A little over a week ago I posted the following remark:
"Changing focal length alone can definitely alter the relationship between
objects as seen in your view finder. This will be readily apparent if you
spend a moment with a zoom lens."
I made the above remark because when sat in my computer room and racked a
zoom lens forth and back, while observing thru the view finder of the
camera it was mounted on, spacial relationships between me and between
objects in the view finder changed. The walls in the corner seemed to move
in my direction and a lamp in the corner appeared to move towards me while
becoming closer to the corner. Note I did not claim the walls or lamp
actually moved nor did I mention perspective.
A reply to my post appeared, differing with mine, and I answered with a
suggestion of a simple exercise and almost simultaneously felt the pain of
a kidney stone as it moved into a new and interesting location in my right
kidney. For those of you who have not experienced this life event I can
only say a kidney stone is about as much fun as a knife fight in a phone
booth. The next six days are a little unclear but I have it on good
authority the liberal (very liberal) amounts of percocet and thousands of
repetitions of the kidney stoned litany ("this too shall pass") sufficed to
win the day. All of which leads to, now, when I've just finished sorting,
deleting, filing, and replying to almost 2000 accumulated E-mail posts.
To get right to it, I'm sure most people know the gist of the replies to my
statement above. With two exceptions I've seen it all before. The main
thrust is that the only way to change apparent spacial relationships is to
change the observers position. That there is no such thing as compression
and it only happens at long distances. There is no such thing as expansion.
That the wide/tel look doesn't exist and this is proved somehow by being
able to duplicate it in the dark room. That a 17mm and 1000mm focal length
lens on the same camera will produce exactly the same result except for a
lot differences. I didn't see much about image viewing distance this time
around but the uninformed should be advized the proper viewing distance is,
focal length of the lens times magnification. I have seen this gem stated
with the example that a 12X18-inch image from a 35mm negative taken with a
1000mm lens should be viewed from a distance of... thats right, 20 feet!!
One new "proof" there is no compression or expansion or change in spacial
relationship via focal length, involves looking thru a rectangle in a card
and noticing that if it is moved back and forth there is no change in
magnification. Frankly I've noticed this lack of magnification when not
looking thru a telescope, before. In fact it is just this lack of
magnification that, many times, prompts me to change focal length. The
other new "proof" appears to be based on the concept that a pinhole lens
has no focal length. This ignores the fact the formula for finding the
focal length for a given pinhole is diameter of pinhole times itself times
750 so the FL of a .4mm pinhole is 120mm. I also remember a power zoom
pinhole assembly offered for sale to the spy types. What does this prove?
Beats me. I didn't bring pinholes into the discussion but I bet I can get
pinhole images that show a wide angle or telephoto look.
None of the above "proofs" disprove what I have observed while watching
Hundreds of NASCAR races and Sunday afternoon football games on TV. Every
tel shot of the fifty yard line from the end zone camera shows
magnification and compression. Every shot from the camera inside the 200
mph race cars show expansion and WA distortion. For an example of telephoto
compression look at:
http://www.netSnapShot.com/pcw/b?KEY=0&ACCOUNT=12042
The frame on the right was taken first at about 400mm and the frame on the
left, within a few seconds, was taken at about 200mm. Note I did not move.
The dog did not move. the window and blinds did not move. And last but not
least the car about forty yards away did not move. Only the focal
length changed!!
For a good example of W/A expansion refer back to Robs excellent shot of
the kids on a hockey rink. That rink looks about twice as long as it really
is. By the way Rob, warping the wall on the right, straight, seems to
improve the shot a bit by removing the keystone distortion. The leaning
wall is accented because it is so close to the edge of the frame.
For some pinhole W/A expansion take a look at:
http://mrpinhole.no-ip.org/harley.php?PHPSESSID=d8f6ad1dbb40cf29f0f8b32c3c53
ca72
This shows just how sharp a pinhole shot can be if it is exposed at the
proper focal length for the size of the pinhole. When you move a pinhole
back and forth you are NOT changing the focal length, you are just moving
it in and out of focus.
When you look thru a card with a rectangle cut in while moving it back and
forth you are NOT changing the focal length of your eye, you are just
blocking out the field of view. There is no more magnification in a hole in
a card then there is in a window. There is no compression without
magnification and there is no magnification with out compression.
Dave
East Englewood
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Every lens has a "normal" format and every format has a "normal" lens.
