Depth of field / Flat field (1 Viewer)

[coonan]

Is flat field the opposite DOF ?

 

[Sancho]

Is flat field the opposite DOF ?

No. DOF refers to the amount of view from close to distant that´s in focus. Flat Field refers to the extent of the Field of View (across the diameter) that´s clearly focussed. (I hope I´ve explained these adequately, there are surely technical definitions out there).

 

[elkcub]

No. Flat field refers to the correction of field curvature, which is an optical aberration. DOF (depth of field) refers to the range of object positions (fore and aft) that appears to be in focus.

In an actual system, of course, both properties influence the observer's perception simultaneously, and may be difficult to parse.

Ed

PS. Didn't see your post, Sancho. Yes, a binocular (properly) corrected for field curvature will appear in focus across the extent of the field (other factors not considered).

Incidentally, the SPIE (Society of Photo-Optical Instrumentation Engineers) "Field Guide to Geometrical Optics" can be downloaded to an iPhone (and probably other smart phones) for free. Everyone can become an instant expert, with definitions and formulae at their fingertips.

Ed :smoke:

 

[ronh]

Sancho and Ed have got it right, but here's an example of how DOF and field curvature can mingle.

My 7x50 Fujinon's dominant optical flaw is field curvature, with a 3 diopter difference in focus between center and edge. As a result, objects at the edge of the view appear sharp at a closer range than objects in the center.

It would be perverse and wrong to call this "depth of field", but I have used the effect to advantage when trying to watch birds (still with only marginal success!) with that binocular.

Ron

 

[coonan]

Thankyou people, I have it now !

 

[elkcub]

Sancho and Ed have got it right, but here's an example of how DOF and field curvature can mingle.

My 7x50 Fujinon's dominant optical flaw is field curvature, with a 3 diopter difference in focus between center and edge. As a result, objects at the edge of the view appear sharp at a closer range than objects in the center.

It would be perverse and wrong to call this "depth of field", but I have used the effect to advantage when trying to watch birds (still with only marginal success!) with that binocular.

Ron

Leaving definitions aside, I tend to agree that field curvature (or lack of a 'flat field') may be more consistent with natural vision than its given credit for. I have two related reasons for saying this: (1) the instrument's image is projected onto a very curved retinal surface, and (2) no account is taken of the normal visual horopter, which is also curved (see horopter here).

I have no hard geometric model to present, but I think the odd feeling I experience from so-called field flatteners is related to subtle dissonance reactions based on horopter distances. Translated, that means that objects at certain distances in the periphery should be more in focus than they are, and others are more in focus than they should be. However, since the horopter is defined by retinal correspondences, not equality of focus, single vision isn't destroyed and I'm just left with an odd feeling.

Ed

 

[elkcub]

Thankyou people, I have it now !

You're welcome. However, you got us started and what follows is out of your hands.

Ed

 

[ronh]

Ed,
Since the OP is squared away now, I'll not feel too bad about leaving the original topic, and imagine there are others here who have heard of the horopter but thought about it perhaps as little, and to as little avail, as myself, and would also be interested, so I will just come out and make this public.

There's something about the horopter I don't get. (OK there's the understatement of the year.) But really...its supposed to be the region in space where the eyes combine their images into an impression single vision, right? For all the hoopla, I'd think there was something really special about it. It seems that it is supposed be a curved surface surrounding the front of the head.

But, when I hold a finger out to the side, but still within the range of both eyes, and view it as best I can with my peripheral vision, and wiggle the finger to make it as easy to see as I know how, it always looks like just one finger, never two, no matter where I hold it, high or low (provided my eyes are focused at about the correct distance, on another finger centered in my view).

What am I missing? Is my horopter just not very picky? Do I not get what the horopter is in the first place? (Say it ain't so, Pa.) Are subtle laboratory measurements required to discover what I'm really seeing?

(Off for a birdout tomorrow at dawn, you have 24 hours to straighten me out!)

Thanks,
Ron

 

[Sancho]

But, when I hold a finger out to the side, but still within the range of both eyes, and view it as best I can with my peripheral vision, and wiggle the finger to make it as easy to see as I know how, it always looks like just one finger, never two, no matter where I hold it, high or low (provided my eyes are focused at about the correct distance, on another finger centered in my view).


Thanks,
Ron

That is amazing! I just tried it, and you´re right! I´d never heard of a horopter, and I´m delighted to know I have one of my own. When I wiggle the off-centre finger without focussing on a finger in the centre field, it looks like two fingers. But as soon as I put another finger into the centre field and focus on it, the peripheral wiggling finger morphs into a single-image finger! I can´t wait to get a whole classroom of 14-year olds trying it on Monday. If the principal walks in, I´ll think of some excuse....

 

[Pileatus]

That is amazing! I just tried it, and you´re right! I´d never heard of a horopter, and I´m delighted to know I have one of my own. When I wiggle the off-centre finger without focussing on a finger in the centre field, it looks like two fingers. But as soon as I put another finger into the centre field and focus on it, the peripheral wiggling finger morphs into a single-image finger! I can´t wait to get a whole classroom of 14-year olds trying it on Monday. If the principal walks in, I´ll think of some excuse....

Run a few dominant eye experiments and you'll see that you are seeing one or two images depending on the location of the target. The brain, seeking simplicity, combines whatever you see into a singular visual experience...after it erects the upside down retinal image(s). Thankfully, our brains do not consult BF before, during or after this miracle is performed.

 

[elkcub]

That is amazing! I just tried it, and you´re right! I´d never heard of a horopter, and I´m delighted to know I have one of my own. When I wiggle the off-centre finger without focussing on a finger in the centre field, it looks like two fingers. But as soon as I put another finger into the centre field and focus on it, the peripheral wiggling finger morphs into a single-image finger! I can´t wait to get a whole classroom of 14-year olds trying it on Monday. If the principal walks in, I´ll think of some excuse....

I'd suggest having them use their index fingers exclusively. |:$|

 

[ceasar]

I had my annual eye exam last month and the Doctor waggled his index finger out on the periphery of my vision and asked me how many fingers I saw. I only saw one.

I assumed this was to check the FOV of my eyes. Was there more involved in it than this?

Bob

 

[brocknroller]

I had my annual eye exam last month and the Doctor waggled his index finger out on the periphery of my vision and asked me how many fingers I saw. I only saw one.

I assumed this was to check the FOV of my eyes. Was there more involved in it than this?

Bob

Bob.

Are you a glaucoma suspect? If it runs in your family, my dad has it, or if your optic nerves are enlarged. or if your eye pressure is abnormal, your eye doctor will give you a peripheral vision test, because glaucoma causes "tunnel vision" once it's advanced enough.

There's a much more sophisticated test than finger wagging. It involves looking into a box where lights are flashed in different parts of the box, center, middle, peripheral.
You have to hit the button in your hand every time you see a flashing light. It's not easy because there's very little time between flashes so you can end up seeing "after images" which can fool you in thinking you just saw a flash where there was none! But I think they take that into consideration by duplicating the same flash in the same area more than once.

Brock

 

[ceasar]

Bob.

Are you a glaucoma suspect? If it runs in your family, my dad has it, or if your optic nerves are enlarged. or if your eye pressure is abnormal, your eye doctor will give you a peripheral vision test, because glaucoma causes "tunnel vision" once it's advanced enough.

There's a much more sophisticated test than finger wagging. It involves looking into a box where lights are flashed in different parts of the box, center, middle, peripheral.
You have to hit the button in your hand every time you see a flashing light. It's not easy because there's very little time between flashes so you can end up seeing "after images" which can fool you in thinking you just saw a flash where there was none! But I think they take that into consideration by duplicating the same flash in the same area more than once.

Brock

No to all of the above. I have a thorough yearly exam around this time every year. This year they added a dilated macular exam and a cell count which I can't remember ever having before. He said everything was normal.

Bob

 

[brocknroller]

No to all of the above. I have a thorough yearly exam around this time every year. This year they added a dilated macular exam and a cell count which I can't remember ever having before. He said everything was normal.

Bob

That's good news. No point and spending big bucks on optics if the last elements in the optical train don't work properly.

Well, the older you get, the more tests they give you. Next up... a colonosope! :eek!:

<B>

 

[ronh]

Thanks for all the responses. Pileatus's was most interesting:

"The brain, seeking simplicity, combines whatever you see into a singular visual experience...after it erects the upside down retinal image(s)."

If this is true, doesn't the concept of the horopter, in some practical sense, sort of fly right out the window? That was my suggestion, after finding myself unable to find a place that did NOT appear to be on the horopter. I know a lot of stuff is goes over my head. But why is the horopter important to visual science?
Ron

 

[elkcub]

Thanks for all the responses. Pileatus's was most interesting:

"The brain, seeking simplicity, combines whatever you see into a singular visual experience...after it erects the upside down retinal image(s)."

If this is true, doesn't the concept of the horopter, in some practical sense, sort of fly right out the window? That was my suggestion, after finding myself unable to find a place that did NOT appear to be on the horopter. I know a lot of stuff is goes over my head. But why is the horopter important to visual science?
Ron

Ron,

I feel your frustration and will respond shortly. It's a good question. Basically, in this case, if we understand how the visual process works we are better able to optimize binocular design.

Double images are somewhat difficult to experience, largely because our brains are designed to suppress them. Yours may work better than mine.

Ed

 

[elkcub]

Well, I guess this horopter thing got started with my speculation in post #6. That's neither here nor there, ... "But why is the horopter important to visual science?" and is there any practical value to it? That's an important question.

Without fear of contradiction, the reason binoculars are collimated is to facilitate the observer's visual horopter! Otherwise, there would be no corresponding image points on the two retinae, and double images would rule both day and night. In short, we would all suffer from instrument induced diplopia — a real headache.

The brain seeks to merge data from each eye to create a single (or 'fused') 3-D spatial image. To do so it utilizes (only) those points in space that appear at corresponding retinal locations, — and disregards the others. The locus of points that it uses defines the "horopter", but only momentarily because the eyes are constantly flitting from one object to another and the horopter changes accordingly.

In order to perceive double images from objects that don't fall on the visual horopter, therefore, it is necessary to fixate on a single point so that the horopter remains stationary. This takes considerable concentration. Only then is it possible to perceive double images from non-horopter locations that are still within the visual overlap region. Usually the images are not of equal intensity and quickly vanish when attention lapses.

I find it most productive to fixate on an object 15-20 deg. off to the side, which requires my eyes to rotate in the same direction. Then, double images for objects near the centerline are easier to discern. Focusing on an object off the centerline requires more ocular muscle effort and helps to maintain the necessary concentration.

A final thought. Looking forward along the natural line of sight, the shape of the horopter is dependent on object distance. Close in, at birding distances, the horopter's curvature is much more concave then far out, at astronomical distances, where it is essentially flat (possibly convex according to some geometric models). It may not be surprising, therefore, that astronomers appreciate flat-field instruments more than terrestrial observers (like me). But, as I admitted in post #6, here I'm alluding to user expectations having to do with focus and not image location. So no more of that...

The drawing below is taken from what many consider the best book to date written about the complex biological interactions and feedback loops within the oculomotor system that maintains vision. Oculomotor Systems and Perception by Sheldon Ebenholtz.

Ed

 

[ronh]

Ed,
I really appreciate your well considered reply. I will have to think about that for a while.
Ron