Exposure 101

*This article is part of a series I’m doing with super awesome fellow film lover and Tumblr(er) The Photon Fantastic. We like to call it: "The Photon Fantastic & I Still Shoot Film Present: The Beginner’s Guide to Film Photography" and you’ll notice there’s now a fancy little link on the right that says “Beginner’s Guide to Film Photography.” Here you will find a collection of articles written by myself and The Photon Fantastic; we hope it will help answer a lot of your questions and get you out shooting film as soon as possible :)

Exposure 101:

Now that we’ve gone over aperture, film speed, and shutter speed, we can take a more in depth look at  exposure. In What is Shutter Speed, we saw that exposure can be expressed as a formula:

Exposure = Intensity x Time (or E=It)

This means that if we increase the intensity, we can reduce the time proportionally and still achieve the same exposure. Let’s have a 2nd look at our exposure triangle to better visualize:

If we use the triangle as a guide, we can see that film speed is effected by aperture, and shutter speed. Shutter speed is effected by ISO and aperture. Aperture is effected by ISO and shutter speed. Meaning: they are all related when it comes to exposure. Increase one and you must decrease the other.

Let me give you some examples:

Let’s say you’re shooting with an ISO of 100 and you know that your correct exposure is  1/250 at f/8. Your correct exposure could also be 1/125 at f/11 (1 shutter speed down, 1 stop up) or 1/500 at f/5.6 (1 shutter speed up, 1 stop down)

How does one decide? This is where knowing about all 3 elements becomes important. First off, we know from What is ISO that if you are shooting 100 speed film, it’s because you have bright light and you want fine grain. Now you need to ask yourself if you want more depth of field (a higher f-stop) or selective focus (a lower f-stop). You also need to ask yourself if your subject is moving, which requires a fast shutter speed, or standing still, which does not.

These factors are the reason why there is no exact answer to the question “What’s the correct exposure?” Any variation is correct, which is why E= It. Are we getting it?

18% or “Middle” Gray:

I am going to (once again) quote Ansel Adams to open this semi-complicated subject:

“A reading made from any uniform luminance surface used directly to determine exposure will give exposure settings that will reproduce that surface as a middle gray in the final print.”

What does this craziness mean? First, it means that there is a middle gray. On a geometric scale going from “Black” to “White,” 18% is the mathematical middle. It also means that that middle gray is ALWAYS the same. In fact, light meters are calibrated to 18% “middle” gray - for an intro to light meters, please head on over to my partner in film crime, The Photon Fantastic. You can use this gray to achieve more accurate exposures, with a maximum amount of information in the highlights and shadows.

When we talk about a “uniform luminance surface” that is referring to a gray card. If you place a gray card into the scene you’re shooting and take a meter reading, you are guaranteed to have a middle value. Why is this important? Because sometimes you don’t have a middle value… for example a black and white checkerboard is only black and white. You need to take a middle value to have sufficient white tones and sufficient blacks when printing. [Anyone wanting to know more on this subject: I suggest you take a look at Ansel Adams “The Negative” and Henry Horenstein’s Black and White Photography: A Basic Manual. I also suggest you take a studio class ;)]

Over- and Under- Exposure

Sometimes we don’t expose correctly, even the best of us, and we finish with a negative that is either underexposed or overexposed. An underexposed negative did not receive enough light, and will appear light with a lot of visible film base. An overexposed received too much light, and will appear black. Each has it’s disadvantages, however underexposed negatives tend to have less or no information in the shadow areas, whereas an overexposed negative retains some information in the highlight areas.

Different types of film have a different range of stops that can be over or underexposed while still retaining information:

  • Black and white film: 5 stops (this means you can over or underexpose by 5 stops and still retain information in your negative… black and white film is very, very forgiving)
  • Color negative film: 3 stops (slightly less forgiving, but still a little breathing room)
  • Color slide film: 1 1/2 stops (here you really need to be precise with exposure, which is often why slide film is considered to be “professional” film)

{Note: I will not be going over reciprocity failure, as that totally defeats the purpose of this series being for beginners.}

Please be sure to head over to The Photon Fantastic for An Intro To Light Meters.