Tom said:
I was meaning with flash. I had assumed that I would have a different reading if I held the meter a foot underwater, as opposed to at the same level outside the tank? Maybe the box of wet stuff would alter the exposure. And with tank lights alone, if I let my Canon 5D expose how it thinks it should every time, I would give up.
Tom I'm not getting this at all mate. It's not clear to me how you arrive at an exposure setting when you use your 5D to shoot a tank. I assume you spot meter the main area of interest, take a shot and then look at the results on your LCD. Camera meters try to make everything look like an 18% grey card, so they find a middle ground between the brightest and darkest element in the frame. In the confines of a tank though, the contrast varies wildly. A digital shot can be fixed easily with curves or levels, but on the film you have to decide what's most important and what element in the shot can be compromised. If you don't want to compromise then the lighting solution with strobes is more complicated. These are the zones and examples of what fits them:
ZONE
0..........Pure black
I...........Near black, with some tone but no texture
II .........The darkest part of the image which shows detail
III ........Average dark materials and low values showing adequate texture
IV ........Average dark foliage, dark stone, or landscape shadows
V..........Middle grey, clear blue sky, dark skin, bark/wood
VI ........Average Caucasian skin, light stone/sand shadows on snow
VII........Very light skin; shadows in snow with acute side lighting
VIII ......Lightest tone that still has texture, textured snow
IX ........Slight tone no texture, glaring snow
X .........Pure white, light sources and specular reflections
The difference between each zone is 1 stop. So lets say you have a scene with dragon stone, sand and hairgrass in the same scene. The dragon stone is Zone III, the sand is Zone VI and the hairgrass will be Zone V. That means there's a 3 stop difference between the lightest element (the sand) and the darkest (the dragon stone). This is completely independent and without special attention to the light. If the scene is lit by a uniform light source, regardless of the intensity, these elements stay in their zones.
So, if you spot meter the hairgrass, the sand will be 1 stop lighter, which is OK, but the stone will be two stops darker, which means it will have adequate, if not spectacular texture and detail. If you wanted better detail in the stone you would have to shine more light specifically at the stone and re-measure.
So the Zone is a sliding scale. You spot meter the elements in the scene, compare how many stops difference each element has and decide which element Zone V should be. When you do that, you have essentially fixed the sliding location of the zone scale and each element then falls into it's zone.
For example, if I decide that I wanted optimal detail in the dragon stone that would mean I'm sliding my zone scale to the stone. The stone becomes Zone V. The sand is 3 stops brighter and therefore would now be in Zone VIII. The hairgrass would be Zone VII. The stone would be exposed perfectly, but you would tend to wash out the rest of the image.
So when shooting film, you need to get more data points within the frame. The more spot metering points you take, the better will be your assessment of the image. You definitely need to spot meter. something like this that has spot metering ability:
Sekonic L758 (this is just the latest model. The part you need, the part that hasn't changed it's design for 50 years, is found in older models.)
I don't see the need for underwater gymnastics here. Camera meters see the world in terms of reflected light. You don't need to measure incident light because everything is relative.
In fact, you can practice this right now with your 5D. The 5D has no idea what it's looking at, so it's matrix metering just compares the scene with it's database and determines a metering solution based on the closest average match. Use the spot metering function and find the zones of the elements. Then take the shot and see if the the LCD confirms what you mapped out.
This is what people did 20 years ago. But, we're living in The Matrix, and like chimpanzees in the lab, we just know how to press buttons and get gratification. It's kind of like the drivers here in USA. No one knows how to drive a manual shift any more because everything is automatic. When I take my car to the shop some of the mechanics freak out because they can't figure out how to move the car.
Cheers,
