to the speed the object is moving, and the direction of motion. A running (or even wildly grimacing) child might require 1/250th to 1/500 of a second. A moving car or bird in flight requires 1/1000th to 1/4000th of a second if it is going across the frame…while 1/250 will catch it coming head on. (It is possible to catch moving objects going across the frame at shower shutter speeds by panning with the object…moving the camera to keep the subject in the same spot in the frame…more on that in another article.)

Show shutter speed blurs pendulum, faster stops it.

Show shutter catches an animated Erin as a blur, faster shutter catches her in mid-gab.

Caught at 1/200 second coming head on.

Shutter speed matters for exposure, but it also matters if you want consistently sharp images.

Aperture

Besides controlling the length of time light falls on the PSS, we can change the size of the hole the light has to pass through to get to the PSS.

Every lens is, at it’s most basic, a hole. Its size (diameter) determines the maximum amount of light that can pass through to the PPS. We can control the amount of light by putting something between the lens and the PPS which has a smaller hole in it than the full diameter of the lens. We call this hole an aperture, and its size is generally controlled by a set of blades which overlap and move so that the size of the hole can be changed. The really confuse matters, the size of the aperture, based on photographic tradition, is given in f-stops. All you really need to know about f-stops is that smaller f-stops numbers represent bigger apertures (more light on the PSS) and bigger f-stops numbers represent smaller apertures (less light on the PSS). Because of this inversion, when talking about f-stops we say we are moving to a smaller f-stop…f5.6 to f8, even though the number is going up. Conversely, we say we are moving to a larger f-stop when going from f5.6 to f4 even though the number is going down. Generally the largest f-stops available on P&S cameras are around f2.8 to f3.5 and the smallest are f8 to f16. In the film world cameras had f-stops as small as f16 and f22, and field-and-view camera lenses went to f32 and even f64. Remember, for f-stops a large number means a small f-stop, a small aperture, and less light…and a small number means a large f-stop, a large aperture and more light. f-stops are computed so that going up or down one, generally doubles or halves the amount of light falling on the PPS. So, going from f5.6 to f8 cuts the light in half. Going from f4 to f2.8 doubles the light.

Don’t ask…f5.6, f6.3, f4.5, f3.5…you do not need to know where the odd numbers come from…only that, for a traditionally marked camera, each smaller f-stop and smaller aperture cuts the light falling on the PSS by half.

(Okay, all right…so I will tell you…because I know that not knowing will drive some of you crazy and you will be googling it right now if I don’t tell you…the f-stop is the ratio between the the physical size of the hole and the focal length of the lens…it has the advantage of being a consistent measure of the amount of light that reaches the PSS, no matter the local length of the lens or the setting on the zoom. f5.6 on one lens admits the same amount of light as f5.6 on any other lens, independent of focal length or the maximum diameter of the lens. Neat trick, pretty simple, when you come to think of it…but it leaves us with all these odd fractional numbers and this inverted system of numbering that look really complicated. Too bad. Life isn’t always simple even when we make it simple.)

To confuse matters further, digital cameras don’t always play by rules. They stick f-numbers in between the traditional ones, which represent, sometimes, quarter steps or half steps, in the amount of light falling on the PSS. Pay no attention. Just remember, bigger f-stop: more light, smaller f-stop: less light.

As with shutter speed, changing the size of the aperture changes more than the exposure.

The size of the aperture also effects image quality. Due to the physics of light passing through holes, and the imperfections of the lens making, you lose some detail in the image at the largest and the smallest apertures. For a lens with an aperture range of, say f3.5 to f16, the sweet spot, where the lens is working at it’s best, is generally in f4 to f5.6 range. Before modern zoom designs it was often at the higher end of that range. Actual testing of the zooms on P&S cameras shows that they are generally at their best, by design, nearer maximum aperture (some are at their best at maximum aperture!).

Then too, the size of the aperture also effects the depth of field in the image…or, to put it another way…how critical the focus is. Smaller apertures mean that more of the image, measured in distance between the camera and subject, is in focus at the same time for any given distance setting. Larger apertures mean that only a very narrow band of objects on either side of the set focus distance is actually in focus at the same time.

Controlling the depth of field changes the way the image is recorded and viewed dramatically. Shallow depth of field (large apertures) can be used to isolate the subject and make it stand out from the foreground and background. Deep depth of field (small apertures) gives a more natural view of the world, with foreground and background just as sharp, or almost as sharp as the subject. This shows the subject as part of the surroundings and elevates the surrounding to all but equal importance in the image.

Focused on the center wise man, large aperture puts foreground and background well out of focus, reducing aperture sharpens them without changing focus.

Lets keep it all in balance…

Remember, to achieve correct exposure, you can 1) change the sensitivity of the PSS (ISO), 2) change the length of time light falls on the PSS (shutter speed), and 3) change the size of the hole through which the light passes to get to the PSS (aperture or f-stop).

In reality, an automated digital camera changes all three for each exposure. Once you have reached correct exposure, changing any one of these settings requires a change in one or both of the other two. Increase the ISO and both the shutter speed and aperture can increase. Increase the shutter speed without changing the ISO and the aperture has to go up to keep the exposure within the sensitivity range of the PSS. Decrease the aperture with the ISO the same, and the shutter speed will have to go down. It is all a matter of balancing those three numbers for correct exposure. Picture it this way.

And to refresh, here is a rough visualization of the effect of changing ISO.

Just so you don’t totally panic about now, let me reassure you: This is an article about automation. You do not have to balance ISO, shutter speed, and aperture (f-stop) on your own. Your camera is going to do this for you. You just need to know what the numbers mean and how they are related so that you can understand what the camera is doing…and intervene when needed, either to correct the camer’s choices…which, trust me, is not often…or to achieve a particular effect in a particular image…which, as you progress in your skills, will happen quite a bit.

When it all comes together: depth of field, exposure, and a bit of post processing to recapture the full range the sensor caught (and more on that in a future article: watch this space).