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Depth Of Field better when diameter is larger? (1 Viewer)

Within this thread mostly the discussion is of the axial rays and depth of field.

I accept that the DOF axially is due to the magnification of the binocular and little else.

However, binoculars are wide field instruments and although it is natural to use the centre to view an object, there is the whole field available.

There are basically two types of binocular, flat field and curved field.

Binoculars with three or four element eyepieces often have curved fields, slight or moderate with some excessive.

I prefer to move to the lower part of the field if something nearer is seen than the centre focus point, rather than refocus.

The DOF is approximately doubled doing this.

2), The circle of confusion is actually the base of two light cones touching at the apex at the point of best focus.
We are looking through these cones.
Whether it is valid to consider the bases of these cones as valid circles of confusion, I don't know as they are not like dots in two dimensions.
Cones are in three dimensions.

Regards,
B.
 
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With my H alpha telescope, bought about twenty years ago, I set it up the day I bought it.

It has to be tuned and focussed.

I used the supplied 12.5mm Kelner eyepiece giving 32x,
This has a distinctly curved field.

I set the tune at mid position.

When observing different parts of the Sun's disc normally the focus has to changed and when viewing prominences outside the Sun's edge focus has to be changed quite a lot.
That is with a flat field eyepiece, which I found to be a nightmare.

With the curved field Kelner in twelve years of daily observing when the Sun was visible and 2,000 sketches I never retuned or refocussed the telescope.
I just used different parts of the eyepiece to see critically sharp detail as I moved around the Sun's disc and the prominences.

For 60x I did use a flat field eyepiece but positioned it differently along the eyepiece holder.
Same for 80x.
But I rarely used other than 32x.

So for 2,000 observations I just used the curved field of the eyepiece and never changed the focus.

Regards,
B.
 
Yes, I suspect this muddle over DOF is mainly a confusion with field curvature, people trying to describe their impression of sharpness over the whole field using the wrong term. This confusion doesn't arise in photography because camera lenses are designed to minimize curvature, for obvious reasons. One can predict that once flat-field designs have become standard in binoculars also, there will be no more arguing about differences in DOF here either.
 
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The Kodak Brownie 127 produced in millions I think has a curved back and simple lens.
The film plane is curved and it takes good photos.

The British Purma Plus also has a curved back in addition to a gravity shutter and square format negatives.
There are three speeds depending whether the camera is held normally or sideways left or right.

Going the other way, large format survey and aerial cameras have pressure plates or either a vacuum back or positive pressure to keep the film flat with highly corrected flat plane lenses.

Some cameras don't keep the film flat because of poor design of rollers.

B.
 
As OP I must confess: Depth Of Field is not better when the diameter is larger...
After reading all your comments I took some time to compare the SLC 8x42 with the Terra 8x25 again. The DOF is not bigger in the 8x42. So why was I thinking that it was the case? I came to two reasons:

  • when I turn the focuser on the SLC 8x42 the thing I was focusing at, stayed sharped for a longer time. I think that is the depth of focus? When I turn the focuser of the Terra 8x25, things get blurred more quickly. I was confusing that with the DOF I think. The SLC 8x42 goes from the closest focus to infinity in 2,25 turns. Whereas the Terra 8x25 it is just 1,5 turns. The SLC has a slow focuser and the Terra has a quick focuser.
  • The quality of the glass does play a role as well. I thought things were still in focus with the SLC, but when I looked better it was not really sharp. I think the view is more forgiving with better quality. The view is still usable when it is not 100% sharp, but with the Terra 8x25 the view was not comfortable anymore. At least, that is what I think now. The larger EP, the larger FOV, it all plays a role in making to think that the DOF is better. That is what I experienced, at least.
 
Hi Tenex, post #66.

Yes.

The 1952 original Brownie 127 had a meniscus f/14 lens and the second version 1960s an f/11 Dakron? plastic lens.
I am not sure if the original lens was glass or plastic.
I got mine in 1952, my first camera.
They were made by Kodak U.K., but maybe half a million went to the U.S.

One took the roll of negatives to the chemist and got enprints.
Only few were enlarged beyond the 4cm square photos, but these photos still look good.
They were black and white.
2x or 3x enlargements are O.K.
The camera had a rotary shutter and one speed about 1/50 second.

I started using colour about 1957.
First Kodachrome, then high speed Ektachrome in 1959 in the U.S.A. 160 ASA.
A Kodak 35 camera, which was actually pretty poor.
The 1967 Minolta 101B was in a completely different class with wonderful lenses.

I doubt that curved field binoculars will disappear.

With very large extra wide angle lenses curved arrays are used. Very expensive, but the best solution.
I think that smaller curved array sensors are also made.

Film was also used bent over a holder for fast Schmidt cameras, as the film plane was curved.

Regards,
B.
 
when I turn the focuser on the SLC 8x42 the thing I was focusing at, stayed sharped for a longer time. I think that is the depth of focus?
No, that's just speed of focus, the ratio of the screw drive. It varies but in general birding-oriented models tend to be faster (to get on the bird), hunting slower (to dial in optimal sharpness on something more static). My SLC 56s for example are quite slow. Thanks for mentioning this, I hadn't thought of it as a source of confusion here.
The quality of the glass does play a role as well. I thought things were still in focus with the SLC, but when I looked better it was not really sharp. I think the view is more forgiving with better quality. The view is still usable when it is not 100% sharp, but with the Terra 8x25 the view was not comfortable anymore. At least, that is what I think now.
This sounds plausible too, if various aberrations are better controlled in SLC... so "all else being equal" covers quite a lot. How does center sharpness itself compare between Terra and SLC?

I doubt that curved field binoculars will disappear.
As do I, not least because they offer such impressive DOF. ;)
 
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Light transmission is the primary factor in perceived depth of field with binoculars and it is a function of the surface area of the objective lenses. It varies more than one might expect if only considering the diameter of the lens.

50mm objective lens area of 1963 sq mm gain of 41% over 42mm

42mm objective lens area of 1385 sq mm gain of 96% over 30mm

32mm objective lens area of 803 sq mm gain of 13% over 30mm

30mm objective lens area of 706 sq mm gain of 44% over 25mm

25mm objective lens area of 490 sq mm gain of 56% over 20mm

20mm objective lens area of 314 sq mm
 
How does center sharpness itself compare between Terra and SLC?
Just a bit better in the SLC, but I find that hard to measure/judge. The view through the SLC is more vivid, bright, contrasty (I think) and way more comfortable. It's a pleasure to look through the SLC. The terra 8x25 is very usefull to determine the species but that's it.
 
I don't understand how that worked with a merely cylindrical back. Was the idea that foreground would look decent enough and sky doesn't matter?
As Binastro indicates, it was only really an amateur snapshot camera which was probably only really good for enprints or maybe something a bit larger if the subject was family or friends, centre field in the 10-25 feet range. Also the Kodak Brownie 127 didn't take square format photos so this correction occurred along the more critical longer width rather than height :

Edit : Ha, typical, seems there was a version of the Kodak Brownie 127 which took square pictures, but that was the later version 3 with such advanced features as double exposure prevention :D. Nostalgia trip :
 
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Thanks for the correction, Norm.

It is rather a long time since I used the camera.

I have been looking at some old photos under our glass top and these are not square, so maybe they were from the Brownie 127.
These cameras only cost a few pounds or less now.

The point is that after 60 or 70 years these photos are as fresh as new and on display.
We may not exist in that time in the future or the photos unreadable.

Regards,
B.
 

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