View Full Version : Light meter with UV filter

27-Dec-2015, 21:06
In the last week I had two light meters calibrated by Quality Light Metric in Hollywood, CA, a Pentax V analog spot meter and a Pentax Digital spot meter.

In an effort to protect my investment I thought I would put a UV filter on each meter.

The Pentax Digital takes a 40.5 mm and the Pentax V takes a 46 mm filter.

I metered the same spot on a target from the same position and the Digital meter is 1/3 stop lower than the analog. I can live with that as each meter was calibrated to manufactures original settings.

I added the UV filter to the analog meter and it metered 1/3 less than without the filter.
I added the UV filter to the digital meter and it metered 1/3 less that without the filter.

I am somewhat surprised as I always thought that a UV filter did not reduce the light passing through it and reaching the film.

As I thought about this I remembered from something I read long ago that light passing through any object no matter how transparent will always block some amount of light because light is made up of both a wave and a particle.

If I leave the filters on the light meters they will always show 1/3 less that with out the filter and I am wondering what effect this will have on my exposure.

Any thoughts on this?


Kirk Gittings
27-Dec-2015, 21:12
The bigger problem IMHO is making them more susceptible to flair with the filters on which will throw off your readings. Notice how the PD has a built in lens hood? By putting a filter at the outer edge of that you are defeating the purpose of the hood.

Jim Jones
27-Dec-2015, 22:05
The purpose of the UV filter is to filter out light at the UV end of the spectrum which film (and the meters) respond to, but are mostly beyond the range of light our eyes can see. If you also protect your camera lenses with similar UV filters, the apparent discrepancy might be canceled. However, as Kirk says, filtering the meters has a down side. Since meters need not form a high quality image as do camera lenses, leaving the filter off of the meter seems logical.

Bob Salomon
28-Dec-2015, 03:10
Heliopan, and I believe B+W have a filter factor of 1.1 which is negligible. But they have always had that filter factor. So do their KR 1.5 skylight filters.

28-Dec-2015, 05:39
As an old friend of mine said about life, and it's a good thought: you can't do something and expect nothing to happen as a result. That sheet of glass you added there has both density and two reflecting surfaces, and it's intended to filter UV; there has to be some effect.

28-Dec-2015, 09:49
Good advice,here, and 1/3rd of a stop is the typical variation of most LF shutters anyway.

28-Dec-2015, 15:25
Thanks to everyone for your thoughts.
All good information.

I will not use a UV filter on my meters.

Thanks again.

28-Dec-2015, 15:49
Modern digital meters have both UV and IR filters in front of their sensor. The Pentax V has nothing, in fact it is not at all linear with respect to color, and it senses far into the IR range. I use it because of that, I bought step-up rings to put my IR filters onto the spotmeter so I can meter my IR directly. Of course I have to guess what it's pointed at, with anything above about 700Nm, and I have to use the light to see the reading. But it works.

I guess my point is: if you're using a filter on your taking lens, it's quite handy to be able to use the same filter on your meter. Any loss from the filter will cancel itself out. And look up the color response curves for whatever meter you're using; AFAIK none of them are linear.