Cover Glass test on a 4x objective

NikonUser · Post by **NikonUser** » Sun Mar 21, 2010 4:27 pm

In a recent post
HERE
Rik commented "In theory, if a dry objective is designed to expect a cover glass then you can always add one. Just stick it on the end of the lens if you need to -- it can go anywhere between the objective and the subject as long as it's properly parallel."

Up until now I have been using a Nikon 4x objective without a cover glass.

Today I followed Rik's advice and taped a cover glass to the front of the lens.

One of the local ants, possibly a New York Carpenter Ant ( Camponotus noveboracensis).

Unfortunately the ant moved during the exposures and so I can't show an actual pixel shot from the 62 frames.

I can show a stack of 5 frames (lower image) where there was no movement.The actual head length is 1700 pixels and I have reduced it to 1000 pixels. Image quality seems OK, appears not to be degraded.
Of course I will have to run the test again to compare 'with cg' vs. 'without cg' - but not tonight!

Nikon CF Plan 4x/0.13 160/- Achromat, 62 frames @ 0.02 mm, Nikon focus block, 2 flashes, styrofoam cup diffuser.
Top image with a few dust spots removed.

NU1011 & NU10012

mgoodm3 · Post by **mgoodm3** » Mon Mar 22, 2010 10:14 am

I think that the coverglass induces a little bit of spherical aberration into the system. I think that it makes more difference as the objective power increases and becomes critical with high power objectives.

I shoot a lot of images through plastic that is thicker (1-2 mm) than a coverglass and I get decent quality even at high magnification.

rjlittlefield · Post by **rjlittlefield** » Mon Mar 22, 2010 10:40 am

It's the NA that really matters. See HERE for examples shooting through a UV filter at NA 0.30 (equivalent to about f/1.5) and at f/2.8 (actually about f/3, considering pupil factor). That doubling of the NA makes a lot more than 2X change in the degradation.

I would be a little surprised if it's possible to see degradation with/without 0.17 mm cover glass at only NA 0.15 .

--Rik

PauloM · Post by **PauloM** » Mon Mar 22, 2010 11:34 am

Actually, if you read the markings on both the 4x Apo 0.20 and the 4x Achro 0.13, Nikon itself didn't stipulate a cover glass thickness.
Both those objectives are marked 160/-, which I take it to mean that they can be used both with and without a cover glass.

According to Nikon's CF N brochure, it's only at N.A. 0.30 and higher that the cover glass thickness is considered important enough to warrant marking it on the objective's barrel.
We have the CF N Plan 10x (N.A. 0.30) marked 160/0.17, and the CF E Plan 10x (N.A. 0.25) marked 160/-.

Out of curiosity, I checked the brochure for the M Plans, and all of them are marked 210/0, even the ones with <0.25 N.A.

Paulo

NikonUser · Post by **NikonUser** » Mon Mar 22, 2010 11:48 am

My interpretation is that

'/-' means cover glass thickness is irrelevant

but '/0' means no cover glass.

rjlittlefield · Post by **rjlittlefield** » Mon Mar 22, 2010 12:54 pm

Agreed. The mystery about "marked 210/0, even the ones with <0.25 N.A." is only that the lower NA objectives are marked /0 instead of /- .

My guess is that Nikon wanted to give up setting a threshold on how much degradation is worth changing the labeling.

It's simpler to just specify the design point. Users will quickly decide if they can live with any deviation from that. With the low NAs they will; with the high NAs they won't.

Besides, if somebody bought two low-NA objectives to get /0 and /0.17 even though they didn't really need to, would that be a problem for Nikon?

--Rik

PauloM · Post by **PauloM** » Mon Mar 22, 2010 1:19 pm

rjlittlefield wrote:would that be a problem for Nikon?

I think that, first and foremost, it would be a problem for whoever had payed for them!