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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/25/2006 at 10:47
ggirino View Drop Down
Optics GrassHopper
Optics GrassHopper


Joined: August/25/2006
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Hello everyone,

     I found this forum in my search for info on binoculars. Planning to buy one and knowing about nothing on the subject, Iím trying to build some knowledge on the matter. Here are a couple of questions to which I haven't yet found an answer.

 

Exit pupil, twilight factor, field of view, depth of field

   Am I right to think that these parameters tell nothing about optical quality as they just follow geometrically from the dxM numbers?

 

- Image quality/brightness

  Given the same power and disregarding any non optical factor such as cost, weight, etc., isnít a larger lens preferable anyways (except maybe for the point below)? It looks to me that larger binos, such as *x50, *x52, *x56, are little considered. Is there any optical reason for this?

 

- Depth of field (DoF) more in depth

   if I'm not mistaking this is another geometric property which depends from focal length, aperture, focusing distance, circle of confusion and ratio between entrance and exit pupils. This may add to the previous point: I tried working out the maths under depth of fied calculation and, if I haven't made mistakes wich I almost always do, it comes out that DoF decreases when the entrance pupil (that should be the front lens diameter, correct?) increases, in a way that looks like:

This feels somehow counterintuitive to me.

 

     Thanks to all those who would take the time to answer. I would also appreciate any reference to good technical sources on optics (binos, camera lenses, telescopes, etc.).

 

Regards,

     Lino

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/25/2006 at 14:22
lucznik View Drop Down
Optics Master
Optics Master


Joined: November/27/2004
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Originally posted by ggirino ggirino wrote:

Hello everyone,

     I found this forum in my search for info on binoculars. Planning to buy one and knowing about nothing on the subject, Iím trying to build some knowledge on the matter. Here are a couple of questions to which I haven't yet found an answer.

 

Welcome ggirino. 

 

Originally posted by ggirino ggirino wrote:

Exit pupil, twilight factor, field of view, depth of field

   Am I right to think that these parameters tell nothing about optical quality as they just follow geometrically from the dxM numbers?

 

Yes, you are basically correct here that the raw numbers follow the same mathematical equations, regardless of make, model, etc.  Thus a 8x40 Tasco that costs $40 has the same exit pupil, twilight factor, etc. as a $2000 Leica of the same configuration.  What you have to be careful about remembering is that the Leica (as well as every other decent binocular) will have numerous types of lens coatings that enhance light transmission to a phenomenal degree, which the Tasco will not - at least not the same quantity nor quality.  Thus while they do have the same exit pupil, the Leica will be many times brighter than the Tasco.  These coatings are so important and so powerful that a Leica with a 25mm objective lens can actually provide a brighter and clearer image than that 40mm Tasco, simply because while it doesn't "collect" as much light, it actually transmits most of the light it does collect directly to the eye whereas most of the light collected by by the lesser-coated Tasco is lost through reflection as it bounces around the inside optic which also serves to ruin any chance of decent image clarity.

 

Field of view is something that will also vary.  Optics manufacturers often place "stops" in their binoculars that will limit the overall field of view.  Presumably this is because they believe the consumer will be more concerned with the degree of image sharpness at the edges of the field rather than providing the absolute maximum field of view possible.  The Bushnell Elite and Discoverer models are good examples of this: The 10x Discoverer has a FOV of 340 ft. whereas the more expensive 10x  Elite has its FOV limited down to 314 ft. Obviously Bushnell has the technical ability to give the Elite the same wide FOV as their Discoverer but, have chosen to focus on the widest FOV that gives really great edge clarity instead.

 

Discussions over Depth of Field tend to cause much contention amoung users as individuals will swear that their favorite make/model gives better DoF than any others.  Reality is that DoF is also mostly controlled by simple mathmatical formulas (primarily based on the magnification of the optic) that simply don't vary for individual manufacturers. Again, a really good binocular will exhibit what appears to be better depth of field over a mediocre binocular simply because its optics are higher quality all around (ie. the glass is ground to more exacting standards providing better resolution, clarity, etc.)  While this may appear to be simply "splitting hairs," it is truly two different issues and should be addressed as such. Also, porro prism binoculars, because of their wider set objective lenses, will provide slightly better DoF than their roof-prism cousins.  Here again however, the quality of the optic trumps all.

 

[QUOTE=ggirino]

- Image quality/brightness

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/28/2006 at 04:20
ggirino View Drop Down
Optics GrassHopper
Optics GrassHopper


Joined: August/25/2006
Location: Italy
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Points: 7

lucznik,

     I must really thank you a lot for your prompt and exhaustive answers. I was aware of the coating issue: being a fan of photography I've tried to learn as much as possible on the subject.

     The Swarovski pubblication also sounds very interesting, I'll check it out.

 

Thanks again,

     Lino

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