Got a silly question for all you binocular boffs.

[Franklin]

When I look at a distant object through my binoculars (8x42 basic roof type) after setting the IP distance and focusing I get a lovely clear, sharp view which is surrounded by a well defined and circular field stop. All is good there, but when I turn my binoculars to an object close up (at the bottom of the garden) I again reach focus but instead of a nice circular field stop it starts to look like two field stops overlapping, a bit like a Venn diagram shape. The view is still focused, sharp and clear and the images are perfectly merged, but I am unclear if this has anything to do with the  collimation of the binocular or is it just an effect of parallax. Thanks.

[PeterW]

Convergence for closer objects. For really close views you need the “reverse porro” designs like the Pentax papillon so the lenses are closer and thus the fields can continue to overlap for really close objects.

 

Peter

 

[Franklin]

Thanks Peter. 

Convergence, so it's kind of like going cross-eyed when you stare at your finger and move it closer to your eyes?

[Stormbringer]

how far away is bottom of garden ?

[Franklin]

5 hours ago, Stormbringer said:

how far away is bottom of garden ?

About 8 m.

[Astrobits]

Putting close-up lenses onto camera lenses allow objects closer to the optical axis to be in focus. The field of view in this case can be much smaller than the diameter of the main lens itself. It follows that a close object in the view of one lens will NOT be in the view of an adjacent lens unless it is pointed at an angle towards the object. Just look at simple stereo microscopes, the two tubes are angled and pointing at one point on the specimen stage. Your binoculars are showing the early stages of the two fields are not fully merging as the two optical axes are parallel and if you could get  to focus even closer the effect will just get worse. Your binocular manufacturers have decided that that is the nearest they are prepared to allow you to focus. If you really want closer focusing binoculars then you will need to purchase some that provide that facility.

Nigel

 

 

 

[Simon2]

On 17/04/2023 at 22:14, PeterW said:

Convergence for closer objects...... “reverse porro” designs

 

That helps me with a question I was going to ask I think - I'm cleaning up a pair of very early porros and couldn't understand why the prisms were 'reversed' - but I guess that's it....

Company that made them associated more with horses and opera glass so that would make sense.

[Coco]

My I have Vanguard Endeavor ED II 8x42 and they focus down to 2 meters with a perfect circle, however my 1960's russian 7x50's show what you describe watching the bird table 30' away beyond that superb.