In Wetzlar's pursuit of perfection earlier M3's incorporated a glass pressure plate while hand loadable Leica cassettes were available.In relation to film flatness do modern felt cassettes or modern thin emulsions affect film flatness? How do we know whether or not our M pressure plates may be a little out?Does film wind on tension have any effect on possible film "curvature" or does depth of focus satisfactorily cover film flatness "errors" for all M lenses at max aperture?

-- Sheridan Zantis (albada60@hotmail.com), May 08, 2002

Answers

I believe that theres still a very slight curvature of film on the pressure plate. Contax tried to overcome this with their RTS3 with a vacuum back plate that sucked the film onto the plate. This was subsequently proven to work, but whether it offers any practical benefits in everyday situations is debated.

-- Karl Yik (karl.yik@dk.com), May 08, 2002.

Many 35mm camera film plane has "film channel" architecture, ie the film is not directly pressed on to the film plane, instead, there is is thin channel, or tunnel, where the film is free to move.

Contax RTS III used vacuum system to maintain film plane flatness

There are other cameras which has no film channel and its associated film plane error for exampe

• All Minox 8x11 camera, a moveable pressure plate press the film direct on the film plane, this contribute to the exceptional sharpness of Minox camera; during film advance, the pressure plate automatically lifted away from the film, leaving the film free to move.
• Minox GT-E 35mm camera, it has no film channel, Minox GT-E channel less film plane

My R5 has typical "film channel" film plane archicture

I don't know how the M series film plane looks like.

I think the screw mount Leicas may have channel less film plane otherwise, I imaging it would be difficult to insert or remove film over tall "guard rail" to lower film plane rails.

-- martin tai (martin.tai@capcanada.com), May 08, 2002.

If I remember correctly, they dropped the glass pressure because it was causing static problems. Glass is not more intrinsically "flat" than metal. Both have to manufactured to a high standard to function as pressure plates.

-- John Collier (jbcollier@shaw.ca), May 08, 2002.

Martin

Is this why the Minox G series are prone to film scratching? This was my one bugbear with the otherwise excellent Minox 35mm I had.

-- Robin Smith (smith_robin@hotmail.com), May 08, 2002.

Robin, I use GT-E extensively, never have film scratch problem. Film scratch is usually caused by dust particles on film fack pulled over pressure plate.

-- martin tai (martin.tai@capcanada.com), May 08, 2002.

Contax RTS III has a ceramic pressure plate

Contax T2 also has a ceramic pressure plate, ceramic being the specialy to Kyocera.

-- martin tai (martin.tai@capcanada.com), May 08, 2002.

The first batch of Lennigrad LTM spring motor drive cameras had glass pressure plates. I read that these were changed to metal due to static buildup while moving the film rapidly past the pressure plate....

-- Kelly Flanigan (zorki3c@netscape.net), May 08, 2002.

Minox 35 cameras are allergic to dust and lint

Due to its small size and light weight, people like to put Minox 35 camera in pocket or purse, without case. The macrolon camera body rubbing fabric causes static electricty which attracts lint and dust on the camera body.

When Minox 35mm camera is open up, the lens comes out of the body however the there is a light tight buffer attach to the lens, when the lens moves, the buffer also moves, turning the film chamber cavity into a dust suction bellow, sucking all dust and lint into the camera body; some dust particles landed on the presure plate, some goes deep in to the inard, clog up the shutter magnet and cause the shutter failure.

The easy cure is always carry Minox 35 in a case, Minox leather case or other case. In this way, the shutter will last a long time and no danger of scratch.

-- martin tai (martin.tai@capcanada.com), May 08, 2002.

As long as the pressure plate shows no sign of damage and is working correctly, Leica M's do not have a film flatness problem. I've only encountered film flatness problems with roll film cameras.

-- Andrew Schank (aschank@flash.net), May 08, 2002.

You can check an M pressure plate by removing the back and looking across it at an accute angle and observing the reflection of something bright - any distortion and you have a problem - it must be absolutley flat! Leica lenses and RF systems are designed to focus .15mm infront of this surface but modern films can be a bit thinner than this by around .02 of a mm therefore a small amount of bulge will be acceptable on some films. Bulge depends, amongs other things, on humidity, temperature and how long the film has been wound on - all variables which make absolute precision impossible. A perfectly flat film plate and an RF system and body thats within Leica tollerances should get you near enough for other factors in sharpness ie camera shake, focusing ability of the eye etc to be more significant. There is a photosite with a large section on film flatness - Monaghan ( spelling?) - could make you lose sleep at night worrying though!

-- Johann Fuller (johannfuller@hotmail.com), May 08, 2002.

and of course new films thiner base is a thing to consider,

-- r watson (al1231234@hotmail.com), May 08, 2002.

Thanks for the information.Hopefully our desire for perfection in Leica's mechanical precision doesn't override our desire to take nice photos.

-- Sheridan Zantis (albada60@hotmail.com), May 08, 2002.

http://people.smu.edu/rmonagha/mf/mfbest.html

-- martin tai (martin.tai@capcanada.com), May 08, 2002.

OK here's a practical thing to SEE the film (un)flatness.

Put a test roll in your Leica, take off the lens, and hold the shutter open on B. You'll notice that the top and bottom of the film is indeed curved. What harder to see is that the middle is curved slightly as well.

The film is against the pressure plate about 1/3 up from the bottom and 1/3 down from the top. The exact center of the film is likely to be bowed very slightly towards the lens.

Complete film flatness is a misnomer.

-- Charles (cbarcellona@telocity.com), May 08, 2002.

Average film bulge in 35mm camera is in the order of 15-20 micron (Leica included )

Contax RTS III has a RTV (real time vacuum) system, which provides genuine film flatness, to the order of 2 micron, a ten fold reduction in film curl

-- martin tai (martin.tai@capcanada.com), May 08, 2002.

I concur... maybe a little more in older (60's and 70's SLRs) and for thinnest emulsions. Maybe aproaching .03mm (30microns) as we measured back about 1978 or so.

-- Charles (cbarcellona@telocity.com), May 08, 2002.

i think its about 26.243 microns

-- grant (lotusphotography@yahoo.com), May 08, 2002.