When 300mm does not equal 300mm?

[gbc]

I recently upgraded from the Canon 70-300 USM non-L to the newer 70-300 L version. Just to make myself feel better about the purchase, I did some quick image comparisons. And without a doubt, the L version is, not surprisingly, way superior in image quality. However, when comparing the images, I also noticed that the focal lengths were not quite the same. Both at 70mm and 300mm, the non-L seemed to be slightly longer in reach than the L version:

Non-L @ 300mm"


L @ 300mm


These are both on a tripod, about 7 feet away, on a 7D.
I don't know a lot about focal length math. Is this common between lenses for focal lengths to not match up precisely? Does it have anything to do with the fact that the non-L's front end extends more? I haven't had a chance to test these outdoors yet, but will this difference lessen as the distance from the subject increases?

Thanks for any info!

[ahab1372]

The focal lengths are always approximately. On top of that, some if not most lenses change the focal length when you focus on a different distance. Focusing on a far away subject makes some lenses shorter, some longer, depending on the design. Exceptions I think are most cinema lenses, which is one of the reasons why they are so expensive. So if you repeat your experiment focusing on a subject 50 or more feet away, the results might be different. I would love to hear if you actually do repeat it.

Arnt

[gbc]

Thanks for the explanation, Arnt. I did just now take some shots at about 100 feet, and though the non-L was still a bit longer, the difference was much less noticeable. And I'll gladly trade the quality of the L shots for the slightly-longer reach of the non-L.

[ahab1372]

no doubt that the L is better, but how did you like the non-L in retrospect? I am renting it next week because I was wondering if it was worth the upgrade from the 55-250.

Arnt

[Dr Croubie]

I've also had the non-L and upgraded to the L, but I didn't have both at the same time to compare side-by-side.

My feelings about them are thus:
The 70-300 USM non-L is a great, cheap, 70-200 IS lens. It's less than the 70-200 f/4 L non-IS, the IQ is almost as good, you get IS but a slightly slower aperture, and it's a lot more compact and portable.
It also has the 200-300mm range, but to be used only in emergency, the performance at 300mm really is disappointing (that's of course subjective). If I still had it i'd like to do a comparison between 300mm and 200mm+cropping. I also still think that my lens may have been a bad copy. Doesn't matter, my sister owns it now.

The 70-300L is a true 70-300 range, I have no problems using it wide-open at any length at any time. It costs about the same (give or take depending on shops) as a 70-200 f/4 L IS lens with a 1.4x T/C, and IS + IQ performance difference is splitting hairs. But the 70-300L is more compact and doesn't require taking the lens off to fit a TC. It's built like a tank and worth every cent imho.


Meanwhile, back to OP. As said, focal lengths can change depending on focus distance, normally the quoted range is true (give or take) for infinity focus. I read somewhere (can't find it, late for work), that the 100mm L IS macro lens is 100mm at infinity, and only 80mm or so at 1:1 macro. Also, either or both or the 70-300 L/nonL lenses could be in the range of 295-305, and marketing probably just decided to call it 300mm because it's a nice number...

[ChadS]

@GBC shoot a picture of some starts. They should come out the same spacing on your photos. Other than that it's a crap shoot.

[Daniel Browning]

Is this common between lenses for focal lengths to not match up precisely?

At close focus distances, yes. The reason it occurs is an effect called "breathing", which is when the field of view changes with focus distance. Many lenses have this issue, so the degree to which they vary can cause the focal lengths to not match up.

The focal length printed on the lens barrel is only for when the lens is focused to infinity. Manufacturers don't claim it to be the same for other focus distances (unless stated otherwise). Furthermore, the focal length is subject to marketing department "rounding", such that 64mm may be rounded up to 70mm, etc. However, I don't think they're ever off by more than 10% (and usually at least half that).

On top of all that, focal length can also vary slightly from unit to unit within manufacturing tolerances.

[ham]

Could it also be that the focal plane is changing as the two lenses are designed differently? Unless you're mounting the lens via a collar at the focal plane.