Is binocular depth of field in focus variable? (1 Viewer)

[Binastro]

Of course there is a bigger change in focus from 20m to infinity than 100m to infinity.

It is unlikely a young person could afford a £2,000 binocular.

It is primarily the type of eyepiece that determines whether the field is curved or not.
Distorted or not.

Fatigue is more likely in older people or those with some eye problems.
Also those with larger prescription dioptres, astigmatism, prism problems.

Also the focus of the eyes changes during the day, and day to day.
Young people may not notice this.

I would think that any difference between roof prism and Porroprism binoculars may be perception or psychological.

Also many binoculars are slightly out of alignment, and as viewing gets longer, more fatigue will set in.
It will also depend on lighting conditions and pupil size.

Lower magnification binoculars, like 5x25 or 6x30 will need less focus changes.
6x18 or 6x20 also.
Also, again, curved field eyepieces change the situation.

B.

 

[Upland]

How can something inside DOF not be in focus?

The same way something in the FOV can not be in focus

 

[Ted Y.]

The same way something in the FOV can not be in focus

What definition of DOF are you using?

 

[MiddleRiver]

The same way something in the FOV can not be in focus

Depth of Field and Depth of Focus

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[Ted Y.]

Depth of field in binoculars is not established in the same way as DOF in cameras. That's because telescopes are afocal devices that have no depth of field. The light that emerges from the eyepiece and falls on the eye is no more focused than the light that originally fell on the objective lens of the binocular. It is the eye's lens and cornea, that bring the afocal light of the binocular's output to a focused image on the retina in exactly the same way that they bring the afocal light that normally falls on the eye from the world around us to focus on the retina.

The binocular does modify the DOF of the eye in two ways.

Firstly, the out-of focus circle of confusion in front and back of a focused point source of light is magnified by the binocular's magnification just as if it were a focused circular object, so the circle of confusion is 25% larger in a 10x binocular than it is in an 8x binocular for an unfocused point at the same distance in front or behind a focused point and is therefore perceived as more defocused. However, all binoculars of the same magnification have inherently the same size circle of confusion and therefore the same DOF.

Secondly, a binoculars exit pupil may decrease the effective size of the eye's entrance pupil if the binocular's exit pupil is smaller than the eye's pupil. Under this condition the focal ratio of the eye is increased just like in a stopped down camera lens and the eye's depth of field increases. In that way a small exit pupil binocular may impose a wider DOF on the eye's optics in low light conditions compared to a large exit pupil binocular.

There are literally no ways at all that a binocular can be intentionally designed to increase its true DOF other than by employing low magnification and/or a small exit pupil. So, why are there so many subjective reports of more or less DOF observed in bright sunlight through binoculars of the same magnification? I think it's simply because other things can masquerade as increased or decreased DOF. Field curvature, as mentioned above, is one. Differences in the speed of focusers is another. Differences in aberration levels and type is another and of course it's always possible that a magnification specification is just inaccurate. If you see what looks like a difference in DOF between two 8X binoculars in bright daylight then it's time to investigate what the true cause of that impression is.

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