Infinity Objectives - magnification, aperture, and image circle

[mroscar]

I am interested in making the leap from macro lenses to infinity corrected microscope objectives. I've used up to 5x macro with the Canon MP-E 65mm but have no experience going beyond that.

I've looked through old posts here and a few other sources but still have a few questions, specifically about the relationship between the objective and tube lens and how changing focal lengths and apertures of the tube lens will impact the image produced by the combined lenses.

For the sake of examples I am considering a Mitutoyo M Plan Apo 10x paired with a Canon R5 and EF 200mm f/2.8L ii prime lens as the tube lens. At this time a DIY tube lens using raynox or others is a tad intimidating so I'd prefer to use more out-of-the-box solutions if possible, plus I already have the EF 200mm lens.



My understanding from reading some helpful posts here is that the magnification is proportional to the tube lenses focal length relative to the objective's. For instance, using a 300mm lens for the tube lens instead of 200mm as designated for the M Plan Apo 10x should result in a magnification of 15x (1.5x greater than the listed magnification on the objective).

• Is that correct?

• If so, what are the drawbacks of altering the tube lens focal length to change magnification instead of using a separate objective?

• Essentially, can I safely change the tube lens focal length to save money on buying multiple objectives?

Edmund lists for the M Plan Apo 10x that the "Maximum Diameter (mm): 30", which I take to mean that the image circle projected on the sensor is 30mm in diameter when the tube lens is 200mm. I don't have an APS-C camera and am seeking a system that will cover the full frame sensor or larger as I also have medium format equipment

• Is that correct?

• If so, how does the image circle size change as the tube lens focal length is increased or decreased? Could I just change the tube lens focal length to get better sensor coverage, or to cover a medium format sensor?

Finally, the relationship between NA of the objective and the f-stop aperture of the tube lens is a subject I don't quite understand.

• What impact does changing the aperture of the tube lens have on the final image?

• Does stopping down the aperture improve image quality or does that increase diffraction?

• Is there a formula or calculator available that will give the effective F-stop aperture of the combined lenses?

Apologies for the many questions, and a big thank you in advance for any insight shared!

[Tim Boomer]

First of all, welcome! I haven't posted much, but have learned so much from this wonderful forum over the years.

As a Nikon user, I don't have personal experience using the EF 200mm, but I think it's a good place to start (Canon users, feel free to chime in if you know that this lens won't make a good tube lens for some reason). Many folks say that macro lenses don't make good tube lenses, but I have been using my 200mm micro-nikkor IF-ED as a tube lens for a couple of years, and I am very happy with the results. I realize your canon isn't a macro lens, but the point is it doesn't hurt to try things out and come to your own conclusions. If your 200mm canon lens has a good tripod collar, it will be much easier to use and make fine rotational adjustments than with the Raynox tube lens.

You are correct that the magnification is proportional to the tube lens length (e.g., 10x mitty on a 300mm lens will give you 15x). This method probably had some merit years ago, but we're not in 2007 anymore, and today's high resolution sensors will outresolve the mitty 10x (the "standard" NA 0.28 version, not the $8500 0.42 HR version). In other words, pushing the mitty past 10x will result in empty magnification; you'd be better off just cropping and upscaling your 10x photo.

Likewise, using a 150mm tube lens with the mitty 10x will give you 7.5x resolution. It should look very good in the center, but you will have very heavy vignetting and very poor corner performance on your FF sensor. Even at 10x with the recommended 200mm tube lens length, the corner performance on a FF sensor leaves something to be desired. Some folks are okay with it, others are not, but I think it's safe to say you would be disappointed with the mitty 10x on a MF sensor.

The Nikon 10x CFI PLAN has good IQ and better corner performance on a FF sensor, and it's a lot cheaper, but I still doubt it would cover a MF sensor. If corner performance is extremely important to you, the Nikon might be right for you, but the mitty is still better over the central ~85% of the frame. The mitty also has better CA correction (even into the corners) and over three times the working distance, which is really helpful for lighting and not bumping into your subject or its substrate.

The formula for calculating effective aperture for microscope objectives is effective aperture = magnification/(2*NA). So the mitty has an effective aperture of 10/(2*0.28) = f/17.86. It's best to leave the aperture on your tube lens wide open. Stopping it down won't increase sharpness or DOF; it just reduces the amount of light hitting the sensor (something you definitely don't need with high mag work!).

I hope that helps. I know a lot of this stuff can seem overwhelming in theory, but it's really not too bad once you start putting it into practice.

Cheers,
Tim

[mroscar]

Tim, thanks so much for your response. It's tremendously helpful.

I suspected there was limited utility to trying to wiggle out of needing multiple objectives, but it's good to have confirmation.

I will do some research on the Nikon objective, my interest in the Mitutoyo was largely based on the working distance and APO qualities, but it's good to know of alternatives. I think I'll mock up a scenario with lights and a ruler and see if the lower working distance is something I can work with or it's better to trade corner coverage for maneuverability.

Your point about upscaling is a good one, it hadn't crossed my mind. That would eliminate both the troublesome effort to make MF work with objectives as well as give more room for cropping.

Thanks again!

[Tim Boomer]

My pleasure! I still think Mitutoyo is probably the way to go for most people who can afford it. Sometimes I cringe a bit when I pixel peep at the FF corners, but then I remind myself that I couldn't have taken many of these pictures at all with the limited working distance of the Nikon.

[mroscar]

Thanks again for your advice Tim

I mocked up a scenario loosely based on photos I've taken at 5x on the Canon MP-E 65mm and then moved the camera and lens in closer to simulate the working distances of the Nikon and Mitutoyo 10x objectives. The Nikon was really pushing the limits of what I could fit with regards to lighting and generally seemed like it would be an added complication. As a result I've opted to go for a Mitutoyo setup for now.

I found a good deal on a set of the 5x, 10x, and 20x brand new so decided to jump in with both feet first and have the three objectives on their way to me now. I figure I'll work my way up to the 20x as I get familiar with the setup.

My mock up experiment also made me rather wary of using a large diameter lens like the 200mm canon as a tube lens as that will also interfere with set ups so I've ordered a setup to use a raynox as a tube lens which will take up far less space. As best I can tell, my WeMacro rail should be capable of small enough steps for 20x magnification.

I expect to spend the next few weeks learning, practicing, and fighting vibrations. I suspect the deeper stacks will mean I need to get better at using Helicon as well.

[Tim Boomer]

That all sounds great! I hadn't thought about the large diameter of your Canon lens. My Nikon lens is only f/4 and therefore rather lean and not much different than a tube lens made with the Raynox. Best of luck to you!

[Lou Jost]

That all sounds great! I hadn't thought about the large diameter of your Canon lens. My Nikon lens is only f/4 and therefore rather lean and not much different than a tube lens made with the Raynox. Best of luck to you!

Tim, congratulations on your Nikon Small World recognition! Great image.

[Tim Boomer]

Tim, congratulations on your Nikon Small World recognition! Great image.

Thank you very much, Lou! It is such an honor to have my work recognized with such incredible imagery from all over the world. =)