Subject: Re: Argus C lenses
                                    Date: Thu, 2 Apr 1998 15:11:52 EST
                                    From: CarlMarin
                                    
                                    In a message dated 98-04-02 14:46:40 EST, you write:
                                    
                                    > 
                                    >    ACG,
                                    >  
                                    >    This brings up an interesting historical thread. My understanding is that
                                    >    the original/early C models had a flawed design in the sense that at full
                                    >    aperture some part of the optical path to the film plane was blocked by
                                    >    the inards of the camera/lens. I have such an old C and am contemplating
                                    >    a crude lens test that would include an evaluation of this vignetting 
                                    >    problem (assuming it exists for now). Any confirmation of this or sugges-
                                    >    tions for a test of it would be appreciated.
                                    >  
                                    >                                                Mike Reitsma 
                                    >  
                                    
                                    Well lets try that again, I sent the clipped part without a message before,
                                    sorry,
                                    
                                    To check for even distribution of light across the film plane I would think
                                    pointing the camera at a diffuse, even light source and taking a series of
                                    exposures at different times to get a step series will give you some frames in
                                    the middle grey zones where you could evaluate whether the edges are
                                    vignetting and will also give you some idea if your shutter speeds are
                                    stepping up in some sort of proportional way.  If they are in the ballpark you
                                    will get a Zone system type step series in a fairly even progression.    
                                    
                                    I've heard of people doing this under the light of an enlarger as these have
                                    pretty even light across the field.  You could also set the camera up to shoot
                                    a grey card out of doors on a diffuse light day.  That would factor out
                                    varying light levels while you were shooting the series.  Use some slow film,
                                    Ilford Pan F maybe.  
                                    
                                    Theres also the View Camera technique of peeking in through the cut off
                                    corners of the ground glass to see if you can see the full circle of the lens.
                                    If not, there will be some light fall off, not uncommon at wide open aperture
                                    on many cameras I guess.  I haven't tried this on something as small as an
                                    Argus C though.  You might cover most of the film gate with tape except for
                                    the corners so when you get your eye right up to the corners you are not
                                    distracted by the light in the rest of the film gate.  
                                    
                                    I'll have to play around with this later today, compare with a C3 perhaps.
                                    
                                    Carl