Tech Thursday #1

I’m a huge nerd. Really, I am. That right side of my brain… barely works. It’s part of the reason why I do photography because it uses a part of my brain I rarely get to. I sit in front of a computer daily and crunch numbers or write reports about the environment… yes, the glamorous life of an engineer! However, being so left-brained does have its advantages. Technical things come much easier to me. So I thought, why not share this technical knowledge with people. Makes sense to me. So every Thursday (or every other Thursday) I’m going to pick a technical or technology related photography topic and talk about it. At least I know I can do that much. πŸ™‚

Topic #1 – Exposure compensation and why everything “goes gray”

Have you ever taken a picture of a snowy scene and found that it turned out dark… or taken a picture of something black and it turns out too light? I have!!!

Well, your camera has advanced hardware that helps calculate what the exposure of a picture should look like. As all photographers know, exposure is a combination of ISO, shutter speed, and aperture but we won’t go into those details today. We’re going to talk mostly about why things go “gray” and how to prevent it from happening.

Now it’s important to note here that what I’m talking applies for those that shoot in aperture/shutter priority mode which I’ve found that a ton of photographers do. It’s also a good starting point for learning how exposure works since you only have one variable to work with (either aperture or shutter speed).

When your camera sees a white scene, it tries to make it gray. Most people will say 18% gray (I have in the past) but it’s actually somewhere between 12-18%. The way to check is to pull up your histogram and see where this spike is.

When you look at your histogram, you’ll notice a spike in the midtone area (middle of the histogram). This is approximately where your camera thinks is 12-18% gray.

And here’s an example for ya. This is an image of an X-rite ColorChecker Chart.

knoxville wedding photography

Each image below was taken in aperture priority mode (meaning no changes were made to the aperture or ISO). The camera calculated what shutter speed was needed in order to develop a “proper exposure.” The images shown below are in order from bottom left corner white square to the bottom right corner black square.

knoxville wedding photography
knoxville wedding photography
knoxville wedding photography
knoxville wedding photography
knoxville wedding photography
knoxville wedding photography

Notice anything? Yes, they all look similar or about the same (small differences caused by human error). Again, this is my camera calculating where 12-18% gray should be. In order to combat this, we need to use exposure compensation. Most modern DSLR cameras will allow you to adjust the exposure compensation by +/-2EV (may vary by camera model). On my Canon 5D Mark II, in order to adjust the exposure compensation, I turn the back wheel on my camera to the right to increase exposure compensation and left to decrease it.

Since everything wants to go to this middle gray, if I’m shooting a snowy scene that is mostly white, the camera will try to make the scene darker in order to get this 12-18% gray. In order to combat this, I’m going to tell my camera to compensate the exposure by turning my back dial +1EV. Now +1EV may be the right amount… it may not be. I’m going to look at the back of my camera at the histogram until I see the big spike move over into the right side of the histogram without it clipping. Another thing I can do is turn on blinking highlights and make sure that I have not lost any highlight detail (usually alternates between blinking white and black).

Now we’re going to do the reverse with a dark scene. Take for example the image above. The beautiful Emily Scheuneman is sitting in a dimly lit room by the window. I’d say 2/3rd of the image is dark and 1/3rd is light. Using evaluative metering, the camera is going to try to see gray… meaning it’s going to try to make the darker areas brighter and brighter areas darker. Since the scene is darker than it is light, the camera will try to make the whole scene a little brighter (in order to make the black go gray)… but we don’t want this. So what I’m going to do is tell my camera to compensate the exposure by turning my back dial -2EV (again I’ll look at my histogram to see where the spikes are). Since I don’t really care too much about what’s in the dark areas, I’ll clip the darks (push the histogram to the far left) in this case.

Now wait a minute, John… how come I just can’t adjust my aperture to change exposure? Well, when you’re shooting in aperture priority mode, whatever changes you make to your aperture, the camera will adjust your shutter speed accordingly. So if you’re at aperture f/4.0 and shutter speed 1/200s and you adjust the camera aperture to f/2.8 (1 stop brighter), the camera will automatically adjust your shutter speed to 1/400s (1 stop darker) to compensate for your aperture change. Hence why you have to use the exposure compensation dial/button/menu option in order to adjust for a dark/light scene.

Sometimes you’ll run into limitations of +/-2EV and this is where shooting in manual comes in handy… but we’ll save that for another day.

Have a question? Post it below. πŸ™‚

Thanks for looking!

John – Contact me!

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