Re: OT high dynamic range photos
- From: Simon <noname@xxxxxxxxxxxx>
- Date: Wed, 08 Aug 2007 20:42:11 +0100
Paul Saunders wrote:
Many high contrast scenes actually look better as silhouettes, and doing HDR versions of them can actually reduce their impact.
As the saying goes "Art is born of restriction and dies of freedom" or something like that anyway.
I envy stills photographers who have these tools. With video it is definitely a case of being creative with the exposure and going for the most dramatic effect.
With stills photography noise isn't much of an issue, but with video (and I know this thread wasn't originally about video...) dark to very dark subjects are often best made black due to noise and compression issues. This might also be the case with some stills cameras, and especially with JPEG compression (not that you'd want to use that of course).
All that said I have to constantly adjust my camera for the range that it has. A decent mid-high end video camera can usually be made to capture around 11.5 stops, sometimes 12. This is achieved generally by adjusting the black gamma (low level gamma curve in the shadow regions), the knee point (the point at which the highlights can be made to 'roll off') and the knee slope (how sharp a roll off it is).
Now the key here is that CCD's and CMOS chips in both video cameras and stills cameras have the same problem. The CCD can only capture a given range at any one time. By adjusting the knee point and knee slope in my camera what I am effectively doing is compressing the extra range that the CCD is capturing into the limited tonal range of video (in an 8-bit colour system that's 256 tonal graduations) that would otherwise be clipped. On a 2/3" CCD it can usually capture 600% (over expusure) input at 100% output. On a standard linear gamma 'curve' the ratio would be 1:1 (100% signal input 100% output). Smaller CCD's are not capable of capturing as much over exposure range (usually only 450% for 1/2" chips, and much less for smaller chips)
Now we start to get to the simularities between HDR and the adjustments I mentioned above. When I compress the tonal range using such adjustments I end up with a very flat image. It MUST be graded in post production. But the rub here is that because I have compressed more exposure into the tonal range subtle graduations run the risk of becoming 'stepped'. So I perhaps don't want to push things too far.
So what does getting the extra range do for me? Much like an HDR image the key isn't to capture an image with a mind to using the entire exposure range in the final image. The idea is to capture as much information as possible so that when you are editing the picture you can make an artistic choice in the comfort of your editing suite/photostudio as to the artistic presentation of the final image, instead of having to make a decision in the wilderness somewhere.
An HDR image, or a RAW image doesn't suffer from the same tonal restrictions as a video camera. But the artistic grading process still remains very similar. Images, unless going for a special effect, need to look natural. Many of the images Paul linked to showed a case of using the features of a tool for the sake of it.
Take what Paul mentioned about black objects. It is true that objects aren't truly black very often (the same with white, but don't get me started on how many people insist on white balancing all the time!). In grading, if you have a very low noise picture, grading such dark areas can be done very subtly. It gives the ability to be able to make them almost black with subtle detail. With a high ISO this may not be desirable though.
I used to be a big advocate of cramming as much exposure range as possible into a shot. However I found that it takes the artistic element out of exposure. Many people become lazy and let the capabilities of the equipment carry them through, before adjusting things afterwards.
Getting a great image on location is paramount. If you have a great image on location then any additional capabilities such as HDR or camera that can cope with more exposure ranges will just help you make them better. Unless a situation has a really high contrast range, such as some snowy landscapes, or a contrasty interior shot that also has a large window that looks out onto a bright and sunny view, or if you are using the HDR image as a lighting map for a 3D renderer, there is not much advantage to using HDR other than creating work for yourself.
Every tool has a use, and every tool has a misuse :-)
Simon
http://www.simonwyndham.co.uk
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