View Full Version : Filter Factors

William Lindley
11-Oct-1999, 19:23
As a 35mm SLR Photographer, I never have had any trouble with filter factors, as the light was always read through the lens. Of course I realize I can hold the filters up in front of my Spot meter and take a reading through them, but would like to know the filter factors for the filters I use, and how to apply this inf ormation to my exposior. I only use three filters as I am an outdoor Photographe r and do not like to use many. The filters concerend are an 81C, A Cokin Medium graduated nuetral density, and a polerizer. If someone could advise me as to fac tors and exposior info for these, I would appreciate it. Also I would like to kn ow if you find these calculations are more helpful them simply holding the filte r befor the Spot Meter???

Thanks, Bill

Ellis Vener
11-Oct-1999, 20:36
Why not make your own list? You'll need an evenly toned wall or large surface that is evenly lit by daylight. Make a reading without a filter and then with the filters in question and take notes.

james norman
11-Oct-1999, 21:07
be careful trying to obtain correct filter factors with some meters - older spot meters can give you incorrect readings, as has been discussed on this forum in the past. much better to do some film exposure testing to determine the actual correction. the manufacturers recommended filter factors are usually the best place to start. go to your local photo store, and look at the boxes of filters like yours - the filter factors are marked on the boxes. or try the manufacturers websites - sometimes you can get that kind of info there. or you can call a good vendor, like calumet (1-888-888- 9083) and ask those guys about your specific filters.

Thomas A. Castelberg
12-Oct-1999, 03:39
With Cokin GND you have to add 1.5 stops for #120 and 2.5 stops for #121. Most polarizers require an additional exposure of 2 stops. Tom

N Dhananjay
12-Oct-1999, 14:22
You wouldn't add exposure with a graduated neutral density (at least if you're basing exposure on the parts of the scene over which you want to place the clear part of grad). The reason you use a graduated ND is because a part of the scene (typically the sky) is so much brighter than the rest that it risks burning out. So you put the dark part of the filter over the sky area to bring down the brightness of that area and expose for the ground or whatever. Instead if you increase the exposure, you will continue to burn the sky and now you also burn out the ground..... DJ

John Hicks
12-Oct-1999, 17:37
Filter factors can be pretty complex and are only really determined by testing.

For example, using my Heliopan #15 filter, which supposedly has a factor of tw o stops, and HP5+ film, photographing a Kodak grey card: the factor for direct s unshine is one stop and the factor for shade is one and a half stops. This is so mewhat different than the published factor and a meter reading through the filte r.

Bill C
13-Oct-1999, 00:48
Hi Bill, I see you don't have info on the 81C, yet. It's called a "light balancing" filter and is rated at 35 mired shift. The Kodak "Filter Handbook" suggests an "approximate" exposure increase of 1/3 stop when your final equiv. color temp is ~3200K to 3400K.

This filter works smoothly over the entire spectrum, transmitting roughly: blue ~60%, green ~70% and red ~80-85% of the light, so it ought to be pretty benign on exposure effects. You're shooting color, I presume. If you were, for example, shooting John's HP5 in bluish light (like shade on a blue sky day), you'd need more exposure correction; maybe over = stop.

Like John said, the factors CAN be pretty complex. Unless you like poring over spectral curves a lot, you should just test shoot your specific combinations.

john costo
21-Oct-1999, 20:53
The best method is to place your filter in front of your spot meter and take a reading through the filter; then place the filter on the lens and shoot. For filters used frequently (for me, a polarizer), you can purchase 40.5mm filters to screw into the front of your Pentax digital spot. Just make sure your meter filter and lens filter are the same brands. While slightly costly, it saves a lot of trouble.