#8
Post
by apochronaut » Wed Oct 17, 2018 1:37 pm
Yes, the image would not be great but not due to empty magnification. Those are non corrective eyepieces, designed to magnify an otherwise perfectly corrected image 15X, with a very wide f.o.v.
You will get considerable chromatic aberration with relatively uncorrected finite tube objectives but also other aberrations the more you look off axis. Those work well with any microscope that produces a fully corrected image to the eyepiece. They are high resolution, ultra wide field 15x magnifiers only.
None of the objectives you have there would be prone to empty magnification with any 15X eyepiece based on the simple rule that the product of the objective mag. X eyepiece mag. must not exceed 1000 x N.A. In the case of your objectives, the 4 ratios are : 35/90 , 150/250 , 645/660 and 645/950
The 43X is closest but it is easy to see in this case, just why microscope optical engineers zeroed in on .65 or so as the optimal basic N.A. for most professional grade microscope 4mm objectives. 4mm objectives, depending on the lens designers optical formula are any where between 40 to 45X magnification , when used with a 160mm tube. Setting the N.A. at .65, gives the possibility of using a 15X eyepiece, instead of adding in a 3mm objective, while allowing for a decent w.d. and fine enough resolution to do most work. Quality manufacturers, further , produced 15X eyepieces of an advanced design, that did not introduce field restrictions. Many cover a field very close or in the case of those FJW, equal, to the actual field covered by a 10X eyepiece.
15X eyepieces were seldom intended to be used with standard achromat objectives over 4mm( 40-45X), nor would standard achromat objectives over 4mm, indicate such use. There are a few exceptions of very high N.A. achromats being made, often as 3mm(60-63X), where N.A.s of 1.0 to 1.25 can be found. In the case of apochromats, with N.A.s often double those of achromats, the use of 15X eyepieces across the board was usually normal. Spencer and Bausch & Lomb for instance produced 15X eyepieces as the default eyepiece with their apochromat systems. They were designed with the widest field and superior eye relief. In the case of the higher power oil immersion objectives, they deliberately limited their magnification a little, to 2mm or around 90x at 1.30 or 1.40, instead of the more common 1.8mm or 100X at 1.25 for an achromat, in order to facilitate a greater, highly resolved magnification.
90 x 15 = 1350 : 1.30 x1000 = 1300. While this may seem to exceed the rule, in the case of apochromats the upper threshold is slightly higher due to the extra wavelength correction designed into apochromats. With modern ones, it is probably even higher.
