DF NA requirements
-
microscopeboi
- Posts: 41
- Joined: Wed Apr 15, 2020 11:26 pm
DF NA requirements
hi, im looking into purchasing a high NA darkfield condenser to work with NA>.65 objectives. If i bought a 1.3-1.4 oil condenser would this work with objectives with .65-.75NA (Nikon plan 40x .65 and Nikon planapo 20x .75) what would be the lower limit of functionality for this condenser?
-
apochronaut
- Posts: 6327
- Joined: Fri May 15, 2015 12:15 am
Re: DF NA requirements
DF condensers produce a thick walled cone of light with an outer wall representing the higher N.A. limit and the inner wall representing the lower N.A. limit. They allow light of an N.A. between those two limits into the optical path. Generally speaking the upper limit determines the field coverage of the condenser as well as it's theoretical resolution and the lower limit determines the maximum N.A. that an objective can have and still obtain a dark field.
I think you have stated only the upper N.A. limit of your condenser but the lower limit is usually around .20 lower, so it will be 1.10 or higher.
In order to get a good contrasty background the objective has to be at least .20 lower than the condenser's lower limit, or usually, .90 and lower, so one of .65 will be fine.
The other factor that needs to be considered is the field coverage of the condenser. Most oil DF condensers of a high Max. N.A. have a fairly narrow field coverage of about 800 microns, so roughly speaking equivalent to a 25X objective viewed through a 20mm f.o.v. eyepiece. A 40X objective should be well within your condenser's field coverage and well below it's lower N.A. limit.
I think you have stated only the upper N.A. limit of your condenser but the lower limit is usually around .20 lower, so it will be 1.10 or higher.
In order to get a good contrasty background the objective has to be at least .20 lower than the condenser's lower limit, or usually, .90 and lower, so one of .65 will be fine.
The other factor that needs to be considered is the field coverage of the condenser. Most oil DF condensers of a high Max. N.A. have a fairly narrow field coverage of about 800 microns, so roughly speaking equivalent to a 25X objective viewed through a 20mm f.o.v. eyepiece. A 40X objective should be well within your condenser's field coverage and well below it's lower N.A. limit.
-
microscopeboi
- Posts: 41
- Joined: Wed Apr 15, 2020 11:26 pm
Re: DF NA requirements
alright, thank you, do you think my 20x .75NA planapo would do well with the aforementioned condenser? the reason i ask is because when i use DF patch stops for my 20x and 40x the resolution goes down significantly.
-
apochronaut
- Posts: 6327
- Joined: Fri May 15, 2015 12:15 am
Re: DF NA requirements
Patch stops are limited in their capacity to enable really good DF. A 20X , even at .75 should give a good dark background. The only time I have run into problems with DF oil condensers is when the N.A. climbs above .80 . Where you may have problems is with the illumination circle. Some condensers, quite a few in fact, cannot fill a wide field at 20X.
Re: DF NA requirements
The oil would be a safe way to go with the 0.75.
Maybe I'm getting a bit lazy, but I try to avoid oiled darkfield unless absolutely needed. Nikon makes an excellent dry darkfield condenser...0.95-0.8. I use it regularly with a 20/0.70 Olympus Plan Apo where it works great. Also works great with 40/0.70. But the 0.75 of your 20x is cutting it close. Too bad there's no way to try it out.
Maybe I'm getting a bit lazy, but I try to avoid oiled darkfield unless absolutely needed. Nikon makes an excellent dry darkfield condenser...0.95-0.8. I use it regularly with a 20/0.70 Olympus Plan Apo where it works great. Also works great with 40/0.70. But the 0.75 of your 20x is cutting it close. Too bad there's no way to try it out.