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DanK
7-Jan-2011, 20:57
Does anyone know if stacking identical or similar filters will affect the filter factor?

I'm looking at extending exposure times on an older lens wide open....and don't have an ND handy....

I was originally looking at Red 25 for the exposure (which will give me a couple stops) and was curious if adding a second Red 25 would give the same appearance and longer exposure, maybe another stop or two....

Thanks in advance,
Dan

DanK
7-Jan-2011, 21:04
Nevermind....

After reading through my own post, I realized simply metering through the filters would answer my question...Which was no change in factor when stacked....

Thanks,
Dan

Leigh
7-Jan-2011, 21:24
One potential problem with filter stacking is reflections.

When you add a second filter you create parallel glass planes that can bounce light back and forth, aggravating flare and other problems.

- Leigh

Brian Ellis
8-Jan-2011, 08:15
I never ever stack filters. One is bad enough, two or more is asking for potentially serious degradation of the image quality IMHO.

Jim Michael
8-Jan-2011, 08:29
I suppose it depends on what you mean by serious degradation. Filters go through a manufacturing process that imposes limitations in terms of flatness, transmission efficiency, spectral purity, and other physical characteristics. Usage of multiple filters simply leads to a summation of the errors of each filter from their nominal values, i.e. it's fundamentally linear. If someone can show some math to the contrary I'd be interested in reading it. I wouldn't hesitate to stack a polarizer and a ND provided I started with a good set of filters.

Daniel Stone
8-Jan-2011, 09:32
having high quality filters certainly helps minimize flaring and de-sharpening of the image itself.

depending on what you're looking to do print-wise(scan, contact print, enlarge?)

also, what size film/sensor are you using? The larger the size material shooting onto, the less magnification there will be to get to the intended print size. So those "flaws" that some mention here that are inherent to the manufacturing process will only show themselves more when higher expansion rates are used(say, a 4x5 negative-->16x20 print, 4x enlargement). Take an 8x10 negative to start with, and enlarge/scan to 16x20 sized print/file. Now its only a 2x enlargement.

I like to use rubber lens hoods when I shoot anyhow, just to cut down reflections on the lens itself. But if using a filter(or filters, I contact print b/w, and only enlarge color 1-2x(so 16x20 from 8x10)), I'd still use the same hood to help tame any possible reflections onto the filters surface.

-Dan

Maris Rusis
9-Jan-2011, 15:23
Does anyone know if stacking identical or similar filters will affect the filter factor?

I'm looking at extending exposure times on an older lens wide open....and don't have an ND handy....

I was originally looking at Red 25 for the exposure (which will give me a couple stops) and was curious if adding a second Red 25 would give the same appearance and longer exposure, maybe another stop or two....

Thanks in advance,
Dan

Yes, adding a second red filter will reduce light transmission and permit a longer exposure; but not by much.

The #25 red filters I use have a peak transmission of about 85% in the red band. The lack of 100% transmission at the colour of interest is taken care of in the filter factor. A second red filter will let through about 85% of what the first one does. Simple maths indicate a net transmission of about 70% with two red filters. This is about half a stop dimmer. Not worth the expense or the risk to image integrity, I think.

Ed Richards
9-Jan-2011, 15:37
Neutral density filters are just additive.

Thebes
9-Jan-2011, 16:10
What happens if you add a yellow and a red? I would guess that you get an orange with ND?

Also, is stacking wratten gels less problematic than uncoated glass? (I've done this = puke)