ND Filter accuracy?

[false_Aesthetic]

Hae Ol,

I have this 3.0 (10 stop) ND filter. I've always used it as a 10-stop filter.

A few days ago I was speaking to a friend of mine in the film industry (He's an old-school cinematographer e.g. always brings his 1500-piece color filter kit to the job and has someone measure color temp every 5 min.) about the project I'm working on. He suggested that I check the actual density of the filter as opposed to whatever is printed on it.

I don't have a densitometer but I do have access to a bunch of light meters. They're all showing that the filter removes 6 stops of light not the 10 that's advertised.

The filter is about 4 years old. I'm wondering if ND filters degrade,r if its pretty common for them to be off, or if I shouldn't be using a light meter to do my "test."

The filter is B+W single coated #110.

Thanks

[ic-racer]

I use a 3.0 ND to test the linearity of my light meters.

[Bob Salomon]

0.3 is a stop. a 10 stop ND has a density of 3.0. A six stop has a density of 1.8.

What does your filter do to your exposures?

[Bill Burk]

My 0.6 reads 0.54 on my densitometer, which does not surprise me.

I think ic-racer is suggesting your light meters may not have enough linearity to accurately judge 10 stops.

Flare could also be throwing off your readings.

Bob suggests a good way to test. Compare a shot without the filter to a shot with the filter where you change the time/aperture by 10 stops. Watch out that you don't get into reciprocity failure territory.

[false_Aesthetic]

Bill and IC,

I'm not exactly sure what you mean by linearity in terms of light meters. Could you explain please.

[Bob Salomon]

Bill and IC,

I'm not exactly sure what you mean by linearity in terms of light meters. Could you explain please.

Is the meter's accuracy the same over a wide range of light? Some are not.

[Bill Burk]

It is common for ND filters to be off. But I wouldn't expect it to be off that much unless it was mislabeled.

That's why your friend advised you to check it. Since you don't have a densitometer you tried light meters and they suggest it is really a 6 stop ND filter.

It's possible your light meters can't read that much light difference accurately, because many can't.

Simplest check now would be to take some pictures with it, maybe slides. Take shots without filter, and same shot with filter and compensate as if it was 6, 8, 10 stops.

Watch out that your exposure does not go much longer than 1 second with the filter because then reciprocity failure could interfere with the outcome of your tests. You probably would have to change shutter speed, so this could also be a problem if different speeds are not accurate to each other.

You could explain to your friend what you found, and get more advice. Chances are you will find the effort was worth the trouble, as knowing how something will work is better than taking somebody else's word for it.

Good luck with your project.

[Sevo]

Is the meter's accuracy the same over a wide range of light? Some are not.

Well, selenium or CdS meters sometimes don't have as much accurate range (but even some of these do). But where relatively current meters are concerned, they tend to be far more accurate across ten stops than ND filters - and cross checking across meters and cameras and my exposed film and digital shots all confirm that.

Back to the OP - B+W ND's are solid-dyed Schott glass, where carbon black or metal pigments are used, and near immune to ageing - unless you have exposed the filter to unhealthy amounts of ionizing radiation (UV-C, x-rays, radioactivity), it will be a production defect, i.e. a filter assembled in a wrong rim. Or, perhaps more likely, some former rascally buyer swapping the filter glass on a more affordable 1.8 with a 3.0 and returning the former in the rim of the latter.

[false_Aesthetic]

Finally heard back from Schneider. Not too bad of a turn around considering that I only sent the filter back to them a week ago.

They "tested" it and verified they put the wrong glass in the filter. Cool thing is that I got a non-coated filter. They're sending me a MC one.

Thanks for your help guys.

[Leszek Vogt]

Many of us don't have densitometers. One can use a digital camera to verify the ND value...along with the meter.

Les