THE HISTOGRAM A PRIMER

While digital photography has many benefits over old school film photography, I would have to say the histogram is one benefit that’s near the top of my list. I have decided to do a large post about the histogram and why it is such a great tool and more importantly show you how and when you should bend it to your will.

The histogram is simply a graphic representation of the tones in your image. With the light tones in your image placed on the right side, the dark tones on the left and all the mid-range tones being in the center area. Your goal is to be sure the tones in your image are in the right place on the histogram. The histogram accompanying the image above is made up of mostly mid tone greens with a small amount of dark and light tones, all of which are properly placed in the attached histogram. So… let’s say you made an image of a field of medium green grass. The histogram for that image should have a band going from the bottom to the top in the middle of the histogram (the mid-tone section). Note it doesn’t matter if the graph touches the top that just means there is a lot of that particular tone in the image. Now let’s say you made an image of a raven on a dark stick with a background of pale beach sand. That histogram would be shaped quite differently it would look like a U with no spike in the middle (no mid-tones) but a spike on the far left representing the dark raven and a spike on the far right showing the light tones of the beach sand.

Back when I shot film I always had my camera set up to shoot in manual mode, using the in camera spot meter. The reason being I wanted absolute control over the exposure, I could never trust the camera to make as good an exposure as I could using my methods. But now since I have a histogram, and more importantly the ability to check that the camera is recording all the tones in the image properly. I now feel comfortable shooting in aperture priority using matrix metering. But only because I have a histogram and can confirm what the camera is doing. If I find a need to fine tune the exposure I dial in an exposure compensation.

Before we get into how and why you might need to dial in an exposure compensation we need to understand one thing. Your camera’s meter is stupid. It has no idea what it is pointed at. So it makes a guess and it guesses that everything it is pointed at is a mid tone, medium green (a football field), medium blue (the blue sky at noon), medium red (a stop sign), etc. This works most of the time except when you are pointed at something that is not a mid tone.

In the shot above of the bufflehead duck, the background and most of the duck are dark. When I made the image as metered the camera placed the tones as mid tones and made the first exposure. Which is over exposed, the blacks are grey and the whites are blown out with no detail. This is reflected in the histogram. The blown highlights are the spike on the far right edge of the histogram. Spikes on the right indicate blown highlights, light tones with no detail.

So how do we correct this? For this shot we need to pull about 2/3 of a stop of light from the exposure. How you do that depends on how you are shooting. If you are shooting in manual exposure mode you would pull light from the exposure by stopping down the lens 2/3 of a stop, or you would go to a faster shutter speed to take away light. If you are shooting in program mode, aperture, or shutter priority modes, you would need to dial in an exposure compensation, in this case minus 2/3 of a stop. This pulls light from the exposure, which will darken the image and bring your whites back, and give you nice rich properly exposed blacks.

How much compensation you dial in depends on how dark your subject is. Let’s say I was making a full frame image of that black wolf face. My camera would try to place that black as a medium grey. So I would need to dial in an exposure compensation of minus 1 1/3 stops to hold light back from the exposure keeping the wolf black. Knowing how much compensation to dial in is something that comes from experience, but thankfully we now have a histogram to double check what our cameras are doing. Because after all, they are just a dumb black boxes that let a certain amount of light in.

Lastly we are going to talk about how to properly expose for light subjects. Remember when you point your camera at a subject it assumes everything it is pointed at is a mid-tone. So when your camera is faced with a situation like the arctic wolves in the snow (a lighter than mid-toned image), it is going to underexpose the image in when your camera assumes it is a mid-tone. You can see this in the first image. The camera’s attempt to recreate a mid-toned image has resulted in an underexposed shot, the colors are muddy, and the snow is gray not white. We need to override the camera. Since we are pointing at something that is predominantly light in tone, we want to tell the camera to over expose the shot to keep the tones light and capture the snow as white. In order to do this we want to add light to the image.

Once again how you do this depends on what mode you are shooting in. In manual mode you would add light to the image by either opening up the shutter or slowing down the shutter speed in this case by 1 stop. In program mode, shutter or aperture priority modes you would need to dial in an exposure compensation of plus 1 of a stop, this will add light to the exposure resulting in a properly exposed light toned image.

Good Luck and Good Light!

Steve