Monthly Archives: October 2010

Understanding Histograms and Exposure

People take 1000’s of  digital images that are perfectly exposed, without ever knowing or even bothering about a thing called a histogram.  Almost every camera can display one, certainly every digital SLR can, so what’s all the fuss about and why do we need to know more about it?

Everyone knows that photography is all about light;  ambient, directional, artificial, or whatever – your camera needs light to take pictures. In an ideal world, or in Florida :-), light is plentiful and is also of a high quality, so you might think why worry.

I like to think that digital photography is about expression and the ability to be more creative with your skills and subjects. When you’re photographing wildlife in particular, there is often very little time to think about what you are doing and to make lots of adjustments. A bird may fly in front of you, then veer away suddenly into the sunlight, or across a very dark background. The early morning Red Deer shot is made better by shooting into the sun and having the deer backlit.  The reflection of the water on the reservoir makes capturing the image of the Swan more difficult, and so on.

So why not just switch your camera to automatic and be done with it?

Well, you can do that and sometimes it will work and yes, sometimes it will work to your advantage. However, it may be better if you didn’t have to leave it all to luck rather than judgement.

You will find that most, if not all wildlife photographers, have a 2-4 second preview of their shot appear on the rear LCD screen of their camera. A few will have the image filling this screen and can spend time “chimping”, or scrutinising the image on this screen. I have regularly done this myself – feeling excited that I’d bagged a good shot – well exposed and sharp, only to find that once on the computer at home, feeling deflated as I saw the poor results of a day’s work.  Except in a few circumstances, you cannot gauge a good picture from the view on the LCD screen.

So let’s change this habit and display the Brightness Histogram, with only a small preview of the image itself, but before we get out and take some pictures, let’s look at some details.

What is a Brightness Histogram and what does it look like?

This is an extract from my Canon 1D Mk3 manual – this is the rear LCD set to show information on the image taken, rather than just the image itself. You will all have something similar on your cameras. When you get a new camera, it’s always tempting to get out and take pictures straight away – nothing wrong with that. But when you get back in the evening, take some time to read sections of the manual, especially the more difficult ones.

As you can see, there’s lots to look at – we are interested in the upper right hand of the screen marked “Histogram”.  Before we go any further,  please remember this one important fact – the brightness histogram is only a graphical distribution of the brightness of an image – it is NOT the brightness of the image looking from from left to right.

Again, from my Canon manual:

What the above is saying, is that it would seem to be better to have an image with a “middle of the road” distribution of brightness – not too many dark areas and not too many light areas. In fact, for most cases that would result in a correctly exposed image. Too much dark will reveal digital noise, especially if you need to lighten the image at all – this noise may then become problematic. Conversely, too much light and the pixels stack up towards the right and you risk “blowing” the highlights, or burning the whites out. In most cases these burned-out areas will show little detail and generally cannot be retrieved satisfactorily.

We will look at how you actually achieve a balanced histogram image later on, but first we need to examine it a little more.

The amount of brightness normally covered by the width of a histogram is approximately 5 F-stops of light, which is about the extent of the dynamic range of most decent camera sensors. So a “normal” centre-peaked distribution will cover approximately 4 stops of brightness. Also, remember that camera sensors see mid-tones as an 18% grey-shade, so this “normal” distribution will render a larger proportion of mid-tones correctly for your image.

Typical “Normally” distributed image – not too much light and not too much dark

Left/Darker” distributed image – note the dark areas are all over the image frame

Right/Brighter distributed image – note that lighter areas are all over the image frame

Hopefully from the above 3 image examples, you can get the idea of the concept of histograms. They represent the overall distribution of brightness in the image, NOT the left to right brightness of the image.

Well that was easy, wasn’t it?

All you have to do is make sure you have a brightness histogram that looks like the top one above and your all set for fantastic images every time – right?   WRONG!

Nature isn’t that simple, the sun shines differently throughout the day, all landscapes aren’t green grass, skies aren’t always blue, birds have a habit of having white AND black feathers, the sun can’t always be behind you, etc., etc., etc..

But don’t give up – most modern digital cameras these days are nothing short of pocket mini-computers – we can get some very good shots in even the most extreme circumstances, with a little thought, care and preparation.

Let’s say your photographing a Kestrel hovering against a lovely blue sky – but the sun is not behind you and in fact shines across the top of the bird, making its underbelly in shadow.  Oh, and it’s moving around a lot as well, oh, and it’s a windy day. Do you give up and try some other subject in a perfect setting? – it’s tempting, but here is a real challenge for you to tackle.

As well as your histogram function, your camera will also have a fantastic exposure system. The settings vary – Spot, Centre-weighted, Matrix/Evaluative and maybe Partial.  In addition to this most cameras have some form of Exposure Compensation, generally -3ev – +3ev, or +- 3 stops of variability. If you don’t know what these are, read your camera manual – it’s all in there. For 95% of the time, I only ever use Evaluative metering.

So back to our Kestrel – its colour is mainly grey/darkish brown, although it’s under-side is very pale cream and remember it is on a bright blue sky background. so how would you set up your camera to take this picture, assuming it’s too far away for flash to have any influence?

Let’s think about it.

For daily use most cameras are set to zero exposure compensation and evaluative/matrix metering. Personally, that’s how mine is set and reset to all day long. In between shots I always reset it to this standard, ready for the next shot. That does not mean my best shots are taken at that setting, just that I know my starting point each time [tip from the great Arthur Morris].

Say we took an image of the Kestrel on these settings ( zero EV), what would the image look like?

Not a bad shot, but the automatic exposure has metered for the whole image and as a result has got the sky almost right at the expense of the bird. The darkness of the bird’s underside has been made worse by the fact the sun was over the bird’s left shoulder, instead of over yours!  The histogram looks like this:

You can see here that histograms rarely conform to standard layouts, but you can hopefully also see that this one is more to the left, giving a higher distribution of dark areas in the image. There is only a small area of lighter area, represented by the thin bright strips on the bird’s wings and belly, as the sun shines on it.

Let’s look at the same bird taken with an Exposure compensation of +1.67 (plus 1 and 2/3) – not quite there, but a much better attempt, considering the difficulty of the subject.

The corresponding histogram would be very different and would look like:

You should be able to see that the image has a higher distribution of lighter areas, a small area of mid-tones and a smallish area of darker tones. Understanding what has happened here is important. By compensating for +1.67 stops of light, you have forced the camera metering to allow more light in, when its natural instinct was to stop the metering down because of the large area of brightness.

With a little bit of tweaking and adjustments of contrast and brightness, together with adjustment of brightness levels/curves, the image can be made into a perfectly presentable one. As the sun was above the bird, then without flash, the face will never be fantastically exposed, but this is the typical pose for a Kestrel and unless you’re lucky to get one with the sun below it, then you have to work with what you’re given. In fact the Kestrel prefers the light behind it, as it makes it easier to see their prey and the sunlight makes it difficult for its prey to see it.

Not bad, but in this case it was ultimately not possible to totally prevent the belly portion being “burnt-out” from the mid-morning sun and still allowing a satisfactory exposure of the Kestrel’s face. In fact this example was an extreme one where the lighting was very difficult, but demonstrates that it can be done, given the right understanding of what is going on inside the camera.

You could have used a centre-weighted, partial, or even spot meter setting here, but I suspect that the sky would have been totally burnt out and unrecoverable, as the metering would exclusively consider the central area of the image – i.e.  the bird itself.  Some photographers would not object to a burnt-out sky, but I feel if your eyes saw the blue sky, then your image should try and reflect that moment.  In some cases however, when the sky is a horrible white cast, it may not be possible to prevent burn-out. The sun can also be your enemy, as well as your friend, so beware in full sunlight – try early and late times of the day  – birds are usually more active then anyway..

When the first shots were taken of this bird, I had already pre-set the EV to +2/3, as I had considered the background was bright and the bird had darker areas and no perceptible white on it. After reviewing the histogram, I quickly decided to up the EV to +1 2/3, to expose for the bird and to “fool” the evaluative metering, preventing it from “balancing” the image, which would have rendered the bird dark. I also took a couple of shots at 0 and -1 EV just so I could have a look and compare when i got back to the house. I know this is wasting a shot, but sometimes bracketing a shot -1 | 0 | +1 can give you a really good idea what’s going on.

There are some basic rules about the use of Exposure Compensation, which are worth noting:

– if the subject is dark or black, then you generally have to over-expose the image by anything up to 1 or 2 stops of EV to allow for the fact that all light is absorbed into black. Beware of pied birds though!

– if the subject is bright or even white, then beware of over-exposure. Generally you should under-expose the image, by -1/3 to -1 EV to prevent blowing the highlights and to retain the detail in the bright areas. Again beware of strong sunlight here – white subjects are best exposed early or late in the day.

– you need to make additional adjustments to the above scenarios, when either the white bird is against a dark background, or if the dark bird is set against a very bright background, and all bits in between. It is not a lottery, but you may have to take a few shots at a variety of settings to make sure you have a good one.

– Remember for bird photography, shutter speed is important and that over-exposing will reduce your available shutter speed by the equivalent number of stops. Conversely, under-exposing a subject will give you a faster shutter speed, but remember that the trade-off here maybe be excess digital noise in the dark areas.

It’s all about compromise and making the best use of the equipment and your conditions, to get the best image you can.

Hope this has helped someone.

There are lots of Internet details on histograms and exposure, so have a good look round and then go out and enjoy your photography!!

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