Shallow depth of field in the 8x32 format (6 Viewers)

[elkcub]

(RonE)... Parallel rays imply infinity focus and I agree that as long as objects are at infinity and the focus is at infinity, there is no DOF.

Oops, there is a depth of focus: the depth of focus of the eye reduced inversely by the magnification of the binoculars. Some "experts," who I haven't located yet, reportedly say it's reduced by the square of magnification, but that may be because they are working with longitudinal magnification (distance) associated with depth of field on the object side.

... The condition falls apart though when objects, or light, is closer or starts diverging. Once divergence is apparent then focus blurs and this is where DOF comes into play.

Yes, moving to positive diopters for terrestrial viewing the binocular becomes focal to assist the eye, as I've mentioned. But, the heavy lifting is still done by the cornea+lens of the eye, which is about 65-70 diopters (if memory serves). In this respect the binocular works like spectacles on an additive basis, and is needed considerably less for a young eye than an old one, which has almost no accommodative power. Except for very close working distances the binoculars contribute less than 10% dioptric power, and the increasing effective magnification of the instrument further reduces the combined DOF more than just linearly.

I guess that's my only point, Ron. We perceive what we see through the combined eye and optical system. Perceived DOF is reduced from normal vision (which we normally are not aware of) as an inverse function of instrument magnification. As a stand-alone device, uncoupled to the eye, its DOF will change with focuser setting, that's true, but why should we care?

I urge you to step back just a bit from the exit pupil and include the optics of the eye into your thinking, even if you don't want to deal with the brain itself, which starts at the retina.

But, if you decide not to I'll still love ya, buddy.

Ed

 

[brocknroller]

The sum of the square roots of any two
sides of an isosceles triangle is equal to
the square root of the remaining side.

Oh joy, rapture! I've got a brain!

Ah, but what good is having a brain if you can't "bench test" it?! :brains:

 

[[email protected]]

The sum of the square roots of any two
sides of an isosceles triangle is equal to
the square root of the remaining side.

Oh joy, rapture! I've got a brain!

Ah, but what good is having a brain if you can't "bench test" it?! :brains:

I pronounce Brock the NEW "MR.OPTICS!". Henry and Dr.Ed you are now students of Mr.Brock. Brock the "Binocular Man!" Go Brock!

 

[lulubelle]

Have been ill for several weeks and now when I have the time & energy to take a look at what I have been missing, I find I have missed alot!

Thanks to all of those who added to this thread - very educational! Have to have a few minutes to back & reread to absorb all the info now!!

 

[ceasar]

Alright! But be careful. Don't have a relapse!

Bob

 

[elkcub]

I'd like to mention, briefly, with regard to my posts #38 and #41 that I just found a companion book to Geo. Smith & David Atchison's 1997 "The Eye and Visual Optical Instruments." It's entitled "Optics of the Human Eye," with the authors in reverse order. A used copy is now on its way to me from Australia.

A snippet from the book is attached concerning the relationship between the DOF of a telescope and that of the eye. Clearly, 1/M^2 is the essential parameter of the telescope, so it's no longer a matter of debate, at least from the user's perspective (and mine).

Cheers, B
Ed
PS. I'm trying to figure out how I missed a factor of 1/M, ... OK, so I'm not perfect.

 

[Kevin Purcell]

Interesting, Ed.

So a 7x bin appears to have twice the DOF** as a 10x bin. I can buy that.

** or more precisely for a given DOF in the eye the 7x bin appears to have twice the DOF of a 10x bin.

 

[ronh]

Ed,
The extra factor comes from the fact that longitudinal magnification is not the same as the lateral magnification. If it was, the factor would be 1/M.

Longitudinal magnification is in fact the square of the lateral. My old copy of Conrady has a nice discussion of this. It is a very important issue in microsccopy, where dof is abysmally small at magnifications in the hundreds, yet the samples have interesting features at significantly different ranges.

On our distance scales, this is the reason that magnification alters perspective. Not that I really understand it for a New York second, though, I find it rather mind boggling.
Ron

 

[elkcub]

Hi Ron,

Yes, I have been aware of axial magnification and the equation you mentioned. It is, indeed, a bit mind boggling. Vergence notation seems to be what I need to learn, so I'm looking forward to the new book.

Regards,
Ed

 

[brocknroller]

Have been ill for several weeks and now when I have the time & energy to take a look at what I have been missing, I find I have missed alot!

Thanks to all of those who added to this thread - very educational! Have to have a few minutes to back & reread to absorb all the info now!!

Lulubelle,

Sorry to hear you've been sick for so long. I guess even the "bugs" are bigger in Texas.

Here's bird 404 to add to your life list when you feel better:

http://www.physorg.com/news74588172.html

You're going to need more than a few minutes if you plan to read Ron and Ed's technical discussion! I took all three of these pills, and I still can't follow it. But I was able to read all 1475 pages of War & Peace (2006 paperback issue) in one hour.

http://www.youtube.com/watch?v=CovO3K1CXkc

Brock