Large format repro lenses as tube lens

[Mendel314]

I have a raynox 150 that is great as a tube lens. I use it with my mitutoyo 10x. The 20x represents a small gain in resolution for the money, so I was wondering if anyone has used repro/process lenses like the rodagon apo ronar, rodagon or gerogon which come in focal lengths like 300mm, 480mm and 600mm, as a tube lens? These are less expensive than new objectives, and are apo corrected at 1:1. If these were optimized for infinity, I wouldn't hesitate, but my question is this:

Has anyone tested them focused at infinity with an infinity objective? How are they?

[Charles Krebs]

I have a raynox 150 that is great as a tube lens. I use it with my mitutoyo 10x. The 20x represents a small gain in resolution for the money,

I'm a little confused by your first sentences. The Raynox 150 has a focal length of 208mm, so it should yield very close to 10X (~10.4X) when used with your objective.

To answer your question, yes, I've used three different "process" lenses as tube lenses... a 210mm Apo-Nikkor, a 150mm Rodenstock Apo Gerogon, and a 240/9 Rodenstock Apo Gerogon (also marketed as the Beseler Apo HD 240/9). In all cases these were used at an "infinity" focus distance.

http://www.photomacrography.net/forum/v ... php?t=9858 (With Mitutoyo 10X)

http://www.photomacrography.net/forum/v ... hp?p=78960

The 210mm Apo-Nikkor was the first one I tried and that was with the 5X Mitutoyo. In all cases I thought the results were excellent (only used APS-C sized sensor cameras), but did no side-by-side testing with other optics as tube lenses.

The magnifications obtained obviously vary from the "marked" objective magnification.

with 150mm:
(150/200)(10)= 7.5X

with 240mm:
(240/200)(10)=12X

if you used a 480mm:
(480/200)(10) = 24X

The numerical aperture remains the same. So the 150mm makes a very viable choice for 7.5X (as long as it will "cover" the sensor format adequately to the corners... OK on APS-C, full-frame???)

The longer ones give you a higher magnification, but since the NA remains the same if you go too far you are better off using the next higher magnification objective since the NA will be considerably higher and thus so will your resolution.

On the longer end I think a 240mm is quite reasonable, a 300mm would be OK for 15X at NA of 0.28 (but you would also be at 15X and have considerably higher resolution, with a 20/0.42 on a 150mm tube lens).

[Mendel314]

Thanks for your response. I think I may have been unclear in my question, which is really two questions, but you seem to have started to address what I meant to ask.
According to Edmund's listed specs for mitutoyo objectives, the resolution limit for the 10x is 1 micron, whereas the resolution limit for the 20x is only .7 micron. I wouldn't expect a 50x on a 100mm tube to cover ff, but maybe I'm wrong.

1. If I assume the sensor (sony a7rii, fwiw) is outresolving the lens*, and I compare a 10x on a 400mm lens with a 20x on a 200mm lens, will the improvement in resolution be on the order of 30%?

2. How do apo process lenses, optimized for 1:1, work when used as tube lenses focused at infinity?
I believe you have begun to address this. Being readily available and much less expensive than almost any other apo lenses, they are of interest to me for this application. Do the very high focal ratios seen on these (as high as f22 wide open for a 480mm lens), or aperture settings on any normal lens used as a tube lens, reduce the NA? Or is the effective aperture of the objective the limiting feature, given the central portion of the relay lens used in this application is so small

*if the a7rii has 4.5 micron pixels, at 10x and resolving 1 micron, the smallest details captured by the lens are 10 microns on the sensor, which is just above the Nyquist cutoff to capture all the information projected by the lens. In reality, the Bayer array reduces this resolution somewhat, so the information projected at 10x is probably about as high or higher than the sensor can record.

[Charles Krebs]

If you are using a full-frame camera I would be very suspicious of corner performance with any tube lens focal length shorter than the one for which the objective was designed.

Your potential resolution will scale directly with the NA.

Play around with this spreadsheet (use "1" in cell B13 since no trinocular optics are involved)

http://krebsmicro.com/relayDSLR/relay_micro.xls

The result box for "3 or 3.5 PIXEL/DETAIL" (desirable if possible) will show how surprisingly high the pixel density needs to be if you wish to squeeze out as much resolution as possible with a low magnification, relatively high NA objective like a 5/0.15 or 10/0.28 (when used at their rated magnification). Conversely, it also illustrates shows the heavy toll of diffraction with high magnification objectives of "modest" to low NA, like a 50/0.55. Overly simplified... for objectives below 20X (used at rated magnification) it is hard to put enough pixels under the projected image. With a good 20X (used at 20X) the higher Mp cameras available today can make for a good match. Over a 20X objective, the objectives we will typically use in this manner are easily "handled" by most of the cameras we might choose from.

Personally I have never used a lens with a maximum aperture as small as f22 as a tube lens so I can't answer one of your questions. Typically the limiting "aperture" is the objective. Too small a tube lens aperture will result in vignetting.