I believe I have a reasonably secure understanding of exposure – i.e. use and relevance of ISO, aperture and shutter speed – and the purpose of and when to use exposure compensation. Nowadays, I use the histogram to act as a guide to exposure – as I don’t have ‘LiveView’ this is a retrospective action when I review the shot on the back of the camera. Nevertheless, I can see what’s going on and can make another exposure if I believe my first attempt is not right. So I’m happy.
Recently, I’ve had need to explain exposure compensation and when to use it to a couple of newcomers to photography. Of course, exposure compensation runs hand in hand with the choice of exposure itself and the way metering works on a camera – 18% grey and all that. As I couldn’t find anything that was particularly useful I decided to carry out some simple bench tests myself. It seemed worthwhile to drop it on my blog for reference/discussion.
For my first test I set up three black chess pieces on a piece of black card (the card was more charcoal than black but that’s all I had). I shot all the image in RAW, converted to jpg, adding text to help me identify individual pics and a splash of sharpening. I have made no attempt to change levels, use curves or mess with the exposure values in any way. All shots were taken using aperture priority at f/11 – the cameras metering then picked up the first 2 stops shutter speed compensation, then I switched to manual and made the relevant shutter speed adjustment myself (hope that makes sense).
Okay. This first shot – no compensation. As we can see my black chess pieces look grey and background/foreground is grey. It’s possible that some people may be perfectly happy with this outcome, I’m in no way suggesting there is a ‘right’ presentation. For me, they don’t represent black. If we look at the histogram we can see the majority of the toning is just below middle grey – for things to be black I would expect the bulk of the histogram to be much further to the left.
Minus 1 stop compensation, (I have a dial that I can turn to move an led marker to a -1 mark). This, as I’m using a fixed f/11 aperture has the effect of reducing the shutter speed by about half. As we can see, the image is definitely darker and there is definite shift away from the central position in the histogram.
Minus 2 stops – another twist of the dial. Now the pieces really do start to look black and there is a significant shift towards black in the histogram.
As my camera doesn’t do minus three stops I had to switch to manual and make the adjustment manually. Now the pieces look black and the histogram is really crowding the left hand side of the graph.
This one is plus two compensation. I did try 3 stops but two much was starting to burn out.
As a complete contrast, I also went for white on white with some seashells on a white card. This first exposure, again at f/11 was with no compensation. A nice grey image – not far from that of the black on black – which is what I would expect as the meter assumes a grey subject.
Plus two stops compensation. I think this is about the limit to where I might want to take the image – any higher and I’ll start to lose texture.
A brief point, which I won’t expand on here – In most of the photography I do I usually expect to see a black and a white in image, but they don’t have to be there, delicacy can easily be lost by putting in too much contrast.
For the black chess pieces, this would be my chosen exposure, i.e. 3 stops minus exposure compensation on that set by the camera’s automatic metering. I ‘final presentation’ I have tweaked the levels a little just to bright the highlights a fraction.
This is my chosen for the white on white – plus two stops exposure compensation over the metering of the camera. Again I’ve just tweaked the levels a fraction for final presentation.
You may notice significant length of shutter speed that I’ve used – light was from a north facing window and it was snowing outside – a great diffuse light, but not a lot of it!
Footnote: I have no idea why but my blog seems to present the same jpg somewhat lighter than the image in photoshop…. not sure what to do about that.