Focus snap (4 Viewers)

[tenex]

We all know and appreciate "focus snap", especially in birding: having a target pop quickly into virtually perfect focus without further doubt or effort. And that some bins seem to have more of it than others. Why is this, and what does it mean? (Let's rule out a few distractions: no concern with speed of focuser knobs, some of which are just too slow to be convenient for birding. Nor with poor quality or defective bins. Nor with diopter, whether it's set properly or drifts. Nor with individual focusing technique; just differences in optics, comparing the same person's experience of each, assuming proper use of good examples.)

Focus snap is widely considered a special sign of optimal sharpness; that is, many think it indicates such striking sharpness of the focused image that one instantly stops there. Is this necessarily true? Some instruments that seem to have more modest snap are very sharp once focused. (MeoStar 7x42 and UV 12x50 among others have been mentioned on this forum.) Another plausible possibility would be that the transition from out of focus to in simply seems more gradual, for some purely optical reason; what would that be? (Could it involve "bokeh", the smooth vs busy character of the out-of-focus image?)

What other high-quality models could be added to this somewhat-less-snappy list, and what else do they have in common? Do they tend to be older or more recent designs? The same ones typically praised for a particularly "lovely view"? Or more carelessly, as having unusual "depth of field"?

 

[chill6x6]

Is it anything more than focus speed/precision and magnification?

 

[GrampaTom]

We all know and appreciate "focus snap", especially in birding: having a target pop quickly into virtually perfect focus without further doubt or effort. And that some bins seem to have more of it than others. Why is this, and what does it mean? (Let's rule out a few distractions: no concern with speed of focuser knobs, some of which are just too slow to be convenient for birding. Nor with poor quality or defective bins. Nor with diopter, whether it's set properly or drifts. Nor with individual focusing technique; just differences in optics, comparing the same person's experience of each, assuming proper use of good examples.)

Focus snap is widely considered a special sign of optimal sharpness; that is, many think it indicates such striking sharpness of the focused image that one instantly stops there. Is this necessarily true? Some instruments that seem to have more modest snap are very sharp once focused. (MeoStar 7x42 and UV 12x50 among others have been mentioned on this forum.) Another plausible possibility would be that the transition from out of focus to in simply seems more gradual, for some purely optical reason; what would that be? (Could it involve "bokeh", the smooth vs busy character of the out-of-focus image?)

What other high-quality models could be added to this somewhat-less-snappy list, and what else do they have in common? Do they tend to be older or more recent designs? The same ones typically praised for a particularly "lovely view"? Or more carelessly, as having unusual "depth of field"?

Im with you. Wondered same. Bino-speak, maybe?

 

[Dennis Mau]

A good focus snap means the image clearly and quickly snaps into focus. To me, it indicates very good optical quality, where the focus point is easy to find, and the image is sharp. Focus snap is determined entirely by the optical quality of the binocular. It is also the hallmark of a very fine refractor telescope like a Takahashi or Astrophysics which have very good focus snap.

 

[CharleyBird]

A good focus snap means the image clearly and quickly snaps into focus. To me, it indicates very good optical quality, where the focus point is easy to find, and the image is sharp. Focus snap is determined entirely by the optical quality of the binocular. It is also the hallmark of a very fine refractor telescope like a Takahashi or Astrophysics to have very good focus snap.

Nah it's just luck

 

[adhoc]

This has long intrigued me. Have not seen an explanation I understood and could accept.

Tenex, why do you ask only for the "less snappy" list and not the other? Is it known, or assumed, that a "snap" bino is optically less than top-tier?

Is there a narrow range of mag, e.g. 8x to 10x only, where this happens? I have found it most noticeable in an 8x.

 

[mwhogue]

I don't know how or why, but the most reliable, consistent snap focus function for me is with a late model Zeiss FL 10x32. Most often when I bring it to my eyes and focus, it's spot on after a single instinctive adjustment and any follow up efforts to fine tune the focus result in a less focused image. Leica UV 8x20 is also noticeably capable w/r/t snap focus.

Mike

 

[tenex]

A good focus snap means the image clearly and quickly snaps into focus. To me, it indicates very good optical quality, where the focus point is easy to find, and the image is sharp. Focus snap is determined entirely by the optical quality of the binocular.

You are explaining a term whose meaning everyone knows, and casually repeating the familiar assumption I'm questioning.

Tenex, why do you ask only for the "less snappy" list and not the other?

Because most bins I've tried (and owned) do seem quite snappy, so it's more efficient to list the exceptions. Again, let me emphasize that I'm talking about high-quality bins that do have excellent sharpness once focused, yet still somehow seem a bit less snappy, so I occasionally find myself fiddling further with the focus (when there's time).

Is it anything more than focus speed/precision and magnification?

That is the question. I'm not sure it involves magnification/DOF, which is why I listed an excellent 7x and 12x as examples of less snap. It's not just focus speed, because 12x50 UV is the only Leica I've felt has a bit less snap, compared with 10x32 BN (and from memory other Ultravids). Or for that matter 10x32 FL, or 10x42 or 10/15x56 SLC, all of which seem quite snappy to me, even given the slower focus on SLC 56s. And there's nothing distinctively different about the focuser on the 12x, yet others have this impression too. Probably the same for the 7x42 Meopta. So the question is... why? How can the transition from out-of to in-focus vary from one optical design to another, in a way that's relevant to how the eyes process it?

 

[Thotmosis]

Hmm, I absolutely don’t now why but also my 10x32 BN has some more focus snap than the 12x50 UVHD.
My FL 7x42 has some more than the UVHD 7x42, which is better than the 7x42 BN. But i like the BN most despite the lesser focus snap.
The Habicht 10x40 has a nicer focus snap than my Swift Audubon 10x50. The Leica UVHD 10x25 is also snappy.
I guess the Trinovid 7x35 has the least focus snap, she is like a Labrador. Always happy 😃.

 

[Dennis Mau]

Hmm, I absolutely don’t now why but also my 10x32 BN has some more focus snap than the 12x50 UVHD.
My FL 7x42 has some more than the UVHD 7x42, which is better than the 7x42 BN. But i like the BN most despite the less focus snap.
The Habicht 10x40 has a nicer focus snap than my Swift Audubon 10x50. The Leica UVHD 10x25 is also snappy.

Why do you like the BN the most? I like BN's also and was just curious what you like about them.

 

[Thotmosis]

Why do you like the BN the most? I like BN's also and was just curious what you like about them.

It’s hard to explain, it’s the “creamy” and “dreamy” image it gives me and maybe also that i got it for 300 euro and got it fixed by Leica Portugal.
For some reason its my most friendly binocular.

 

[mwhogue]

Again, let me emphasize that I'm talking about high-quality bins that do have excellent sharpness once focused, yet still somehow seem a bit less snappy, so I occasionally find myself fiddling further with the focus (when there's time).

Nikon EII 10x35 and Curio.

Mike

 

[GrampaTom]

That is the question. I'm not sure it involves magnification/DOF, which is why I listed an excellent 7x and 12x as examples of less snap. It's not just focus speed, because 12x50 UV is the only Leica I've felt has a bit less snap, compared with 10x32 BN (and from memory other Ultravids). Or for that matter 10x32 FL, or 10x42 or 10/15x56 SLC, all of which seem quite snappy to me, even given the slower focus on SLC 56s. And there's nothing distinctively different about the focuser on the 12x, yet others have this impression too. Probably the same for the 7x42 Meopta. So the question is... why? How can the transition from out-of to in-focus vary from one optical design to another, in a way that's relevant to how the eyes process it?

Many say Swarovski's focus is slow, while Zeiss tends to be described as fast. The terms slow and fast as I read them, sort of infer a bit of good/bad, depending. I prefer the focus on my Swarovski, as it allows me to come to best focus and not blow past and have to fiddle back/forth most of the time. Whereas I do tend to blow past and back up/go forward more with the Conquests. Mostly Ive just thought these are different, not one better than the other. Its what Im used to. The opinions I read, therefore I interpret as preference, something the writer is used to.

I never have the sense of "snap," that is the view suddenly coming into focus and locked in, as I think the descriptions of focus snap infer. So I wonder...

 

[tenex]

I never have the sense of "snap," that is the view suddenly coming into focus and locked in, as I think the descriptions of focus snap infer. So I wonder...

Perhaps an indirect definition is better: focus snap is when you easily get satisfactory focus, very seldom doubting or fiddling afterward. (I'm avoiding the word "hunt" which indicates a more serious problem.)

 

[tenex]

Well, this doesn't seem to be getting very far. I can't seem to find the button that calls in the experts.

Do some optical designs have a more gradual approach to focus? (Imagine a graph of size of circles of confusion as focus approaches optimal... could it be concave?) Can high contrast make it harder to tell when optimal resolution is achieved? Is there some other optical property that could make certain models seem a bit less sure or snappy to focus, despite actually being sharp once focused? If not, what else could explain it, or is this just some sort of misimpression in the first place?

 

[Hermann]

I also find some binoculars have "better" focus snap than others even if they are equally sharp once I've achieved "optimal focus". However, I don't quite understand why this is so. I've been experimenting with different binoculars over the past few days, and didn't get to some satisfactory conclusion. There are so many variables to consider:

• I find some binoculars quite "snappy" in some light conditions but not in others. High contrast works better for me in most cases. Other binoculars always seem me to be snappy, for instance the Nikon 8x32 SE.
• What I also found was that I generally find slower focusers "snappier" than fast ones; with fast focusers I almost always need to go back and forth a few times.
• Magnification OTOH doesn't seem me to be important. I used binoculars from 6x to 12x magnification. DOF doesn't seem to be a factor.
• Interestingly in many cases I found I mostly hadn't got optimal focus even though I thought I had. I experimented with putting the binoculars on a tripod a few times, and even with the "most snappy ones" I found I often could get an even sharper image by (very) slightly refocusing when I put them on a tripod.
• Exit pupil size seems me to play a role: I found none of the pocket binoculars (exit pupil <3mm) as snappy as binoculars with larger exit pupils, neither handheld nor when I put them on a tripod.

Hermann

 

[GrampaTom]

I also find some binoculars have "better" focus snap than others even if they are equally sharp once I've achieved "optimal focus". However, I don't quite understand why this is so. I've been experimenting with different binoculars over the past few days, and didn't get to some satisfactory conclusion. There are so many variables to consider:

• I find some binoculars quite "snappy" in some light conditions but not in others. High contrast works better for me in most cases. Other binoculars always seem me to be snappy, for instance the Nikon 8x32 SE.
• What I also found was that I generally find slower focusers "snappier" than fast ones; with fast focusers I almost always need to go back and forth a few times.
• Magnification OTOH doesn't seem me to be important. I used binoculars from 6x to 12x magnification. DOF doesn't seem to be a factor.
• Interestingly in many cases I found I mostly hadn't got optimal focus even though I thought I had. I experimented with putting the binoculars on a tripod a few times, and even with the "most snappy ones" I found I often could get an even sharper image by (very) slightly refocusing when I put them on a tripod.
• Exit pupil size seems me to play a role: I found none of the pocket binoculars (exit pupil <3mm) as snappy as binoculars with larger exit pupils, neither handheld nor when I put them on a tripod.

Hermann

Yup, me to...

Wonder then, what role might our individual eye/brain, maybe even focusing fingers play in all this?

 

[tenex]

Interestingly in many cases I found I mostly hadn't got optimal focus even though I thought I had. I experimented with putting the binoculars on a tripod a few times, and even with the "most snappy ones" I found I often could get an even sharper image by (very) slightly refocusing when I put them on a tripod.

I'm sure this is generally true. I used both terms "optimal" and "satisfactory" above, but we should probably stick to "satisfactory" for handheld use. In any case, I'm curious why I notice myself sometimes second-guessing focus with one bin and not others. At least it helps to know that others also do.

 

[Patudo]

Interesting topic. My impression is that most modern birding binoculars, certainly the better ones, but even less expensive models, "come to focus" (the term I prefer to use) pretty quickly. To "get on the bird" quickly is important in a lot of birding situations, and no doubt a lot of effort has gone into binocular designs that achieve that. The binoculars I have, or have tried, that did not come to focus so easily were not really designed for birding.

I agree with chill6x6 that focus speed has a lot to do with it. In situations where you have to rapidly refocus a fast focuser will, or at least should, get you on the bird faster (although maybe by only a second or two, but that can seem a long time when trying to zero in on fast-moving critters). It doesn't need to reach ultimate sharpness (which as Hermann says almost always needs some tweaking) at the first attempt, just get into what one might call the zone of sharpness where the user has a good clear view of the bird. That said, comments on, for instance, the Conquest 8x32 HD suggest that in some cases a focuser can be almost too fast and result in overshooting. The other important factor is probably depth of field (or perceived depth of field , which would be influenced by factors such as one's individual eye accommodation). On the one hand lower mag has more depth of field, so gets you into that zone of sharpness quicker. But high mag can isolate the bird better because of the shallower depth of field - and that, combined with the larger image size that say 10x gives you, can give an effect that one might call "snapping the bird into focus" too. Exit pupil, as noted in H's post, is probably also quite important: the larger the EP, the more comfortable the eye is and the more able (I feel) to perform its functions eg. accommodation naturally and easily. And mechanical quality comes into play too, ensuring consistency, repeatability, and feedback via touch/feel.

I get the feeling some binoculars need to be in perfect or near-perfect focus more than others; if they are not, the image becomes visibly less satisfying. The zone of sharpness is more restricted in those models, while others appear more tolerant in that regard. The further away a target is, the more important getting perfect focus seems to be - if looking at a target close by I won't need to get the binocular in as perfect focus as I would if I found a raptor on a high perch over 1km away and settled down to wait for it to fly - but even in the latter type of situation, some binoculars can be not quite perfectly focused yet still seem pretty good (the Swaro 8.5x42 Fieldpro seems a very good performer in this respect). There are probably optical design factors at play here (eg. larger "eye box"). I like binoculars that have that ease of view and consider it a valuable quality. But a binocular that needs more careful tweaking can be a good performer optically, and useful in the field; just less ideal for "fast action birding".

 

[has530]

Do some optical designs have a more gradual approach to focus? (Imagine a graph of size of circles of confusion as focus approaches optimal...

This is exactly the mechanism behind depth of field/focus

could it be concave?)

Yes and no. For a perfect lens all rays will converge to focus and then diverge in a perfect cone. In reality things like spherical aberration cause them to converge in a neck rather than to a point which may be sort of what you were thinking of? (A "perfect" lens vs one with spherical aberration from Wikipedia).