Yet another filter question

Sorry for continuing to pick all your brains in this fashion but there is a big upside: I am currently working in a camera store and have become known as "the resident film guy". All the local university/college art students come to me with questions and I pass on all information I can plus the forum's address as often as possible.

I recently purchased some Hoya filters and was testing them out yesterday by setting up a gray card in sunlight, taking a reading with my spot meter (known to be reliable) and then holding the filter in front of the spot meter and taking a second reading. The change in readings are quite different than the filter factors published online. The yellow K2 had less than 1/3 stop change (as opposed to the supposed filter factor of 2 or 1 stop); the red 25 had a full stop change (compared to the 8 filter factor or 3 stop) and the circular polarizer had a 4 stop change (compared to the factor of 4 or 2 stops) regardless of position.

Any ideas which adjustment to make?

A couple of things come in to play here. The filter blocks some wavelengths of light and passes other. The meter is more sensitive to some wavelengths then others. The gray card is in full ( white ) light. So with all these factors the reduction in light as read is not as you expect. The filter factor is computed as an average. Then to add one last item to the mix for shooting, is the fact that shadows are generally illuminated in blue light. So even the filter factor can be off when you may be expecting details in shadows. 'Understanding the View Camera' has a good section on filters, and how Gordon Hutching has some added adjustments.

Instead of using a meter take a series of shots with the different filters on camera with your favorite film. Since my archive file is 5 negs wide I take the some shot with No Filter, Red,Orange,Yellow,Green; as the meter reads then the same shot, same order, but this time hand metered and filter factor added. I do this three times on different shooting situations. Any set of filters can be used but it is often very telling the difference in what is metered TTL and filter factor produces. Doing this for each film helps determine a personal filter factor for the filters you use and the camera, how it see the light.

TR

Please don't think you need to apologize. It's a good idea to pick people's brains on tech subjects like this. In your work, you'll be talking to many people who need facts, and it's a good thing to get whatever info you can. This is a good place to ask.

The published filter factors are the best starting point. The film companies base their filter factors on metering with no filter over the meter and applying the factor to the resulting reading. They also presume one is metering "plain vanilla" midday daylight incident upon a neutral colored surface. Since both film and meters have varying spectral responses, once you start getting away from the neutral colors, things can get complicated.

Your experience metering through strong filters shows why many people, myself included, are hesitant to meter through filters. That being said, I do have to add that some people do so, and many of them add a correction to make the process work, and do achieve good results.

Strongly colored light and strong filters can make for "interesting" problems, so I try to meter off neutrals in the subject - or gray cards or do incident readings.

Best,

C

The idea of associating a fixed filter factor with each type of filter is not so useful. The stated filter factor is just a suggestion that is based on a certain film spectral response and certain light and a certain meter response, as CBG said.

There are many situations in which a manufacturer's suggested filter factor is not appropriate. You have to ask yourself a few things:

*What is the colour temp of your light; is it significantly different from 5000K? (Recommended filter factors you refer to are probably based on ~5000 K)
*Does your film have typical spectral response, or is it extended pan or IR or ortho or something with an atypical response that might require an unusual factor? If colour, is it balanced to 5000 or 3200 K?
*And does your meter accurately represent the sensitivity of your film? (We usually assume that it does)

The filter factor of a filter depends on the spectral response of whatever is "looking through it". Meters are often rather unlike film in their spectral sensitivity. Film spectral sensitivity curves vary as well, note that Kodak gives distinct filter factors for each of their B&W films in the data sheets, and they are not the same from film to film.

Kevin Kehler said:

I recently purchased some Hoya filters and was testing them out yesterday by setting up a gray card in sunlight, taking a reading with my spot meter (known to be reliable) and then holding the filter in front of the spot meter and taking a second reading.

Click to expand...

I assume that you used a hand held meter, right? How exactly did you put the filters in front of the meter? I doubt that you can get a meaningful reading this way. Try metering with any camera (center weighted metering will do just fine with a grey card) and see what the difference is. I think you'll get something very different.

Thanks everyone for you replies, I would have replied yesterday except for some internet connection issues. I was planning on doing a full-shoot/print test but was hoping I was just doing something wrong that I could have delayed doing that till the summer. I am using a Minolta F spotmeter that I know from testing to be accurate, or rather, that I accurately interpreting its' readings for the shooting I do. I was just holding the filter up over the meter, as close as possible to avoid extranious light, and then taking my readings. My primary B&W camera does not have any metering so I will have to use another but I never thought of that.

Again, many thanks and I will burn through a couple of reels soon. (Poor me, have to go take more pictures)

I would echo trexx's comments with a couple of other things. First, there are meters and then meters. Some are well corrected for average film sensitivity, but many are not. CdS meters (not often found these days) can be quite accurate, but their sensitivity peaks sharply in the yellow-green part of the spectrum. Silicon cells, more often found now, are much more sensitive in the red area than in the blue. Sometimes they are corrected with filters, but usually they are not. This can cause false readings with filters, and it is why you should generally take an exposure reading without the filter and then apply the filter factor (unless you know the meter will give you proper results when reading through the filter). Another consideration is that filter factors are determined for average scenes. A gray card may or may not be equivalent. Some scenes can really throw off your measurements. For instance, red rocks through a red filter may well require less than a filter factor of 8. Bracketing is needed in such tricky situations. Also, filters from different manufacturers may require different filter factors or may just perform differently. Hoya is a big and well respected manufacturer, and using the filter factors they publish for their filters should be a good starting point for their filters.

Also see this thread:

(there was a url link here which no longer exists)