40-150 PRO focus breathing?

whumber wrote:

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.

I am well aware that it is based on a thin lens model but that doesn't make it useless for the purpose at hand. Moreover, the formula is useful precisely at close focus distances. At infinity, there is no real need for it.

What the results of the formula tells us is that the focal length of the 40-150 when set to 150 mm and shot at its minimum focus distance is equivalent to that of a thin lens (a single lens element) with a focal length of 100 mm. If there were no focus breathing, it would instead be equivalent to a thin lens with a focal length of 150 mm (provided that the 150 mm specification is correct in the first place).