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A few days ago I left my blog readers [URL="http://dpnow.com/forum2/blog.php?b=60"]pondering on a couple of images[/URL] that had been, respectively, slightly under exposed - as you might do to play safe and protect highlights from being burned out, and the second image slightly over exposed.
I didn't reveal which image was which.
Here are severe crops from the same images, and I hope you will be able to see a clear, if subtle, difference between them:
[URL="http://dpnow.com/galleries/showphoto.php/photo/17149"][IMG]http://dpnow.com/galleries/data//766/P6140724-2.jpg[/IMG][/URL]
Image 1
[URL="http://dpnow.com/galleries/showphoto.php/photo/17148"][IMG]http://dpnow.com/galleries/data//766/P6140722-2.jpg[/IMG][/URL]
Image 2
The answer is that Image 1 was over exposed and Image 2 was under exposed.
What I hope you can see is that Image 2 exhibits more noise, especially in the darker areas of the image, than Image 1.
In order to recover the lighter areas of the under-exposed image, the levels across the brightness range of the image have had to be boosted in post production. Doing this always risks amplifying noise, but especially so in the darker areas.
This is because most digital cameras don't have the same sensitivity across the brightness range of an image. Have a look at this graph:
[CENTER][IMG]http://dpnow.com/files/blog/dynamic_range.png[/IMG]
[I]Camera sensor recording level across brightness range (simplified illustration)[/I][/CENTER]
[LEFT]Basically, the recording ability of the camera sensor to detect and record different brightness levels is biased heavily to the lighter end of the density scale. This is because the eye places most importance on details in the mid to light tones. [/LEFT]
[LEFT]If you maximise your exposure histogram to the right of centre in your images, when they are taken, you will ensure the more sensitive end of the camera's dynamic range capability is utilised. [/LEFT]
[LEFT]Any corrections that may be required will usually be towards darkening the highlights and mid-tones, and this does not risk amplifying image noise. It also means that much more the of the sensor's capability is focused on recording highlights, so there is more highlight detail to recover than you might think, and certainly more than in slide film, for example. [/LEFT]
[LEFT]This is why, in theory, there should be no need to set your camera to under-expose slightly, by default, in order to protect highlights. This was good advice in the days of slide film, but unnecessary, now, with most decent digital cameras.[/LEFT]
[LEFT]I'd recommend experimenting with your own camera and getting to know just how far you can push the exposure without really losing valuable highlights.[/LEFT]
[LEFT]As always, I'm keen to hear what you think - so don't hesitate to post a response to this blog here![/LEFT]
Why not retry the experiment with an abundance to white in the picture, for example, a wedding dress?
In my opinion, you would then come to the conclusion that underexposing would be a better route.
Also, underexposing would obviously speed the shutter, and so, maybe when there is a need to have a faster shutter speed, it would benefit.
(I have to say, I am suprised there havent been more responses to this!)