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# Thread: Filter Factor

1. I just wanted to stress to the OP that when people here say built-in they actually mean through-the-lens which is only one class of built-in meters. I shoot sometimes an olympus 35SP, great camera with good meter, and I have to compensate for the filter since the meter is not behind the filter. As for calculating the number of stops from filter factor, if you have a calculator just take the log2 (the logarithm to base 2) of the filter factor. If your calculator lacks the log2 and has only log10 (or just log) then use the following formula instead: No. of stops = log (filter factor) / log (2)

ex: filter factor = 8, then No. of stops = log2(8) = 3 = log (8) / log (2)

2. Kodak put out a couple books back in the day. The first pub is Transmission of Wratten Filter, and the second is KODAK Photographic Filters Handbook. Another great pub to refer to is the Kodak Reference Handbook from the 1940s. They all give the amount of light transmitted for a wide variety of filters. For instance the red 25 passes about 12.5% of light at 590nm wavelength, hence the filter factor of 8, or 3 stops. At above 600nm, it passes in the mid 80s range. The total light passed by a #25 filter runs about 15% of all light averaged, hence the generic factor. Find a good color/wavelength chart to go with the pub, and you can compare your lighting and make your adjustments according to your scene. Relying only on a published factor can often be disappointing.

The books can be found on Amazon for a reasonable price, and will help you get the most from your filters.

?Here's a link to some tables for a few common filters http://www.mat.uc.pt/~rps/photos/other-filters.html

3. Hey Bill! Thanks again for the advice. Specifically in reference to the Hoya 25a filter, if you look at this page http://www.bhphotovideo.com/c/produc...d_25A_HMC.html and click on the specifications tab and go down to the filter factor it says: 3 (+2 stops). This has me slightly confused because I am being told 3 stops by most for a 25a. What do you think?

4. It really depends on the exact filter you are using and quality of the light at the scene. It'll fall somewhere between 2 and 3 stops though.

5. Originally Posted by mporter012
Hey Bill! Thanks again for the advice. Specifically in reference to the Hoya 25a filter, if you look at this page http://www.bhphotovideo.com/c/produc...d_25A_HMC.html and click on the specifications tab and go down to the filter factor it says: 3 (+2 stops). This has me slightly confused because I am being told 3 stops by most for a 25a. What do you think?
Ah, 25a they say 3 (+2 stops), while another 25 filter is 8 (+3 stops)... That's just my recollection that red is 3 stops - your filter may be 2 stops.

6. Originally Posted by tkamiya
It really depends on the exact filter you are using and quality of the light at the scene. It'll fall somewhere between 2 and 3 stops though.
I've been reading some 1800s articles, it's trippy that yellow filters, we are accustomed to being 2 (1 stop)... were 10 (more than 3 stops) with blue-sensitive film.

7. Originally Posted by mporter012
........... it says: 3 (+2 stops).
A 3x factor does not add up mathematically to a 2 stop compensation-----a 2 stop compensation is a 4x factor, adding twice the amount of light with one stop, then adding twice the amount of light again with two stops. I don't know what the best factor actually would be for the 25A, I'm only commenting on the factor as it is being advertised in f/stops. It has to correlate to the fact that opening up one stop adds twice the amount of light as the previous stop----just something to keep in mind.

21 = 2x factor
22 = 4x factor
23 = 8x factor

A 3x factor is an intermediate factor that adds 1 2/3 stops.

It's the 1:2 ratio that exists between each f/stop and each shutter speed on the lens. Opening up one stop adds twice the light, stopping down one stop cuts the light in half, therefore, if you open up 2 stops with the 25A filter, you are adding a 4x factor to the film.

8. Originally Posted by CPorter
A 3x factor does not add up mathematically to a 2 stop compensation-----a 2 stop compensation is a 4x factor, adding twice the amount of light with one stop, then adding twice the amount of light again with two stops.

21 = 2x factor
22 = 4x factor
23 = 8x factor

A 3x factor is an intermediate factor that adds 1 2/3 stops.

It's the 1:2 ratio that exists between each f/stop and each shutter speed on the lens. Opening up one stop adds twice the light, stopping down one stop cuts the light in half, therefore, if you open up 2 stops with the 25A filter, you are adding a 4x factor to the film.
Right.

I think they only said 2 stops as a suggestion, since you can't practically set 1 2/3 stop.

But you can carefully note meter reading to nearest 1/3 stop, add 1 2/3 stop filter factor. Then sometimes the correct exposure (including filter factor) will fall exactly. Or you can include filter factor in the EI that you set on a handheld meter.

When I go out, I often pick the filter in advance and use it the whole day. I set the EI on my handheld meter to include the filter factor. When I go inside, then I take the filter off and set EI on the meter to rated speed.

9. Originally Posted by Bill Burk
Right.

I think they only said 2 stops as a suggestion, since you can't practically set 1 2/3 stop.

But you can carefully note meter reading to nearest 1/3 stop, add 1 2/3 stop filter factor. Then sometimes the correct exposure (including filter factor) will fall exactly. Or you can include filter factor in the EI that you set on a handheld meter.

When I go out, I often pick the filter in advance and use it the whole day. I set the EI on my handheld meter to include the filter factor. When I go inside, then I take the filter off and set EI on the meter to rated speed.

If you're using a modern era bells-and-whistles (35mm or even medium format) camera, setting 1.6 stop filter compensation is easy if there is provision for choosing stepping among 1 stop, 0.5 stop or 0.3 stop. I regularly do 1.3 and 1.6 stop compensations for filter-unrelated scenes on an ancient EOS 1N.

10. Originally Posted by Poisson Du Jour
If you're using a modern era bells-and-whistles (35mm or even medium format) camera, setting 1.6 stop filter compensation is easy if there is provision for choosing stepping among 1 stop, 0.5 stop or 0.3 stop. I regularly do 1.3 and 1.6 stop compensations for filter-unrelated scenes on an ancient EOS 1N.
True, but in that case, with through the lens metering, the "Hutchings" factor is the one you will want to apply, not the manufacturers' factor.

It's all so simple once someone points it out. But it's OK if you forget it all and have to look it up when you need it.

I was out at the telescopes last Saturday night looking at Jupiter and several of its moons. And it made sense to me then and over the next few nights I could look up and see Jupiter still, getting closer to the moon. But wait a month and I won't be able to tell Jupiter from Venus.

Maybe filter factors are like that.

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