EF 24-105 f/4 L at full zoom produces same picture as EF 85 f/1.8 - what am I missing?

[justthisguy]

Hi all,

I feel like I am going crazy and I need an adult.

I have a EF 24-105 f/4 L and a EF 85 f/1.8 as well as a Canon EOS 650D (Rebel T4i). I wanted to compare their images at the same zoom level and an aperture of f/5.0, on a tripod.

I took a picture with the 85 mm and switched lenses to the 24-105 mm. I zoomed in until the live viewfinder matched the first photo, and I was flummoxed to see that the lens was at full zoom - 105 mm.

I took the photo and compared the EXIF data of the two identical photos, and yes, the first one was at 85 mm focal length, the second one was at 105 mm. How can this be? What am I missing? Something obvious, I guess.

I know about the crop factor of the camera, but the lenses are both EF, so I can't see how this would be a factor.

The two images - 85 mm on the left, 24-105 on the rightEXIF data of second imageEXIF data of first image

[Waddizzle]

Welcome to the Canon Forums!

The EF 24-105mm f/4L was released in 2005.  It was not uncommon for some zoom lenses to not faithfully cover their entire listed range.  Nikon had a 70-200mm that seemed like it could no goo longer than 135mm or so.

What you have observed looks to be an accurate observation.  There is nothing that can be done to the lens to correct that behavior.  Sorry.

[justthisguy]

Thank you so much, Waddizzle.

That's an amazing reason, but also the only one that really made sense for me. It's pretty funny that I wanted to compare the minute differences in photo quality between two supposedly great lenses, and what I found was that one of them doesn't even do what it says on the tin.

So do you mean that the decreased focal length is consistent across 24-105, or that it varies from actual lens to actual lens?

[Tronhard]

Hi and welcome to the forum:
The phenomenon you refer to is called focus breathing and usually occurs when a lens apparently changes its field of view (often confused with focal length) depending on the distance to the subject.

See What is Focus Breathing in a Camera Lens? | PetaPixel

[Waddizzle]

Yes, it is called focus breathing.  Trevor is correct.  Part of the problem is how close your subject is to the lens.  Had you focused on a subject at or approaching infinity focus, then the angle of view would seem not as wide, which would suggest a lens with a longer focal length.

The more costly lenses contain corrective lens elements to create a parafocal lens.  The best cinema lenses exhibit parafocal properties.  

[EDIT]. In other words, the angle of view is NOT constant for all subject distances.  At shorter subject distances, the angle of view on a focused subject may seem wider.  At longer subject distances, the angle of view on a focused subject may seem narrower.

{EDIT 2} Redo your test shots.  But this time focus on a subject that is significantly further away from the lens.  Something 2-3 car lengths away might suffice, if not a bit farther away.