Ok, let's get everything straight here.
1) If your image sensor area (film or CCD) is bigger, you need a longer
focal-length lens to cover it, so a 24mm lens is normal for 110, a 50mm
lens is normal for 35mm, an 80mm lens is normal for 6x6 a 165mm lens is
normal for 4x5 (need I go on). If you have a 1/3" ccd an 8.4mm lens
would be considered "normal"
2) Although these normal lenses will fill the different sized frames
with the same object at the same distance they are delivering very
different subject magnifications
3) Depth of Field is controlled by two things and two things only.
Subject magnification and aperture. It doesn't matter whether you
increase your subject magnification by moving closer or by using a lens
with a longer focal length your depth of field will be reduced. In
addition, if you use a camera with a larger format you are magnifying
the image more so you will have less depth of field. Now most people
think that if you switch to a wide-angle lens you will get more depth of
field, that's wrong. If you switch to a wide angle lens and do not move
closer to your subject, you are decreasing subject magnification and
your depth of field will increase. However, if you move closer to have
the same subject fill the frame, you depth of field will be the same.
This explains why digital cameras have a lot of depth of field, there
CCD's are so small, subject magnification is very low and depth of field
is great.
4) Lens/film to subject distance controls perspective. So if you are
close you will get exaggerated perspective, far away you will get
compressed perspective. Now, it just so happens that with wide-angle
lenses you tend to work in close so they get blamed/credited for the
exaggerated perspective effect. However, if you look through a
telephoto lens and see the compressed perspective, take the camera down
and look at the same section with just your eye, the scene will have the
same compressed perspective. Your lens to subject distance didn't change.
All for now,
Mr. Bill
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