Depth of field (1 Viewer)

[BoldenEagle]

Hi,

I'm wondering what factors affect to the depth of field on spotting scopes (or binoculars also). It's clear that magnification is one of them but is it the only one? Does aperture or focal ratio have any effect?

Are there some optical aberrations or defects (miscollimation?) that can narrow the dof?

Juhani

 

[Binastro]

Hi Juhani,

A curved field can increase apparent DOF.

My small 6x18 waterproof cheap roof prism binoculars with IF are left set at one distance.
I just use the lower part of the field for nearer objects.

In fact I do this with several binoculars.

Also as one approaches the edge of the field the vignetting brings closer objects into view.

I have minimal accommodation.

The 4x21 Bushnell Xtrawide is a fixed focus binocular. The centre field is blurred for my far sighted eyes.
However, the side edges are nicely in focus and I can view two planets or stars well separated.
The field is about 18.5 degrees.
The binocular is actually 3.5x21.

Regards,
B.

 

[jring]

Hi,

proportional to the square of the focal ratio. Slow objectives have a lot more depth of focus and thus depth of field.

https://www.cloudynights.com/topic/455942-what-factors-affect-depth-of-focus/

Joachim

 

[Tringa45]

Hi Juhani,

As you probably know, depth of field in an afocal device like a telescope is inversely proportional to the square of the magnification, i.e. the depth of field at the same focussed distance would be twice as great with a 7x binocular as with a 10x binocular.
The focal ratio is irrelevant and although a smaller aperture results in a smaller exit pupil (for the same magnification) this would only give a theoretical improvement in poor lighting, where the eye's acuity is poor anyway.
At scope magnifications the user's ability to accommodate doesn't play any significant role as the DoF is so small anyway. For a focussed object in a scope different users will have to adjust the focus unless they all have perfect vision or are all wearing glasses.
IIRC, you have an ATX85 and will probably have noticed that focussing at 60x is very sensitive. Even the coarse/fine ratio of 1:3 on my Kowa 883 is a mistake, as the DoF at 60x is only about a sixth of that at 25x.

John

 

[henry link]

Aperture has an effect in the sense that an exit pupil smaller than your eye's pupil will increase DoF by changing the focal ratio of the eye. An 80mm scope at 60x will have less DoF than a 60mm scope at 60x just because its exit pupil is larger and that causes the eye's entrance pupil to be larger and that causes the eye's focal ratio to be lower.

Spherical aberration can confuse the appearance of defocus and cause an impression of less or more DoF because with spherical aberration there is no point where all the rays from the inner and outer parts of the objective come to the same focus. Changing the focus in one direction will defocus all rays, but changing focus in the other direction will defocus some rays while bringing others into focus, so that the appearance of defocused objects is different depending on whether they are in front or behind an object at best focus.

 

[Tringa45]

Hi,

proportional to the square of the focal ratio. Slow objectives have a lot more depth of focus and thus depth of field.

https://www.cloudynights.com/topic/455942-what-factors-affect-depth-of-focus/

Joachim

Joachim,

The OP's question related to depth of FIELD not depth of focus.
The depth of field of a 100 mm f/6 with 15 mm eyepiece (40x) will be the same as a 100 mm f/12 with 30 mm eyepiece.
With the same Crayford or rack & pinion focusser, the latter would be easier to focus.

John

 

[Tringa45]

Aperture has an effect in the sense that an exit pupil smaller than your eye's pupil will increase DoF by changing the focal ratio of the eye. An 80mm scope at 60x will have less DoF than a 60mm scope at 60x just because its exit pupil is larger and that causes the eye's entrance pupil to be larger and that causes the eye's focal ratio to be lower.

Henry,

We posted almost simultaeneously and I suggested that any theoretical gains in DoF at small exit pupils would be negated by light loss.
Diffraction is going to diminish the eye's resolution at 1 mm exit pupil and Wikipedia suggests that maximum resolution is achieved at pupil diameters as high as 3-4 mm.
My somewhat futile attempts at boosted resolution measurements with a backlit USAF 1951 glass slide have shown the importance of brightness in perception, and for birding with the scope I don't seem to get any significant gains at exit pupils below 1,8 mm and their attendant magnifications.
YMMV and I recall that a dealer here conducted some tests with customers and a Diascope 85 and astro eyepieces. I don't know under what conditions and it was probably not very scientific but the conclusion was that for most, the minimum usable exit pupil in daylight was around 1,3 mm.

John

 

[Tringa45]

Here's an old thread and an exchange between Holger and my alter ego (John Russell), post #s 31-39.
https://www.birdforum.net/showthread.php?t=258235&page=2

John

 

[henry link]

Henry,

We posted almost simultaeneously and I suggested that any theoretical gains in DoF at small exit pupils would be negated by light loss.
Diffraction is going to diminish the eye's resolution at 1 mm exit pupil and Wikipedia suggests that maximum resolution is achieved at pupil diameters as high as 3-4 mm.
My somewhat futile attempts at boosted resolution measurements with a backlit USAF 1951 glass slide have shown the importance of brightness in perception, and for birding with the scope I don't seem to get any significant gains at exit pupils below 1,8 mm and their attendant magnifications.
YMMV and I recall that a dealer here conducted some tests with customers and a Diascope 85 and astro eyepieces. I don't know under what conditions and it was probably not very scientific but the conclusion was that for most, the minimum usable exit pupil in daylight was around 1,3 mm.

John

Hi John,

Yes, my mileage does differ. I'm sorry, I don't have much time to discuss this at length, but I just tried a simple experiment, focusing just on the question of whether DoF can be seen to increase at very small exit pupils.

I focused a pair of 16x40 binoculars on a target about 8 meters away under moderately bright interior light, quite a bit dimmer than daylight. I then looked at some print about 6 meters away without changing the focus. As you would expect the print was quite out of focus. I then stopped down the binocular aperture to 12mm, thus changing the exit pupil from 2.5mm to 0.75mm and making the same change to my effective entrance pupil. The image was dimmer, but the letters were certainly sharper and more readable.

Henry

 

[etudiant]

What is the advantage of increased DoF?
From experience, very limited DoF has real benefits when birding in areas with vegetation. My Canon 10x42 has a very shallow DoF, which is really helpful when trying to get details on a bird that is in the weeds. The more adjacent out of focus stems seem to melt away somewhat.
Of course, the Canon's focus speed is correspondingly slow, three full turns lock to lock, so no free lunch absent some sort of variable speed focusing mechanism.

 

[Binastro]

Probably twenty times a day I stop my eye down to about 0.4mm to read small letters.
I make a pinhole with my fist.
I am far sighted and cannot read fine print without glasses.
But I normally don't use glasses when just ambling around.

With my 317mm Dall Kirkham I could measure distances of over two miles by seeing the difference in focus between the Moon and the target in daylight.
I cannot remember the magnification. Perhaps 200x.

With an 80mm to 90mm spotting scope my preferred magnification in daylight is 120x.
I find that 60x is totally inadequate for fine detail. This has been the situation for years and my eyes were then good.

Horace Dall's camera obscura table magnifier gave 135x for 108mm aperture.

However my 150mm Maksutov was left at 95x for daylight views as the best compromise outdoors.

Regards,
B.

 

[Tringa45]

What is the advantage of increased DoF?
From experience, very limited DoF has real benefits when birding in areas with vegetation.

Why would you want to eliminate the third dimension, aesthetic reasons?
Disregarding accommodation, the human eye offers enormous depth of field and we are used to perceiving objects simultaneously in different planes.

A higher magnification binocular not only has a restricted field of view in the x-axis and y-axis, but also in the z-axis due to the restricted DoF.
Putting the bird in the FoV of a 10x binocular is not a problem for most of us, but at the closer distances you are forced to search for it in the third dimension with the focusser.

The use of a 10x binocular for woodland birding is a self-imposed handicap.

John

 

[etudiant]

Why would you want to eliminate the third dimension, aesthetic reasons?
Disregarding accommodation, the human eye offers enormous depth of field and we are used to perceiving objects simultaneously in different planes.

A higher magnification binocular not only has a restricted field of view in the x-axis and y-axis, but also in the z-axis due to the restricted DoF.
Putting the bird in the FoV of a 10x binocular is not a problem for most of us, but at the closer distances you are forced to search for it in the third dimension with the focusser.

The use of a 10x binocular for woodland birding is a self-imposed handicap.

John

Tringa45, thank you for your thoughts. We all agree that expansive views are a very desirable feature for binoculars.
I was simply pointing out that a shallow DoF can be a significant help when looking at birds through clutter such as reeds, weeds or shrubbery.
Separately, a 10x is no handicap for woodlands imho. The incremental FoV provided by a top tier 8x surely helps tracking nervous warblers, but the greater detail seen at 10x is not to be dismissed.

 

[Tringa45]

With an 80mm to 90mm spotting scope my preferred magnification in daylight is 120x.

David,

I don't think many birders would be interested in that sort of magnification, with the possible exception of David Rodrigues, who has a very specific application. Finding the bird would necessitate a change of eyepieces (too slow) and even with an extender the bottom end of the zoom range would be 50x. Heat haze also often puts limitations on usable magnification.

I suspect that birding scopes get most of their use at distances under 300 m.
The longest distance I can recall using mine terrestrially was to view a White-tailed Eagle (according to the locals at about 1500 m IIRC). I could just make out the white tail feathers, indicating a mature bird.

Aesthetics play an important role for many birders and the best views are often at moderate magnifications. One of my most memorable birding experiences was training my scope on some Red-capped Plovers on an Australian beach at a mere 15 m!

John

 

[Binastro]

I was just relating to depth of field.

Also to sub millimetre eye pupils.

I think with my 317mm Dall Kirkham the difference in focus at 6 miles and the Moon was about 2.4mm or a tenth of an inch.
Probably noticed in actual use.

The laser artificial stars used by adaptive optics on large telescopes projects a star at either 90km or 90 miles, I cannot remember which.
There is a difference in the focus position of the artificial star and real stars.

Regards,
B.

 

[Tringa45]

Hi John,

Yes, my mileage does differ. I'm sorry, I don't have much time to discuss this at length, but I just tried a simple experiment, focusing just on the question of whether DoF can be seen to increase at very small exit pupils.

I focused a pair of 16x40 binoculars on a target about 8 meters away under moderately bright interior light, quite a bit dimmer than daylight. I then looked at some print about 6 meters away without changing the focus. As you would expect the print was quite out of focus. I then stopped down the binocular aperture to 12mm, thus changing the exit pupil from 2.5mm to 0.75mm and making the same change to my effective entrance pupil. The image was dimmer, but the letters were certainly sharper and more readable.

Henry

Henry,

With some delay I have repeated your test and achieved similar results under similar conditions. I set up my 65 mm Swarovski with a 28 mm RKE for 16,5x magnification and 4 mm exit pupil and was also viewing print under artificial lighting. Focussed at 6,5 m I could not read the print at 5,9 m. With a 25 mm stop for a 1,5 mm exit pupil it was just legible for me.

Yesterday I stumbled across a passage in the first edition of Holger Merlitz' book, "Handferngläser". He quotes Albert König (?) who postulated that a circle of confusion of 3,4' is still perceived as sharp and that the eye can contribute an additional 1/d diopters of depth of field, where "d" is the diameter of the eye's pupil. This however only applies to pupil diameters above 2 mm. Below that, diffraction has a negative effect .

Personally, as an afficionado of large exit pupils, I find it hard to understand how some amateur astronomers can use 0,5 mm exit pupils for lunar or planetary observing.

John

 

[Binastro]

A colleague has recently produced lovely disc drawings of Mars using a rather low priced 90mm f/10, maybe 910mm focal length, doublet refractor.
Many colour drawings on many days at 165x.
So, about 0.55mm exit pupil.
No ED glass etc.

Horace Dall's 8 inch Maksutov gave exquisite views of Mars at 400x.
0.5mm exit pupil. Horace was in his 80s, I was in my 40s.

In the last month or so another colleague using a 350mm Dobsonian used 200x on Mars in poor conditions.
But 400x or more in good conditions.
To see Deimos he used 550x.
550x and higher to see Uranus moons.

Dawes used 400x with his 160mm refractor for Mars, Jupiter and Jupiter's moons.
0.4mm exit pupil.

With my Pentax 100mm f/12, 200x was standard, with 250x and 300x used in very good Seeing.
So 0.5mm standard, but sometimes 0.33mm exit pupil.

Opinions expressed here are opinions.
But they don't equate to actual observations by planetary observers.

Regards,
B.

 

[Tringa45]

Yes, David, I know they all do it and don't dispute that many can profit from high magnifications and small exit pupils. I certainly could not (floaters).

I just see a contradiction, when everyone is interested in diffraction limited scopes and Strehl ratios but ignores the diffraction limitations of the human eye.

Regards,
John

 

[Binastro]

Hi John,

I just read the September reports of Mars observations.

One of the very best observers with very fine eyesight normally uses 190x and 300x with a 12 inch Newtonian.

But in very fine Seeing (Ant 1 to 2) most of the night he also used 600x to see detail previously unseen.

Personally I am not the slightest bit interested in Strehl ratios.
I am just interested in how good the telescope is in actual use.
Telescopes do not have to be perfect to give very fine detailed views.

It rather amuses me how optics mavens discuss the quality of their scope while usually never making any meaningful observations that are reported and become part of the astronomy record.

I doubt that birdwatchers need perfect telescopes or binoculars.

We don't have perfect eyes either.

Regards,
B.

 

[Tringa45]

I doubt that birdwatchers need perfect telescopes or binoculars.

Yes, we're all geeks here. Some really competent ornithologists use battered old wrecks of bins.

Regards,
John