I use a Pentax spot meter. I am wondering if rather than calculating filter factors, I can just place the filter over the spot lens and take a reading. Does that work?
Thanks!
I use a Pentax spot meter. I am wondering if rather than calculating filter factors, I can just place the filter over the spot lens and take a reading. Does that work?
Thanks!
It won't deliver a linear result across the range of colors, so the theory is that you shouldn't do that. For example, if the meter's sensor is overly blue sensitive and you meter on something blue, you might get inaccurate exposures. That said, I've been doing it for years and get pretty consistent results.
The Zone VI modified meters are supposed to be accurate when you take a reading through the filters. The regular non-modified meters are not calibrated for that use.
While back I had 2 Soligor Spot Sensor meters, one stock the other Zone VI modified. Both read within a 1/4 f stop of each other. Both read within a 1/4 f stop of each other reading through the same filter, even with a Wratten 25 (red tricolor) and a Wratten 12 (deep yellow - minus blue). FYI neither agreed with my SEI which should have been the standard to match, but in the end all were so close that I got perfectly acceptable negatives using any of the 3 meters readings.Originally Posted by Willie
I have 2 of the modified meters, and they work through filters.
I've got two modified and one unmodified Pentax digital spot meters. None of them are 100% accurate when reading through filters, especially the stronger ones like the 25A. No meter really can be; there will always be some mismatch with the spectral response of the meter vs. the film. Even the Zone VI modifications didn't fix this entirely (despite Fred Picker's claims).
However, a couple of quick tests through the two or three strongest filters you have yields an E.I. adjustment for reading through the filter that's easy to apply. Some stronger filters also affect the contrast of the negative. That's not so hard to test for either for a particular meter and film combination.
Yes, that's a bit like having a filter factor, so "why not just use the filter factor?" you may ask. Well, when reading through the filter with a spot meter, I can compare values from adjacent areas and see quickly if there will be mergers in tone or just how much separation I can expect. That's the advantage.
With weaker filters like the Wratten nos. 8, 11 and 15, the discrepancies are small and you can simply meter through them and use the reading. With the 25A and my #44A and #58 filters, I find I need an E.I. adjustment and, often, changes in development times to compensate for contrast changes when exposing through the filter.
Of course, one could just forget the adjustments, err on the side of overexposure and probably be able to deal with the resulting negatives just fine with today's VC papers (or digital adjustments), but I like to try for more accuracy, at least.
Best,
Doremus
Or you can dial in a new ISO setting with the filter factor (Don't forget to change it back, later!!!)
But even with my fairly modern Leica R6 TTL meter, B/W filters won't jibe with the factors... Then there's that spectral sensitivity curve on the film thing...
Steve K
If you use a Green, Yellow, Orange filters over the spot lens it should give a more consistent result. With Blue and deep Red filters reading should be less consistent.
If you look at the SPD spectorscopic sensitivity chart of the Pentax:
http://www.pacificrimcamera.com/rl/00042/00042.pdf
you see that spectral sensitivity for blue is 50% than with green.
So with a deep blue filter over the meter you will block a lot of long waves, and as the SPD of the film is different that the meter the reading will tend to overexpose (I guess is like this, not the counter), also the same will happen with deep red, but perhaps with a lower extend.
I'd suggest you to try it, just look what exposure indicates the meter with the filter, and what is calculated without the filter and applying the correction factor. Then just write in a paper the difference you have with each filter, and form then you know if the yellow match or if with the deep red you have to stop down 1/2 from what the Pentax says with that filter over it.
All also depends on the WB of the light source and the color of your subject...
Also the filter correction factor is orientative, as it do not account for SDP of the light, the color of the subjects and the film SPD.
Regards.
All my meters are precisely calibrated and matched Pentax digital spotmeters. I find it far more predictable to bracket-test for filter factors relative to SPECIFIC films and your representative subject matter and lighting conditions, then use the filter factors, rather than metering through a filter over the meter itself, which just opens a writhing can of worms. After awhile, filter factors get downright second-nature, and with most black and white applications you can round a bit, to the nearest full stop. Generally the filter factors published by the specific filter manufacturer are a perfectly usable starting point for typical panchromatic films in
daylight. Just beware of films with peculiar spectral sensitivity like ACROS, which is actually orthopan.
I've written a bit here and on APUG about how I use my Pentax spotmeters to read directly through filters. I'll elaborate a bit later, but first, here's a quote from the manual that comes with the Pentax digital spotmeter:
"By mounting different filters in front of the objective lens of the Pentax Spotmeter and comparing the readings taken of the various colored areas with each filter, it will be easy to determine which on provides the best contrast for black and white film."
So, obviously the manufacturer thinks metering through the filter a viable option.
However, there are a couple of problems with metering through filters. First is the mismatch of sensitivity spectra that has already been mentioned. This can be overcome by doing a bit of testing with the most extreme filters you use on a given film and arriving at some exposure factors.
The second, and less discussed, problem is that extreme filters that cut all but one wavelength of light (e.g., #25 red or #58 green, etc.) can affect the contrast of the film. This is not a property of the filter, rather a property of the film and the way it is sensitized and will occur whether reading through the filter or applying factors. For example, with TXP, a #25 red filter increases contrast for me the equivalent of about N+1. However, with TMY, the same filter reduces contrast somewhat. Again, a bit of testing with the films you use will give you a development factor if you wish.
There is a distinct advantage to metering through filters instead of applying factors that makes it a no-brainer for me. Only by metering different areas through the filter can you really see and approximately quantify the difference in exposure for areas of different color. This will show you if there is enough tonal separation between these areas or not. It's really the only way you can visualize a scene when using filters and a great tool for us Zone System users. I can easily spot a convergence between say a blue sky and a green roof and change filters if needed. Just applying factors doesn't allow this.
So, what I have done is made a chart for selected filters and the films I use (as Pere mentions, only for the more extreme filters) that includes an exposure factor and a development factor. Example: when metering through a #25 filter and using TXP I'll give 2/3-stop more exposure and develop N-1 from the normal indication.
Search on the topic and my name here and on APUG to turn up more discussions on the subject if you're interested.
Best,
Doremus
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