My experience with light meters over the years is that the readings they give for the same subject vary widely. I recently did the following experiment with three SLRs and two light meters. Using an 8x10 gray card under midday sunlight I took a reading with each SLR, making sure that the card filled the viewfinder. Readings with the light meter were taken with the meter held one foot from the card.
None of the five readings agreed. The three SLR meters were close, covering a range of about one stop. One of the light meters indicated about one stop more light than the highest reading from the SLRs. The other meter added yet another stop.
Why the discrepancies? I don't know but I have some theories.
- Calibration methods of camera and light meter manufactureres just suck.
- Camera manufacturers calibrate their meters to match the camera in question, zeroing out inaccuracies in shutter speed and aperture. Since the aperture is in the lens the manufacturers have no control over that, meaning manufacturers suck when it comes to building good shutters.
- Light meter manufacturers calibrate their meters by pointing them at a gray card. If the reading falls somewhere in the middle, it's good.
- Manufacturers assume that you'll be shooting negative film. Since most emulsions are forgiving enough to allow most exposure errors to be corrected when the print is made, no problem. Those who shoot reversal are left to their own devices.
I think all of the above are true to some degree. The conclusion I reach is that correct exposure is not as critical as you might think. With negative film you can be off by one, two or maybe even three stops and still get good results, but there will be times when you'll lose a good picture due to incorrect exposure.
I shoot 8mm and 16mm movie film. The cost of a print or telecine from negative movie film is expensive so I limit myself to reversal. Some years ago Kodak dropped K40 from its inventory and substituted E64. Super8 cameras sense the film speed from notches on the cartridge. Many cameras will only sense ASA 40 and ASA 160, the only speeds Kodak manufactured for many years. My cameras all sense E64 as ASA 40, a two stop overexposure.
To get some idea of how this would affect my movies I went down to the camera store and ordered two rolls of 35mm E64T. I shot these in my SLR, bracketing each shot from ASA 40 to ASA 64 to ASA 100. ASA 40 was clearly overexposed. ASA 64 and ASA 100 were good.
This clearly indicates that reversal film is not as forgiving as negative, at least if you're projecting the original. Perhaps scanned/telecined images could be improved with software.
