Filtering for a deep red filter

**Bob Salomon** · 24-May-2022, 04:08

Originally Posted by Alan Klein

Changing the ASA setting on the meter is chancy when you forget to change it back after the shot and you change or remove the red filter.

Just to be slightly picky, it hasn’t been ASA for decades. It’s ISO.

**Tin Can** · 24-May-2022, 04:46

Yes it is, but I prefer as I first learned ASA as they are the same

While looking this up to be correct, I found a problem with Dentists

Dental Radiography: Doses and Film Speed

Originally Posted by Bob Salomon

Just to be slightly picky, it hasn’t been ASA for decades. It’s ISO.

**nitroplait** · 24-May-2022, 10:44

From the Pentax Spotmeter manual:

"When taking black-and-white pictures of a colored subject, you know that the areas adjacent to each other having little difference in light reflection, although different in visible colors, will not be reproduced on the film in proper contrast. In such a case, a proper filter is selected and used to properly reproduce the contrast.

It is difficult, however, to select a proper filter to be used for that purpose. The Asahi Pentax Spotmeter helps select proper filters. Put the filter over the objective lens of the Spotmeter and read the difference in the L.L. readings.. . use different filters until the difference in the L.L. readings (contrast) of the picture areas in question will become greatest. Select that filter which gives the greatest difference in the L.L. readings to properly reproduce the contrast.

The Spotmeter will also help correct illumination not only in color but also in black-arid-white photography. The reproducible contrast of a picture area is limited within a certain range as stated earlier. The Spotmeter readings of the various important picture areas will indicate which areas should be additionally illuminated."

In other words, just put the filter in front of your meter.

If you happen to disagree from experience; then compensate accordingly.

**Drew Wiley** · 24-May-2022, 17:54

nitoplait - that odd advice has little or nothing to do with conventional filter factors, but about manipulating overall contrast through specific filter selection. Perhaps there was some application in black and white movie or TV studio sets where that method was hypothetically used, but I've never heard of anyone doing it in still photography where simple development changes work best.

**Bernard_L** · 25-May-2022, 00:21

Also depends on film type. Some have a response falling to zero ~650nm, some ~700nm. Just like for the light meter's response, a small difference when unfiltered, a large difference when restricted to "red".