Using my M6 TTL for black and white with a red filter (#25), I got very underexposed images. Then I found in the instruction manual a statement that the meter doesn't respond accurately to a red filter. Seems consistent with my experience, but pretty amazing as I have never encountered this with any other camera. Two questions: any idea why Leica would use such a problematic sensor/meter for a camera often used for B&W film; and has this been corrected in the M7?? Thanks. r.a.harris

-- r.a.harris (porsche993@austin.rr.com), May 18, 2002

Answers

where in the manual does it say you can't use a red filter and get accurate meter readings?? i missed that. i have used a red filter (8x) on my m6 many times with good results. what an odd thing!!

-- roger michel (michel@tcn.org), May 18, 2002.

The M6 TTL instruction manual states on p. 92: " orange filters generally require one stop more exposure, red on average two stops more....." This is what I have seen in using these filters on the M6 - but I have never had this problem on other cameras. Seems pretty dumb for Leica to use a meter that does not respond accurately to common filters(?)

-- raharris (porsche993@austin.rr.com), May 18, 2002.

This is a common problem with light meters..Many are overly blue light sensitive .. Here is an Ilford PDF on special extra red sensitive B&W movie film.. It gives the usual warning that one may have 1 1/2 stops under-exposure when using dark red filters..It rightfully states that one should do ones own test with your TTL meter when using dark filters...

The 150 foot roll of HIE 421 (High Speed Infrared Film 2481 of mine expired in Nov 1975..The few wacky photos I got were thru brute trial and error....We got all the Kodak books available..We were using the stuff to photograph people without their knowledge..We placed our electronic flash/(or flashbulb) at the focus of a 12x12 square fresnel lens; which was covered with nine 3x3" wratten filters that only let Infrared pass...This unit would flash a narrow infrared beam for our camera...We bought 3 different types of kodak wratten IR filters..we settled on the middle "darkness one"; that allowed some light; but was barely visible.....A flashbulb allows alot more light thru; it has alot of outout in the IR region....The IR film was grainy as hell...

-- Kelly Flanigan (zorki3c@netscape.net), May 18, 2002.

OUTPUT So much for spellcheck!

-- Kelly Flanigan (zorki3c@netscape.net), May 18, 2002.

Many cameras will give inaccurate readings through a red filter, not just Leica. The inaccuracy may not be linear across the range of brightness, so a compensation of a set number of stops may not be applicable at all lighting levels. When shooting with a red filter the only really accurate way is to meter without the filter and apply the proper factor.

-- Jay (infinitydt@aol.com), May 18, 2002.

Or, to bracket!

-- George C. Berger (gberger@his.com), May 18, 2002.

Silicon photodiodes, the basis of most modern meters, have a spectral sensitivity which is heavily skewed to the red end of the spectrum. For use in photographic light meters, they fit a red absorbtion filter, but to keep the meter reasonably sensitive and useful there is an inevitable amount of oversensitivity on the red side. This means that metering while looking through a deep red filter will underexpose, often by 1-2 stops.

This is true for nearly all modern cameras, but in varying amounts.

The key to determine what the red filter factor is for any given camera is to meter a scene without a filter, then add the red filter and meter again, preferably under the same lighting conditions that you'll be shooting under. The second meter reading should be the same as the first plus the red filter's exposure factor. If it is any less than that, the difference is the adjustment you need to make to obtain the correct reference exposure.

With a manual camera, you make it by tweaking the shutter speed and aperture, or adjusting the film speed. With an automatic camera, the sensible way to adjust it is to tweak the EV Compensation setting.

This is hardly a fault for Leica. All cameras are this way, it's just that most manufacturers don't list it, they presume that film latitude will carry the day.

Godfrey

-- Godfrey (ramarren@bayarea.net), May 18, 2002.

ra

The late Fred Picker explained this in "The Zone vi Workshop" book and is why his modified Pentax spot meters had special filters to respond linearly to the entire spectrum. That is one of the pitfalls of TTL metering and must be compensted for by testing or the general rule of 1 stop for yellow, 2 for orange and 3 for red. Works for me.

Cheers

-- RICHARD ILOMAKI (richardjx@hotmail.com), May 18, 2002.

Gallium arsenide photocells don't suffer from this problem -they respond linearly across the spectrum. Alas they are considerably more expensive than silicon cells, so they are seldom seen. Are they used in any of the Leicas? Nikon used them in the original FM, but dropped them in the FM2 and subsequent models.........

-- david kelly (dmkedit@aol.com), May 19, 2002.

richard: about your rule: going up or down?

-- stefan randlkofer (geesbert@yahoo.com), May 19, 2002.

get an M7, set the exposure compensation dial to +2 and you are all set.

-- Tristan (emulsion71@hotmail.com), May 19, 2002.

Stefan;

One stop MORE exposure for yellow, 2 MORE for orange and 3 MORE for red.

This follows the absorption and apparent density of the filters. I must say that these should be the basis for your own test. At the expense of one roll of your favourite B&W film, you can take full frame photos in standard light-overcast sunlight- of violet blue green yellow orange red pieces of coloured cardboard, all at the measured setting for 18% grey. Then compare the resultant densities on the negs. That is easily done by meauring the light thru the neg with a hand-held meter or thru a TTL setup with a slide copier. Or, you can make a contact print of all 6 frames on the same sheet of paper and see the varying densities. This will tell you teh relative sensitivity of the film.

Then take an 18% grey card and take shots with the filters as measured by the TTL meter and compare the resultant densities. This will give you the exact effect of the filters when you make the contact sheet.

You may want to get a white board and a sky blue board and take photos of the two-colour boards side by side with the three filters, thus giving the effects on clouds in the sky.

Certainly not professionally quantitative but will give you a very good idea of the effects of filters.

Cheers

-- RICHRRD ILOMAKI (richardjx@hotmail.com), May 20, 2002.