Some Diatoms Visualised with Objectives Needing Extenders

Cactusdave · Post by **Cactusdave** » Thu May 29, 2014 1:10 pm

This thread http://www.photomacrography.net/forum/v ... hp?t=23756 turned into a bit of a discussion of short non-DIN standard length objectives that need some kind of extender to reach focus, or be approximately parfocal with other objectives in the turret. I promised I would post some pictures of diatoms imaged with high power objectives needing an extender to show that in my experience pretty good results can be obtained without horrendous spherical aberrations. Most shots are converted to black and white as firstly there isn't much natural colour in fossil diatoms, and secondly by the time you have Photoshoped out the significant chromatic aberration, which does occur in some images, there isn't much colour left anyway. Some of these images have been shown before in this thread http://www.photomacrography.net/forum/v ... 725#112725 Most of the images were obtained on a Zeiss Standard with the Goerz condenser and a Lomo 7X photoeyepiece coupled to a Canon 40D using a Pentax adapter. The Goerz condenser is described in this thread http://www.photomacrography.net/forum/v ... 3258#53258 The condenser had a NA 1.2 top but was not oiled to the underside of the slide. The lens That I have used most for high magnification examination of mounted diatoms (strews of cleaned specimens from Oamaru and elsewhere) is the Lomo X70 1.23 water immersion apochromat lens. This needs an extender to reach focus in the Zeiss Standard and I used a simple extender without an internal lens to correct for the increase in tube length that results of about 12mm in this case.

This shows a couple of pictures of the classic test diatom Pleurosigma angulatum.

A couple more pictures showing the very good resolution that this lens can achieve even under suboptimal conditions.

I'll post some more pictures in this thread taken with lenses with a simple extender. I have used the Lomo X85 1.00 water immersion achromat and Lomo and Nikon short X40 Apochromat objectives as examples.

Charles Krebs · Post by **Charles Krebs** » Thu May 29, 2014 5:02 pm

Look very good (surprisingly so). I know these can be excellent optics but adding to the tube length at those NA's would normally not be recommended. It would be interesting if you have the ability to try a comparison with a Leitz Plezy (or the Zeiss equivalent)

Jacek · Post by **Jacek** » Thu May 29, 2014 11:20 pm

Very nice

Cactusdave · Post by **Cactusdave** » Fri May 30, 2014 6:21 am

Thanks Charles and Jacek.

It would be interesting if you have the ability to try a comparison with a Leitz Plezy (or the Zeiss equivalent)

Yes I'd like to do that. It's on my 'to do list' I do have a PLEZ-Flu adapter with nice clean glass in, but I've only tested it with Leitz objectives with not very challenging test subjects. My impression from this was that the extra lens didn't have as much effect as I would now expect. If it will play nicely with these objectives (it many not offer enough extension) it would be good to try.

Cactusdave · Post by **Cactusdave** » Fri May 30, 2014 6:41 am

A few more pictures taken with high magnification lenses and a simple extender with no compensating lens for the increase in tube length.

This and the following picture were taken using the Lomo X85 1.0 water immersion achromat. This image can be compared with one in the earlier post which shows the same diatom. The image is magnified in comparison, but little if any extra detail is revealed. This is generally my experience with this lens. It lacks the real quality of the X70 Apochromat which is a better and more useful lens.

Lomo also made a rather nice X40 0.95 dry objective. This image is taken with the X40.

I did a small comparison of X40 objectives at one point for my own interest. This is a 100% crop of an image of the test diatom Stauroneis phoenicenteron taken with the Lomo X40 0.95 dry

Brian Matsumoto · Post by **Brian Matsumoto** » Fri May 30, 2014 9:26 am

This is a fascinating discussion as it illustrates the difference between theoretical and practical limits of performance. I thought a quote by Roger Loveland from his book Photomicrography: A Comprehensive Treatise is appropriate.

“When the acceptability of an image of a specimen rather than the star test is the criterion, the tolerance may become about eight times greater. In a photomicrograph to be made with the 4 mm 0.95 NA objective this allowable variation from the strict optimum would be about plus or minus 8 mm.”

Loveland has a graph on page 66 plotting the effects of variation of tube length. It should be noted that immersion objectives are far more tolerant of tube length variations than dry lenses.

It strikes me that we, as photomicrographers, have significantly more latitude in “pushing” the limits of the boundaries established by optical designers.

Brian