Using the Histogram for Exposure Control

This article can also be found in Swedish here.

One of the most useful features of the digital camera is the histogram. This is a diagram that shows how you shot was exposed after you take it. It is invaluable to avoid over- and under exposure, but also when shooting in manual mode because just looking at the LCD is most of the time very difficult to see exactly how the exposure works.

How to find the histogram

This is different on different cameras. You may have to consult your manual. On most NIkon digital bodies you can do the following. Take a picture and press the ”view” button so that it shows on the LCD. Then use the 4-way button, keep pressing the ”right” on this button and it will cycle through various modes showing different information. When you see the following you have found the histogram:

What the histogram shows

D70 Histogram Page
D70 Histogram Page

As you can see the histogram shows the distribution of dark and light pixels in the picture. Some camera models has separate histograms for red, blue and green. My D70s has a single histogram showing the combination of these colours.

Notice the vertical red lines in the histogram? They are ”f/stop” indicators. An f/stop is a doubling or a halving of the light that comes in to the camera. Looking at this histogram you can see that we are exposing pretty well. If the yellow curve is squashed to the left hand side it means that the picture is under exposed. If the histogram is squashed into the right hand side then you are over exposing the picture and need to decrease the shutter time or stopping down the aperture a bit.

How to compensate when you read the histogram

The way it is organized is that usually three ”ticks” of the wheels that controls exposure / aperture is equal to the distance between two red bars. If you want to move your exposure curve up one ”f/stop” (or 1EV which is a terminology I like better, it is the same thing, EV means Exposure Value).

If your histogram shows 1 EV underexposure then you can compensate by either

  • Increasing the shutter time 3 ”steps”, this should double the exposure time, for example if your exposure time is 1/250 s then 3 steps longer means 1/125 s.
  • Opening your aperture, this means lowering the f/stop value, if your aperture is f/8 then three steps down is f/5,6.

There is also a third way to compensate for under exposed pictures. You can increase the ISO setting of your camera. The D70 camera normally works at an ISO setting of 200. Increasing this to 400 represents 1 EV also. Increasing to 800 is 2 EV.

How to work in manual mode

Start by setting reasonable settings. On a sunny day you may wish to start with say f/8 and 1/500s. When you look in to the view finder there is a coarse over/under exposure meter. It looks a little like this:

 3     2      1    + 0 -    1      2     3
< | | || | | || | | || | | || | | || | | >

There is a bar that shows how your current setting reflects the cameras own meter. If the bar is growing to the left (+) then you are over-exposing. Rotate your thumb or index finger wheel to change the shutter time or aperture until you are close to the middle (0) where exposure is judged to be correct by the camera. The numbers above the bar indicated EV or f/stops.

Take a picture.

Then take a peek at the histogram. How does it look? Does it look nice and centered or is is definitely sqashed to the left or right? If it seems that the exposure is wrong, try to judge by the red vertical lines how much. Then decide if you want to compensate by using shutter speed, aperture or changing the ISO setting.

When you have found the right settings this way take the picture you want. After each exposure check the histogram, compensate as you go, keep shooting. This way you have full control over the exposure in the camera.

This may take some practicing to get used to and to understand properly but after a while it will become second nature to you.

When to use manual metering

If you wish to take infrared pictures then you must practice this because with the IR filter on the camera it is not possible for the camera meter to work properly and this means that you can not use the cameras automatic metering. And because in difficult conditions when you are shooting the camera normal exposure compensation functions may be wrong, manual whooting will give you better pictures once you are used to it.

The only drawback here is that it may take a few seconds for you to find the correct setting for a certain scene to photograph. So if you are shooting some action shots you may be better of using the camera own metering.