Focal length of macro lenses at 1x magnification

These days I measured and calculated that my Sigma 105/2.8 Macro acts like 65.88 mm focal length when focused at 1x magnification. I'm wondering what the other macro lenses' focal lengths are at 1x.

P.S. To calculate it I used extension tubes and this formula:
focal length = (mm of extension tubes) / (measured magnification - 1)

Interesting subject.
I believe the 100mm macro (as well as most/all other macro lenses with internal focus) get shorter as well. Around 70mm, but I've never actually measured it.
When working with 1:1 macro between different lenses, you have to decide on the trade off between shorter focal length, smaller actual aperture and the lens extending.

Yes all the internal focus lenses will do this, the reduction in focal length will be greater the closer the focus. It is easy to caculate from the high school lens equation.

Lester Wareham wrote in post #13968481
Yes all the internal focus lenses will do this, the reduction in focal length will be greater the closer the focus. It is easy to caculate from the high school lens equation.

Lester - AFAIK all macro lenses do this whether they are internally focusing or not. Did see an approximation suggesting the quoted minimum focus distance/4 gave the focal length at min focus although in practice this value stillm seems to be a shade high.

Brian v.

LordV wrote in post #13968530
Lester - AFAIK all macro lenses do this whether they are internally focusing or not. Did see an approximation suggesting the quoted minimum focus distance/4 gave the focal length at min focus although in practice this value stillm seems to be a shade high.

Brian v.

Hi Brian,

I can't see any reason for the focal length to change for overall linear extension, by definition really.

Of course something like the MP-E 65mm although noted as OLE focus it has a floating element which might confuse the issue. I have not been able to measure the pupilary magnification accurately because of the internal light baffle. A changing pupilary magnification is a good way of seeing if the lens objective/image focal length ratio is changing. You see this on IF lenses.

Can you explain what you are thinking here?

Of course both methods give light loss, for OLE it can be deduced as simply the inverse square law assuming a unity pupilary magnification (ie a symmetrical focal length design which few actually are).