B&W contrast filters

[clcolucci58]

I have some filters and I see each has a different filter factor. My question is if I am using a red filter #25A which has a factor of 3 and I am shooting at say f8 @ 1/125 with the filter it would change to f22@ 1/15. Am I correct with this that with a filter the factor number changes f-stop and shutter speed down with the coordinating factor number.
Regards
CLC🙄

 

[ReeRay]

Take a look at this link which may help

http://www.fineart-photography.com/bwfilter.html

 

[Ko.Fe.]

Have a exposure meter, smartphone or TTL camera? You could use it to measure the stop of exact filters you have.

 

[rodinal]

No, you are using the correction factor in a wrong way.

If the 25A filter asks for a factor of 3 stops (or 8x), then you have to increase the exposure, either opening the diaphragm or slowing the shutter speed three steps. Always the change must be in the direction that increases exposure. In your example, the normal exposure is f8@1/125, then the correct exposure to be used with the filter is f8@1/15 or f2@1/125 (or any equivalent, like f4@1/60).

The "corrected" exposure you calculated f22@1/15 is exactly the same (in terms of exposure) as f8@1/125 because you decreased exposure with the diaphragm and increased it by the same amount using the shutter speed. In addition, you corrected twice, you need to correct three steps in total. In your example, you corrected six steps (three in the correct way and three in the opposite way).

MP

 

[clcolucci58]

No, you are using the correction factor in a wrong way.

If the 25A filter asks for a factor of 3 stops (or 8x), then you have to increase the exposure, either opening the diaphragm or slowing the shutter speed three steps. Always the change must be in the direction that increases exposure. In your example, the normal exposure is f8@1/125, then the correct exposure to be used with the filter is f8@1/15 or f2@1/125 (or any equivalent, like f4@1/60).

The "corrected" exposure you calculated f22@1/15 is exactly the same (in terms of exposure) as f8@1/125 because you decreased exposure with the diaphragm and increased it by the same amount using the shutter speed. In addition, you corrected twice, you need to correct three steps in total. In you example, you corrected six steps (three in the correct way and three in the opposite way).

MP

Thanks, that makes it easier to understand.

 

[Dwig]

... I am using a red filter #25A which has a factor of 3 ...

Not, it doesn't have a "factor of 3".

To add to rodinal's excellent post, "factors" are not "stops". A #25 filter* has a filter factor of 8x, which is equivalent of 3 stops. "Factors" are the number that you multiply the exposure time by to achieve the corrected exposure.

* The is no true #25a filter designation although some manufacturer's have drifted into using the designation out of ignorance. The original Wratten designation was "A". It was later changed to "#25" as the list of filters expanded. Filters often came with a dual designation "#25 / A" to provide both the "new" and "old" designations. There are Wratten designations that contain numbers with a letter modifier (e.g. "23a") and such correct designation should not be confused with the "25a" b a s t a r dization (oh, don't we love the prudish auto-censor...)

 

[rpsawin]

I use this chart from the Schneider Optics site:


http://www.schneideroptics.com/info/faq/bw.htm#qu21

 

[rpsawin]

I use this chart off the Schneider Optics site:


http://www.schneideroptics.com/info/faq/bw.htm#qu21