The primary tool on my camera is by far the histogram and the reason for this is that it shows the graphics of the image in the best way possible. There will always be a difference between the screen on your camera and the actual image regarding color and light, what is seen on the screen may be off for a number of reasons as direct sunlight, screen settings, quality on the screen itself etc.
However the histogram will give you the detailed information to evaluate the shot and help you settle with what you have or trying for a more optimized shot. So the question is not when to use the histogram, it should be used on all images captured.
In the images below take a close look at the white histogram (light), it will be difficult to get this all the way to the right side without creating a burnt out red color. The color channels may be adjusted in most software, so the blue and green may be adjusted later on if needed.
In the image above I have chosen to use the separate Red, Green and Blue color histograms. This way it is possible to detect if any of the colors are “burnt out”, the left side shows the dark colors/areas of the picture and the closer to the right edge the brighter the image/colors. In most image softwares each color channel can be adjusted separatly to make a more acurate image.
If the bars of the histogram climbes all the way up the left side it shows that the image do have areas so black that a image software will not be able to recognize details in the areas. If the bars goes all the way up on the right side, the situation will be the same although for the bright areas on the image. If there are no grey left in the areas, a image software (photoshop or similar) will not be able to show details within the areas. The software will try it’s best however structures (like the trees in the image background shown above) will be seen as a grey area without contrast rather than separate trees if any light is adjusted after taking the picture.
To show you the difference in increasing the exposure by 1.33EV this is the difference:
The first image have an exposure that is to dark and it will be difficult to get the colors correct at any time. If I had made the histogram look like the second one when making the picture, it would have had a much better base for print and color correction.
So my solution is always to make sure to frame the image correctly, and check the histogram on the camera. The further the histogram is to the right side, the more color information will be available to a software. Be really picky about this if shooting in a auto program and adjust the EV+/- on the camera before taking another picture. There will also be a different experience with different camera. My D3 was pretty much spot on with measuring light, however the D800 has to be adjusted. My D800 normally puts the histogram to far to the left (dark image), so it is almost without exeption that I need to add light to the picture by adjusting shutter, aperture or ISO.
This is also the first explanation regarding file format, I will get back to file format in another blog, a camera with the ability to catch more gray tones and this way give a software better chance to adjust more details in the image.
After taking a picture like the one above, my priority is to: Check the Histogram and then check to see if the camera settings gives me the expected effects in the picture. If I decide on calm water without waves and the screen on my camera shows water with the waves then I need to change the settings before trying again.
My approach to photography has always been to do more work when taking a picture rather than base my outcome on time spent in front of a computer. So a perfect histogram is always a goal since it gives a image that in some cases may be printed directly, rather than spending my time on a imaging software. And since the true outcome does not show until printed (unless your monitor is very well calibrated), the histogram will show you the truth before you need to waste paper, ink and money on multiple prints before the good print shows up.
The easiest way to adjust more or less light will often be with the exposure compensation button on your camera. (normally shown with a +/- symbol on the camera. If the histogram is to far to the left, adding a bit of light slides the histogram towards the right side and by doing so your work will be easier. So when adjusting the settings, make it a goal to get histogram heavy on the right side, or if perfect, looking like a mountain.