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FOV in classic vs flat field designs (1 Viewer)

tenex

reality-based
I was about to ask a question about differences in field edges and width in classic vs flat-field designs, then found a good post about linear and angular distortion that's already answered it, and is worth calling attention to. (Thanks Henry.) link

For my purpose I'll just add on the obvious consequence for apparent FOV: classic designs can "stretch" it a bit at the edges, while flat designs "shrink" it and appear to have a bit less.

This subject has often confused me because it doesn't seem immediately obvious how binos with the same magnification and actual FOV can have different apparent ones, and because manufacturers use two different formulas for calculating FOV in degrees, and seldom even state AFOV.
 
I need to look at something that has a straight vertical edge located at the edge of the view in my binocular to notice "pincushion" distortion. My Nikon 8x30 E2 makes it very obvious. It can be dialed into sharp focus. But this is best found when looking at static objects.

When I am following small birds flying through the branches of nearby trees while using it, it is not noticeable.

I don't know how to recognize field curvature.

Bob
 
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I need to look at something that has a straight vertical edge located at the edge of the view in my binocular to notice "pincushion" distortion. My Nikon 8x30 E2 makes it very obvious. It can be dialed into sharp focus. But this is best found when looking at static objects.

When I am following small birds flying through the branches of nearby trees while using it, it is not noticeable.

I don't know how to recognize field curvature.

Bob

You do, now:

The field starts getting progressively softer as one nears the edge. Notice how the focus changes from axial to periferal. :cat:

Bill
 

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You do, now:

The field starts getting progressively softer as one nears the edge. Notice how the focus changes from axial to periferal. :cat:

Bill

There you go again Bill, making things easy to understand.
(psst... don't stop)
Mike
 
There you go again Bill, making things easy to understand.
(psst... don't stop)
Mike

Sorry, Mike, it's a sickness. I have been treated with ever heavier doses of BS over the years. But, so far no luck. Some people appreciate my sickness. However, some who can think faster than they can reason take offense. It seems reality is not appreciated as it was in my day.

Bill
 
Thank you Bob and WJC... now I don't have to ask the stupid question!!! (scratching my head emoji/emoticon goes here)

CG
 
I need to look at something that has a straight vertical edge located at the edge of the view in my binocular to notice "pincushion" distortion. My Nikon 8x30 E2 makes it very obvious. It can be dialed into sharp focus. But this is best found when looking at static objects.

When I am following small birds flying through the branches of nearby trees while using it, it is not noticeable.

I don't know how to recognize field curvature.

Bob

Hi,

field curvature is the effect that the edge of the field is not in focus but can be brought to focus (if it can't, then other aberrations like astigmatism are present too).
So the fact that the usually not so straight line is not in focus but has to be brought to focus is the effect of field curvature.

The reason is that the focus plane of most objectives is not flat but a spherical surface with a radius of 1/3 of the focal length. The focus plane of most simple eyepieces is flat and will either touch the objective focus plane at the center spot (not exactly a single point as it would be mathematically but a larger sweet spot due to depth of focus).

So you can either have the sweet spot in focus or the edge but not both with these instruments. Or the designer has provided some extra lenses to get the two focal planes to match, which is called field flattening.

Joachim
 
Maybe I will run this by my son when he comes home during the Spring vacation?:king: He got his PhD in Physics recently and is an Assistant Professor of Physics at a College not very far from Pennsylvania.

The Upper Division Physics course he is teaching this semester is Optics. This sounds like a practical problem that could be a subject of discussion although I don't know how deep they get into things like this in a basic Optics course.

I gave him my Swarovski 8x30 SLC B as a gift recently. It was made in 2010. He can compare it to my Nikon 8x30 E2 while he is home. I have others he could compare also.

Bob
 
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