Reading the histogram

From the reactions to my previous posting I got that I went a bit too fast with the information. So here I come back to the histogram. This is a very important tool for the serious photographer: it gives information on the exposure, and much more than a light meter can.

In situations where we work with available light, the automation of a modern camera gets it right in most cases. Only when we want something outlandish like the photo of Vivianna further down the page, the meter does not understand what we are doing. It will see a lot of black, and decide that the photo is too dark. Then it will compensate, and the photo that comes out is exposed average, which is not what we want, we want something very dark. The same goes for very bright photos like the lake in morning mist on this page, in the opposite way: the automation will make them average, which is darker than we want.

When we use on-camera flash, the flash and the camera communicate. We can put the flash on ttl, and the camera on an automatic mode like P (for program), S (for shutter speed), or A (for Aperture) and the light meter and flash together will calculate an acceptable exposure. A modern camera considers the distance you have focused (it presumes that is the distance you want to have average flash exposure) the available light, and if you use auto-focus it also looks at the area where you have focused, presuming that area should have average exposure. Then it sends out a pre-flash, and meters how much light comes back from this pre-flash, quickly calculates an average exposure and about 1/10 of second later makes the photo. You hardly notice this, but if you observe carefully, you can see that the camera sends out several flashes. The last one is the one for the actual exposure. If you trigger your off camera flash with an optical trigger, the off-camera flash will fire at the first (pre-) flash, which is before you make the photo. So if you use an optical trigger, you must set your on-camera flash on manual. Then it does not use a pre-flash, and your off-camera flash will fire when you make the photo.

Also, the camera’s light meter cannot know how much flash your off-camera flash is going to give. So it cannot calculate exposure correctly in this situation. That means you will have to set the correct exposure manually. For this you make a few test photos, to check the exposure.

histogram

On the lcd screen on the back of the camera you get an impression of the way the photo looks. The image quality there is much less than a computer screen, and a computer screen has much less image quality then a good paper print. So you get only an impression. On top of that, the lcd screen looks different in different available light: in bright light the lcd screen will be overpowered. If you try to see your work in bright sunlight, your lcd screen will not be very clear. You need to move to the shade. If you view your lcd screen in dark circumstances, it will look even brighter. This means, from your lcd screen you cannot judge if the exposure is correct. For this the camera manufacturer has given us a great tool to judge exposure: the histogram. This tells us much more than a light meter does, so digital is much better for off-camera flash than film. (Off course: many photographers with lots of experience have made amazing work with off camera flash on film; digital makes it faster, easier and more user friendly)

The histogram shows the dark tones on the left. If the histogram reaches up high, there are a lot of them, if the histogram is low there are few of them. It does not show where in the photo these tones are, and it does not show colour. If you want to know about the colour, many cameras can show separate histograms for red, green and blue, I will not go into that now, you do not often need those.

I made a photo of two young giraffes. When we look at the histogram, we see that it touches the left side of the diagram at a low level. This means there are a few flat blacks in it. They are in the eyes of both giraffes, in the horns of the front giraffe, and the tail of the back giraffe. These are small areas, so the histogram is low on the left side. The histogram just tapers out on the right, which shows there are few very bright tones in the photo, and no flat whites. The brightest tones are in the white lines on the back, neck and ears of the front giraffe. The histogram is high in the middle, which shows there are a lot of middle tones in the photo. This is a fairly typical histogram.

lake in morning mist

In the photo of a lake in morning mist, you see there are no dark tones. The histogram shows values on the right only, which represent the light tones. The histogram shows the bright character of the photo. If you process the image through photoshop to give it a full range of tones, the feel of early morning mist disappears, and the whole point of the photo is gone. In this case the histogram with values only on the right shows correct exposure.

Vivianna

In the photo of Vivianna on a dark background, the histogram shows values mostly on the left. There is a large black background, and black areas in the pants. It shows some middle tones, which represent the grey floor, the shirt, and the skin. On the right the histogram shows a few values, which are the hi-lights on the side of her face, arm and body. These bright areas are small, so the histogram stays low on the right side.

fishing boat

In the photo of the fishing boat returning in the sunrise, you see most values on the left or the right with few in the middle. This shows a high contrast photo. The values on the left are the black boat and dark water in the foreground. The high values on the right represent the bright sky and bright reflections in the water. The values in the middle are the bluish clouds and the middle tones in the water.

In the next installment I will explain how to use the histogram with off-camera flash.