You might get less contrast because there might be more light bouncing
around in there, but the lens doesn't care what it is focusing on.

-Jack

Medium Format lenses usually trade some resolution for coverage, or make
other compromises for the requrements of the format.

Otherwise, it'd probably be a slow 55mm lens, if you weren't expecting
16x20 enlargements from the Amateur Format film.

If you own a Pentax 645 and a Pentax 35 SLR, you can find out. They sell an
adapter that allows you to mount the 645 lenses on some of their 35mm SLRs.

I doubt you'd see a difference.

I agree with the other posts that larger format lenses generally trade image
area covered for less resolution (line pairs per mm)

50 or 55mm is not normal for 35mm, 43mm is.

THOM

Probably not.  The 35mm frame would use only the center of the field of
the MF lens, which is good, but MF lenses don't always achieve the
resolution of 35mm lenses, which is bad.  So it's a wash overall, and
certainly not worth the effort to adapt the lens.  It's best to use
lenses designed for the format you have in mind.

Nearly everybody on this newsgroup has answered this so I will as well.

You won't get any coupling between the MF lens and the 35mm camera unless
you had a adapter specially made for the purpose, if it exists. It might not
exist.

The main problem will be the resolution of the MF lens. This will be worse
in the centre, if you look hard for it,  although you might not see any
difference at the edges. Another problem will be flare. I'll explain...

A 55mm lens for MF will be a very different design than a 50mm lens for 35mm
because the MF lens is designed to give a flat image over a much greater
area. The image must be sharp and evenly illuminated across a width of 6cm
rather than the 3.6cm for 35mm. To achieve this, the design must be
fundamentally different. A 50mm 35mm format lens will only just cover the
area it is supposed to and even then will show some darkening at the corners
on the wider apertures. This becomes logical when you look through the back
of the camera. If you look "out" of the lens from the corner of the film
plane you will see this vignetting in the form of the lens being obscured by
the housing in front of it. Light from that extreme angle will have
spherical distortion and so the lens designers deliberately cut out this
light in a physical way so it does not fall on the film plane and blur the
image. But your 55mm MF lens will be designed in such a way as to accept
light from these extreme angles and still keep it sharp over the area it is
supposed to cover. It will be a "wide angle lens" in the sense that it can
focus light from these extreme angles, even though fitted to a 35mm format
camera it appears to have the same focal length (or nearly so) as a standard
lens. Typically such a lens will have a bulging from element that focuses
the light in to an element close behind that sort of gathers in all the
light in a short distance and then passes the light through to the back of
the lens where it gets sort of spread out again and focussed on the film
plane. They are an ingenious design and if you look through the back of the
camera at the corners of the film plane and out through the lens then you
will see that the aperture circle sort of bends in your direction. This is
to get the same light intensity (or nearly so) at the corners. So the MF
lens is very different. It will have more elements in it, most likely, and
will be a lot more expensive to make. And the more complex the design with
these extra elements then the more the resolution will be compromised. You
will get less resolution in the centre of the image in terms of the number
of line pairs per millimetre that can be resolved, compared to the standard
lens for the 35mm format. But on the other hand, since the MF lens gives a
larger coverage then it will still be producing a similar sort of resolution
at the edges of the 35mm film plane whereas the standard lens for the 35mm
format will not be. So the MF lens won't resolve as well in the centre but
might be just as good as the standard lens or even better in the corners and
with no vignetting as well.

But then there is the problem with flare. If you fit one of these lenses
onto the front of a 35mm camera then you have got to bear in mind that there
will be a lot of light surrounding the film area because this lens was
designed to give a greater coverage. This light will need to be very
effectively blocked out or absorbed otherwise it will show up as fogging or
lack of contrast on the film as some light leaks out and falls on the film.

So it is not recommended.