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Understanding Photo Histograms
They're more useful than they look!
|By David Fleet|
Photo histograms might appear fairly technical and complicated on first glance. But actually they couldn't be simpler and are a really handy way to help you get the right exposure in your shots.
Often one of the questions at the front of people's minds when they're taking a photo is, what's the correct exposure? The subject of exposure can sometimes be needlessly overcomplicated. Ultimately, getting a technically 'correct' exposure simply means capturing a scene in a way that represents what you saw.
So there's no universal gold standard to aim for. But the histogram is an incredibly useful tool to have at your disposal - providing a quick and clear visual insight into the exposure of a photo.
What is a photo histogram?
The histogram is a graphic, which you can view both on the LCD screen of your camera and in post-processing software. It represents all of the information that you captured in your photo ranging from dark tones - on the left - all the way through to the highlights (the brightest parts of the scene) - on the right.
A 'well' exposed image will generally have captured information in the dark areas, the mid-tones and the highlights. So you would see a nice even spread of information, as seen in the image below.
What does the above photo histogram tell us about the image? Well, on the left you can see a peak in the graph, which represents the patch of black in the photo.
There is a lot of information in the middle of the histogram, representing the mid-tones - which include the grey colour of the box, the blue patches and the left and right side of the flooring at the bottom.
This photo doesn't have particularly strong highlights, as you can see by the way that the graph descends on the right hand side. The small amount of highlights are represented by bright patches on the word 'Canson' and in the middle of the flooring, where bright light is reflecting back towards the camera.
What does an underexposed shot look like on the photo histogram?
You can quickly see when a shot is underexposed because
all the information in the histogram will be on the left hand side of the graph, whilst there'll be a lack of information in the brighter tones on the right hand side. An example of this can be seen below:
Here you can see that the histogram is bunched up to the left, with no information captured in the highlights. Notice the white on the box in the image, which if better exposed would show up on the right hand side.
Here's the same shot properly exposed:
The key to getting a good exposure is to try to ensure that you capture as much information as possible, from the whole range of tones - highlights through to shadows.
This actually gives you the maximum room to manoevre later on in post-processing. The more information in a photo, the more you have to play around with when fine tuning things in Photoshop or Lightroom.
Plus, if you capture a nice range of tonal information, there will be smoother gradations between different colours. This helps to avoid ugly 'banding', where the transition between different colour shows up as an obvious line.
Avoid clipping the highlights
'Clipping' the highlights refers to overexposing the brightest parts of your image, so that no detail is recorded at all. These areas are often referred to as 'burnt out'.
For most shots, clipping the highlights is a big no-no! It's well worth avoiding if you can, because higlights simply show as lurid bright patches without any detail whatsoever.
To make sure that you are not clipping highlights in a shot where there's a risk of doing so - have a look at the LCD photo histogram after taking the shot. If the graph shows lots of information bunched up to the right edge, then you will want to adjust your exposure accordingly.
In the above example you can see that there is a spike of information on the extreme right hand side, which looks like it has been pushed up against a wall. This is a sure sign of clipped highlights! When you take a look at the image you can see large bright areas, on both the side of the house and in the sky, where little to no detail has been recorded.
Expose to the right
So, for a good exposure with a range of tones, we're looking for an even spread of information across the photo histogram. But there are, in fact, some quite good reasons to err towards exposing to the right.
This means capturing as much information as possible from the brighter parts of the scene, without ever actually clipping the highlights. Why...?
Firstly, digital cameras are built with the ability to capture much more information from the brighter areas than the darker parts. So exposing to the right will allow you to maximize the details in a scene.
Secondly, it gives you a much better chance of fine tuning exposure effectively in Photoshop or Lightroom. If you have to slightly darken bright areas that's no problem. But lightening dark patches very often produces the side effect of 'noise' - or graininess.
So I hope that helps you to understand what photo histograms are and how to use them. To sum up:
- There's no such thing as correct exposure. It's your creative choice, and completely depends on the 'mood' you want to create.
- The photo histogram can be viewed on the back of the camera and in post-processing software. It's a really handy way to quickly get an idea of the tonal range in your images, and whether you might need to make any adjustments (such when highlights have been 'clipped').
- Generally speaking, you'll get the best results when you slightly 'over-expose' a shot, or expose to the right. That's down to the way cameras are build to record information, and the risks of lightening shadows in post-production.