This weeks blog i'm going to take a look at the Histogram, What is it ? Why should you be using it ? When to use it ? And how it can improve your photography.
First of all don't be scared by the histogram, It really is your friend and can tell you a wealth of information about your photos and what is missing and how to improve them.
Yet it seems to be rarely used by a lot of people as it sounds and looks fairly complex, But in reality its far from that. I rely on it to give me feedback about the complete range of information in any image.
What is a Histogram ?
Ok so the basics of the Histogram are actually quite simple, It enables you to see exactly where the information in your file is.
You will generally have four Histograms on your camera, Luminosity (the value of light) The Red Chanel, The Green Chanel and the Blue Chanel or RGB as you will hear them more commonly referred too.
Most digital cameras will actually have the ability to show all of these on one Histogram so the information is visible for easy reading much like the above image.
The far left of the Histogram represents true black and slightly further in, the shadow information, The far right shows the true white information and slightly further left the highlights, everything in-between is your mid tones.
It can give you all the details you will need to tell you if your exposure is indeed correct and what you're looking for. But wait, why not just look at the image on the review right ? Well the human eye is good, but it's not that good that it can give you as much feedback as the Histogram. You can tell so much more and it will help you when you come to process the information you have.
For example the graph below will show you a few scenarios of an image that has been either over or under exposed or the holy grail, correctly.
How To Use A Histogram ?
If we look at the over exposed Histogram you will see it is all the way over too the right and hardly any information in the left.
This means that your whites have "clipped" meaning they have gone past true white and have blown out, Thus there is going to be a big ugly white patch somewhere in your image. The under exposed Histogram shows that you have no highlight detail in your image and some serious clipping on the blacks, so typically some very large completely black areas with no information in.
The normal image shows that you have a good deal of information in your mid tone section, and your white/highlights & your black/shadow information is good, You haven't clipped them and your image should have a large amount of information available to you.
Now each scene will dictate how the histogram behaves, if you are shooting into direct sunlight you will have very contrasty image, depending on how you expose it of course.
The camera will see areas of heavy bright luminosity and therefore will try to under expose to allow for it, So you might get the sunlight exactly how you want it but you will be left with heavy clipping of the shadows as it under exposes everything else. Of course camera metering has come on in leaps and bounds over the years but it still can't get it bang on every time. The Histogram will be your equaliser and tell you if you need to up the exposure or lower it.
How To Read A Histogram ?
The above pair of images show a pure black and pure white oblong and a Histogram that matches it, you will see there is information on the left side (Blacks) and the right side (whites) only and nothing in-between.
I have used pure black and pure white as this example, your camera will never really produce a Histogram just like this as its and extreme situation.
The above image here shows a progressive gradient from pure black to pure white with all the greys in between and you will now see the Histogram is showing a lot more information, Now you will start too see why this is such a valuable tool !
Above you will see an actual picture and the Histogram that relates to it, you will see that there is detail all the way through from left to right. The black information isn't clipped (it could in fact have some more darks in it) and the highlight information isn't clipped, there is no pure white in this image.
If anything it could probably benefit from a slightly higher exposure. The mid tones have a good amount of information in them and they don't clip the top.
This is typical of a scene and an exposure when shooting with the directional light behind you.
The image above is the opposite end to the spectrum, it shows an image that has far less detail in and some highlight clipping (not afraid to show you these mistakes) You will see from this Histogram that on the far right there is some clipping, That is represented by the sun and its reflection on the water. It has blown out and therefore has no detail in it at all, it is nothing more than a white mess on the picture. The mid tones are showing nicely and as i'm sure you will appreciate shooting directly into the sun is an exposure nightmare but I've used this to give you and example. The image could have also down with some more detail in the shadows as it's missing from the left, So from the information in the Histogram we know I should have under exposed slightly to give me a better overall image.
Clipping a Histogram
You will have to accept that some scenes will cause clipping if you insist on shooting them and lost information that no matter how heavily you process it there will be no recovery as the information is simply not there to extract.
These scenes from past experience are better avoided, but we all know the sweet temptation of shooting directly into the light ! It can produce amazing results but be prepared for clipping at either end of the Histogram.
Most digital cameras will have a warning system for this and when you switch it on your clipped highlights will flash or blink at you (as with the clipped shadows) I highly recommend you turn this on as it's an instant indication that you have clipped and lost all detail in that area, In print it will appear as pure white or pure black only and you won't be able to recover it.
Very tiny areas of clipping are ok really and won't ruin an image but your aim is to have a perfect exposure, and if you have any of the dreaded blinks then you won't have achieved that.
A point worth noting here is that if you shoot in RAW (which you absolutely should) then it has a higher dynamic range than a JPEG and whilst it might show a minor clip in RAW it possibly wouldn't show it if it was shot as a JPEG. A little known fact is that your cameras Histogram is actually based on a JPEG that is hidden in the RAW file, not the actual RAW file itself. This means that you might show a fraction of clipping on your cameras Histogram but you might actually be able to pull the detail out of a RAW file but as always, try and get it right at the time of shooting and not rely on post processing to correct the image.
ETTR - Expose To The Right
You may well have heard of a technique called ETTR or Expose To The Right, This is standard practice for a lot of photographers although it isn't without its own controversy.
The general principal is that you expose the scene as brightly as you can without blowing out the highlights, then in post processing you can reduce the exposure thus adding towards the dark area of the Histogram and pulling in a better overall image. You will reduce noise by doing this, you should have a better tone to the image too and richer more vibrant colours and a better dynamic range.
A point worth noting here is that ETTR is based around using the lowest possible ISO on your camera i.e. 100 on most and on a few that have expanded ISO capabilities then ISO 50 (not actually recognised on the scale) so it is referred to a different name on some cameras, for example Canon refer to it as ISO L (love you Canon)
Exposing to the right has it's own issues, obviously you don't want to clip your highlights and pure whites, if you do there is no recovery and it was a pointless exercise, the idea is to gather the absolute maximum information without taking it too far.
Anyway I hope this gives you a bit more of an idea about the Histogram and the benefits it can bring to your photography.
I highly recommend you use it when you next shoot and understand what its telling you, and when you can push your images a little bit further.
As always, Happy shooting.
Daniel Wretham
