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Metering Question


Well-Known Member

I just bought a B+W filter for my 21mm lens. It says on the filter the filter factor is 4x which i understood to be +2 stops. I've just checked this out and found something strange.

Without the filter, I get a "correct" exposure reading of f8 and 1/2 second.

When I put the filter on the lens, I get a "correct" exposure reading of f5.6 and 1/2 second.

(by "correct" I mean the two triangles appear in the viewfinder)

I would have thought that leaving the shutter speed at 1/2 second, I would need to change the aperture to f4.

Or am I being stupid and not realising f5.6 IS 2 stops away from f8.

Any suggestions would be greatly appreciated.



You are finding a discrepancy between the recommended filter factor (2 stops) and your measured exposure (that shows 1 stop change). Depending on the filter and the situation, either the recommended filter factor or your camera may be right. This is what the B+W site has to say in their FAQ:

10. Now that I know the filter factor for my filter, do I ever have to take another meter reading?

Yes! Filter factors are useful to estimate the optical density of a filter across the entire visible spectrum, but they are just generalizations. A filter might have a filter factor of 2, meaning that half the light that enters the filter never makes it out, but every scene is different and requires a meter reading to determine correct exposure. A filter might pass about 60% of red and blue light, but only pass about 25% of green light. It measures as a filter factor of 2, but if you photograph a head of lettuce, it will be underexposed by a stop. Always take a meter reading to be sure you are getting a good exposure.

Maybe someone else on the list may be certain of which one is the right exposure. I would take 2 pictures until I know for sure (after developing the film!)

Good luck.



Well-Known Member
I agree with Juan. The camera's meter can be biased (confused) by the scene or color spectrum.

If the filter factor is 4x, I would meter the scene (usually with my incident meter) and stop down two stops. It is because when you use a filter, in black and white as least, you want certain color be darken (the opposite color) and the samilar color (of the filter) to be lighten. To achieve the correct effect, you need to stop down to what the filter factor suggested. However, when I shoot negative, especially color, I don't really care unless it is very important image (I would bracket it anyway.) You can always compensate in printing or in PS.




Filter factors are indeed a generalization. I use an ambient meter most of the time so I bracket my shots. But on my Canon 1V which has a very accurate meter I have found that most of my exposure compensation with a B+W circular polarizer has been about 1 1/2 stops. I tend to use this as a basis point for all my cameras; 35mm, C645 and 4x5. If you shoot and bracket in half stops you will be pretty sure you cover yourself and any minor variance can be compensated for in PS.



Well-Known Member
mmm... I'm still confused and disappointed. Confused becuase I thought that TTL metering on the G2 was supposed to take the filter effect into account. Disappointed because I've just sold a damn good light meter as I found I didn't need it as the G2 meter was fine with out filters AND I've just ordered some more B+W filters.

Ah well, back to the drawing board.

Thanks for your input.


PS Does anybody want to buy some 46mm B+W filters for B&W photography. I got 3 lovely red, green and yellow-orange ones arriving in the post.


I would not be disappointed before seeing the results. I can tell you that when I used colored filters for black and white on TTL cameras (the Contax RTS, many many years ago) I just trusted the TTL meter to take care of it, and it did. It never crossed my mind to do the experiment you just did, so I never worried (ignorance is bliss!)
Trust the G2 until proven wrong with the pictures.


Hi Stuart,

I'm assuming your pictures came back incorrectly exposed? If not at least experiment with the TTL system and determine which settings give you the best results. I'm not familiar with B&W photography (I'm guessing that was the reason for these filters?) so I don't know how these filters affect the gray tones nor what you would be looking for in deciding what is the "correct" exposure. I would think that after a handful of experiments you will learn how the camera's meter handles the scenes. If in doubt follow Kodak's or Fuji's mantra by bracketing.

I don't rely on the G's meter very often, not because it is inherently wrong but because it does not see a scene like my eyes. My optician would no doubt agree. I find a hand-held meter better at dealing with most situations. I would always have an incident meter or a gray card handy.

Sorry, I can't give you an answer to your problem but I find G cameras to be hard taskmasters. They need more attention than other cameras but the results are worth it.


Jim Hully Boston, MA


Well-Known Member
Hi Jim,

No, I actually haven't shot any film yet, I was just experimenting with the filter (as one does!).
I will in the next couple of days.

I must admit, I've never really had a problem with the G2's metering, which is why I sold the light-meter. Mind you I am quite a conservative photographer at the moment in the sense that I tend to shy away from very contrasty scenes.

This filter stuff quite interests me, so I think its some time for some more experimentation - eek! I might try and shoot a backlit subject.

All the best,



Well-Known Member
>stuart keeton said "Does anybody want to buy some 46mm B+W filters for B&W photography. > I got 3 lovely red, green and yellow-orange ones arriving in the post."

Please contact me off line. Dave


Well-Known Member
Albert, I think you really meant to say "open up 2 stops" If you "stop down" e.g. f8 to f16 you will be vastly underexposed. One should open up to f4. Don't wish to confuse poor Stuart.


Well-Known Member
Colin, You are right. Sorry for the confusion. I was multi-tasking when I posted the message. At least someone is paying attention. :)