filter factors: measuring them using a lightmeter?

[Maris Rusis]

....
All this leads me to ask, for example, how to deal with metering a blue subject through a blue filter. Anyone? Guessing the only way to really know is to test it with a favorite film ... like Maris says.

I've done this. My dark blue filter has been film tested and it eats 2.3 stops of light. When I take a light measurement with my Sekonic L-758D and then interpose this blue filter the meter reading drops by 2.3 stops. Bingo! The Sekonic reads correctly for blue light (but as previously discovered it is over sensitive to red and underestimates the density of a red filter).

When exposing through this blue filter the prior meter reading I take through this same blue filter will indicate a middling exposure, an informal Zone V or grey card equivalent, that will put a middling density on a piece of panchromatic film. And it doesn't matter what the colour of the original subject was, blue, green, red, whatever, because the exposure and the meter reading use only the same blue light.

Now to complain. Why can't makers of photographic light meters, even expensive ones, calibrate the darn things to see light the same way as panchromatic film? How hard can it be?

[Drew Wiley]

Metering through filters is just looking for trouble. Not only do they differ with respect to wavelenth sensitivity, but so do different flavors of even pan film. I do own a couple of expensive darkroom meters that were designed to be color-neutral, but that's a different story. Start with mfg tech sheet recommended filter factors per specific film, and then run bracketed gray card tests on roll film to find out what matches your preferred meter.

[Pere Casals]

Pere...do keep in mind that I'm speaking of the Pentax spot meter version which was modified by the (now-defunct) Zone VI company...to specifically address what are otherwise some spectral biases of the factory version of this meter, as reflected in the chart which you've posted above. Not saying the modified version is perfect...but measurably better. Richard Ritter could give you more info on this modification. In the meantime...yes - I've heard amazing reports of the accuracy of the F-5's meter!

John, regarding spectral sensitivity of meters, I wouldn't say that the original Pentax Digital is not good. IMHO it can be considered more or less suitable depending on film we use, subject color, illumination SPD, filters and what we want, so IMHO there is no other way than experimenting with our meter in real field conditions.

I would like to say something more about the F5 meter in matrix mode. The sensor fills all screen with 1,005 elements RGB Matrix, so it knows if we are to saturate one of the color layers, it also considers the particular film latitude known from DX code, so it exposes very different Velvia than Portra. The RGB information is processed in a Neural Network that was trained with 30,000 real examples that were correctly exposed, so at the end by knowing the color of each pixel it may understant if the good exposure policy is the one for shots in the snow, if we have tugnsten or daylight illumination, or where is the sky.

The impressive thing is that it never doubts, by changing the framing we can have a lower average/ponderated reading, but as the matrix meter "understands" the scene then the reading rather remains stable.

I usually use the F5 in spot mode, in special to meter LF scenes, when I can I also check what says the meter in matrix mode, if a discrepance is there normally the F5 pointed the best choice.

I point all that because IMHO it shows how complex has to be an smart meter, to the point that a neural network algorithm is required, and amazingly it knows how to TTL meter perfectly when a color filter is on the lens...

http://www.mir.com.my/rb/photography...onF5/metering/

[Corran]

The F5 weighs as much as my Chamonix so it stays at home when I'm shooting LF and the Pentax spotmeter goes in my pocket.

[Doremus Scudder]

We've veered far from the original topic...

But, heck, I'll chime in here too.

Most of you here know that I'm an advocate of metering through filters. Yes, I'm aware of the spectral-sensitivity differences. These need to be dealt with by testing. Once that is done, however, you have a tool that allows you to visually compare different areas of a scene with the meter. This is enough of an advantage for me that I'm willing to a) do the testing and b) live with the inherent inaccuracies that still exist.

Just applying a filter factor won't tell you if that blue sky is going to be rendered lighter or darker (or boringly the same) as the green copper roof, or whatever. Reading these values through the filter is the only way to get even an inkling of how various colored subjects in a scene will be rendered in the final print. Just applying a filter factor, no matter how well tested using a grey card, is just a crap shoot in this case. I think that metering through the filter at least "loads the dice" for me a little.

So, I do the testing (both exposure and contrast, since strong filters affect the contrast of different film emulsions differently) and come up with a set of "fudge factors" for the meter, filters and films I use. Fortunately, only the fairly strong filters really need much compensation.

Given that most light meters aren't matched to the spectral sensitivity of films, there is an inherent inaccuracy for color-saturated subjects in a scene anyway. I imagine that with my "fudge factors," metering through a red #25 filter with my (unmodified) Pentax digital meter may be more accurate than metering without a filter...

Best,

Doremus

[Pere Casals]

Doremus,

Let me point an "heretic" metering way : a cheap DSLR. A D3300 weights a bit more than a Pentax meter, but it shows RGB histograms, and it makes BW instant conversions after placing a filter on the DSLR lens, so we can guess very well the tonality we are to obtain.

Once the DSLR-Film matching is understood this delivers a crazy amount of control, perhaps beyond what's necessary with negative film, but with a Velvia sheet one cannot make jokes, anyway for that job I like metering with an F5.

With color film (this is a bit off topic) we have 3 color layers, the metter may see a right exposure, but as light is concentrated in a color then that "small" amount of light may burn a layer if subject is saturated, destroying detail. That's easy with blue sky.

IMHO an experienced&good photographer may nail exposures just smelling the scene but, at least for learning, a DSLR is best meter we can dream with, I think, specially when filters involved.

[Pere Casals]

The F5 weighs as much as my Chamonix so it stays at home when I'm shooting LF and the Pentax spotmeter goes in my pocket.

Bryan, you are right, but as my CAMBO 810 backpack floats in the air then an F5 brick in the bag always heps stability while running uphill. Just training to attack Hornbein Couloir next winter

[jon.oman]

With regard to filter factors. I have a Promaster yellow Y2 filter. They say the filter factor is 2, and the exposure factor is 1 stop. Am I correct in taking a film of 400 iso, and setting my meter for 200 iso to account for the 1 stop difference? I like the idea of just setting the iso on the meter if this is the case.

[cowanw]

Assuming you are metering through the filter or camera, this is incorrect. You are accounting for the filter factor twice. The meter will read one stop less light, thus requiring an extra stop of exposure and the setting will cause the meter to require an extra stop because you are saying to the meter the film is less sensitive.
If you are not metering through the camera or filter, then this is fine.

[jon.oman]

Assuming you are metering through the filter or camera, this is incorrect. You are accounting for the filter factor twice. The meter will read one stop less light, thus requiring an extra stop of exposure and the setting will cause the meter to require an extra stop because you are saying to the meter the film is less sensitive.
If you are not metering through the camera or filter, then this is fine.

Thanks! I am not metering through the camera or filter.