Oly 45 1.8 with Pana GTC1 converter... works !!!

Started May 17, 2012 | Discussions thread
kenw Veteran Member • Posts: 5,350
Re: Rather different than how you put it.

Great Bustard wrote:

Interesting. So a front end TC changes the diameter of the entrance pupil? Makes sense, I guess, since the entrance pupil (virtual aperture) is the diameter of the iris (physical aperture) as it appears through the front element, thus it makes sense that a front mount TC changes its size.

Yep, you got it exactly. It effectively magnifies the entrance pupil.

The FE would have to be 50mm to make the 45 / 1.8 into a 90 / 1.8. Even then, however, it would be equivalent to a 180 / 3.6 on FF. Still pretty good, though!

Yeah, and from the picture of the converter it is questionable that it is really that large. The outer diameter is 53mm and the front element significantly smaller than that. Which means it would "vignette" even on the optical axis. Though a more accurate interpretation would be that the aperture stop is now at the front element if you have the 45/1.8 wide open (the iris of the 45 is no longer actually doing anything optically). In general that means a lot of other optical parameters may now be suboptimal as well (CA, coma, field curvature, etc.) as they vary with aperture stop position.

Not surprising since it was designed for a 42/5.6 lens! Trying to use it with an entrance pupil three times larger probably won't work as intended.

EDIT: I cross posted with your addendum. Good job on the diameter estimate. So yeah, wide open the front element has definitely become the aperture stop of the optical system.
Ken W
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