160mm achromat as a tube lens

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

Moderators: rjlittlefield, ChrisR, Chris S., Pau

JayMcClellan
Posts: 124
Joined: Thu Nov 24, 2022 8:50 pm
Location: Saranac, Michigan
Contact:

160mm achromat as a tube lens

Post by JayMcClellan »

I've been using my Raynox DCR-150 (208mm) and DCR-250 (125mm) for tube lenses that give about 1X and 0.6X times the nominal magnification of my 200mm-tube-lens infinite objectives, but I wanted another option about halfway between them. I could adjust the tube length to alter the magnification as discussed in this thread, and my 5X/0.14 and 10X/0.28 objectives should tolerate significant changes to the tube length but the 20X/0.42 is expected to be much less tolerant of such changes due to its higher numerical aperture, although I have not tested that experimentally. I am using a micro-4/3 sensor with a 3-micron pixel pitch and I prefer the level of sharpness I get with a resolution of around 3 pixels per line pair:

PixelsPerLinePair = 0.275 / NA * Magnification / PixelPitch

Most of the time I push my nominal objective magnification down by 0.6X - 0.8X to get acceptable sharpness, and the PPLP that I find acceptable also depends on the subject and lighting and on how well I control vibration etc. so it's nice to have different options. I found this Melles Griot 160mm achromat on eBay and decided to give it a try as a 0.8X tube lens:
160mm-a.jpg
The lens has a 40mm diameter, which fits very neatly into one of my short M42 extension tubes. Then I add another short extension tube to lock the lens in place, for a compact assembly with M42 threads on both sides. I tried it in both orientations and didn't see much difference, but the image quality looked very slightly better with the less-convex surface facing the sensor so that's how I have it mounted.
160mm-b.jpg
160mm-c.jpg
I've been using this without any extra infinity space between the acrhomat and objective. I tried it with an extra 50mm extension in that space, and didn't notice any difference in image quality but of course it makes the assembly longer and more affected by vibration so I just kept it short. My M42 adapter has a flange focal distance of 45.46mm from the sensor and by adding two flocked 50-mm extension tubes plus the short tube holding the lens, I get the lens (strictly speaking its secondary principal plane, which is typically near the surface of the glass at the edge) close to 160mm from the sensor and thus infinity focus. I didn't try to get it exactly at infinity focus, just close. Measurements with a calibrated scale confirmed it gives very close to the expected 0.8X nominal magnification of the objectives. The tubes and clamps are from WeMacro:
160mm-d.jpg
I used this tube lens for many of the shots in my latest macro video, including this shot that used a 10X objective for 8X optical magnification:
Papilio_maackii_1k.jpg
See viewtopic.php?f=27&t=46560 for that video and discussions about it. Note that I've added some synthetic depth of focus to this shot and some others, so the noticeable falloff in sharpness is not related to this tube lens.

JKT
Posts: 420
Joined: Fri Oct 28, 2011 9:29 am
Location: Finland
Contact:

Re: 160mm achromat as a tube lens

Post by JKT »

Hey, come on - how did it do compared to the two Raynoxes? :D

JayMcClellan
Posts: 124
Joined: Thu Nov 24, 2022 8:50 pm
Location: Saranac, Michigan
Contact:

Re: 160mm achromat as a tube lens

Post by JayMcClellan »

It's hard to compare directly with the raynoxes because with a different magnification it selects a different fraction of the objective's image circle and spreads the features across a different different number of sensor pixels. I could vary the tube lengths to make the magnifications the same but I haven't done that experiment. Subjectively I don't notice any difference in image quality that I can attribute to the tube lens.

JKT
Posts: 420
Joined: Fri Oct 28, 2011 9:29 am
Location: Finland
Contact:

Re: 160mm achromat as a tube lens

Post by JKT »

Considering that the longer Raynox is pretty good, that is good enough for this. Thanks!

ray_parkhurst
Posts: 3416
Joined: Sat Nov 20, 2010 10:40 am
Location: Santa Clara, CA, USA
Contact:

Re: 160mm achromat as a tube lens

Post by ray_parkhurst »

A bit late but I have been using Thorlabs SM1 mounted achromats as tube lenses for my 20...50x work. They give a great result and integrate well in M26 systems. I bought a 200mm and 250mm and am planning to get a 150mm and maybe 100mm so I have a range of mags to work with.

JKT
Posts: 420
Joined: Fri Oct 28, 2011 9:29 am
Location: Finland
Contact:

Re: 160mm achromat as a tube lens

Post by JKT »

That looks like an interesting series. Same question - any comparison to Raynox DCR-150? The 200mm one is pretty close to that.

ray_parkhurst
Posts: 3416
Joined: Sat Nov 20, 2010 10:40 am
Location: Santa Clara, CA, USA
Contact:

Re: 160mm achromat as a tube lens

Post by ray_parkhurst »

JKT wrote:
Mon Jan 15, 2024 11:56 am
That looks like an interesting series. Same question - any comparison to Raynox DCR-150? The 200mm one is pretty close to that.
Sorry, never tried the Raynox. I have tried several achromats though and the Thorlabs seem best-corrected. I still need to try the RRMs I bought a few years ago. All my old tests need to be redone now that I am on FF, and I am finding some unexpected results.

JKT
Posts: 420
Joined: Fri Oct 28, 2011 9:29 am
Location: Finland
Contact:

Re: 160mm achromat as a tube lens

Post by JKT »

ray_parkhurst wrote:
Mon Jan 15, 2024 7:13 pm
Sorry, never tried the Raynox. I have tried several achromats though and the Thorlabs seem best-corrected. I still need to try the RRMs I bought a few years ago. All my old tests need to be redone now that I am on FF, and I am finding some unexpected results.
Having used FF for quite some time, I know the feeling.

Post Reply Previous topicNext topic