23.2 vs 30mm

[Scarodactyl]

Bit of a tangent but I am still amazed Nikon's cheapest E objectives on my alphaphot fill the entire UW eyepieces with a very decent image.

[david_b]

And the better Olympus UIS2 optics for the BX line will support optional 26mm fields (so, incidentally, will your SPlans with a UW Olympus finite head).

Yes! Now if only I could find one..

[apochronaut]

The F.O.V. designation in microscope optics is a pretty inconsistent measurement. If you look up what it is supposed to be , you can find information from various sources that is somewhat contradictory. About the only thing you can be sure of is that a 22mm f.o.v. will be bigger than a 20mm f.o.v. if the eyepieces are from the same series but that could change , if two different mfg. are involved or if the eras of manufacture are wide enough apart. To confuse matters further, some makers marked the eye relief on the eyepiece, instead of the f.o.v., or the diameter of the eyelens, which in some information is the f.o.v. Telescope eyepieces are really plain, with the angle of view measuring how wide the field is. Telescope eyepiece fields of view, relate to apparent field or actual field. Microscope manufacturers would do well to follow suit.

The microscope eyepiece conundrum, seems to have improved of late but it is still difficult when dealing with some older eyepieces and eyepieces that are not 10X. It seems that some sort of standardization may be taking place but it is very easy for a mfg. to claim a wider f.o.v. than is actually the case, based on a specific formula that suits their purpose.

There are two methods I use to evaluate a f.o.v.. One is to get an actual measurement of what the eye is really seeing at the eyepiece field stop. If an eyepiece has a 20mm f.o.v., the eye should be seeing a 20mm wide disc in front of it, as though a 20mm disc is right at the eyeball. This relates to the apparent field.
F.o.v. can become a bit confusing when the magnification of the eyepiece is not 10X, because it seems that a 10X/20mm field has become a reference point. With many 15X eyepieces, to use telescope jargon, the apparent field is greater than that of the 10X/20mm from the same series, yet they might be designated as having a 16mm f.o.v. What the mfg. is actually saying , is that they are seeing a field as though they have a 16mm f.o.v. if they were 10X eyepieces. So, they are using an actual field measure as the 20mm reference but that actual field measure would only relate to the use of a 1X objective. Any other objective would maintain that actual field measurement ratio between the 10 and 15X but the 20mm becomes 2mm with a 10X objective and so forth .

I have 15X eyepieces that present a greater than 30mm disc in front of my eyes; the field is out around my ears, yet in the literature they are designated as having a 20mm f.o.v. No! They are seeing the same field as a 10X/20mm eyepiece is seeing but they in fact have a 30+mm f.o.v. They see a much broader angle of view, than a 10X/20mm, so have a wider apparent field.

The second way I measure field is by measurement of the actual field. a 10X/20 eyepiece will see 200 microns with a 100X objective. Other eyepieces will see relative diameters, based on their apparent field but clearly a 10X/18 eyepiece will see 180microns.

I don't like the f.o.v. systems used in microscopy at all. Apparent field is a much more useful measure to describe the breadth a field of view.

[wporter]

Well said, Phil. Nice summary.

I, too, would prefer standardization on apparent field. FOV in mm seems somehow extracted from some sort of optical design calculations, with not much users can relate to.

It would be nice to have an easy way to determine apparent field of an eyepiece (relative field sizes can of course be determined by putting different eyepieces (of the same mag.) together in a binocular head.)

[apochronaut]

Glad you got it , Bill because in rereading I was trying to figure out what that guy was saying! . Hopefully the edit will clarify . Hope I didn't make it worse!

[Dmi3n]

There is a formula to calculate apparent field of view of eyepiece in degrees but I cant't find it in my notes. If I find it I'll post it here. I remember that it has magnification*FOV in mm in the upper part so for example 12,5x*16mm eyepiece will have wider field than 10x*16mm.

[Scarodactyl]

I have usually heard it described as the FoV you'd get at 1x, though that's a lot easier to measure on a stereo for obvious reasons.

[wporter]

"I have usually heard it described as the FoV you'd get at 1x..."

Yes, that's right; thanks, Scarodactyl.

It is still a cumbersome number. So you have a real FOV with a 60x objective, e.g., that in microns will be (FOVmm*1000)/60). Fine; you want to measure a critter's length for a scale bar on the image you're taking. You know the real width of the fov in microns, so all I have to do is guess how many critters' lengths it would take to stretch from one side to the other of the field; shoot, that doesn't sound very precise.

Plus, if you already have an image, the field of the microscope is not the same as the field in the camera or on screen, so...

Aw, the heck with it, I'll just put the critter, or one like it, on a micrometer slide, and measure it directly as it scoots over the scale bar. Or, I'll drop in my microscope's built-in calibrated reticle slider for a second and determine the length that way.

[einman]

Good answers Pete! The move to 30mm eye tubes even for the "teaching" microscopes has been happening for a while. I went brain dead initially when responding to this thread. I have an Accu-scope with the larger eyetubes as well, although the FOV is only 22. I just never really gave any thought to the newer eyepieces being "30 mm" and the relationship to the research scopes of the past.

Great thread...everyone!