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So say I am in a church shooting f/1.4, ISO 3200, 1/100. Is there a way that I can calculate how much light there is in the church? I've heard of EV (exposure value), which seems close to what I want but it appears to be independent of ISO setting. Since ISO is not included, it doesn't really capture what I want. Obviously this is assuming that whatever exposure settings (aperture, shutter speed, ISO) are the correct ones for a good exposure.
The idea would be I'd get a number like 16 EV or something and then I could say...Show more →
You can do what you want to do implicitly with a gray card.
First, you meter a gray card. In manual mode, the center of the exposure meter display represents the current EV level of the scene. Meanwhile, the exposure indicator represents the EV of the aperture/shutter combo. By changing aperture/shutter, you move the exposure indicator along the exposure meter display. When the exposure indicator is in the center of the exposure meter display, the EV of the aperture/shutter combo matches the EV of the meter. This situation is called Standard Exposure and is represented by the APEX equation that was already mentioned...
Exposure Value has two meanings. First, it is a combination of aperture and shutter. Second, it is a combination of scene luminance and ISO. When the two EV values match, you have Standard Exposure. In your camera, the center of the exposure meter display always represents the EV of the scene luminance/ISO combo, and the moving indicator always represents the EV of the aperture/shutter combo.
Technically, you could look up your aperture/shutter combo and get the numeric EV value of the scene. However, as a stand-alone piece of information, it's not very useful. What's important is how EV relates...not what EV is. All that really matters is that the EV of aperture/shutter match the EV of scene/ISO...and you don't actually need to know the EV in order to do that. After metering a gray card, you simply center the exposure indicator in the exposure display, and you're done.
A long time ago...before cameras had built-in meters...knowing the EV value was important. Camera lenses had aperture, shutter, and EV scales on them. Using a meter or exposure calculator, and considering film sensitivity, you would determine the EV of the scene. You'd then set that EV on the lens. Now, when you changed the aperture, the shutter would automatically change to maintain the set EV. And when you changed shutter, the aperture would change. In a way, these lenses were the precursors to the modern Aperture priority and Shutter priority modes. After all...maintaining EV equivalence is what the "Auto" of auto modes is all about.
In manual mode this is very limiting. The only time you have the correct EV is when you're actually metering the gray card. When you remove the gray card, you're left with the correct aperture/shutter, but no good way to change them easily. This problem is addressed in different ways, depending on the camera maker. Pentax cameras have a function where pressing the AE-L button will allow you to change both aperture/shutter with one control dial. Nice, but leaves ISO out of the equation. The better solution is to use AE Lock. To do this on a Nikon means that you have to change your "Auto Meter-off Delay" (now called "Standby Timer" on the D600) from the default 6 seconds to 10 minutes or longer (I use 30 minutes.) Also, change your AE-L button to "AE Lock (hold)"
Now, you simply spot meter your gray card, press the AE-L button, and you've locked in the EV. It's just like setting the EV value on those old lenses. You can now set the camera to any mode (I prefer A mode) and turning the dial will change both aperture and shutter, maintaining your locked exposure. If you change ISO, then shutter (in A mode) will change to maintain the locked exposure.
So you see, by utilizing the functions of the camera you can achieve the same goal. You wanted to know the EV level so that you can select exposure values. By locking your exposure in the camera, you implicitly "know" the EV level. And when using auto modes, the only choices you get for aperture/shutter/ISO are those that provide your locked exposure. And don't forget that you can always tweak that exposure using the Exposure Compensation function. All-in-all, using an extended AE-Lock with a gray card represents a significant amount of EASY control over exposure.