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I have recently acquired a 10-stop ND filter for my Bronica ETRS, but I have run into a problem while metering. I use the Bronica with the AE-II prism viewfinder with integrated light-meter, and so far I am quite satisfied with the readings it gave. But now the following situation: I set the scene (without filter), and the light-meter gives me an suggested exposure of 1/60s at f8. When I put on the 10-stop ND-filter, I would expect to have to expose for 16 seconds, as this is 10 stops more or 1024 times 1/60. But when the filter is attached, the light-meter in the camera (which works TTL) gives me 4 seconds. In different situations the measure of the camera varies, but it mostly is just 8 stops more, sometimes 7, sometimes 9, but so far never 10 stops. The filter is from Haida, so I would trust it to be rather accurate.

Should I rather meter without filter and then add 10 stops, or meter with filter, to get accurate exposure? As I want to use slide film, this will already be an important difference...

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    \$\begingroup\$ Have you performed a test. Put the filter on the lens and meter, shoot three shots with a bracket of one stop on each side of the recommended exposure then take the filter off of the lens meter and Subtract your 10 stop filter factor and perform the same three shot test. Take notes and then compare the two sets of photographs. \$\endgroup\$
    – Alaska Man
    Commented Jun 15, 2019 at 21:14

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Should I rather meter without filter and then add 10 stops, or meter with filter, to get accurate exposure?

Neither.

Film doesn't respond linearly at very long exposures. This is known as reciprocity failure or The Schwarzschild Effect. For most films, any exposure longer than about one second are affected by this.

You should meter without the filter and then adjust for the actual density of the filter and for the Schwarzschild Effect.

As I want to use slide film, this will already be an important difference...

Each film will have different characteristics for long exposures, and thus long exposure times should be modified based on the product data sheets published by the maker of each specific film.

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With my long exposures, I meter for the scene and then add whatever ND amount needed and disregard the camera’s meter from there on out. That being said, it wouldn’t hurt to do some testing to determine more about your particular filter’s characteristics (I’d recommend using a digital camera for the testing). Take a regular exposure, manually adjust 10 stops and shoot, then go 8 and 9, 11 and 12, and see which is the most accurate LE.

At those slow of speeds, don’t forget to adjust for reciprocity failure.

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