Re: [Leica] LUG -- what's the point?

["Bryan Willman" <bryanwi@seanet.com>]

for what it's worth, Edmund Scientific, of all
people, sells some little free-standing scopes
very suitable for looking at film.
They are made by Peak, and sort of pricey
($300) but much easier to apply to the problem
than a "real" microscope.  I can hunt up
part numbers if anybody cares.

And of course, sharpness as delivered on
the film is what really matters.  a pure mechanical
rangefinder checker wouldn't find problems
like the film plane not being where the otherwise
correct rangefinder thinks it is.

bmw

- -----Original Message-----
From: Paul and Paula Butzi <butzi@halcyon.com>
To: leica-users@mejac.palo-alto.ca.us <leica-users@mejac.palo-alto.ca.us>
Date: Thursday, June 04, 1998 3:17 PM
Subject: Re: [Leica] LUG -- what's the point?


>At 12:54 PM 6/4/98 , Harrison McClary wrote:
>
>>If you want to check the focusing accuracy of your range finder the only
>>way I know is to focus with the camera on a tripod on something then flip
>>open the back and put some vellum over the focal plane and see if it is
>>in focus there.
>
>If I were going to try this (and I'm not)
>I'd worry a lot about the following:
>1. Depth of focus will be really minimal
>    with the wide apertures available on
>    Leica lenses.  Rather than use vellum,
>    I'd use something that I could ensure
>    would be FLAT, like a small piece of
>    ground glass that would rest against
>    the film rails.
>2. even so, it would be difficult to get the
>    ground glass ground surface exactly
>    registered where the pressure plate
>    and the film rails would put the front
>    surface of a piece of film. Film flexes.
>    I assume that Leica take this into
>    account when building cameras.
>3. Very small errors in focus would be
>    very hard to detect on the GG but might
>    be quite apparent on film.  At the very
>    least, you'd need to check the focus
>    with a loupe.  My preference would be
>    to focus on the GG with a fairly high
>    power loupe, then check the rangefinder,
>    rather than the other way 'round.
>    Looking at the GG with your naked eye
>    would be pretty much useless.
>4.  Absolute accuracy probably requires
>    focusing a microscope on the *aerial*
>    image formed at the film plane, since
>    even a fairly high power loupe on a
>    ground glass will not allow you to
>    resolve all the detail in the aerial image.
>5.  All of this fooling around at the film
>    plane would make me nervous if the
>    cable release let go and let the shutter
>    close.
>
>If I really had some reason to believe
>that the rangefinder was out of whack,
>I'd stretch out a tape measure for 30
>feet or so, and then expose multiple frames
>of various marks on the tape, with the
>aperture wide open.  I'd do it on
>really fine grained film like Tech pan,
>or maybe Ektar 25, and then I'd
>examine the image of the tape measure
>on the negative with a low-power
>microscope.  Focus each frame separately.
>Remember when evaluating the actual
>plane of focus that the depth of field is
>asymmetric about the plane of focus -
>one third in front, two thirds behind.
>So if you focused on the ten foot mark,
>you'd expect that the marks at 9 feet and 12 feet
>would be equally out of focus, with the
>ten foot mark being sharpest of all.
>
>All of that sounds like a lot of work to me.
>In the end, I'd probably just send the camera
>off and have it checked/adjusted.  All right, I confess.
>Maybe I'd buy another M6, and send the
>suspect one off to be checked/ adjusted.
>That way, I'd not have to go without one.
>
>-Paul
>
>
>