. . . For sure, the sharpened image is the winner by a mile (kilometer). With the nominal aperture at f/10 (I assume the camera has taken into account the teleconverter), at 4X the effective aperture is f/50, way into diffraction territory. This is where modern sharpening algorithms come to the rescue. . . .
A note about the OMS 90mm . . . Oneforone25 is shooting at f/16, not f/50, with 4x on this lens.
The 90mm's minimum effective aperture at 2x -- without the TC -- is f/8, as described in the OM documentation. (Note that this lens manipulates both focal length and pupillary ratio to produce its light loss profile.) Adding the 2x TC brings the final effective aperture to f/16 for 4x. My understanding is that in calculating final effective aperture, this is correct:
Final_effective = Primary_lens_effective + TC_loss
Oneforone25 is into diffraction territory with his MFT setup, but he isn't anywhere near f/50.
Enjoyed your books . . . Steven
I doubt it's actually an f16 effective aperture at 4x, the combination is too soft to have a similar effective aperture to the Laowa 25mm. Or the focal length/tc glass have a higher impact on the sharpness than I would expect, though we are already diffraction limited even with the MC14.
You'll still have to factor in the magnification change when calculating the effective aperture. The OM 90mm with the MC20 at 2x has an effective aperture of f16 at a nominal aperture of f10, but not at 4x.
Hmmmm? I'm confused and always willing to learn more. Accepting the fact that this lens truly is f/8 effective at 2X (per Olympus instruction manual, a bit unusual among macro lenses where an f/3.5 lens would be f/9.5 at 2X - f-effective = f-nominal (M+1), adding the 2X TC to f/8 brings the effective aperture to
f/16. I think, indeed, now that we're at 4X, that is the effective aperture, with attendant DOF and diffraction. Am I missing something? Or...is the f/16 effective just due to loss of light in the TC, and then we have to add even more effective-efffective for the increased 2X magnification from 4X????
There shouldn't be confusion. Shooting the 90mm wide open (at nominal f/5 in S-macro), at 2x
without TC brings us to f/8. (This can be easily verified by simple testing.) The 90mm apparently achieves this lower-than-expected effective aperture by manipulating both pupil ratio and focal length. (See also the reference below.) Adding the TC loses two additional stops (only) and brings final effective aperture to f/16.
Macrophoto_markus' comment, above, on the TC's effect is incorrect. The TC magnifies an existing image; it doesn't generate additional magnification through lens extension. The loss with a 2x TC is two stops; there is no additional magnification penalty.
But here's another way to look at it: The EXIF data on oneofone25's photos reports f/10, which would be what we would expect from an f/3.5 lens (wide open) at infinity when a 2X TC is attached, assuming the lens/camera knows the presence of the TC. Though not an Olympus user, I believe like all cameras (except Nikon) the EXIF data is unaware of any lens magnification.
The f/10 only tells us that Oneforone25 is shooting in S-macro wide open at nominal f/5, or f/10 with the TC. Using nominal f/10 as a base to calculate final effective aperture runs into trouble, however, because you're now double-counting the TC's light-loss effect.
Again, the applicable formula is:
final_effective = primary_lens_effective + TC_loss
Normally, a nominal f/10 aperture would be (M+1) = f/50 effective 4X. Even with Olympus' adjustment of the entrance pupil, I doubt it would gain more than about one stop lower effective aperture.
F/50 double-counts the TC's effect and isn't correct. Also, where the exit and entrance pupil diameters don't match, which seems to be true in this case, the applicable multiplier is (m/p+1), where p=pupil ratio.
For those interested, a good discussion of the pupil ratio's effect on effective aperture appears in this photomacrography.net thread. I would suggest reading through the entire discussion:
https://www.photomacrography.net/forum/viewtopic.php?f=29&t=8895
(The thread references your 1979 book
.)
So we can get two significantly-different results for effective aperture. Can some smart person out there explain what the heck is going on?
There is one correct result; see above.
I wish I had that 90mm Olympus lens to test: doing 4X with the TC and 4X with a long bellows. That's always better than a theoretical discussion.
This lens is very popular in MFT land (and my favorite lens) in part because it's the only 2x auto macro on the market. Also, the 8.6mm field width at 2x means that, unless you're into extreme macro like Oneforone25, you really don't need to attach any TC. You're already at 4x FF FOV equivalent without one.
A lot of people are producing good results with this lens. I suspect, however, that most are using this lens in the 0.5x-1x range.
By the way, is oneofone25 always shooting wide open in is tests?
If his nominal aperture is f/10 with 2x TC, that means that he has to be shooting at marked f/5, the widest aperture available in S-Macro.
. . . Steven
