Walters Extra Close Focus 6.3x25
Posted 19 January 2013 - 10:13 PM
Is lousy, short eye relief inherent such short scopes? I suspect so. This is probably the same issue afflicting those nifty, miniature binoculars. I can not wear my prescription eyeglasses while using this scope, at least not without killing the field of view and straining my eye against an image attempting to focus well in front of my pupil. Worse, upon removing my glasses and stuffing my eye into the eyecup, I can not focus sharply at infinity. That's strange, actually; given that this scope is designed to help patients with low vision, one might expect it to have a vast range of over- and under-focus to accommodate wide variations in vision, but it does not.
Next question is about depth of field. On the extra-close end, I can focus with my eyeglasses removed, but the depth of field is so narrow as to make it impossible to see anything not exactly in a plane perpendicular to one's line of sight. Is this a flaw in the design, or is it, again, an inherent problem with small scopes, or perhaps those with so broad a range of focus?
Might there be a pocketable, extra close focus monocular with long eye relief and good depth of field, or is the Walters Extra Close Focus going to be as good as I could find, despite its limitations?
Posted 20 January 2013 - 02:24 AM
Posted 20 January 2013 - 06:24 AM
depth of field scales with magnification
Meaning what? The higher the magnification, the lower the depth of field?
The 15mm eye relief is impressive. Could use that with eyeglasses.
Posted 21 January 2013 - 11:38 AM
The eye relief is good enough that I can see nearly the full FOV with my thick eyeglasses and that's really saying something. I have few other binos that I can say that about.
They have the same short depth of field as above; that comes with the territory. Consider when you look through a microscope, the DOF is really short; just a plane of focus. That's a function of magnification and there's no way around that.
Posted 21 January 2013 - 12:36 PM
I have a Zeiss West close-focussing 8 x 30B monocular (looks like half of the squat 8 x 30B Porro) which was designed to be used in conjunction with the Zeiss Contaflex SLR, and this has a very shallow depth of field at the close-focus end - I think it's all down to physics and nothing to do with instrument design.