exposure with filter and spotmeter

[fralexis]

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!

[Michael Graves]

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.

[Willie]

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.

[LabRat]

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

[Pere Casals]

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!

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.

[Drew Wiley]

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.

[Kevin Crisp]

I have a Zone VI modified one. It has worked fine with yellow, orange and green filters. Film (Tri-X, usually) is very underexposed with a red filter. So I use the filter factor instead. Would an unmodified meter give me nearly the same results? Could be, I haven't tested.

[Drew Wiley]

No, they won't give the same result. I don't want to get in a debate over whether Fred Picker improved the meter or ruined it with those modifications. About all I can say is that he "standardized" it to Tri-X, and hence "unstandardized" it to anything significantly different. Hollywood cameramen also have their own debates over what this does to color film metering. I am accustomed to its out-of-the-box peak sensitivity to green, just like our own human vision, and know that panchromatic films are slightly depressed in green sensitivity, but otherwise differ from one another in the degree of blue and red sensitivity.

[Doremus Scudder]

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

[Pere Casals]

"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."



I've got some great ideas from your comment...

When one masters the spot metering with the filter on the spot lens... he can know with precission how the sky is darkened by the filter, for example, in order to determine a N-2 development or not... !!!

This is a powerful tool.

Anyway one has to know how the meter reacts to colors. Spectral sensitivity of film is not as critical as it is mostly flat, a 20% more sensitivity in a band it is a tinny fraction of an stop.


For consistency for now I'm using a Nikon F5 to meter LF in spot mode, if has a RGB photometer and things are pretty consistent.

With an F5 you can meter with any filter on the lens and results are ultra precise. For LF I spot meter with a prime lens (50mm) i the F5 that has same or close optical transmision than an LF lens. I tested it by placing the F5 i the back of the view camera.

[Drew Wiley]

Even with Nikons I prefer to hand meter and apply a filter factor. I find it's far more reliable, but do realize there are a helluva lot of models of Nikon out there,
and I certainly don't have any experience with all of em.

[Pere Casals]

Even with Nikons I prefer to hand meter and apply a filter factor. I find it's far more reliable, but do realize there are a helluva lot of models of Nikon out there,
and I certainly don't have any experience with all of em.

Drew, there are only 2 Nikon models there: the F5 and the others :)

I collect Nikons, all are nice machines, single problem of the F5 is weight. I use it as a TTL spot photometer for the view camera, placing it in the back, so it includes bellows extension, then also I make a bracketing with same film than the sheet, after developing the roll I know how I've to develop the sheets, and what exposure was the best, useful to me to learn new films...

[alen]

How about spot metering a grey card (and white and black cards too if you wish) through the respective filter?
As long as the grey cards orientation is the same angle as the subject and in the same light, i think that would be ok.

[Greg]

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.

[Pere Casals]

How about spot metering a grey card (and white and black cards too if you wish) through the respective filter?
As long as the grey cards orientation is the same angle as the subject and in the same light, i think that would be ok.

It is a partial solution, I think. It can be good if subject it is not very "saturated" in color and it has a wide spectral band as the grey card has.


This do not solves the case that the spotmeter has not a flat spectral sensitivity curve and that a color filter will darken some colors more than others. For example if we have an sky that will be everexposed and we want to know the effect of the filter, to know if the sky will drop 2 zones or 1 zone relative to the earth with a yellow filter. Some scenes have a high dynamic range that it is difficult to capture.

if we look the spectral sensitivity chart of the Pentax 5 (http://www.pacificrimcamera.com/rl/00042/00042.pdf) we see that sensitivity for blue is 50% of what it can be expected. So sky will be more overexposed than hat Pentax V says by pointing up, and this effect will be increased with a yellow filter on.


So it is not easy, and we can go to the problem by two ways. One is with real field experience, knowing is a Zone 7 sky is nice. The other way is by using charts to figure what will happen.

I think best option is combining both ways: knowing the theory of what happens, and looking what happens in practice.

Anyway I think that Doremus Scudder is right in what he says in post #9.

A Spotmeter it is very usefull know what contrast we'll have in a photograph, and it is also useful to know how contrast will change with placing the filter on the spotmeter, but corrections are needed.

[McAir]

I am just returning to LF black and white after a number of years. I used to meter through filters with my Pentax spot. Now as I refresh some of my techniques and reading this thread, my head is about to explode. Now, I'm not sure yet how I will be going forward. Seems like it might be ballpark except Red #25, which maybe should use recommended factor for the filter as a starting point. Maybe end up doing some testing but I sure didn't realize this was such a complicated issue.

[Drew Wiley]

Welcome back. The older threads mainly addressed the fact that meters do not have equal sensitivity across the whole spectrum. So if you take a reading through a filter it might not be correct. But another reason to apply a filter factor instead or reading through a filter is fact no. 2 : different kinds of panchromatic film themselves also differ somewhat in spectral sensitivity. For example, by careful testing with gray card exposures and densitometer readings afterwards, I discovered there was half a stop of difference in filter factor between Delta 100 and TMax 100 using a med green Hoya X1 filter.

Some people might shrug their shoulders about what seems to them a small difference. But those are apt to be the same people who shrug off several other factors under pretense of exposure latitude; and all those seemingly little things soon add us to a serious exposure error.

[John Layton]

...about those grey cards - be very careful of the cheap, shiny cardboard ones - as they can exhibit just a tad too much specularity, (depending on light quality) which can really throw readings off. About the only grey card that I've ever really trusted is called "The Last Grey Card," which is made from a tough plastic and which has a very slight, almost velvety texture.

[Alan Klein]

Curious how readings work with a polarizing filter since the stops change depending on how much you rotate it?

[BrianShaw]

Curious how readings work with a polarizing filter since the stops change depending on how much you rotate it?

It's an easy experiment, Alan. :) There might be difference between linear and circular polarizer, so 2 easy experiments perhaps.

[neil poulsen]

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. . . .


Respectifully, I tend to be skeptical about these kinds of manufacturers' statements. For one thing, it's out of context, since it refers to conventional exposure determination, which is fundamentally flawed, versus Zone Systems exposure determination (for the shadows). Also, one doesn't know whether it might be quasi-advertising, representing more aspirational thinking, versus realistic thinking.

There are two reasons for using a meter that are worthwhile to bear in mind. One is to get an idea of how one area of the scene will compare with another in the final black and white photograph.

A second is to determine exposure (exposing for the shadows) and to determine development (through metering highlights). I have both an analog Pentax V modified meter (where I can at least estimate tenths of stops for film texting and calibration), and a Pentax Digital modified meter for use in the field. I tend to think that modified meters are more useful for the former reason (above), where one is compariing how different colors will appear in the final black and white photograph. If one is using a color filter, then one can make these comparisons by metering through the color filter.

In contrast, by evaluating shadows and highlights, one is metering areas in the scene that are relatively colorless. I think that using a non-modified meter in these cases is acceptable. If one is using a color filter, then one can compensate the meter reading by applying a filter factor.

When I'm in the field, it might be nice to compare how different areas in the scene will copare in the final black and white photograph, with or without a filter. But, my primary purpose is to determine both exposure and what development time that I'll be using. After these have been determined for the negative, it's pretty much job-done.

Chris Burkett is a master color photographer, especially when the method of capture is via color transparency. And I've had the good fortune to have twice participated in guided tours of his studeo. Its interesting to note that he used a Pentax modified meter to estimate exposure in the field. Just in case, he also maintained a second Pentax Zone VI modified meter as a backup.

[Drew Wiley]

If Chris Burkett has his reasons for using a modified Pentax meter, I have equally good reasons for using using the un-modified version. And one of those is the fact that modified meters contain internal filters which fade or degrade over time, and need a specialized kind of service, whereas factory meters are all calibrated to an industry standard which can easily be compared one to another to see if adjustment is due. The functional debate one style over the other, has been known to occur even with Hollywood cinematographers working in color exclusively. But all the modified ones are getting old by now, and I sure wouldn't buy one personally. But other than that arcane debate, Pentax and similar Minolta spotmeters are a well-known precise products known to be consistent for a number of decades now. I've have four of them, one so mauled from hundreds of trips the mountains that it's held together with masking tape, the others in fine condition - but all still read the same over the full range, given a minor recalibration tuneup about once per decade.

[neil poulsen]

Just as an aside, I was speaking in past tense to the time of the tours. (Probably 15 years ago.)

I just had both my meters calibrated by Richard Ritter; I'll check with him on this. I know that both meters are currently reading corrrectly after calibration.

[Robert Opheim]

I just keep a notation of the filter factor in each filter case.

[Jim Noel]

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.

I have 2 of the modified meters, and they work through filters.

[Doremus Scudder]

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