Neutral density filter

[Weldon]

I aquired a 4X neutral density filter and thought it would increse exposure time by 4 stops...I was wrong. Each neg was underexposed. Shoud I have tried 4 times the exposure time?

 

[glbeas]

Was the exposure long enough for reciprocity failure to occur? Or were you opening the lens by 4 stops instead of extending the time? A few details will help here. A 4x ND does cut the light by 4 stops.

 

[Nick Zentena]

4 stops is 16 times the time. 2x2x2x2. I thought the ND filters labeled 4x were actually 2 stops. I'm confused how you ended up with underexposure unless it's reciprocity failure.

How did you meter?

 

[Weldon]

I was using a old Rollie T1 that has a meter on the side. The exposure was 15 seconds so I don't think it was reciprocity. The straight exposure should have been F11 at 1/30 sec. I stepped it down to F22 at 15 seconds using the nd filter plus a red filter that I step down 2 stops for. I use the red filter very often at 2 steps down but the nd filter is new.

 

[fschifano]

I was using a old Rollie T1 that has a meter on the side. The exposure was 15 seconds so I don't think it was reciprocity. The straight exposure should have been F11 at 1/30 sec. I stepped it down to F22 at 15 seconds using the nd filter plus a red filter that I step down 2 stops for. I use the red filter very often at 2 steps down but the nd filter is new.

A 4x ND filter will cost you two stops of light. You should have opened up to f/5.6 to account for the two stops of light lost to the ND filter. Then you need another 2 stops to compensate for the red filter. Keeping the shutter speed the same at 15 sec, you'd need to open up the aperture another two stops to f/2.8. You were going in the wrong direction! Don't feel too badly about this. Happens to all of us at one time or another. Keep at it.

 

[Nick Zentena]

I don't think that's right. He said the meter read F/11 at 1/30th. Going to F/22 should have moved the time to 1/8th. Then the ND filter moves it to 1/2. Then the red filter to 2 seconds. I don't know what film was used but from 2 seconds to 15 isn't the worlds best long exposure response. The film was still underexposed? I'm thinking it's something else. No idea what.

 

[gainer]

At 15 seconds indicated exposure time you were well into the region where reciprocity isn't reciprocal. You don't say what film you were using, but the exposure would have to be 21 seconds for 100Delta, 24 econds for 100 or 400 TMX or HP5+ and 30 seconds for 400TX. These are the latest emulsions.

 

[Weldon]

TMax 100 metered at ISO 100. Please explain how you came to 24 sec Gainer.

 

[Kirk Keyes]

I aquired a 4X neutral density filter and thought it would increse exposure time by 4 stops...I was wrong. Each neg was underexposed. Shoud I have tried 4 times the exposure time?

What are the marking on the ND filter. I believe that ND filters are usually marked with their optical density. If you have a ND filter, and it says "ND 4.0" then it is not 4x, and not 4 stops, but 4.0 density. So 4.0/0.3=13.3 stops. That would lead to some very underexposed negs. Using this system, a filter that is 4 stops, would say ND 1.2.

If yours is a 4x, then it may have ND 0.6 on it. If so, then that needs only 2 stops additional exposure.

So what exactly does your filter have marked on it?

 

[Weldon]

It is a Rollei filter that was made in Germany. It says "grau 4" and -4 on the filter and the box.

 

[Huub S]

My guess is that the ND-filter is indeed a 4-stop filter, when the negatives are not way underexposed. In that case it's Kirk made the right analyses.
The underexposure is probably caused by not compensating for the reciprocity factor of your film. Starting with F11 and 1/30, closing down to F22 would give 1/8. Adding a red filter (2.5 stops) would give something like F16.5 at 1/2 sec. Adding the ND filter for 4 stops should give F16.5 at 8 sec, or F22 at 12 sec. This time should be compansated for to about 20 sec. using TMax100.

 

[Weldon]

Thank all for responding. Did a very simple test this morning...tmax 100/120 at F16 metered without filters at 1/125. My red filter is optimum at -2 stops or F16 at 1/30. Adding ND filter (bayonet mount) to red and assuming the -4 stop assumption; F16 at 2 sec. Bracketed roll, same image (contrasty sunny and shadow) went from -4 stop to -12. -6 stops (using both filters) was detailed but would be difficult to print (underexposed). -12 was still detailed though overexposed. I found -10 to be dead on. Which would make it a -8 filter in my test...I guess. Dev in Rodinal at 1:50 for 13 min.