160mm vs infinity objectives

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henryr
Posts: 94
Joined: Mon Aug 31, 2015 8:48 am
Location: Connecticut

160mm vs infinity objectives

Post by henryr »

Hi all,

I've been absent for a few years and I've missed the back and forth. Last time I posted was when I was converting my Leitz dialux from 170mm TL to 160mm TL. It went well due to helpful responses on this site. Now I have a new question about objectives. What are thoughts about 160mm objectives vs infinity objectives? Is one considered better than the other?
Thanks,
henryr

Ichthyophthirius
Posts: 1112
Joined: Thu Mar 07, 2013 5:24 am

Re: 160mm vs infinity objectives

Post by Ichthyophthirius »

Hi Henry,

For what purpose? Biological objectives for brightfield transmitted illumination? Or more specialised contrast methods?

Most major manufacturers switched their biological objectives from 160 mm to infinite tube length in the late 1980s / early 1990s. The early infinite objectives are sometimes just modified designs of the last 160 mm models. From then on, there were incremental improvements over the following (no 30!) years.

So a 1990s objectives might be a slight improvement over a 1980s 160 mm; a 2020 objective might be significantly better.

If you compare a 1980s Plan Apo to a 2020 Plan Apo (like for like), there will be better colour correction (more colours corrected), less autofluorescence, higher contrast through better coatings and better DIC compatibility. However, flatness and resolution will be the same, so in simple transmitted light microscopy, you will probably notice very little improvement.

Regards, Ichty

Scarodactyl
Posts: 572
Joined: Sat Apr 14, 2018 10:26 am

Re: 160mm vs infinity objectives

Post by Scarodactyl »

In terms of inherent advantages, the reason everyone switched over to infinity correction is that it makes it easier to insert things between the objective and the head since you don't need to compensate for the added length--things like mag changers, illuminators, polarizers, beamsplitters etc. In a finite system you need lenses to compensate for that extra length and that's always going to be worse.
Other than that, infinity-corrected objectives don't give an inherently better image. However, since most infinity systems are newer than most finite systems they may have a few decades of optics advancements built into their designs, giving wider fields, better contrast and aberration correction, that sort of thing--but that isn't universal. For some objective series the top end has become better and better over time, but other things have been pretty stable. If you get a high-end finite system kitted out with high-end optics it's going to be excellent, and in most cases you'd need to get a very good infinity-corrected system to beat it out.

That said, when we're talking about the things that are most often photographed here most of the best options are infinity corrected. The late 80s and onward saw a lot of advances in reflected light objectives (no doubt spurred by demand from the semiconductor industry) and many of those moved to being infinity corrected even before biological systems did, possibly because it makes coaxial illumination easier. And particularly for the type of work most done here the best suited objectives are almost all infinity corrected: the Mitutoyo m plan apos, later Nikon epi objectives, Olympus epi objectives and a few others.

edit: got beaten to the punch.

Lou Jost
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Joined: Fri Sep 04, 2015 7:03 am
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Re: 160mm vs infinity objectives

Post by Lou Jost »

One advantage of infinity objectives is that you can use a zoom tube lens to easily change magnification without violating the objective's design parameters. Zooms usually vignette at low focal lengths but a zoom for a format one size larger than your camera will have a wide enough non-vignetting zoom range to be very useful. Example: 80-200 Nikon zoom on MFT body. This is my main MFT tube lens.

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