Lomo 3.7x finite vs 120G

[ray_parkhurst]

I shot the Lomo 3.7x in both finite and infinite modes to compare performance. As we all know, if indeed the infinite tube lens complements the objective well, the performance "should" be better based on the FL multiplication making a slightly wider effective aperture. In this case, the improvement is 1-3.7/4.7 = 21%. However, with the 120G, I see both improved sharpness and contrast. Makes me wonder if the Lomo was designed for infinite use? Anyway, here are the comparison shots. You won't see much difference in the overall, but there are subtle but noticeable differences in the center and corner crops, with the 120mm Gretag tube lens showing both improved contrast and sharpness.

Lomo with 120mm Gretag tube lens Overall


Lomo in finite mode Overall


Lomo with 120mm Gretag tube lens Center


Lomo in finite mode Center


Lomo with 120mm Gretag tube lens Corner


Lomo in finite mode Corner

[Miljenko]

Ray, my MTF tests confirm your findings. Lomo 3.7x produce higher resolution figures across 2x to 5x magnification range when paired with tube lenses compared to finite mode. Even the CA was lower. However, performance below 3x and beyond 4x drops rapidly no matter the coupling. My 120G is still with Robert and I can't wait to lay my hands on that nice find. I believe it will improve performance of some objectives I used "shortened" with 125mm Raynox DCR-250.

Best,
Miljenko

[ray_parkhurst]

Yes, the 120G seems to be working well. I bought a bunch of these a while back, thinking of integrating them into larger coin photo systems, but the mounting difficulties stumped me. My recent work with Raf to produce the 50mm protection ring adapter solved that problem, so now I could consider it as a coin lens. Of course the adapter costs 2x what the lens cost.

The 120G seems apochromatic and produces a nice image on its own. When Robert started looking at stacking, I tried it out and it worked "OK" but I was already working on trying to get 3x, and the 120 tube length would not support it with long working distances, so I only attempted a few combos.

Robert's latest reminder about the Lomo working well with tube lens prompted this latest test. I was actually hoping that the flatness performance would improve, but alas it's about the same, just with improved sharpness and CA. I'll take it!

[Macrero]

I believe the 3,7/0.11 was intended for use in Fedorov (Universal) Stage.
I have tried quite a lot of finite objectives with tube lenses and the only one (aside from the Lomo 3.7)
that not only works well, but actually gets improved, is the Nikon U10/0.22, designed for the Nikon Universal Stage
(curiously, the U5 and U20 does not improve when used with tube lenses).
All the other finite objectives I've tried as infinity performed worse than when used as intended.
Other "U" objectives I tried (e.g. Lomo 11/0.40) didn't do well either.

So the type of objective is not a guarantee. Something in the optical design it must be...
What? I have no idea. The only method that works for me is trial and error.

Best,

- Macrero

[ray_parkhurst]

I believe the 3,7/0.11 was intended for use in Fedorov (Universal) Stage.
I have tried quite a lot of finite objectives with tube lenses and the only one (aside from the Lomo 3.7)
that not only works well, but actually gets improved, is the Nikon U10/0.22, designed for the Nikon Universal Stage
(curiously, the U5 and U20 does not improve when used with tube lenses).
All the other finite objectives I've tried as infinity performed worse than when used as intended.
Other "U" objectives I tried (e.g. Lomo 11/0.40) didn't do well either.

So the type of objective is not a guarantee. Something in the optical design it must be...
What? I have no idea. The only method that works for me is trial and error.

Best,

- Macrero

My experience is similar, and when Robert posted the improved performance in infinite mode I was surprised, but indeed this lens is doing very well with the 120G. I have affixed an RMS adapter to the front of one of my 120Gs, and will leave it there. I plan to test a few more RMS objectives which were designed for long extensions, such as the Nikon 35mm, some Mikrotars, etc. In any event, I'm delighted that the very strong performing Lomo is even better with the 120G, and that's how I will use it.

[ray_parkhurst]

I've been mounting the Gretag 120mm on my Pentax Auto Bellows, and the min extension is longer than infinity. I just tried a 90mm Mikrotar, and the minimum mag is 1.8x instead of the expected 1.3x if it were able to focus to infinity. I extended this a little to get 2.0x, and the image looks really good. The Mikrotar is f6.3, so what is the effective aperture of this strange combo? Is there a simple way to calculate?

[Macrero]

I've tested the Lomo and the U10 with various rear lenses, and they works well will all of them. Difference is negligible, mostly in CA, with the Raynox, for instance, there are more CA than with more complex optics, such as Apo-Gerogons, Rodagons, etc.

John Hallmen posted years ago tests with old Leitz Summars that worked well in combos. I found the Photar 2/25 to work better in combo as well.

I am not aware of a formula to exactly calculate the effecive aperture of a given combo. It seems that trending is to calculate it by the standard formula, but removing the "1 +" from "1 + Magnification".

I am not sure how accurate that formula is. I guess it will depend on various factors: FL of both lenses, optical formulas, distance between lenses, pupil ratio, etc, etc.

Best,

- Macrero

[Lou Jost]

Just like with magnification, it is often easier to just measure EA directly rather than trying to calculate it. Using transmitted light, look at the exposure difference relative to a lens of known EA.

[ray_parkhurst]

Jiust like with magnification, it is often easier to just measure EA directly rather than trying to calculate it. Using transmitted light, look at the exposure difference relative to a lens of known EA.

Maybe so, but accuracy of that method is not all that great.

Could we consider the EA of the combination lens operating "properly" at m=1.3, then linearly decrease the aperture by ratio of the actual magnifications? That would give

EA1.3 = 1.3 * 6.3 = 8.2
EA2.0 = 8.2 * (2 / 1.3) = 12.6

[Lou Jost]

You can do it very accurately, though it is time-consuming. With a reference lens of known EA, take a series of photos at different shutter speeds centered around a given speed X set for perfect exposure. You can read the RGB value of the gray at each exposure and graph it. Then take a picture of the same with your unknown lens. You can match the RGB value exactly on the graph and see exactly how many stops it is away from X, that's how many stops it is from the reference EA.

[ray_parkhurst]

OK, that sounds like it could give reasonable accuracy. Maybe worth a try to compare with the simple formula.

[chris_ma]

You can do it very accurately, though it is time-consuming.

there is another way which would be faster:
since sensors in cameras react pretty linearly to light, if you take a RAW photo and bypass any contrast/gamma curve, your RGB values are linear as well so you can simply find the multiplication factor.

note that most RAW converters will apply a contrast curve to make things prettier, and a gamma curve to make things look good on monitors, so the hardest part is to actually get a linear RAW conversion. I created a linear profile for my S1R for Lightroom and export with ProPhoto profile, then reverse the gamma correction of ProPhoto later which works pretty well.

yet another option would be not to worry and to just use the exposure slider in Lightroom and read the EV difference there - you'd probably get less then 1/3 stop error this way as long as the reference lens has a similar f-stop.

chris

[ray_parkhurst]

You can do it very accurately, though it is time-consuming.

there is another way which would be faster:
since sensors in cameras react pretty linearly to light, if you take a RAW photo and bypass any contrast/gamma curve, your RGB values are linear as well so you can simply find the multiplication factor.

note that most RAW converters will apply a contrast curve to make things prettier, and a gamma curve to make things look good on monitors, so the hardest part is to actually get a linear RAW conversion. I created a linear profile for my S1R for Lightroom and export with ProPhoto profile, then reverse the gamma correction of ProPhoto later which works pretty well.

yet another option would be not to worry and to just use the exposure slider in Lightroom and read the EV difference there - you'd probably get less then 1/3 stop error this way as long as the reference lens has a similar f-stop.

chris

I would worry about not using the same lens for the tests. I've seen fairly significant differences in exposure between lenses of same aperture. T-stops? Anyway, using same lens would fix that, but then I'd need to rely heavily on the curve. I'm not sure how to linearize the demosaicing/raw conversion.

[ray_parkhurst]

I measured the exit pupil of the combination at 10mm. The extension from rear of lens to sensor is 110mm, so I believe the EA is f11.

Now the question is if I measured the exit pupil correctly. I held the lens at arm length, and looked through it at my monitor while holding a ruler up to the back of the lens, measuring the diameter of the bright circle. Does that give a correct number?

[rjlittlefield]

I measured the exit pupil of the combination at 10mm. The extension from rear of lens to sensor is 110mm, so I believe the EA is f11.

Now the question is if I measured the exit pupil correctly. I held the lens at arm length, and looked through it at my monitor while holding a ruler up to the back of the lens, measuring the diameter of the bright circle. Does that give a correct number?

That's close, but you also need to know the distance between exit pupil and sensor.

You can accurately determine both size and distance by looking into the rear of the lens, using a camera equipped with a macro lens. Focus and photograph the exit pupil, then remove the lens under test, swap in a ruler placed so that it's perfectly focused too, and take a second photo of that.

The ruler now gives you both the location and the size of the exit pupil, so you can calculate NAatSensor = RadiusOfExitPupil / DistanceFromPupilToSensor in the target system.

Effective F-number is then just Feff = 1/(2*NAatSensor) as usual.

--Rik