is 100 mm 100 mm?

[John Chardine]

I was interested to see how my Canon 24-105 mm f4 zoom matched up to my Canon 100 mm f2.8 macro, in macro mode. The results are another story, but here I show images taken with each lens. I set the 24-105 at a hair off the 105 mm setting and shot at f8 with each lens, of course not moving the tripod between lenses.

I was very interested to see that the field of view at 100 mm on the 24-105 is substantially wider than the 100 mm macro, so 100 mm is not always 100 mm! The white square shows the approx. field of view of the 24-105 lens @ 105 mm on the 100 mm macro image. This is a bit of a revelation to me but may be old news to BPNers. I wonder how many zooms out there advertise a wider or narrower range than they actually give? Or maybe I'm being unfair to the zoom and it's the 100 mm prime that is off?

[Roger Clark]

Hi John,
That is quite a difference. One issue that comes to mind is that you fixed the camera and thus the distance of the entrance pupil of each lens is probably a different distance to the subject. You might redo the test on a landscape scene so the subject is effectively at infinity. Then the entrance pupil to subject distance will be essentially the same. If you redo the test, also include an image at max zoom to see if it matches closer, as it is always hard to know when you back off a little just how much that is. Your exif data probably provides a more accurate focal length than the lens barrel.

But it is also likely that the zoom lens is a little different than the specs, but I would be surprised if it was more than about 10%. Even the fixed focal length lens could be off, but usually they are closer than zooms, at least for the data I've seen.

[John Chardine]

Thanks Roger! EXIF shows 105mm so I guess I didn't back off at all!

I think you must be right about the distance to subject being different between the two lenses, based on where the entrance pupil is. I had always thought the important distance was subject to sensor/film plane, which of course did not change between lenses. Can you explain why it's the entrance pupil that is important.

[arash_hazeghi]

As Roger pointed out it is the distance between subject the principal plane that is very different between the two cases, the principal plane of the macro lens is very close to the front element in order to produce high magnification versus for the zoom lens the principal plane is closer to the rear element, thus at a given working distance between camera and subject macro lens will provide a higher magnification (narrower FOV). The focal length for different lenses are usually measured at infinity and that is what the marking on the lens indicates.:D

http://en.wikipedia.org/wiki/Cardinal_point_(optics)

[John Chardine]

Thanks for this. Makes sense. I'll now look up "principal plane"!

[Roger Clark]

John,
Check out http://hyperphysics.phy-astr.gsu.edu/HBASE/geoopt/priplan.html

[Christopher C.M. Cooke]

Thanks for that link Roger, I know know why I failed maths.:)