40-150 PRO focus breathing?

Anders W wrote:

Arctra wrote:

Today my Olympus 40-150 PRO lens arrived and while it does feel like a great lens, there's something that I noticed. I was comparing it against my trusty old Minolta Beercan and I was noticing that the Olympus zoomed all the way in at 150mm was producing a similar result as the Beercan at 210mm.

I ran the test again, but this time with APS-C crop mode turned on my Sony cam to simulate ~300ishmm and I had to get significantly closer to my test subject (in this case a old Charmander piggy bank) with the Oly to produce a similar FOV. This isn't a deal-breaking issue for me but it wasn't something I was expecting to see as I hadn't seen any reports of focus breathing issues with this lens.

Is this something that is known about this lens? Is mine the special one on the block? Or am I just fundamentally misunderstanding equivalency here? Like I said, not a huge deal-breaker but was definitely interesting to see.

It's very easy to calculate the focus breathing from the lens specs. The 40-150 PRO has a max reproduction ratio of 0.21 (at 150 mm obviously) and a minimum focus distance of 0.7 meters. The focal length, F, at that reproduction ratio, R, and focus distance, D, is calculated as

F = D/(1/R + R + 2) = 700/(1/0.21 + 0.21 + 2) = 100 mm

so yes, certainly some focus breathing going on, just as one might expect.

This equation is based on a thin lens model and while it may be fairly accurate at infinity focus, it's going to fall apart at close focus distances with thick lenses (i.e. any real lens you can imagine), especially for a lens where the distance between the principal planes is on the order of 10% of the subject distance.