ND filter factors/metering

[Mark Sawyer]

I need a fairly dark filter, preferably in the Cokin P series size or larger. I've tried the Fotodiox brand ND8, which supposedly cuts three f/stops of light. But on three separate ND8 filters, they only cut 1.5 f/stops of light. (grrrr...) This is as tested by metering through the filter with both Sekonic and Pentax meters.

I've searched the past threads on metering through filters, but nothing applies to this situation. Am I missing something? Or are Fotodiox ND8's really only ND3's? (I'd like to know before spending more on another brand of ND8 filter...)

As always, thanks muchly!

[Ron Marshall]

Normally metering through a filter is not a good idea, since most meters are not panchromatic. But with a ND filter this should not be a problem.

[Mark Sawyer]

Normally metering through a filter is not a good idea, since most meters are not panchromatic. But with a ND filter this should not be a problem.

Agreed, Ron. All I can figure out is this company is waaaay of on their filter factors, especially unforgivable (und unlikely) in a neutral density filter, which is all about the density.

I just feel like I'm missing something...

[Eric Leppanen]

For what it's worth, I just metered through my Lee 0.9 ND grad using my Sekonic L-608, and I'm coming up with a metered difference of 1.5 to 2.0 stops, depending on the target. Maybe I'm not able to hold the filter perfectly flush to the meter, so that reflections creep in?

I'd suggest shooting a test frame to actually see what is going on, before drawing any final conclusions about your Fotodiox filter.

[Helen Bach]

I've just metered through a Lee polyester ND9 (not a grad) using the retracted dome of a Sekonic L-508C and an L-758DR, and the difference in readings is either 2.9 or 3.0 stops every time, the difference occuring at different lighting levels - ie rounding differences. I use Lee ND filters to test for meter linearity (after checking the density with a densitometer), particularly at low light levels, and so I expect to see a three-stop difference with a Lee ND9 when used with an incident meter.

Using the L-508C as a spot meter with the same Lee filter I find that I get a difference of between 2.3 to 2.8 stops, depending on whether I am reading a dark area surrounded by lighter areas - less difference - or a uniformly lit surface with the same Lee filter - less difference if there are light leaks. Using it on a camera with TTL metering (Leica R6.2, 50/1.4, 'spot') I get back to a consistent 3-stop difference as near as I can tell (it's difficult to read to better than about a quarter of a stop) with less sensitivity to scene contrast than with the L-508C as a spot meter (My Pentax Digital behaved more like the R6 than the Sekonic, showing a fairly consistent 2⅔ stops difference). These various differences in behaviour, if they are real and not experimental error (not least from being so close to the limits of precision of the measuring instruments!), suggest to me that reflections between the lens and the uncoated rear surface of the filter (better controlled by the Leica lens than by the spot meter optics?) have some effect on the apparent filter factor, and that there is some small amount of light-piping in the filter even with the edges covered. There are a lot of things going on.

I used the same clip-on filter holder and barn doors for all the tests, with the filter right against the meter to prevent light leaks round the side. The edges of the filter were also covered to reduce light piping. When I get an opportunity - maybe later today - I'll try it on LF with a Sinar Booster TTL meter.

The Callier Q coefficient of ND filters must vary a little depending on whether they are dyed or made of colloidal carbon or whatever.

As Eric suggests, a test frame would be the best thing to do - choose something with a wide contrast range to see if there is an increase in flare.

Best,
Helen

[Mark Sawyer]

Just as a follow-up for anyone ever searching the archives for information on neutral density filters...

The Fotodiox brand ND filters are just nowhere close to what they claim. I made a test with and without the filter, making a 3-stop shift (the correct amount for this filter), and the negative with the filter was significantly denser. Also, I talked to the Fotodiox people directly, and while they're refunding my purchase, they had zero interest in whether they were continuing to sell the wrong filters.

I've ordered the "real" Cokin filters (as in made by Cokin) for quite a bit more money, but I think this time the filters will be what they're supposed to be. Lesson for today, sometimes things are cheap for a reason...

And thank you very much for checking your filters so carefully, Helen. It sounds like using pretty much the same methods I used, your filters were behaving as they should. I was trying to figure out if the fault was with my methods, but it was just cheap filters...

[GSX4]

Mark, I'd been watching this thread with some anticipation as I had ordered my Fotodiox ND-8 filter prior to it starting. Anyway, I received it this afternoon. First thing I did was line up my Pentax spotmeter onto a tripod, and point it at a target, took a reading, then placed the ND-8 filter in front, I a consistent and repeatable 2 stop decrease in light reaching the meter. So there maybe some quality control problems with these filters. If it's any consolation, I can see a noticeable increase in density at the edges of this thing which is a little worrying. However I placed the filter in the Cokin holder and the edges are no where near my largest lens diameter that this will go onto. The meter reads 2 stops decrease across the entire lens coverage area required. So I will try this out sometime soon.

[vinny]

I use fotodiox step up rings for all of my lenses but i'd never put a cheap filter on an expensive lens, it doesn't make sense to spend thousands on cameras, lenses, travel, and film to chance it all with a cheapo filter. B+W or Heliopan.

[Mark Sawyer]

I use fotodiox step up rings for all of my lenses but i'd never put a cheap filter on an expensive lens, it doesn't make sense to spend thousands on cameras, lenses, travel, and film to chance it all with a cheapo filter. B+W or Heliopan.

Agreed, but this is for my high school class next year, (a few of kids who will be in the advanced class want to do some long daylight exposures...) I'm buying them out of my own pocket, and a teacher's salary only goes so far...

But I'm springing for the Cokins, just to get it over with...