Filter Factor

mporter012 · Jan 22, 2013

Can someone explain to me precisely how the filter factor works. Today was the first day I've ever used a filter. I used hoya 25a and the filter factor is 3 (2 stops). When I put the filter on, the light meter picked up the 2 stops automatically. I don't need to open up more stops do I?

Also, with my polarizer, a hoya linear, the filter factor is between 2.3 and 2.8 (1.3 stops). I'm not really sure what to do here. How do I open up 1.3 stops on camera that only has full stops?

Thanks -

tkamiya · Jan 22, 2013

If you are using an in-camera meter, filter factor does not matter. Your camera will be measuring the light coming through the filter.

If you are using a hand-held meter, the light reaching the meter is different from light reaching the film through your camera lens and through your filter. In that case, you take the meter reading and OPEN whatever number of stops necessary.

Alex Muir · Jan 22, 2013

The polariser filter factor varies, depending on the setting of the filter on the lens. The camera meter sees the difference and adjusts the reading accordingly. This peculiar feature of polarisers makes it difficult to use them with a seperate exposure meter. Whether or not you can dial in 0.3 or 1/3 stop of a difference depends on the camera. If it has aperture priority auto, the shutter speed is likely to be continuously variable, and will therefore set the exact speed for the aperture you have set. The readout may show the closest marked speed. In manual, you are usually stuck with set speeds, but the aperture can be tweaked to 1/3 or 2/3 between settings. I hope that makes sense!

Chuck_P · Jan 22, 2013

A 3x filter factor is a 1 2/3 stop compensation.

John Koehrer · Jan 22, 2013

Several things, There's nothing to stop you from setting the aperture between stops. There just aren't clicks and it's not precise. It is "close enough for government work" though.
The factors are suggestions from the manufacturer. just like film you may find you need to apply a little "windage"
Often reading through the filter will not give what you need because of the spectral response of the metering cell.
Try a few test exposures using the ttl method and the using the marked recommendation.

RalphLambrecht · Jan 22, 2013

i agree with all of the above, ignore the filter factors if you use a built-in meter or adjust the meter recommendation with experience. some hand-held meters allow you to measure a filter's filter factor

Gerald C Koch · Jan 22, 2013

I would like to second the previous stateement. Using the camera's built in light meter assumes that its sensor is equally senbsitive across the visible spectrum. This may or may not be true especially for dark colored filters. Testing may be required.

polyglot · Jan 22, 2013

Firstly, filter-factor is a gross approximation. If you take a photo of a mostly-red scene through a red filter, the change in exposure will be small whereas if you shoot a blue scene through a red filter, the required exposure change is quite large.

A 25A has a filter factor of 3 stops, which is 8x. In theory if you use a TTL meter, it will read the already-corrected exposure but that makes the assumption that the meter and film have the exact same spectral response, which in general they will not. I've found that many meters are quite red-sensitive, so they will report only +2 stops with a 25A whereas that filter has a factor of 3 stops with most films in daylight conditions. If your film lacks red sensitivity (e.g. Acros), then it's even worse. But if the film has extended-red sensitivity (e.g. SFX200), then the filter-factor will be lower!

So you need to be aware of the interaction between lighting spectrum (red/orange filters have little effect under tungsten light because it is already missing a lot of blue), scene spectrum, filter absorption spectrum and film sensitivity spectrum. They all matter, and bundling them into a single number is just a handy oversimplification that works maybe 80% of the time.

Bill Burk · Jan 22, 2013

mporter012,

Your Hoya 25a has an 8x filter factor. They also say "3" which is simpler to think - 3 f/stops.

So with a Red filter you open up 3 stops normally (from a meter reading without filter).

I may have opened the can of worms in your mind when I described the different effects you can get with a filter and the different exposures that would lead to those results. I said if you want to make green normal while really brightening red you would open more. Strictly speaking you can perform any of the exposure variations in the darkroom when printing. You don't have to wrangle with the crazy thought I threw out there.

For your example, the in-camera meter told you 2 stops. Well it was off the filter factor because it was about to make you underexpose 1 stop. You probably should open up at least one more stop.

There is a refined "Hutchings" method in "Steve Simmons Using the View Camera" - Amazon will serve the specific page if you are lucky but avoid untrustworthy spyware download sites in Google results. In the Hutchings method you meter through a filter and then compensate according to a short chart.

In that chart a 25a red filter says open up yet additional two stops after reading through the filter.

Reason: shadows have a lot of blue light - and the blue light is severely cut by red filter. So to get sufficient shadow detail you need much more light than the meter indicates.

So a factor quoted as 3 meant 3 f/stops. Your meter was a little off. Steve Simmons would say a lot off.

Reality: You are fine following the meter reading but the Hutchings method is easy to use (for many filters it says use reading through the meter).

Strictly speaking: I think it was for Tri-X, different factors might need to be "worked up" to be accurate with tabular grain film.

Back to reality: I carried the Hutchings chart in the wilderness and when I needed it, I was too lazy to turn to the page I had written it on. If you like the concept, tape it to the back of your camera.

henry finley · Jan 22, 2013

John Koehrer said:
Several things, There's nothing to stop you from setting the aperture between stops. There just aren't clicks and it's not precise. It is "close enough for government work" though.
The factors are suggestions from the manufacturer. just like film you may find you need to apply a little "windage"
Often reading through the filter will not give what you need because of the spectral response of the metering cell.
Try a few test exposures using the ttl method and the using the marked recommendation.

Agreed, absolutely. Metering cells do not see colors to the same degree. That is, they can be quite color-blind. And when they age, it can be even worse. My advice is to think in therms of the meter being daylight-sensitive only, and apply the correct filter factor afterwards. You've go enough possible other color-sensitivity issues with the different films you're bound to be using. When I was a teenager in the early 1970's I'd slap on a filter and let my meter handle it. I suppose I got by, but I'm sure I never was able to arrive at a dependable darkroom standard. Use the filter factor. You want to be making test prints with Polycontrast filters on everything till the end of time? Or do you want to pull the wet film off the developing reel and see a whole roll of pretty negatives that all look the same, and easy to print? These people manufacturing these filters pegged the factors pretty well.

Poisson Du Jour · Jan 23, 2013

All polarisers set to their maximum affect cause a loss of at least 2.0 stops (actual EV stops). Onboard TTL meters will observe this and make the necessary compensation (not necessarily always the right amount of compensation), but with a manual meter (hand-held metering) it's up to you to factor in the compensation value and keep that in mind when the shot has full polarisation applied.

Be aware that a polariser must be matched to the type of camera meter to avoid derangement of the reading; most modern-day evaluative/matrix/multipattern meters require circular polarisers; plain TTL meters can use either linear or circular. Additional correction based on your experience is often necessary above what the camera automatically compensates because very often a polariser can 'flatten' the scene making it look very dead. A little over-exposure is better than under exposure.

Salem · Jan 23, 2013

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)

kintatsu · Jan 23, 2013

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

mporter012 · Jan 23, 2013

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/product/23526-REG/Hoya_A5225A_52mm_Red_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?

tkamiya · Jan 23, 2013

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.

Bill Burk · Jan 23, 2013

mporter012 said:
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/product/23526-REG/Hoya_A5225A_52mm_Red_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.

Bill Burk · Jan 23, 2013

tkamiya said:
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.

Chuck_P · Jan 23, 2013

mporter012 said:
........... 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.

Bill Burk · Jan 24, 2013

CPorter said:
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.

Poisson Du Jour · Jan 24, 2013

Bill Burk said:
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.

Bill Burk · Jan 24, 2013

Poisson Du Jour said:
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.

Mr Bill · Jan 24, 2013

mporter012 said:
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/product/23526-REG/Hoya_A5225A_52mm_Red_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?

Hi, I'm not going to spend a lot of time trying to convince you, but I think you are mainly being mislead by the B&H numbers. My guess as to what "3 (+2 stops)" means is this: +3 stops for daylight, or +2 stops for tungsten light.

Hoya's own information (p 47 of their catalog) says the filter factor is 8X (3 f-stops). They also say, "The precise filter factor is determined by considering the film type and specific light source."

The reality is that you should be getting your filter factors from film data sheets. A sharp cut filter like this simply does not have a filter factor on its own. The filter factor is based on a combination of the film's spectral sensitivity and the light source. And it assumes a neutral item in the scene; this is the basis of the corrective effect of the filter factor.

I would personally start with the presumption that Hoya's "25A" is roughly the same as a Wratten #25. (I would somehow double check this.) Then, if you look at some Kodak data sheets, you'd find that Tri-X filter factors should be (roughly) 8X for daylight, and 5X for tungsten light. T-max is similar, except only 4X for tungsten. If you look at an obsolete film, such as Tech-Pan, you'd find factors of 3X for daylight and 2X for tungsten light. From this, it should be obvious that metering through the filter is a bad method - the meter doesn't know if your film is more like Tech-Pan or Tri-X.

The Ilford site isn't loading for me tonight, so I can't say what's there, but it's worth checking if you shoot Ilford films. Same for other brands.

As a few other people have mentioned, the exact colors present in the scene are affected differently. So this is something to keep in mind. Likewise for any oddball light sources, such as energy-saving fluorescents, LEDs, etc. So a first roll should maybe be considered partially as a test. Good luck.

polyglot · Jan 24, 2013

both Bills are giving excellent advice here.

Bill Burk · Jan 24, 2013

Thanks polyglot!

Mr Bill said:
Hi, I'm not going to spend a lot of time trying to convince you, but I think you are mainly being mislead by the B&H numbers. My guess as to what "3 (+2 stops)" means is this: +3 stops for daylight, or +2 stops for tungsten light.

Hoya's own information (p 47 of their catalog) says the filter factor is 8X (3 f-stops). They also say, "The precise filter factor is determined by considering the film type and specific light source.".

Mr Bill,

You might be onto something. This is where having a wealth of different answers can be confusing, I saw Hoya's and B+H contradiction and didn't know what to make of it.

Film type, light source, meter type, how you meter, what you want. All play a part in black and white filter factors.

Film type: Depends on the film you are playing with. Some differences are significant and might be several stops (Ortho/IR). Other films (Technical/Tabular/Traditional) differences are subtle but might lead factors to vary about a stop .

Lighting: (Tungsten/Daylight/Open Shade). Could be a stop just as you pointed out Mr Bill.

Meter type: Have you seen how super-responsive a selenium meter (Master II) is to Tungsten. This is a documented reason why manufacturers used to recommend lower EI in Tungsten... With tungsten less blue, less actinic... And with meter jumping wildly to the light bulb... Old-timers had two problems going opposite directions - so something had to be done. They recommended lower EI in Tungsten. Now we have better meters.

How you meter: Simply does your setup or habit make you meter through the filter or do you meter without filter?

What you want: If you want detail in shadows then "Hutchings" factors help you obtain that.

Filter Factor

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