What else determines Depth of Field? (1 Viewer)

[tpcollins]

I just received a Vortex Fury 8x28 (supposedly moved to the Viper line now) and it's a very nice little binoc for $229 but the Depth of View is really shallow. There's a large school sign about 200 yards behind my house and although the view is clear and very, very sharp, there is no "range of adjustment" when focusing whatsoever. It's back and forth, back and forth, and then sharp. Seems like if I set it down, pick it back up, it's out of focus.


I understand that magnification reduces Depth of Field but this is an 8x28 binoc. On my Leica 8x20 I have alot of adjustment range while still in focus as well as my Zeiss Victory 8x42 FL - so there has to be something else that affects Depth of Field besides magnification. Can anyone explain? Thanks.

 

[ceasar]

Your focusing knob on the Vortex is probably much faster than the one on the Leica. The Leica, being slower, (I have one.) is easier to "fine tune" when focusing back and forth and easier to get (and keep) a sharp view.

The Zeiss 8 x 42 has a much longer focal length and so it's DOF will be greater than the short focal lengths your 8 x 20 and 8 x 28 have.

As a personal example, I can see a big difference in DOF between my Leica 7 x 42 Trinovid and my Vortex 7 x 36 Diamondback.

Bob

 

[Kevin Purcell]

The change in f/number (even for a long bin like the ZR 8x43) doesn't have a perceiveable impact on DOF. It feels like it has more DOF because it has a slower focus rate.

Focuser "speed" i.e. focuser rate and focuser stiffness contribute to what people "perceive as DOF" though. It's very clear in 10x bins: I have a Zeiss 10x42FL which had very low friction focusing that was a pain to use in the field. Increasing the focuser friction made it a more usable bin (these bins already have a pretty high "rate" a bit more than half a turn from close to distant).

Field curvature (and amount of accommodation in the user) may also have an impact on perceived DOF. Older folks may feel a bin had less DOF than younger folks (as their eyes can't do the work for them).

After some argument the technical basis shows that only magnification impact the true DOF. There are heavily argued thread on this here as I'm sure Bob knows

 

[Tero]

28mm roofs are as hard to make, it seems, as are 20mm and 25mm. I have one pair with OK optics, a Bushnell. The same thing in 10x28 is just awful.

With Vortex I would stick to 32mm and up and the reverse porros.

 

[henry link]

It's true that magnification is usually the one and only thing that determines optical DOF in binoculars, but there is an exception when the exit pupil of the binocular is so small that it effectively stops down the eye compared to another binocular. So, an 8x20 could show wider DOF compared to an 8x42 if the light is dim enough. For instance, in low light the eye might be open to 5mm when looking through an 8x42 giving the eye a focal ratio of about f/5, but it would be effectively stopped down in the same light to about f/10 when looking through an 8x20.

 

[tpcollins]

Thanks Kevin, I may be confusing "focus speed" with what I thought was DOF. I'm going to re-evaluate how to determine the DOF. Thanks.

 

[ceasar]

The change in f/number (even for a long bin like the ZR 8x43) doesn't have a perceiveable impact on DOF. It feels like it has more DOF because it has a slower focus rate.

Focuser "speed" i.e. focuser rate and focuser stiffness contribute to what people "perceive as DOF" though. It's very clear in 10x bins: I have a Zeiss 10x42FL which had very low friction focusing that was a pain to use in the field. Increasing the focuser friction made it a more usable bin (these bins already have a pretty high "rate" a bit more than half a turn from close to distant).

Field curvature (and amount of accommodation in the user) may also have an impact on perceived DOF. Older folks may feel a bin had less DOF than younger folks (as their eyes can't do the work for them).

After some argument the technical basis shows that only magnification impact the true DOF. There are heavily argued thread on this here as I'm sure Bob knows

Kevin:
I'll compare my 7 x 42 Trinny's with the 7 x 36 Diamondback again (for DOF only-otherwise there is no comparison!) from my deck which overlooks my back yard. The 1st 50 feet of the yard is clear. The rest is heavily wooded. I'll do it tomorrow about 9-10 AM. It promises to be bright so the 5.2mm and 6.0mm exit pupils will not be a factor. I've done this in the past a couple of times. It won't hurt to do so again to see if my memory is OK.
Bob

 

[tpcollins]

Kevin was right

You were right Kevin -

I was misinterpreting the fast focus for a lack of Depth of Field. I took the Vortex, my Leica CRF rangefinder, Leica 8x20, Zeiss Victory 8x42, and my Leupold GR spotting scope to the field with two McCain-Palin yard signs (they were good for something at least) and set them up at 100 and 110 yards to see if both signs would be in focus. Everything passed including the spotting scope at 12x and 20x.

I kept moving the back sign further out in ten yard increments all the way to 150 yards and I was impressed with what I saw. The Leica 7x rangefinder was ok out to 100 & 140 but needed the diopter adjustment when the back sign was at 150 yards. The Leica 8x20 was great out to 100 & 140 respectively but dropped off slightly with the rear sign at 150. The Zeiss 8x42 was flawless at every yardage and I didn't have to center each sign in the middle of the view - they were just clear as a bell. The little Vortex was equally good in all of the yardage differences but I did have to center each sign due to a slight edge distortion. Actually my Leupold GR at 12x was great in all of the yardages but started dropping off at the 110 & 140 distances at 20x.

I don't know if a 50 yard variance is a good test but I was impressed with what I saw, especially with the Vortex. Now that I know it doesn't have a DoF issue, I'm still not sure if I'll ever get comfortable with the fast focus but thought I should correct my original post.

 

[Tero]

Apparent depth of field is different from those measured by optics tests. Many 28mm models do have small apparent depth of field.

 

[tpcollins]

Apparent depth of field is different from those measured by optics tests. Many 28mm models do have small apparent depth of field.

What would have been the appropriate way to make this test?

 

[henry link]

Kevin:
I'll compare my 7 x 42 Trinny's with the 7 x 36 Diamondback again (for DOF only-otherwise there is no comparison!) from my deck which overlooks my back yard. The 1st 50 feet of the yard is clear. The rest is heavily wooded. I'll do it tomorrow about 9-10 AM. It promises to be bright so the 5.2mm and 6.0mm exit pupils will not be a factor. I've done this in the past a couple of times. It won't hurt to do so again to see if my memory is OK.
Bob

You may see some kind of difference, but it won't be a difference in DOF unless the binoculars have different true magnifications. The best test I've found for detecting differences in de-focus (while eliminating other variables that confuse the issue) is to use a glitter point as the out of focus object. Focus the center of the field on a distant object, then center a nearby glitter point (reflection of the sun in a small round shiny object). The size of the diffraction disk formed by the unfocused glitter point indicates the amount of de-focus.

 

[BobinKy]

In my photography days, I measured the various DOF on my various camera lenses by going to the country and using a straight wooden rail fence. With the assistance of a friend, I would tie a string at the near and far focus points for each lens. I used the same near focus point for all lenses, to make it easier to measure.

Some other photographers used to go to football fields and use the 100 yard white lines on the field to measure DOF.

I don't know if the same targets (straight wooden fence or white lines on football field) would be any assistance for measuring the DOF of your binos.
Measuring DOF in camera lenses or binoculars can be a sticky wicket.

. . .

And yes, as said above, quick focus vs. slow focus differences in binoculars can easily be misinterpreted for DOF differences.

A shallow DOF with a quick focus--now that is one pernickety bino.

 

[henry link]

Bob,

I wouldn't use this method for binoculars. It introduces at least two variables that you don't want. First the fence posts occupy different points in the field which inadvertently introduces field curvature to confuse the issue and using your eyes at the very edge of focus causes your eyes to strain to accommodate. Success at that varies from second to second so it can't be relied on for accuracy.

The beauty of the glitter point method is that it doesn't require trying to judge how far out of focus something is. Only the size of the diffraction disk matters.

Henry

 

[BobinKy]

Henry...

We actually used a pair of focus cards along the fence. The white rail fences were pretty straight--you would know if you ever toured the horse farms around central Kentucky. The focus cards were not the USAF color chart, but something similar. The depth of field focus points (near and far) were then marked on the fence and strings with tags tied onto the top rail in the fence, which was at the same height as the camera.

Not fool proof, but also not dependent upon a bright sun reflecting ontp a diamond ring, which I am sure we would have tried, but none of us could get our wives to loan us their jewelry.

. . .

It would be interesting to develop some kind of focus grid (Excel worksheet) of the in-focus DOFs at various distances from the observer for various binoculars. It also would be nice if the manufacturers provided DOF specs on the binoculars they make and distribute.

 

[henry link]

Bob,

The analogy between DOF in a camera lens and binocular breaks down because the binocular is afocal. The light that emerges from the eyepiece doesn't have any depth of field. The DOF we perceive is in the eye's optics when the afocal light from the binocular is focused by the lens of the eye on the retina. There is a mathematical formula for DOF vs magnification. Essentially the circle of confusion grows larger in step with magnification. Maybe Ron (Surveyer) or Ed (elkcub) posted it here at some point. A manufacturer's spec for binoculars would be superfluous since magnification is the only variable - all binoculars of the same magnification would have the same spec. But, the experience of individual users will vary with focus accommodation and pupil dilation. Even a single user will experience different DOF with the same binocular at different light levels as the focal ratio of the eye changes with pupil dilation.

For the glitter point you need a round object, light bulb, Christmas tree ornament, marble, ball bearing, etc.

 

[Tero]

What would have been the appropriate way to make this test?

There is really no test. You could easily rank binoculars in order, good to bad.

 

[njlarsen]

After some argument the technical basis shows that only magnification impact the true DOF. There are heavily argued thread on this here as I'm sure Bob knows

Mathematically you might be right: my experience says something else. I tested two bins by looking at a fence positioned in an angle to me, focused on the same spot in the fence, and counted how far (how many wooden sticks) to each side I could see the fence in focus. I moved the bins to the side so that the in or out of focus was at center field. There was a clear difference between two different alpha priced 10x42 bins.

Niels

 

[henry link]

I would caution again against using the fence type target and trying to arrive at the exact point at which the eye can no longer see a sharp image. That is always a fuzzy place with the eye constantly struggling to achieve better focus. The size of an out of focus diffraction disk works much better because it's determined by defocus only and the eye doesn't struggle against it.

 

[spitfiretriple]

I understand (or at least I think I do) the mathematical argument that links DofF to magnification and to magnification only. But I have a theory I'd like to air that might explain why perceived DofF is not always the same as theoretical DofF. This theory (I doubt it's a new one) appears to explain why DofF seems to vary not just with magnification, but with other factors.

It's this: thinking like mathematicians is all very well when considering a camera (at least, a film camera, before digital sharpening was invented). But what we perceive in our mind's eye when looking at the world (whether through binoculars or through the eyes we were born with) is not the result of just a glass-and-metal optical instrument, but also involves our eye and our brain. I'll expand on that.

I'm 48. My eyes no longer have the focusing ability they had when I was younger. Yet if the light is good,I still have a reasonable range of focus. If the light is poor, and/or if my eyes are tired, my range of focus is smaller. Yet all this time my eyes are working to a fixed magnification of... 1x. So much for the idea that only magnification affects DofF. So what is happening here?

The brighter the light going into my eye, the wider a DofF my naked eyes can achieve. So, already we have a suggestion of something that will affect perception of DofF through binoculars: How bright an image does a particular binocular allow to pass through it? The brighter the better. That might explain why large objective binoculars do better; why porros do better; why high-quality expensive binoculars do better.

There's more. Information useful to the eye-brain is not just about brightness (which is a way of saying quantity). It's also about quality. The better the quality of information going into the eye-brain, the better the ability of the brain to send accurate signals back to the eye-muscles that control focusing.

And, the better quality the binocular, the better quality the information going to the eye-brain. Millions of years of evolution have directed the brain to use as many cues as it can when processing images. We have two eyes for a reason: binocular vision is superior to monocular vision, especially when it comes to perception of distance. Binoculars that can give our eye-brain a better ability to perceive distance will allow our eye-brain more and better quality information. And, old-fashioned porros, having their objective lenses set further apart, deliver better distance perception. So, all else being equal, porros will again tend to allow greater DofF.

Most of the posts on this subject have been focusing on the binocular alone; we need to focus on the optic system as a whole. That includes the eye. And the brain.

 

[henry link]

I'm 48. My eyes no longer have the focusing ability they had when I was younger. Yet if the light is good,I still have a reasonable range of focus. If the light is poor, and/or if my eyes are tired, my range of focus is smaller. Yet all this time my eyes are working to a fixed magnification of... 1x. So much for the idea that only magnification affects DofF. So what is happening here?

spitfire,

I'll just address this one idea. Your eye's DOF does indeed increase in bright light even though there is no change in magnification because the pupil aperture decreases and that changes the focal ratio, just like a camera lens. In very dim light your eye may be open to 6mm and operate at about f/4 and in very bright light it may close to 2mm and operate around f/12. The test I suggested using a glitter point can show this happening because the out of focus diffraction pattern you see through a binocular is actually an image of your own pupil unless your eye is open wider than the exit pupil of the binocular. In very bright light you will see a smaller diffraction disk (and DOF will be wider) than in dim light even when you are using the same binocular.

Henry