Nikon PlanApo2

ray_parkhurst · Post by **ray_parkhurst** » Fri Apr 27, 2012 8:01 pm

I feel stupid asking this question, but I'm having a heck of a time getting an image with a Nikon PlanApo2 that I just received. I set it up on bellows with 160mm total extension, but it won't give me a focused image at any working distance. What should I expect for working distance with this objective? Thanks...Ray

rjlittlefield · Post by **rjlittlefield** » Fri Apr 27, 2012 8:17 pm

What specifications are written on the lens? Can you post a picture of it?

--Rik

ray_parkhurst · Post by **ray_parkhurst** » Fri Apr 27, 2012 8:29 pm

The lens says:

Plan Apo 2
0.08
160/

Ray

rjlittlefield · Post by **rjlittlefield** » Fri Apr 27, 2012 11:37 pm

That sounds like an ordinary DIN-spec objective. With 150 mm total extension from sensor to mounting flange, it should focus at 45 mm from that same mounting flange. I don't know how much working distance that is, but just measuring the objective and subtracting should give the right number.

--Rik

Craig Gerard · Post by **Craig Gerard** » Sat Apr 28, 2012 12:24 am

Ray,

Could you take a snapshot of the objective for reference?

Craig

Chris S. · Post by **Chris S.** » Sat Apr 28, 2012 12:31 am

The Nikon CF lens brochure lists a Plan Apo CF Plan Apochromat 2x/0.08, with a tube length of 160 and parfocality at 45mm. If it is this lens, the working distance is listed as 5.3mm.

ChrisR · Post by **ChrisR** » Sat Apr 28, 2012 1:16 am

The short working distance of low magnification objectives can come as a bit of a surprise! Nice image in only the small circle, then darkness outside.
The CF Mplans are more usable WD-wise - "unusual" WD versus focal length numbers compared with what we're more used to:

The note (2) refers to vignetting when used with some UW eyepieces. If that's because the field inside the tube is too wide, perhaps that's encouragement for use with direct projection onto a sensor.

The effective apertures of these aren't encouraging, of course

ray_parkhurst · Post by **ray_parkhurst** » Sat Apr 28, 2012 7:48 am

ChrisR wrote:The short working distance of low magnification objectives can come as a bit of a surprise! Nice image in only the small circle, then darkness outside.

Surprising to say the least. The working distance is almost zero compared with my expectations for a 2x objective, so short in fact that I did not even test it that close before I wrote for help. Looks like it will be going in the eBay pile. Thanks for the info all...Ray

rjlittlefield · Post by **rjlittlefield** » Sat Apr 28, 2012 10:11 am

The problem with low-mag objectives is that they still have to work within the microscope frame and be parfocal with other objectives. Given the 45 mm parfocal distance of most microscopes, the lowest magnification that you could possibly reach without elaborate designs is about 4X. Beyond that it's a matter of fitting a long lens into a short body, so the objective has to start looking sort of like a reversed telephoto. The lower the magnification, the worse the problem. Add in the need for a large field to go with the low mag, and the problem gets harder yet.

Low-mag objectives make great sense when you want to switch quickly between low and high mag. Otherwise they're more expensive and less effective than ordinary lenses.

--Rik

Charles Krebs · Post by **Charles Krebs** » Sat Apr 28, 2012 11:20 am

Ray,

I have both that objective and the non-Apo version of the same time period. As has been mentioned, the working distance is very modest (with "biological" objectives like this, 4X and 10X are about the only ones that can be found with usable working distances... and if you want "Apo" probably only 4X).

As soon as you start to go over the "designed-for" 150mm extension that short 5.3mm working distance drops very quickly, to the point where you can't achieve focus at all.

In my experience the Plan Apo (2/0.08 WD 5.3) does not seem to provide a significantly superior image to the Plan Achromat (2/0.05 WD 5.8 )