Saturday, September 15, 2012

The Science of Photography -- Part Five

Now that we’ve conquered both shutter speed and f/stops, and are well versed in the idea of “double / halve” the light, we are ready to apply these concepts, along with film or sensor speed (ISO), to determine a correct exposure setting.

We will establish the meaning of the often spoken “stopped down” or less often spoken “stopped up,” and will learn to apply that concept to f/stop settings as well as shutter speed and film or sensor ISO values.

Finally we will discuss various concepts and ideas such as half stops and one-third stops. There’s more to be said about exposure control, and in this installment we will say it.

Exposure Control

Suppose you have determined that the correct exposure for the current film or sensor setting (ISO) is 1/125 at f/8. You may have determined that with a hand held light meter, or -- in this day and age -- your camera’s automatic exposure control has indicated these values.

(Now the whole point of all this discussion is to not set your camera on full automatic, but rather to have some control of the settings. This may be done by setting the camera in “Aperture Priority” mode, where you set the f/stop and the camera automatically sets the shutter speed; or you may use “Shutter Priority” mode which is the opposite. There is even a “Manual” mode where you set both. But, typically, the camera automatically sets the other exposure parameters or values to match the one you set manually. In simple point and shoot cameras, these are often called “scenes” which automatically set shutter speed or aperture to extremes.)

(But the point of this series of articles is for you to understand the implications of shutter speed and f/stop, even if you aren’t controlling it manually. So let’s assume you are.)

As I was saying, suppose you determine that the correct exposure setting is 1/125 shutter speed and f/8 aperture. BUT ... there’s more of that “but” -- you want a faster shutter speed. You want to capture the beating of a bird’s wing, for example.

The answer is simple. If you decrease the shutter speed by one stop, to 1/250, then you just open up the aperture by one stop, to f/5.6. (Remember, the smaller f/stop number means more light. You need a bigger “stream” of light to compensate for holding the hose over the bucket for less time -- bucket analogy.)

Or, suppose you’re taking a picture of a moving brook with water splashing over the rocks and you want a very artistic view where the running water is blurred into a steady white froth and only the actual still items in the scene are crisp focus.

Answer: slow the shutter down by five stops to 1/4 a second, and stop the lens down by five stops to f/45. Oh, oh, you say. Your lens only goes to f/22! And you want to set the shutter speed even lower!!  Well, there’s a fix for that too, but let’s hold off on that for the moment.

At least the first example showed, you can adjust either the aperture or the shutter speed up or down as long as you adjust the other control the same amount, in the opposite “direction,” to compensate. That was the whole point of all the half light / double light description of both controls. Here is a chart that you could use in the above example. This chart actually came right off my old hand held light meter.  I would dial in the film ISO on a sort of circular slide rule, and then read off various settings based on the value in the light meter. The slide rule calculator then gave me the combination of shutter speed and f/stop to properly expose the film under those measured light conditions.

  Shutter Speed    1/4     1/8     1/15    1/30    1/60    1/125
  F/Stop               f/45    f/32    f/22     f/16     f/11     f/8

  Shutter Speed    1/125    1/250    1/500    1/1000    1/2000    1/4000
  F/Stop               f/8         f/5.6      f/4       f/2.8        f/2        f/1.4

Basically, you can match almost any shutter speed with an appropriate f/stop to go with this particular light condition and obtain a proper exposure, at least within the limits of the values of aperture available on your lens.

I think you can imagine reasons to adjust shutter speed. You might want a very fast shutter speed if your taking a picture of something in motion which you’re trying to freeze. I remember taking pictures of cars on the drag strip as they passed me back in high school. Unfortunately, I didn’t set the shutter speed high enough and they are all blurred.

Or, maybe you do want a blurry effect like in the artistic photo of a small stream I mentioned earlier or maybe you want the cars to be blurred to represent the speed in contrast with the still background.

Those would all be reasons to adjust the shutter speed either up or down to obtain the effect you wish in the photograph. You then adjust the f/stop to get the exposure correct with that particular shutter speed.

But there are also reasons to adjust the f/stop up or down and then adjust the shutter speed to compensate. You see, the f/stop determines a focus characteristic called “depth of field.” If you want a scene containing near objects and far objects to all be in focus, you use a high f/stop number (stopped down lens), and if you want the opposite, only items close to the lens to be in focus and the background to be blurry -- for example in a portrait of a person, then use a wide aperture setting such as f/1.4 or f/1.8.

I’ll explain this depth of field issue in more detail in a later installment of this series, but -- for now -- realize that you may wish to adjust the f/stop to a particular value and then adjust the shutter speed to set the exposure. They were designed to work together and that is the whole point of all this half / double stuff.

Speaking of half or double, what if you want a finer adjustment and don’t want to reduce the light by as much as half or double? Well, in the old days, due to limitations of mechanical devices, shutter speeds were typically fixed at the values I’ve noted so far. But it was relatively easy for the camera manufacturer to have the f/stops be adjusted by incremental amounts. Usually these values were half of the regular amount, which would be half of a half equal to one-quarter reduction or, on the larger aperture, instead of double (200%) the light, you want to get 150% more light. These were called “half stops,” not to be confused with the fact that the standard f/stops cut light in half. This meant half of the standard stop.

Even finer control was to be had with 1/3 stops. And some cameras even had continuous adjustment with the f/stops just marked on the lens.

In modern digital cameras you can usually adjust the aperture by half or third stops and the notation is usually something like -1/2, -1/3, +1/3, and +1/2. Although I suspect that modern digital cameras can also adjust the shutter speed continuously, I don’t think that is common. Besides, as we’ve learned, either the shutter speed or the aperture can equally adjust the final exposure amount.

(It is also possible to set the overall exposure to higher or lower values when using automatic exposure control. I won’t get into that here, but that is also shown as +1 or -1 or even +1/3 or - 1/2. With automatic exposure control this would mean one stop or a partial stop brighter or darker, regardless of if it uses f/stop or shutter speed to make the adjustment. It’s all automatic in the little computer in the camera.)

Now let’s finalize our nomenclature. When we say “stop down” we mean reduce the amount of light. When you stop down a lens, you are going to a larger number/smaller aperture and therefore less light.

You can do that by changing shutter speed to a faster setting or closing down the aperture ... or even by adjusting the ISO value. Typically, photographers will say “stop down” to mean “one stop down.” You could reduce shutter speed from 1/125 to 1/250. Or you could change f/stop from f/8 to f/11 -- remember: higher f/stop number equals less light. Or you could even change ISO from 200 to 100. That is one stop down in film speed.

Film Speed

About the only thing left to discuss is the ISO numbers. With film, you had little choice but to change the film in the camera to change ISO. You could get film developed in a special way that made it higher ISO, but the typical way to change film speed was to load the appropriate ISO film.

Film came in a variety of ISO values such as 100, 200, 400 and 800. There was also slower film in ISO 50 and 25 and even 16. Just from these numbers, you may have guessed that ISO values are just like shutter speeds. If you double the ISO number, the film is twice as sensitive. (A very popular Kodak film was ISO 64, so the “double/half” relationship wasn’t always true!)

So, in our earlier 1/125 at f/8 problem, let's assume we want to slow the shutter speed down for an effect. We want to change the shutter open time to one-half second. That is six stops up from 1/125. If we assume that was with ISO 400 film, we can change film (or sensor ISO) down four stops to ISO 25 (400 -> 200 -> 100 -> 50 -> 25) and stop down (increase) the f/stop an additional of two stops to compensate: f/16.

We increased the light by six stops with shutter speed, decreased the effect of the light by four stops with ISO, and added two stops of light reduction by closing down the aperture to f/16. That would be 6 - 4 - 2 = 0 change in exposure! With digital cameras which have adjustable ISO, this is common. (Just be careful turning the ISO up over 1,000 or 2,000 because the picture can become noisy, displaying colored confetti in a grainy effect.)

Now we have three parameters to control exposure, while one parameter, shutter speed also controls how moving objects are photographed, and the other parameter, f/stop, controls depth of field. We can “tune in” almost any photograph result we wish.

Now get out there, set those cameras to manual mode, and start adjusting. Artistic photographs have blur and movement and crystal clear focus. All are under the artist’s control.

We’re not done yet. I actually have more to say about f/stop and a lot to say about depth of field. But this is all for now. See you next time in installment six of “The Science of Photography.”

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