Same Focal Length, Different Field of View?

[slejhamer]

Please explain why two different zoom lenses, both set to the same focal length, show a different field of view. I recall this having been discussed before, but I can not seem to find the thread which explains why this happens.

Example:
I set up a comparison between my Canon 70-200 f/4 L lens at 70mm, f/4 against my Tamron 28-75 Di also set to 70mm, f/4. (I was comparing AF accuracy and sharpness.) All shots were taken with the camera mounted on a tripod at roughly 1.5m from the subject. Exposure was identical for the shots (f/4 at 1/60s, ISO 100), and I used mirror lock up with the camera's timer.

The subject appears notably larger (maybe 15-20% larger) in the images taken with the longer Canon lens. In fact, in later shots I zoomed to 75mm with the Tamron, and the subject still did not fill the frame as it had with the Canon. I had to move the tripod forward several inches to get a similar FOV.

What causes such differences in field of view? Why doesn't 70mm on one lens look like 70mm on another?

Your insights will be much appreciated.

[Ballen Photo]

Mitch, I read about this comparison somewhere a few years back. Sometimes a manufacturer will cheat/fib a little about the actual focal length of a given lens in their line. This would appear to be the case in your finding. Your advertised 70mm may actually be a 65mm or less.:shock: I wish I could remember where I saw these test results.
-Bruce

[maderito]

This is the thread you may be thinking about: http://www.photography-on-the.net/forum/showthread.php?t=34235&

I haven't designed any lenses lately, but I believe the issue is related to where the focusing lens group is located in the lens body relative to the image sensor plane.

So much to learn ... :wink:

[Ballen Photo]

This is the thread you may be thinking about: http://www.photography-on-the.net/forum/showthread.php?t=34235&


While this is a very good example and comparison, No, this is not the article I was thinking of. It may even have been in a magazine. I remember where they posted the advertised focal lengths of the tested lenses, then posted the actual focal length as measured. Like I said, I wish I could remember where. :oops:
-Bruce

[CyberDyneSystems]

. I had to move the tripod forward several inches to get a similar FOV.



ASny chance that several inches is the same inches differece in lenght of those two lenses?

IE.. you hadn't moved the tripod.. so the measurement from the front lens element was different for each photo as the Tamtron lens is physically shorter?

[slejhamer]

This is the thread you may be thinking about: http://www.photography-on-the.net/forum/showthread.php?t=34235&

I haven't designed any lenses lately, but I believe the issue is related to where the focusing lens group is located in the lens body relative to the image sensor plane.

So much to learn ... :wink:

Thanks; that link does contain a discussion on the topic, but I still don't understand the mechanics involved. If I set a lens to a focal length of 70mm, isn't that where the focus group would then be located?

The Tamron is an "IF" lens - internal focus. If I read the other post correctly, so is the Canon 70-200 f/4 lens. Argh, so confusing!

[slejhamer]

. I had to move the tripod forward several inches to get a similar FOV.



ASny chance that several inches is the same inches differece in lenght of those two lenses?

IE.. you hadn't moved the tripod.. so the measurement from the front lens element was different for each photo as the Tamtron lens is physically shorter?

Quite possibly ...

[slejhamer]

Mitch, I read about this comparison somewhere a few years back. Sometimes a manufacturer will cheat/fib a little about the actual focal length of a given lens in their line. This would appear to be the case in your finding. Your advertised 70mm may actually be a 65mm or less.:shock: I wish I could remember where I saw these test results.
-Bruce

Bruce, in this golden age of corporate honesty, social conscience and do-gooding, I find it hard to believe that a lens maker would cheat like that! ;)
( I really hope you are wrong ! )

[slejhamer]

I found this in a Google search, and something similar was also mentioned in one of the threads within the thread that Woody posted:

Using the Leica Manual as a reference, the focal length of a lens is defined as the distance from the principle plane (optical center) of a lens to the film "when the lens is focused on a subject at an infinite distance."

and this on photonotes.org:

Finally, note the technical definition - the focal length is determined given infinity focussing. Some lenses have shorter focal lengths than their stated focal length when focussing to a distance less than infinity.

So focal length is not absolute, though I thought it was just a physical measurement. Apparently this does not apply if an subject is at a close distance. Once again I do not pretend to understand the mechanics involved, but at least this seems to be the culprit.

[slin100]

Just to back up slejhamer, just look through the viewfinder as you twist the focus ring. You will see the FOV change as you vary focus.

[slejhamer]

Just to back up slejhamer, just look through the viewfinder as you twist the focus ring. You will see the FOV change as you vary focus.

Cool, thanks Steven.


Oh, P.S. - my conclusion on the lenses: while the Tamron is a very good $300 lens, it is not quite as sharp as the Canon. However, a dash of USM evens things up nicely. Anyone surprised?