View Full Version : Rule-of-thumb exposure measurements
03 March 2006, 10:53 AM
Probably want to go on a photography course to find this out, but does anyone know a good resource for general rule-of-thumb exposure levels for different things?
For example, grass is about middle grey, human skin is about a quarter stop above middle grey, a white sheet of paper is about 2 stops up etc.
03 March 2006, 05:33 PM
The main thing to remember is that the light-meter in your camera assumes that the average scene is 18% gray in terms of illumination and that's what it regards as correctly exposed. This is why if you take a photo in the snow, and take your picture at the exposure suggested by your camera it will appear dark (about 18% gray) so you need to open up a stop or two to get it to read as white. The simplest way to guage what to set you camera to is to guesstimate whether the scene is brighter or darker than 18% gray on average and dial it in from there.
In terms of actual combinations of f-stops and shutter speeds that is all dependent on the ISO you use and to an extent the lens. F-stops are a theoretical calculation of the amount of light that will transmitted onto the capture medium but 2 different lenses set at the same f-stop will yield subtley different exposures because of manufacturing differences. T-stops, as used on movie cameras, are actually calibrated per lens so you can change from one lens to another and be confident of consistent exposure. Once you've got used to a certain lens on your camera you will begin to get a feel as to when you should believe your light meter and when you shouldn't.
Hope that helps a bit.
PS As always i recommend Michael Langford's "Basic Photography" as the best technical introduction to photography.
03 March 2006, 11:48 AM
Hi Andrew, I'll have to pick up that book!
I realise that it all depends on how you expose the image, I was asking about relative exposures to middle grey. Was just wondering if any guidelines were documented anywhere, or whether it's one of those bits of knowledge that's only held in the wisdom of experienced photographers :)
How's the new job?
03 March 2006, 09:17 PM
OK I see what you mean. I think it is better to think of shades relative to the 18% gray average and then place visual experience relative to that. I've never found thinking about grass being a bit darker than 18% for example terribly helpful as is that grass in sunlight? Shadow? It's easier to train your eye to recognise how bright a scene is relative to a known constant and that's really a matter of experience. I think it's almost impossible to make a generalisation about a surface for this reason. The only exception is snow as you pretty much always want that to register as white at its lightest so that's a pretty consistent open up a stop and a half scenario.
I don't know if that's helpful or not but that's how I've been taking pictures for about twenty years and it's (mostly) worked for me.
The new job's not so new now but I'm enjoying being back at D Neg and it's a doozy of a project. ;)
03 March 2006, 08:50 AM
Hehe yep should be fun :)
Seems what I was after was Ansel Adams's zone system (http://www.normankoren.com/zonesystem.html)
What I'm getting at is that say you expose something so that grass is middle grey. You then know that skin should be a quarter stop above that, snow nearly stops above that, etc. I'm thinking more about CG than about photography (as all those relative values are provided by nature for you :)).
Maybe I should pick up a bloody camera instead of concentrating on the maths :)
03 March 2006, 03:20 PM
I could be talking out of my fundament here but I thought Adam's zone system effectively meant, "expose for the shadows, develop for the highlights" where you'd choose the shadow area to be mid-exposed on the camera, work out how much brighter the highlights were and if there was little difference (< 4stops) give it a longer development time to build up contrast in the neg and if there was a great difference (>4 stops) give it less the idea being to use the same tonal range on the neg regardless of the lighting conditions, which makes printing the little blighters much easier? Since we don't have the development functionality in CG (well not without mucking about in Shake) I'm not sure I understand how this helps us, especially since we're all EXR users these days which has massive dynamic range?
Can you be more expansive about what you want to achieve because I feel I must be missing something fundamental here?
03 March 2006, 10:54 AM
Yes the zone system itself isn't exactly what I was after, but the page I linked to had some of the kind of information i wanted on the zone chart.
The sort of thing I'm after is relative brightness information for shaders and texture painting, particularly.
For example, you have a 18% grey sphere reference for your plate, you light a cg grey sphere to match, then you put your character in. He's got pale skin. How bright should that skin appear under the lights? If you know that pale caucasian skin is .25-.5 stops above middle grey, then you know that your character should do the same relative to your grey sphere. This is the sort of thing that comes naturally with experience, but being able to sanity-check the values and make sure your in the right ball-park is useful :)
03 March 2006, 10:54 AM
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