On Nov 4, 2005, at 3:32 PM, keith_w wrote:
The format gate is a rectangular cutout inside the film transport
rails. It does not sit against the emulsion, it stands in front
of the film by a couple of mm. As the nodal point of the lens
becomes closer to the film rails and the light path to the film
'overshoots' by a small amount. An exaggerated picture of how
this happens is seen here:
>
http://homepage.mac.com/godders/fchange.gif
The emulsion is the black line, the edges of the gate are in red.
Green lines indicate the light path from a short lens, blue lines
indicate the light path from a long lens. As you can see, light
paths from short lenses with a nodal point close to the film tend
to grow the format.
Godfrey
Precisely as I had envisioned it, but the delta must be very small,
no?
As I said, it's most obvious when comparing actual frame sizes
between lenses of widely disparate focal length, and of course the
lens design has a lot to do with it. How far in front of the film the
gate stands also has a big effect. SLRs require relatively
telecentric lens design in wide angles anyway, due to the flapping
mirror in the way, so the effect tends to be smaller than with
rangefinder bodies.
With the Leica M, comparing frames from a Heliar 15mm vs an Elmarit
90mm, the difference was almost 3mm linear dimension change in H and
V. Remember that that's +1.5mm measured from the center of the frame,
which is pretty easy to miss. With some old Nikon negatives,
comparing negs from a 200mm lens and 20mm lens, I see about 1.6mm of
growth.
It's easier to see the spacing between the frames decrease with wider
lenses than to realize that the frames are actually larger.
Godfrey
