Strange issue: Canon EF 70-200mm f/4 L IS USM acts as 85-250mm. How?

Steve Balcombe wrote:

Nightly wrote:

Dear photographers, I faced a strange phenomenon or issue and need your support with your experience.

Today I purchased a used Canon EF 70-200mm F/4 L IS USM lens and realized a strange issue with it. While I tried to compare it with EF 24-70mm L IS USM @70mm on my 5D camera, I realized the focal lengths are different - the photo taken with 70-200 was more zoomed. I was confused about this.

Two separate issues. Above - it's common for the zoom range to be a bit less than the name implies, so the 24-70 is probably a bit less than 70 at the long end, let's say 67 mm, and the 70-200 is a bit more than 70 at the short end, let's guess at 73 mm. Only 3 mm but together they add up to almost 10%, which will obviously be visible.

And below - yes, I agree with the other posters. Focus breathing is when the focal length noticeably reduces at close focusing distances, and it's not just a few mm this time - the EF 100-400L IS II is one I happen to remember as it has been discussed a lot, and it's only 173 mm at the long end when focused at its minimum focusing distance. So less than half the 'claimed' focal length. Focused at infinity it is close to 400 mm - but the first issue applies and it is a bit less than 400, I forget the actual figure.

Focus breathing is mainly a symptom of modern rear-focusing zoom lens design, although it does happen with primes as well. The EF 70-200/4L IS is a much older design and shows much less focus breathing.

If you ever watch Dpreview TV, you may notice that Jordan mentions focus breathing sometimes - it's a PITA for video, because the size of the background changes as you focus in/out.

[snip]

... And slightly I am thinking that's awesome to have a free 1.25x extender with still f/4 . However, the fact that focal length doesn't correspond to the real focal values is frightening.

Unfortunately, it's the 55-250 which is much less than 250 mm at MFD - not the 70-200 which is more

I agree that the actual focal lengths, in reality, may differ a bit - which I expected to be. But this one was beyond my expectations thus I got cautious about this instance. Here the difference was >20%, which is more than acceptable.

Shooting close to MFD can be the answer to the phenomenon, and possibly the correct way to test them would be doing it on long distance rather than closeup as was mentioned earlier. I am aware of focus breathing from macro shooting. But I have doubts it can be so big to affect the result in such a way.

However, I did a test at @70mm in almost the same distance with EF-S 55-250 and EF 24-70 on 70D. The results were somewhat similar to earlier ones, but probably with a 10% difference. 55-250 had a closer view than 24-70. While 70-200 had even closer than 55-250. The distance between the subject was short again, so might be still affected by the breathing. Thus not attaching the images for comparison now, but sharing with you the results.