Magnification changes while focusing a prime lens?

[OldBodyOldSoul]

I bought this lens about a month ago that turned out to have some problems (couldn't focus farther than about 20 feet, soft in weird ways etc) so I returned to the seller. He took it to a repair shop and asked me if I wanted to give it another try and I said yes.
Apparently, they fixed the focusing and infinity is there now. Sharpness is another story, but it's definitely much better than before.

But I noticed something I don't remember noticing in any other lens I've had (never had a macro, I know they do this): while focusing, the magnification changes albeit very slightly. Is this even possible with properly functioning prime lens or is it just another sign that there is still something wrong with it? I've had about 20 primes and can't for the life of me remember seeing anything like that.

 

[polyglot]

Yep, that's normal. Should be a very small effect though and only evident at close ranges / higher magnifications.

That's not to say it ain't still broken.

 

[OldBodyOldSoul]

Thanks polyglot.

 

[lxdude]

The magnification does increase with lenses which unit focus-i.e., move the whole lens forward to focus at close distances.
Let's say you put a 50mm lens set at infinity and its smallest aperture on a 35mm camera, and close the aperture with DoF preview, then turn the focus ring to the closest focusing distance. You will see the image get slightly bigger and slightly cropped (and of course, not as sharp). The reason is that as the lens is moved toward the close focusing position, the distance to the image plane increases, effectively resulting in a longer focal length.

 

[btaylor]

Yes, this is one reason cinema lenses cost so much, this so-called "breathing" has to be corrected to a great degree. The early Russian anamorphic lenses used on such features as "Solaris" had such severe breathing that when there was a rack focus it actually looked like the lens was zooming-- the focal length change was that noticable.

 

[darkosaric]

This is the problem in rangefinder cameras: viewfinder in most rangefinder cameras are showing frame for 10 feet, or closest focus - and on infinity you will have much more on negative than in viewfinder.

Regards,

 

[ic-racer]

The farther the lens is from the film, the greater the magnification, the narrower the angle of view and the larger the image circle.

 

[Ihmemies]

Yes.. focal length increases when focusing closer. In many Nikon primes for example this is readily noticeable from viewfinder. 50/1.4, 85/1.4, 135/2 etc. do it. I don't pay attention to it since I'm not a cinematographer