When I was starting out as a photographer, a fledgling beginner trying to make some sense of all those buttons, knobs and wheels on my DSLR, I had an interesting discussion with a senior of few years. He asked me, “Do you understand what a histogram is?” And I replied in all my honesty “not really.” What I got as a reply was a 10 minute lecture on what these graphs meant and how we can use them to reassure ourselves of a properly exposed picture even when the rear LCD says something different.
Just so that you know what these really helpful graphs are and can use them in real life situations, I have prepared this short write-up for beginners.
What are histograms?
Histograms are the graphs that appear when you check the details of any picture on the rear LCD screen. Most DSLRs would have them and these days even the higher-end compacts have them too. When you look at them they appear like a wave that extends from left to right of the LCD screen.
The left side of the screen represent the blacks and the right side represent the whites. By black and white I mean the extremes of tonal range that a standard DSLR can capture. A sensor which can record images in 8-bit can in effect record 256 levels of brightness, starting from 0 and ending at 255. It is like the same numbers on Photoshop, 0 meaning absolute black and 255 meaning absolute white.
The x-axis of the graph represents the tonal range from 0 to 255 (absolute black to absolute white) and the y-axis of the graph represents the extent of the image that has a particular tone. If the graph is more right-sided, it means the picture is over exposed. On the other hand if the graph is more left-sided it means that you have more underexposed areas in the image.
Let’s imagine that your camera sensor can record only one pixel (hypothetically). And that one pixel records a tone which is absolutely black. Where do you suppose that pixel will be represented on the graph? If you guessed it right, congratulations! Yes, it will appear on the far left just where the two axis coincides.
How to turn them on?
Every camera has a different way to turn on histograms. Nikonians and Canonites will find their info buttons set differently. So, refer to your manual and find out how to reach it. Pressing the button will immediately pop-out more information about the picture you have taken.
Why it is risky to rely on the rear LCD all the time
Constantly relying on the rear LCD to confirm whether we have taken a proper exposure is a risky habit, one which the sooner you drop is better. The reason is many times the brightness levels of the rear LCD can miss-represent a picture to be properly exposed, when the fact is that it is actually under-exposed.
Sometimes this over-indulgence on the rear LCD can create embarrassing moments after we have completed the shoot and went home. When the images are finally downloaded on to the computer we fail to make two and two together as to why these images, which looked perfectly okay on the LCD screen, now appears visibly under-exposed.
Why relying on the histogram is good idea for all your everyday photos
Your everyday photos are probably shot using nothing more than the Auto mode. You may even use the ‘convenient’ scene modes to accurately expose for a scene. This is a good thing especially when manually setting the right exposure may be difficult and time consuming.
Let’s say you are photographing your kids playing in the yard in the light of the setting sun. The light will be changing every minute, so much so that it would become impossible to manually make the changes in settings and then properly expose for the scene. In such circumstances it would be ideal if you set the camera to Av or even Auto mode and let the camera decide on the shutter speed or of all of the exposure details. The histogram will tell you if the camera got the right exposure for the scene.
Limitations of the histogram, when it is a good idea to ignore them
However, when you want to deliberately capture an underexposed or an over overexposed scene, you may want to avoid what the histogram reading says and rely more on your eyes. Let’s say when you are photographing a bride getting ready on her big day. There is a large window in the room and it has an abundant quantity of sunshine. The histogram will be right heavy in such conditions, which in fact you want to. This is a classic case of when you should disregard the histogram and instead choose to go by your own intuition.
Again when you are shooting a predominantly white or black scene (let’s take the example of the bride above) the scene can be metered incorrectly by the camera and that means the accompanying histogram would also be incorrect.
Why this happens? This happens because the camera sensor is designed to make everything 18% grey. 18% grey is the name given to a tone that is right in the middle of the luminosity chart, absorbing all but 18% of the light that falls on it.
When you shoot a predominantly white scene the camera will meter the scene to be too bright and try to underexpose it to achieve the 18% grey. Ideally what you need to do in such situations is to judge the scene by your eyes and overexpose a stop or two (as required) so that you achieve the white you are looking at.
The reverse happens when you are shooting a predominantly black scene. In such a situation the camera will try to compensate for the all the black it sees and try to achieve middle-grey. The solution is to disregard the histogram (which will show middle-grey) and underexpose the scene so that you get the black you are looking at.