Exposure values vs filter factors
Posted: Sat Mar 19, 2016 3:58 pmI have to remind myself frequently that original Rolleiflex filters have a number engraved on the rim that refers to an Exposure Value (EV) rather than a filter factor. The two are very different, and confusion arises because it is much more common in photography for the exposure adjustment required for different types of filters to be expressed as a filter factor.
A case in point is the Rolleiflex polarizer filter -- the Rolleipol -- which shows a required exposure adjustment of -1.5 on its rim. I see frequent references to this number in online discussions as a filter factor. If this were so, then the Rolleipol would require increased exposure of about three-quarters of an f stop. That would be wrong, and result in under-exposure. The correct adjustment requires increased exposure on one and a half stops.
I always liked the EV system, and not just because it uses the initials of my name. Exposure value adjustments are quicker and easier to calculate than filter factors, where mistakes are easily made. (It is not intuitively obvious, for example, that a filter factor of 2 requires one stop of extra exposure; 4 requires two stops; and 8 three stops.) The EV adjustments are made easier still if you use an old hand-held light meter like the Gossen Luna-Pro, which shows all corrected aperture-speed combinations after a simple turn of an EV dial. My main point, though, is that many photographers familiar with filter factor numbers might not realise that the number engraved on any original Rolleiflex filter is an EV number. It would have helped if that had been clearly marked on the filter, but one cannot change history.
Eddie Vaughan
A case in point is the Rolleiflex polarizer filter -- the Rolleipol -- which shows a required exposure adjustment of -1.5 on its rim. I see frequent references to this number in online discussions as a filter factor. If this were so, then the Rolleipol would require increased exposure of about three-quarters of an f stop. That would be wrong, and result in under-exposure. The correct adjustment requires increased exposure on one and a half stops.
I always liked the EV system, and not just because it uses the initials of my name. Exposure value adjustments are quicker and easier to calculate than filter factors, where mistakes are easily made. (It is not intuitively obvious, for example, that a filter factor of 2 requires one stop of extra exposure; 4 requires two stops; and 8 three stops.) The EV adjustments are made easier still if you use an old hand-held light meter like the Gossen Luna-Pro, which shows all corrected aperture-speed combinations after a simple turn of an EV dial. My main point, though, is that many photographers familiar with filter factor numbers might not realise that the number engraved on any original Rolleiflex filter is an EV number. It would have helped if that had been clearly marked on the filter, but one cannot change history.
Eddie Vaughan