Questions about principles of flash and gray card metering

[Tom Camilleri]

I hardly ever use flash, but I want to supplement my hot lights with flash since they don't seem to be strong enough and I've sometimes been getting "dingy," poorly saturated pictures. Generally, they can be greatly improved in PP (CS2 camera raw) with a little added contrast, but I have noticed that flash can yield better colors and contrast. Problem is, I don't know what the "idea" is for flash metering so I'm not sure what the system is trying to accomplish.

My understanding of gray card metering in ambient light is that the "knowns" are the reflectivity of the gray card and the measured intensity light it reflects. From these the camera knows the brightness of the scene/ambient light and can calculate the amount of light (mistake: I was confusing amount of light with amount of exposure, thanks Doug!) necessary for the system to produce a result that will approximate what the eye is seeing under this lighting and will give aperture/time values for a standard exposure. Am I good so far?

Assuming the above is correct, what's going on when flash is used? If I meter the gray card in Av or Tv priority mode with the flash active, is the camera calculating how much flash (if any) to use in order to provide that amount of light which the system requires to produce a result that matches what the eye sees? Since the non-priority parameter will be controlled by the camera, how does it decide how much flash to use?

If I am shooting in M mode, will it simply use as much flash as it needs to achieve this amount of light up to the limit of the flash?

In all cases, how does one meter the grey card and then compose the actual shot which is not seeing the card yet will use the metered values?

Am I even asking the right questions, or is my understanding fundamentally deficient?

Any insights appreciated.

[Doug Pardee]

If I meter the gray card in Av or Tv priority mode with the flash active, is the camera calculating how much flash (if any) to use in order to provide that amount of light which the system requires to produce a result that matches what the eye sees?

No. It is determining how much exposure will be needed for the ambient lighting.

In Av and Tv mode, the only effects from having flash active are:
1) unless you also have activated High Speed Sync on the flash, the shutter speed is limited to the camera's X-Sync speed (1/200 on the DReb/300D); and
2) if you're using Evaluative metering (forced in Av and Tv modes on the DReb/300D) and are shooting in reduced light where the flash would probably be the main light source, the exposure will be reduced by a bit to allow for the extra light provided by the flash.

Since the non-priority parameter will be controlled by the camera, how does it decide how much flash to use?If not using Flash Exposure Lock (FEL): just before the mirror goes up, the camera fires a preflash. It compares the brightness results at all 35 metering sensors with and without flash and determines how much flash power will be needed. On the original E-TTL system used on the original DReb/300D, if you're using autofocus the metering sensors associated with the AF sensor(s) that locked are considered to be looking at the subject and are given considerable priority in this calculation.

If using FEL: when you press the * button, the camera fires a preflash. It compares the brightness results at the center metering sensors with and without flash and determines how much flash power will be needed.

In all cases, how does one meter the grey card and then compose the actual shot which is not seeing the card yet will use the metered values?Flash Exposure Lock—the * button. Page 83 of the DReb/300D manual.

[Tom Camilleri]

Thanks Doug. This clears things up a bit, and now I need to study the manuals.