ND Filter Exposure

[Roughcollie]

I have a Pentax 645n and have recently bought a 10 stop screw in ND Filter so I can blur water and clouds,couple of questions what is the best way to get the right exposure,I am aware that it is 10 stops more exposure,do I take the exposure with no filter first then add 10 stops more exposure ie: if exposure is 1/60s with no filter, extra 10 stops would be 16sec more exposure,I am not sure if the camera will take an exposure reading with the ND filter attached,as I have not yet tried it.
The other question I have a remote shutter release, if I need to do an exposure of 16sec do I set the camera in bulb mode and hold the release for the 16 sec, timing it by a stop watch held in the other hand?
I am aware that I have to be careful with long exposures and have to take in account reciprocity failure, am possible considering using Fuji Acros film as it has extremely fine grain a good tonal range,not sure which would be the best colour film to use,at the moment I am using Kodak Ektar 100.
Any advice would be greatly appreciated.
Nigel

 

[jim_jm]

Nigel -
The 645n has TTL metering, so you should be able to get a fairly accurate reading with the filter attached. Try it both ways just in case - use the exposure determined by the meter with the filter on, or just add 10 stops to the meter reading without the filter.
I don't believe this camera has manual speeds as long as 16 sec, so just use a cable release on bulb, and time it with a stopwatch. Auto and Program speeds will go as long as 15 or 30 sec if you don't want to use manual, but if you take a lot of long exposures this will run down the batteries quicker. Not sure if it has a mirror lock-up, but I'd suggest using it if it's there.

No suggestion regarding color film, but Acros 100 is great for time/night exposures. No reciprocity correction up to 2 mins and I've had some really nice results.

 

[Laviolette]

Yes a 10 stop ND filter is very dark so you are probably better to take the exposure reading without the filter first, then add 10 stops. You are correct when you say that for 1/60 you would need to expose for 16 seconds with the filter on.

To take the photo, place the camera in bulb mode and hold the cable release button for as long as the exposure requires. Some cable releases have a lock for very long exposures.

Acros is a good choice for B&W as is Provia 100F for color work. Both don't requirement any compensation if the exposure is under 2 minutes. Any film can be used, but the slower ones (iso 100 and below) are habitually better in regard with reciprocity law's failure.

About reciprocity failure : if you have a smartphone, there's an app called Reciprocity timer that is very useful. You enter your film choice, filter type and exposure and the app will calculate how much time you have to add to compensate for reciprocity law's failure. There's even a timer so you don't need a stopwatch. The app has other useful features like bellow compensation for large format shooters.

 

[Dwig]

Yes a 10 stop ND filter is very dark so you are probably better to take the exposure reading without the filter first, then add 10 stops. ...

+1, most TTL meters can be influenced by light entering through the eyepiece. This is normally of little or no consequence, but when the light entering the lens is massively dimmer than the light entering the eyepiece (e.g. when using a very heavy ND filter, ...) the influence may be significant. Metering without the filter and adjusting the 10 stops is likely to be more reliable.

 

[Laviolette]

+1, most TTL meters can be influenced by light entering through the eyepiece. This is normally of little or no consequence, but when the light entering the lens is massively dimmer than the light entering the eyepiece (e.g. when using a very heavy ND filter, ...) the influence may be significant. Metering without the filter and adjusting the 10 stops is likely to be more reliable.

A very good point.

I forgot to say that the filter might not be exactly 10 stops. I have a B+W and it is more like 11 stops.

 

[Colin Corneau]

When I bought a Lee 10x Big Stopper, it came with a table to show your exposure, ie. regular metered exposure + 10 stops. It doesn't account for reciprocity, so that app is a GREAT suggestion.