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

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

Alright. Will try it soon, whenever I get a chance for it.

I doubt it can be so different at MFD, but will the technique mentioned above, to get the correct results.

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.

Nightly wrote:

Alright. Will try it soon, whenever I get a chance for it.

The relationship between focal length and angle of view only holds good at infinity. A simple mathematical thin 200mm lens focussed to give a ¼ life size image will have to be 250mm from the sensor, and the focussed distance from subject to sensor (the only consistent way you can measure focussed distance with different camera lenses) will be 1250mm. That simple 200mm lens will have the same angle of view at 1250mm focussed distance as a 250mm lens focussed on infinity, so there is considerable focus breathing at that sort of distance with a unit focussing lens.

To avoid focus breathing and to keep the same angle of view as they focus, lenses have to zoom to a shorter focal length when focussing closer.

John Crowe wrote:

Now try it with a subject about 100 feet away or even at infinity.

Seems to me that lenses will change focal length based on focus distance. You must have been at or near minimum focus distance for these shots.

I did it. Posted an update below.

Comparison update: Some photographers criticized that the measurement was done wrong - at close distance. Though I was aware of focus breathing of the lens from macro, but the amount in zoom was very little, so I had small doubt it to be the reason.

Here are the images from two lenses taken during sports event with 5D camera. The difference might be not as big as in EF-S 55-250mm case but is still quite visible at 70mm. And again 70-200 seems to be zoomed.

70-200m f/4 L IS USM @70mm

24-70mm f/2.8 L II USM @70mm

I doubt the difference in both lenses are -/+2mm. This set of the images had less difference in framing because of distance probably, but still recognizable. I took only 5D with me, so could compare with 24-70 only. I think days ago I compared 24-70mm with 55-250mm with 70D - there was no visible difference in framing - this is how should the lenses behave.

I sent the lens to Canon service to check it. Probably the next week will receive the laboratory results.

Alright. Waiting for the lab test. Let's see what they found out.

UPDATE: I received a call from Canon Technical service: They called me and told me that it might appear as 75 (compared to 24-70mm) and the tele end as 230mm. But it may differ based on distance, and focus breathing. Focusing at a longer distance might lower the difference, focusing to infinity might lower even more but still be visible. So the answers given here mostly matched the answer from technical service.

Thank you all.