Exposure adjustments when using filters on a Hasselblad 500 c/m question.

[Graham_Martin]

This question actually applies to any camera that does not have a built in TTL meter. I use the Cokin P filters such as ND, graduated ND, red, orange, polariser etc. How do I know how much extra stops of exposure time I need to add after having checked the exposure with an external meter such as a Seconik?

Thanks.

[Exeter2010]

I went through this too, but the solution is an easy one, especially if you have a Sekonic meter. Each and every filter has a "filter factor" ranging from zero stops change in exposure, all the way up into 10s of stops for some neutral density filters. Other filters fall somewhere in between and you can look this up in the mfg data for your particular filter. Filter Factors are expressed in increments from 0.0 to 16 and beyond and can be converted into stops, or EV values that you can plug into your exposure meter to compensate for your filter. There are tables for this all over the internet - here's a link to one from Wikipedia:
http://en.wikipedia.org/wiki/Filter_factor

If your exposure meter doesn't allow you to set a Filter Factor, you can just change the ISO setpoint to match the change in EV value with your filter on, for example:
400ISO FIlm
Filter Factor of 2.0 = 1 Stop
Change ISO in meter to 200 to compensate
Shoot and develop normally for 400ISO film

It's a little more difficult when you are using a meter in another camera for your exposures, but the principle is the same, just change the ISO per the filter factor. If you are guessing exposure, or if the filter factor is some fraction of a stop, it becomes even more complicated, but I do it pretty regularly and if I can do it, you certainly can!

[jeffreyg]

You can also take your readings with the filter over the light meter. in some situations the filter factor is not the correct adjustment. Try checking with reading through the filter from a gray card and off other subjects to see how your setup is. It may not be an issue with ND filters but can be with some color filters.

[Exeter2010]

Yeah, what jeffreyG said...that would be the easy way. The obvious method he just mentioned didn't occur to me just then! I do everything the hard way anyway - builds character my old man used to say

[Graham_Martin]

Thanks for the helpful info. I was wondering about simply placing the filter in front of the meter. I was somewhat familiar with filter factors. I have an IR filter for which I have to add 4 extra stops of exposure.

[jeffreyg]

I haven't used IR film for a long time but I am quite sure you have to adjust the focus. The lens should have an "R" on the distance scale. You focus then move to the "R". Perhaps someone can confirm this. There used to be both B&W and color IR film. I'm not sure what exists in today's market.

[Sirius Glass]

Filter factor 2 = 1 stop more light [- 1 EV]
Filter factor 4 = 2 stops more light [- 2 EV]
Filter factor 8 = 3 stops more light [- 3 EV]
Filter factor 16 = 4 stops more light [- 4 EV]

Steve

[Q.G.]

No "R" on Zeiss/Hasselblad lenses.
The focus compensation depends on what the dominant wavelength is that filter and film allow you to capture, so it differs anyway.