[MF3D.FORUM:33] Re: Apparent image size, MF verus

[Richard Rylander <rlrylander@xxxxxxx>]

Paul Talbot wrote:

> Here is something I just dreamed up.  I'm intereseted in any
> comments as to the validity, or lack thereof, of this approach.
>
> Format   Size     Viewer FL    Mag Factor        Apparent size
> ------   ----     ---------    ----------        -------------
> MF       50 x 50  80 mm        3.125 (250/80)    156.25 x 156.25
> Realist  23 x 21  44 mm        5.68  (250/44)    130.68 x 119.32
> 35-FF:   23 x 33  44 mm        5.68  (250/44)    130.68 x 187.50
>
> If this is true, then 35mm full frame images have a significantly
> wider (20%) field of view than do medium format images mounted
> in the 50 x 50 mounts.  (Note, I understand the other advantages
> of MF like much less visible grain; at the momemt we are just
> talking about apparent image size.)
>
> I seem to remember John B saying angles had to be factored
> in, not just height, width, or area....but of course it was
> all way over my head.  ;-)
>
> Thoughts?
>

Magnification in practice depends on your eye accommodation as well as
the focal length of the magnifier lens.  The "250/f " formula for
magnification assumes that the virtual image is at infinity and the
magnifier lens is just in front of the eye.  If you move the object
(slide) closer so that the virtual image appears to be at the near point
of the unaided eye (which is assumed to be 250mm for a "standard
observer"), the angular magnification increases to  1 + 250/f .  The "1
+ " factor is more significant for lower "infinity-specified"
magnification lenses as are used in MF viewers.

Taking 250mm as the near point, the [horizontal] angular field of a 50mm
wide MF slide will be about 11.4 degrees.  With a magnifier of 80mm
focal length and the virtual image at infinity (your 3.125x original
case), the angular field becomes about 35.7 degrees.  If the slide is
moved closer so that the virtual image appears to be 250mm away, the
angular magnification is 4.125x and the field is about 47 degrees.

A FF 35mm slide, of horizontal width 33mm at a distance of 250mm has an
angular field of only 7.55 degrees.  A 44mm focal length magnifier
arranged so that the virtual image is at infinity (5.68x) increases the
apparent angular field to about 43 degrees.  Moving closer to place the
virtual image at 250mm, the angular magnification is 6.68x and the
angular field appears to be about 50 degrees.

If a slide viewer is adjustable, most people will focus to place the
image at their near point, where it will be largest.  In this case, the
angular fields of MF and FF-35mm slides will be very close (47 vs 50
degrees).  I think this is a more realistic comparison than the case
where the virtual images are at infinity (35.7 vs 43 degrees).  Of
course, all this depends on your particular eyesight (YMMV).

Richard Rylander