Focal length variations, different lenses

I just picked up a new ef 70-200 f4 IS yesterday and I was keen to see how much sharper it was over my ef 24-105L. I set up my tripod and took sample shots from the same position with the same camera. I set the 24-105 to 105 and I set the 70-200 as close to 105 as I could. The EXIF's show the 24-105 @ 105 and the 70-200 @ 100 but in the actual shots, the 70-200 looks to be at a longer focal length than the 24-105. Is there any reason for this?

First shot is the 24-105:

IMAGE LINK: https://flic.kr/p/B9UC​ss

IMAGE LINK: https://flic.kr/p/ByJy​NW

FOV of two lenses at the same focal length can vary but the difference in your pictures are substantial and I guess something else is the cause.Did you take images also say at 70m with both lenses or using a different camera to check the same exact 105mm as the end point on one lens and report of EXIF on the other? to try to eliminate the variables.

I tried them both at 70mm with the same results. I repeated the test at 105, this time managing to get 106mm on the 70-200. The results are the same again, see below. The only other Canon body I have is an EOS 3.... no EXIF there

24-105 @105

IMAGE LINK: https://flic.kr/p/ADGE​1u

IMAGE LINK: https://flic.kr/p/Ba6e​2Y

I've just checked the 24-105 @ 50mm against my ef 50 f1.8 and both shots look identical so its the 70-200 that is looking odd.

IIRC, the JCII (who set standards) allow a certain tolerance in actual vs. claimed focal length; it's somewhat larger for zooms (maybe 5%) than for primes. I read this quite a while ago, so I might have misremembered the tolerances. The other thing I would point out is that intermediate focal length settings on a zoom don't have "click stops", so "100 mm", either by the marking on the lens barrel or as reported by the EXIF, might be somewhat off.

As Jon said, the focal length markings on lenses are not precision markings by any means.

In addition, and far more importantly to answer the OP's question - all lenses have their focal lengths stated for when the lens is focused at "infinity". The focal lengths can change radically when focused closer than "infinity" and various models of lenses therefore can have significant apparent focal lengths when they are focused significantly closer than "infinity" even though the markings on the lenses indicate identical values.

On top of all this, manufacturers often round off the focal lengths advertised for their lenses. I suspect this is for marketing purposes. For example, if manufacturer "X" made a lens that was measured at an actual 189mm (at "infinity" focus), it would likely be sold as a "200mm" lens because of the competitive niche for that focal length. The same goes for zoom lenses. A zoom lens that was measured at 27mm to 67mm would very likely be marketed as a 24-70mm lens.

This could be the result of "Focus Breathing" where the actual focal length is less as you focus on closer subjects.

Repeat the test on a distant subject and see if it is closer.

msowsun wrote in post #17799322
This could be the result of "Focus Breathing" where the actual focal length is less as you focus on closer subjects.

Repeat the test on a distant subject and see if it is closer.

this ^

set the 24-105 to 105, manual focus to infinity, then look through the lens while turning the focus ring to the close end. You will see the framing changing.

some lenses do it more than others. the 85 1.8 moves a lot.


what skip said also has a lot to do with it.

SkipD has the answer. Focal length is defined only when the lens is focused at infinity. Still, your difference is HUGE. I'll be right back with some shots from my 70-200 f/4L IS.

24-105 STM vs. 70-200 f/4L IS @ 100mm. Focused at 15'.

24-105 STM vs. 70-200 f/4L IS @ 100mm. Focused at infinity.

Close to a decade ago, I had my Tamron 28-70, Canon 18-85, and Canon 70-200. I had to set all three lenses differently than the FL setting shown on the zoom ring, in order to get same FOV from a tripod-mounted camera. According to an old post of mine on POTN, it ended up something like indicated value vs. lens FL setting, the 17-85 had to be set to 85mm to get the same FOV as the Tamron at 75mm as the Canon 70-200 at 70mm FL
The links included below was about differences in color rendition, not about FL claim vs. AOV, but AOV differences was mentioned in my post, and the differences existing in spite of the EXIF confirming that the same FL value was used for each shot.

Wilt wrote in post #8172915
Indicated FL is not always going to produce the identical framing with different lenses! I had to crop one of these shots, although both said 16mm in the EXIF...one Canon, one Tokina...

https://photography-on-the.net …hp?p=8163287&po​stcount=29

And once I tried to compare IQ from Canon 17-85 vs. Canon 70-200 vs. Tamron 28-75 at 70mm, and trying to match FOV at the indicated 70mm (lens marking verified via EXIF) was troublesome.

And here is a comparison of some non-L Canon again Canon L against Tamron. The Tamron does have a warmer appearance, but it can be adjusted with WB correction alone.

https://photography-on-the.net …hp?p=8072739&po​stcount=55

Thanks to everyone who replied. I'm reassured and will now get on with enjoying my new "pride and joy".

Right, the 70-200's are known for focus breathing at close focus distances (i.e. actual FL is much shorter than indicated as you are seeing). It is how they get the 70-200's to focus down to 1.2m. In addition the 70-200 f4 IS is known for much poorer IQ at MFD, especially at the long end. While IQ improves rapidly as you focus further than MFD, I'm not surprised that it still has some focus breathing.

If you do want to use the 70-200 near MFD, I would advise using a short (say 12mm) extension tube if you can. This will enable you to set the focus further out and improves IQ quite a bit. Or stop down to f8 if you don't need f4.