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View Full Version : Is Focal Length not absolute?

Radtech1
5th of June 2004 (Sat), 23:03
Yea!!!

I just brought home the 70-200 2.8 L IS!!

So far, I am a little distressed to report that it does not seem any sharper than my old 28-135 nonL IS. I will post results of comparisons soon.

But the question at hand is this: I have carefully duplicated the same tripod shots with both lenses, and verified with the EXIF data that I had the same exact zoom on both lenses, yet the FOV is quite a bit different. The 70-200 is by a wide margin "closer" to the subject than the 28-135.

I thought that focal length was absolute. If not, what is it relative to? If it is not absolute, when I get my next L, will the 70 of the 28-70L be "shorter" than the 70 on 70-200? If that is so, then there is an "empty spot" in the coverage.

Any thoughts?

Rad
AzzKicker
6th of June 2004 (Sun), 11:37
L glass is over-rated :)
gcogger
6th of June 2004 (Sun), 11:51
2 lenses of the same focal length on the same camera will give the same FOV. Something is wrong somewhere!

On the issue of sharpness, the 28-135 is a very good lens already. If you take pictures at, say f/8 or f/11, there will probably be little difference, especially at the centre of the frame. Differences should be much more noticeable at wider apertures and at the edges of the frame.
SDK^
6th of June 2004 (Sun), 12:06
Will the magnification factor make any difference?
Tom W
6th of June 2004 (Sun), 12:06
2 lenses of the same focal length on the same camera will give the same FOV. Something is wrong somewhere!

On the issue of sharpness, the 28-135 is a very good lens already. If you take pictures at, say f/8 or f/11, there will probably be little difference, especially at the centre of the frame. Differences should be much more noticeable at wider apertures and at the edges of the frame.

Perhaps reported focal length is not actual focal length. Also, I believe that the location and operation of the focusing mechanism can have some effect on the effective focal length. Someone else has posted on this effect before.
Guillermo Freige
6th of June 2004 (Sun), 13:20
This is already answered in other thread. 28-135 is a rear focus lens, so actual focal lenght changes for close objects. 70-200L isn't a rear focus lens, so focal lenght is constant.