Exposure correction for filters

[johnnywalker]

Some time ago I read here (I think) that one should use the filter correction for a specific filter rather than try to meter through the filter for the correction factor. I don't remember who said it, apart from the fact it was someone who should know what they're talking about. I've been having a few issues with filters lately, including finding out what the filter factor is supposed to be on some used filters I've bought for my RB67 and 4X5. So, I'd like to know what the problem is metering through the filter with my hand-held meter. After all, in-camera meters meter through the filter.

 

[DamenS]

Even in-camera meters only provide an "approximately correct" light metering with coloured filters (pretty much all filters apart from UV/Clear - Skylight isn't too bad) as they are not uniformly sensitive to all visible light, and using a filter changes the quality/colour of this light - I don't know if this analogy is appropriate but I guess it is similar to them judging the reflected light from a dark surface and a light surface differently (in their misguided attempt to produce a "neutral gray" tone). It has been a while since I researched this so I can't provide an immensely clear technical reason - but trust me that the GIST is right at least !!

Having said that, metering THROUGH the filter should (as with in-camera meters) at least be in the right ballpark - I guess it depends on how accurate you need to be.

Should also point out that the darker the filter colour (the higher it's filtration and compensation factor) the more likely it is to "fool" the exposure meter behind it (ie. an 81a or light Yellow will fool the meter less - as it is changing the colour spectrum less - than a deep Red filter).

 

[Ian Grant]

Companies used to publish data-sheets of filter factors because they vary slightly depending on the film used and also the type of light source. It's easy to test whether a camera's meter is fooled or not by metering with & without the filter and comparing the difference to the filter factor.

I acquired an Ilford Filter factor data sheet, tucked inside a 1940 Kodak Professional catalogue, and this shows the slight differences between films and also Daylight/Tungsten light sources. (See below)

In practice the differences between films will be slight except for the Ortho films and the Adox/EFKE films particularly KB/R/PL25 with their reduced red sensitivity.

Ian

 

[Rick A]

I've checked all my filters against the published data for each on my OM-1. My meter does not compensate as much as needed for the dark orange and red, it only compensated 2 stops. My orange filter requires 2.5 stops added exposure and the red needs a full three extra stops. I havent tried metering through them with my hand held meters, I just do the math, or set the meter for the desired adjustment and take my reading.

 

[polyglot]

As ralnphot implies, the spectral sensitivity of the meter may not match the film. Say in the case of a meter with lots of red sensitivity (pretty common) and a red filter, the meter would claim there is more light than the film could capture, leading to under-exposure. When you combine this with a film with poor red sensitivity (e.g Efke-25) then you need even more than 3 stops compensation and metering through the filter will be very wrong. Once you get to IR films, the meter is basically meaningless because they're calibrated for the visual band not the IR band.

For things like ND and CPL, metering through the filter should be fine.

 

[epatsellis]

Also, be aware that spectrum differences between tungsten and daylight can cause filter factors to change. Your best bet is to download copies of the technical data sheet for your intended film, if they are available. They typically list the filter factors for both tungsten and daylight light sources.

 

[RalphLambrecht]

Also, be aware that spectrum differences between tungsten and daylight can cause filter factors to change. Your best bet is to download copies of the technical data sheet for your intended film, if they are available. They typically list the filter factors for both tungsten and daylight light sources.

I agree. Short from conducting extensive tests with different light sources, different films and differently colored subjects, that is the best approach.

 

[johnnywalker]

So the jist of this is that I shouldn't trust my TTL metering if I have a filter on the camera (which I usually do, a yellow #8 at least), and will have to remember to correct the exposure, especially for darker filters. This explains a lot, but is a Royal PITA and something I should have questioned years ago. Some of the filters I have I haven't been able to find the manufacturer's recommended correction factor and they are a slightly different colour than the ones with a standard designation written on the filter. That is, sold to me as a yellow-green, but not quite the same colour as another I have that has XO written on the filter.

 

[Carl V]

It's possible TTL meter readings with filters attached may vary from camera to camera with regards to how well they cope. I use all the various filters for black & white films, ie. yellow, orange, red and green and have made comparison readings with and without the filters attached when viewing the same scene.

On my Nikons, they appear to read correctly when attaching the yellow filter which would require one extra stop of exposure. With the orange filter which requires two stops, my meter would underexpose by half a stop. With the red filter which needs three stops, the meter would underexpose by one and a half stops.

 

[Rick A]

I normally have a yellow 8 on my OM-1 for shooting B&W, and always exposes properly. The yellow 8 only needs one stop extra exposure(generally) for daylight, tungsten is different, and I usually remove the filter for that type of light. You can test your filters for yourself and note your findings for later use.

 

[Carl V]

I normally have a yellow 8 on my OM-1 for shooting B&W, and always exposes properly.

Yes, I tend to leave a yellow filter on the lens all the time and only change to one of the other colours when I want to achieve a desired effect. As with your Olympus, I have no problems metering with the yellow filter.

 

[Ian Grant]

I agree. Short from conducting extensive tests with different light sources, different films and differently colored subjects, that is the best approach.

It would be if Film manufacturers published detailed Filter factors for their films.

However with such a wide variation in the filters available Kodak only give data for their own Wratten gelatin filters and Ilford say to get the data from the filter manufacturers instructions. Foma give no recommendations.

Then that's flawed because Filter manufacturers don't give data for different films or light sources.

So it is a case of giving the factor you think is closest and a bit of tweaking with experience.

Ian

 

[johnnywalker]

Rick, how do you test them if you can't use a light meter to do it?

 

[Ian Grant]

Rick, how do you test them if you can't use a light meter to do it?

Test them on a 35mm camera one shot no filter and then bracket with the filter and compare negatives when processed.

Ian

 

[johnnywalker]

It would be if Film manufacturers published detailed Filter factors for their films.

However with such a wide variation in the filters available Kodak only give data for their own Wratten gelatin filters and Ilford say to get the data from the filter manufacturers instructions. Foma give no recommendations.

Then that's flawed because Filter manufacturers don't give data for different films or light sources.

So it is a case of giving the factor you think is closest and a bit of tweaking with experience.

Ian

Right. And, I can't find the manufacturer's information for Tiffen, Sunpak or Marumi filters on their respective websites at least.

 

[Ian Grant]

Right. And, I can't find the manufacturer's information for Tiffen, Sunpak or Marumi filters on their respective websites at least.

Use the nearest Hoya or Cokin equivalent as the staring point, should be reasonably close, and both manufacturers give reasonable data.

Ian

 

[DWThomas]

There's an interesting compilation of filter info on Jack's Photography and Chemistry Site. At least it's another set of data points.

"It's on the Internet, it must be true"

 

[Rick A]

What Ian said. Thank you Ian.

 

[Ian Grant]

I solved all my filter issues by switching almost entirely to Cokin P series in the 1980's, I got fed up with needing stepping rings, and different size filters depending on lens choice.

Now I have a small set of filters that fit all but one of my lenses, not sure yet what I'll use on my 165mm Super Angulon it needs huge filters

Ian

 

[2F/2F]

Yet another one of the many reasons to completely ignore your in-camera meter. Save money on batteries, and get better exposures. What's not to like?

 

[ic-racer]

For hand-held rollfilm photography with a TTL average meter, if you have been using the camera without the filter and getting good results, just meter through the filter. Don't complicate things, because you will never get it exact unless you know the spectrum of the film and sensitivity spectrum of the meter. If you are using a spot meter, you will have to estimate exposure based on the filter factor, because, unless you include a gray card to meter from, any meter reading through the meter will be influenced by the color of whatever the spot meter is viewing.

 

[Ian Grant]

With a Red filter you can easily be 1.5 stops under exposed if you use a cameras meter so that's very poor advice.

Ian

 

[RalphLambrecht]

Test them on a 35mm camera one shot no filter and then bracket with the filter and compare negatives when processed.

Ian

... shooting a gray card, I suggest.

 

[RalphLambrecht]

I solved all my filter issues by switching almost entirely to Cokin P series in the 1980's, I got fed up with needing stepping rings, and different size filters depending on lens choice.

Now I have a small set of filters that fit all but one of my lenses, not sure yet what I'll use on my 165mm Super Angulon it needs huge filters

Ian

You buy expensive lenses and put plastic in front of them???

 

[RalphLambrecht]

Yet another one of the many reasons to completely ignore your in-camera meter. Save money on batteries, and get better exposures. What's not to like?

That's what I settled on, by the way. No more in-camera metering here.