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Greeting all,

I have a bit of a problem trying to get my head around the subject of "filter factors". I can understend the basics and can convert a factor number to an increase in f-stops. however I can not work out the increase in the ISO.

I recently found a web site which I thought may have answered this question however the imformation given appears to be incorrect, if the ISO is divided by the factor number, as sugested, and a reading taken then a smaller aperture will result. I have pasted the relevent section from the web page below.

"An easy formula for determining exposure correction when you are using a filter is to divide the film speed by the filter factor and use the results as a corrected film speed on your exposure meter. As an example, when you use a black-and-white film with an ISO of 400 and a No. 11 filter (filter factor four), divide four into 400 and use the results, 100, as the film speed on the exposure meter. When using this method, make sure you return the meter dial to the correct film speed after using the filter".

I have asked my college tutor to explain the principal to me and he has advised multiplying the ISO by the guide number, I tried a test shot using this method however when processed the negative was over exposed.

I am hoping that a wise and venerable member of this forum has a simple equation with which to calculate the required ISO setting for the light meter.

Kind regards,
Rob.

Rob. That quote you give from the website is Ok. Filter factor 2=1 stop, and 4=2 stops. 2 extra stops with 400 film = reading it as a 100 speed. I always think in terms of stops, ie my red is an extra 3, orange 2, yellow 1, green 3. Sometimes the factor makers give is a little out. Never gone down the route of adjusting a meters iso to accomodate a filter factor - just seen things in terms of extra stops needed. I'm sure you will get a more scientific reply than mine!

Hello from France !
I am hoping that a ... member of this forum has a simple equation

Being probably neither wise not (yet) venerable, I have, nevertheless, found this in my repository of posts to photographic forums.

You said : equations ? Here they are

First, the simplest is to start from examples of commonly used filters.
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Data extracted from the Heliopan catalogue (disclaimer as usual, I am not affilitated, etc, etc..)
http://www.heliopan.de

FF = multiplying Filter Factor for exposure time

minus sign for f-stop clicks means : open the iris by ... f-stops

Classical B&W contrast filters
pale yellow (like wratten #5), FF 1,5 - 2x, f-stops -0,5 to -1
medium yellow [like wratten #8], FF 2,5 - 3x, f-stops -1,3 to -1,5
medium+ yellow like wratten #12 ), FF 3 - 4x, f-stops -1,5 to -2
strong yellow (like wratten #15), FF 4x, f-stops -2
yellow-green (like wratten #11), FF 2x, f-stops -1
green (like wratten #13), FF 3x, f-stops -1,5
orange (like wratten #22), FF 4 x, f-stops -2
pale red (like wratten #25), FF 8x, f-stops -3

Polarizing filters (caution : actual factor actually depends on the subject)
filter factor between 2x and 3 x ; f-stops -1 to -1,5

Neutral Grey Filters (for Waterfalls in Long Exposure Times )
transmission factor = 1/(filter factor)
optical density = log_10 (filter factor) = -log_10 (transmission factor)

(Neutral) optical Density / transmission factor / filter factor FF / f-stops
ND 0,3 / 50,00% / 2x / -1
ND 0,6 / 25,00% / 4x / -2
ND 0,9 / 12,50% / 8x / -3
ND 1,2 / 6,25% / 16x / -4
ND 2,0 / 1,00% / 100x / -6,66
ND 3,0 / 0,10% / 1000x / -10


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You said : equations ? You should not, too bad for you
More equations below !

Number of full f-stops = Log_2 (muliplying factor) = Log_2(10) x Log_10(muliplying factor) = 3.32 Log_10(muliplying factor)

muliplying factor for exposure time = 2^{number of full f-stops} = 10^{0.30 x number of full f-stop}

Correspondence between...
half f-stops -> FF multipliyng factor
minus sign means : open the iris by ... f-stops

6 > 0.0156 = 1/64
5.5 > 0.022
5 > 0.031X = 1/32
4.5 > 0.044X
4 > 0.0625X= 1/16
3.5 > 0.088X
3 > 0.125X = 1/8
2.5 > 0.177X
2 > 0.25X = 1/4
1.5 > 0.353X
1 > 0.5X = 1/2
0.5 > 0.707X
0 > 1X
-0.5 > 1.41X
-1 > 2X
-1.5 > 2.83X
-2 > 4X
-2.5 > 5.66X
-3 > 8X
-3.5 > 11.3X
-4 > 16X
-4.5 > 22.6X
-5 > 32X
-5.5 > 45.2X
-6 > 64X

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Same by 1/3 of f-stop. for the uncompromising user of Compur and Prontor Professional shutters !

(Since Compur & Prontor view camera shutters have been discontinued, Copal shutters mounted by Schneider are supposed to have clicks by 1/3 f-stop)

clicks by 1/3 f-stop -> multiplying factor
minus sign means : open the iris by ... f-stops

6 > 0.0156 = 1/64
5-2/3 > 0.0197
5-1/3 > 0.0248
5 > 0.0313 = 1/32
4-2/3 > 0.0394
4-1/3 > 0.0496
4 > 0.0625 = 1/16
3-2/3 > 0.0787
3-1/3 > 0.0992
3 > 0.125 = 1/8
2-2/3 > 0.158
2-1/3 > 0.198
2 > 0.25 = 1/4
1-2/3 > 0.315
1-1/3 > 0.397
1 > 0.5 = 1/2
2/3 > 0.63
1/3 > 0.794
0 > 1
-1/3 > 1.26
-2/3 > 1.59
-1 > 2
-1+1/3 > 2.52
-1+2/3 > 3.17
-2 > 4
-2+1/3 > 5.04
-2+2/3 > 6.35
-3 > 8
-3+1/3 > 10.1
-3+2/3 > 12.7
-4 > 16
-4+1/3 > 20.2
-4+2/3 > 25.4
-5 > 32
-5+1/3 > 40.3
-5+2/3 > 50.8
-6 > 64

same but much quicker to read as an analog bar !
to be downloaded here
http://www.cijoint.fr/cj200903/cijGce0Kw5.pdf

Emmanuel, thanks for the pdf link. The BTZS software uses filter factors rather than f-stop changes so it'll come in handy. I remember the common ones but there's always one that slips the mind...