View Full Version : center filter at non-optimal aperture

john costo
23-Oct-1999, 05:30
When using a center filter on, say, a 75mm lens, where the filter spec states th at use is optimized for f/16, what is the effect when shooting at f/22 or f/32 ? What sort of correction or adjustment, if any, needs to be made at these small er apertures ?

Bob Salomon
23-Oct-1999, 06:10
None it will work fine but the lens performaance will be reduced due to diffraction.

What you need to bear in mind that a center filter can not be seen by the film until the lens has been stopped down at least 2 stops.

So using it wide open or one stop down would result in its' having no effect.

Michael S. Briggs
23-Oct-1999, 18:46
There are basically two kinds of light falloff from a lens: falloff due to optical laws and falloff because the lens elements aren't big enough or the lens barrel gets in the way of some of the rays. The first reason will exist at all stops, the second reason with the lens at wide apertures. The widest opening is primarily intended for focusing--making the lens illuminate the corners of the negative to the extent allowed by the optical laws would be too expensive in terms of weight and cost.

What does this have to do with center filters? To get the most even illumination possible, you need to stop down to or past the first aperture in which only optical falloff pertains. Stopping down further shouldn't make any difference for light falloff, though there will be consequences for depth-of-field and diffraction. I would take the f16 spec as the first f-stop at which only optical falloff exists. I don't think you will need any exposure adjustments for f22 or f32 vs f16, nor will you see any difference in light falloff.