Sharpness and sharpness impression, details and contrast (1 Viewer)

[Troubador]

If I can see the texture of the plumage of birds such as Blue Tit and Chaffinch, and the detail of plumage markings such as wing bars and supercilium and the whiskers of Otters and Seals, at my normal viewing distances, then I am happy with the perceived sharpness of the binoculars.

Lee

 

[chris6]

I believe that 'focus snap' is only evident when the binocular is capable of presenting a sufficiently sharp image to the eye.
While operating the focus wheel, the image is adjusted somehow independently and automatically,
and this is what I take to be the effect.

Presumably a camera's AF uses algorithms and motors to improve the definition like this.
When it doesn't work, as when the degree of zoom is too great in macro mode,
I guess the required adjustment is beyond the scope of the system.

Perhaps the eye works along the same lines, while in addition it requires a certain potential for detail
for the brain and eye muscles to accomplish 'accommodation' at speed,
so it can be appreciated while turning the wheel:-

.... can't do it.... still can't do it..... now it can = focus snap!

 

[Troubador]

I believe that 'focus snap' is only evident when the binocular is capable of presenting a sufficiently sharp image to the eye.
While operating the focus wheel, the image is adjusted somehow independently and automatically,
and this is what I take to be the effect.

Presumably a camera's AF uses algorithms and motors to improve the definition like this.
When it doesn't work, as when the degree of zoom is too great in macro mode,
I guess the required adjustment is beyond the scope of the system.

Perhaps the eye works along the same lines, while in addition it requires a certain potential for detail
for the brain and eye muscles to accomplish 'accommodation' at speed,
so it can be appreciated while turning the wheel:-

.... can't do it.... still can't do it..... now it can = focus snap!

Canon's DSLR autofocus presumes the image is in focus when maximum contrast is achieved. This is not a bad guide for the naked eye looking through binos either.

Lee

 

[chris6]

Thanks Lee, and would you go as far as "absence of decent focus snap suggests poor sharpness"?

 

[Jessie-66]

I believe that 'focus snap' is only evident when the binocular is capable of presenting a sufficiently sharp image to the eye.
While operating the focus wheel, the image is adjusted somehow independently and automatically,
and this is what I take to be the effect.

... and would you go as far as "absence of decent focus snap suggests poor sharpness"?

For a sharp image, binoculars must first be functional. Differences in centre sharpness are difficult to detect with the naked eye, as long as we are not talking about defective or/and 20 euro binoculars. The increased angle of view compared to the naked eye by the magnification leads technically to a reduced sharp depth of field. Search in your native language for websites on depth of field in photography and the effect of apertures. Furthermore, binoculars with higher magnifications (reduced depth of field) and a high ratio of the focusing drive ("fast focus") need to be adjusted more carefully than others. The (little) diameter of the focus wheel is part of the (faster) overall gear ratio (with the same angle of rotation). This all is popularly understood as "snapping" in focus.
An interssant biologic-technical aspect in this context: Technically there is only 1 sharp focal plane. The resolution of the eyes of humans (and animals), limited by the density of the sensory cells, leads to an "invisibility" of small blurs and to an acceptance of visible little blurs. This gives rise to depth of field. In calculations for sharp depth of field for photographic objective lenses and for low-magnification binoculars (daytime), points of observation objects are not in focus plane will be (unsharp) areas of circle of confusion that the eye can no longer separate. This are the calculated 2 limits of the by humans perceived sharp depth of view. Dispersion circle areas arise from points of the observation object that lie outside the focal plane. An "earthly" analogy to astronomically used criteria for airy disks (Dawes or Rayleigh criterion). The airy discs are created by refraction, which every optical instrument has, but are not visible at low magnifications.
Hopefully, I have not oversimplified. My translation of technical terms can be awkward. ;-)

I still have the question of whether the eye's ability to accommodate has any influence at all on the size of the depth of field. After all, with the help of the eyepiece, one observes a real intermediate image that is generated by the lens. Different depths of focus of the eye probably influence the perception of the intermediate image, which has depth and field curvature: We usually do not observe surfaces (walls) but bodies (landscape and animals etc.).

 

[Troubador]

Thanks Lee, and would you go as far as "absence of decent focus snap suggests poor sharpness"?

Well, you need to be careful with the terms used. I prefer to use a term introduced (I think) by Typo and that is 'perceived sharpness' and this is caused by a combination of resolution and contrast. Personally, like the Canon DSLR auto-focus system I find good contrast aids quickly achieving a well-focused image. I am a little wary of the term focus snap as the quick transition from out of focus to in focus can appear to be facilitated by a higher magnification due to the reduced depth of field which means the view more suddenly goes from blurred to in focus in a 10x than an 8x or 7x.

To more directly address your question I would say that given that most binos have decent resolution (ie better than the human eye), for me it is harder to achieve a quick focus with instruments that have poor contrast.

Lee

 

[chris6]

.... Furthermore, binoculars with higher magnifications (reduced depth of field) and a high ratio of the focusing drive ("fast focus") need to be adjusted more carefully than others. This is obviously popularly understood as "snapping" the focus ...

Certainly slow adjustment, at just the right stage, does help the 'snapping' to take place, while this probably improves with practice. However I am not so sure that careful adjustment is really what is popularly understood as "'snapping' the focus".

Instead, statements included phrases like '(binocular xxx) shows good focus snap'. It used to mentioned much more regularly and still seems to be a very useful, perhaps essential, feature.

As as above I understand that it is the function of the operator's brain and eye which probably produce the seemingly independent and automatic effect. i.e. the focus seems to 'focus snap' by itself, like magic, rather than it being 'snapped' consciously and intentionally by the operator, in the process of careful adjustment.

The point was intended to be that a binocular which is capable of sharper focus would be more likely to, and more readily, exhibit the phenomenon of Focus Snap.

That begs the question as to whether, when adjustment in the right direction approaches the eye's accommodating range, a binocular would be equally likely to, or most readily, do the trick:-

A. when most of the field were about to become sharp (flatter field?)
or
B. only a small sweet spot were about to become sharp

I think not, and A. would be the favourite, but at the moment I don't have binoculars with a small sweet spot to try it.

 

[chris6]

Well, you need to be careful with the terms used. I prefer to use a term introduced (I think) by Typo and that is 'perceived sharpness' and this is caused by a combination of resolution and contrast. Personally, like the Canon DSLR auto-focus system I find good contrast aids quickly achieving a well-focused image. I am a little wary of the term focus snap as the quick transition from out of focus to in focus can appear to be facilitated by a higher magnification due to the reduced depth of field which means the view more suddenly goes from blurred to in focus in a 10x than an 8x or 7x.

To more directly address your question I would say that given that most binos have decent resolution (ie better than the human eye), for me it is harder to achieve a quick focus with instruments that have poor contrast.

Lee

Very good Lee, and point taken about 'perceived sharpness' being a better term when defined as a combination of resolution and contrast. Also the point about effect of magnification, to which Jessie-66 also referred.

 

[Jessie-66]

Re: post #27: During refocusing, I concentrate strictly on a central, detailed object of observation, possibly a central detailed sub-area. For me, the size of the sweet spot for focusing is therefore unimportant.

 

[Troubador]

Re Post 27: Like Jessie, when I am focusing my binos I am almost always concentrating on a subject that occupies a small area of the field of view and which I have centred withing the fov. This could be a bird, butterfly, dragonfly etc so like Jessie the size of the sweet spot doesn't have any relevance for me at this point.

Lee

 

[Gijs van Ginkel]

Jessie, post 1,
Some nice studies about resolution and contrast in binocular/telescope images can be found in the following sources:
-1- "Telescope optics" by Ruttten & van Venroooij, Wilam-Bell inc, 2002, chapter 18.
-2- L.P. Osipova and V.V. Polikhonova, Sov. J. IOpt. Technology 58, sept. 1991, pages 542-544
-3- Francis B. Patrick, Optik 33, heft 5, 1971, pages 494-500
-4- L.P. Osipova and V.V. Potikhonovan Sov. J. Opt. Technology, 58, 1991, pages 88-90
Gijs van Ginkel

 

[Brink]

This is a good topic. I think that unless resolution is very poor, contrast is probably the more important component of an optical system. I have done a lot of work with radiographic systems, and a lot of these systems have been moving toward measuring MTF as opposed to resolution. Modern digital imaging systems tend to have much worse resolution than old film systems, but the "image quality" is vastly superior due to better contrast. Example: in conventional mammography, film screen systems can display 14 or 15 lp/mm resolution, whereas high quality digital systems rarely hit 9 lp/mm. For this reason I would trust the perceived image quality of a knowledgeable person over a spatial resolution test any day of the week if making a blind purchase.