From: "Izzet" <izzet@vizo.com>
>
> Assume I have a 35 mm. SLR with a 28 mm. lens on it. I am looking though
> the viewfinder to the subject. Then I change the lens to a 300 mm, and
> look at the same subject from the same distance. Now the subject is
> filling more of my view and also seems magnified.
>
> 1- Is the reason for that magnification ONLY the decreased
> field-of-view?
No. The reason for the magnification comes from the optics of the lens,
specifically the focal length. Changing focal length changes the physical
size of the real image at the focal plane. The size of an object at the
focal plane is:
Sf = Df / Do * So
where: Sf = size of image at focal plane,
Df = distance, lens to focal plane,
Do = distance, lens to object, and
So = actual size of image.
> 2- If this is so, do I get the same image if I use a
> a)300 mm. tele and print full-frame to an 8*10 card,
> b)100 mm. tele and print full-frame to a 24*30 (if it exists) and then
> crop to an 8*10?
Yes, but the imperfections of the system (grain, sharpness, distortions,
etc.) will be magnified by three, sharpness will be reduced by a factor of
three, all other things being the same.
