Relative focal lengths - canon ef lens and olympus om lenses

escalinci · Joined: 04 Jul 2013 Posts: 5 Location: United Kingdom

So, I've been using a 5dmkii for a while for video, exclusively with olympus OM Primes, but I had a few projects where I'd just stayed on one lens (usually the 35mm or the 50mm), because I found that on balance, I was covering more, although I was creating problems later in the edit, as different shot sizes are often much easier to cut together!

So I kind of needed a zoom lens to shoot 'actuality.' I didn't want to get a mf zoom lens, so I got an old canon zoom pretty cheaply. It's a 28-85, and I felt, a decent choice. This also gave me a chance to do like-for like comparisons with the old and the new. I haven't got the 85mm prime (yet), so I compared the 28 and 50mm primes with equivalent focal lengths on the ef zoom lens, and the ef lens was... much wider?

These shots are all at f4.5, which is the widest aperture on the zoom lens past 50mm. This is the 28mm prime.

And the Canon at 28mm. Quite a bit wider!

The 50mm Prime. Thankfully, this test also showed that there's no colour cast on the old lenses, which makes it much more likely I'll mix footage between the two lenses.

The Canon at 50mm. Not massively wider but a bit weird.

So I could get out a measuring tape and charts to work out which is correct, but does anybody have any thoughts on this?

escalinci · Joined: 04 Jul 2013 Posts: 5 Location: United Kingdom

Perhaps I need to be an approved member or something before images show. Still would like to get your thoughts, do any of you find that legacy lenses seem to be a slightly longer focal length than their modern equivalents?

:edit: well the pics seem to be working now.

casualcollector · Posted: Thu Jul 04, 2013 2:18 pm Post subject:

Images are not shown for first time posters as a SPAM prevention measure.

RE: Focal lengths. Due to production tolerances and other conditions, the 50mm marking on a zoom lens is probably not precisely 50mm. Using your camera and fixed focal length lens as a standard for comparison, you could adjust the zoom to match the field of view of your 50 and make a mark for that position on your lens. You could do the same with other primes on your zoom. At the extremes, your zoom may not be exactly 28mm or 85mm. Also keep in mind that focal lengths are measured at infinity focus.

Prime lenses are not always exactly the marked focal length. Tolerances for focal length are usually about 5 percent IIRC. Tolerances are often wider for zooms. That means your 50 could be between 48 and 52mm and your 28~85 zoom may be a 31~77.

Primes and zooms focus differently. Primes, with some exceptions, focus by moving the whole optical assembly further from the film/sensor plane. Zooms focus by moving just the front group forward or by varying the spacing between groups.

escalinci · Joined: 04 Jul 2013 Posts: 5 Location: United Kingdom

Yeah, I would've thought that, but they showed up when I previewed the post.

Good knowledge, thanks, I imagined there are tolerances for such things, but what you've said about that is interesting, and the differences in focusing mechanisms too. None of these lenses breathe too much so I think there's still some discrepancy. I don't think it'll cause any problems though.

Sirrith · Joined: 17 Sep 2010 Posts: 215 Location: Hong Kong

Focal length is also measured at infinity, I believe. There is plenty of room for focus breathing.