Astigmatism (1 Viewer)

Not sure - after as good a search on Bird Forum as I can manage with a phone and large fingers - if there's a more suitable general sub-forum for my question. As I am largely a Zeiss user I'll post it here till corrected...

I'm looking for the definition / explanation of the term astigmatism as applied to binocular design. When I look it up the only explanations I have found apply to the human eye rather than glass optics.

Thank you if you can give me an idiot's guide, or more.

Tom

Hi,

astigmatism is a term for a specific optical defect which can be found in various optical systems, the human eye being one of those.

It is caused by parts of the optical system like lenses or mirrors not being completely rotationally symmetric which results in its inability to get a cross to optimal focus - either the horizontal line is sharp and the vertical or vice versa.

If you look at landscape or birds it will mean that you cannot get a clearly defined focus point.

Joachim

With top quality binoculars centrally placed stars are usually round if ones eyes have no astigmatism.

With binoculars that have poorly made optics central stars are small lines or ellipses. (Sometimes just distorted stars of all shapes).
This is what I look for when selecting cheaper binoculars.
I test 6 or at least 3 different similar binoculars using the sun's reflection in ball bearings or actual stars, and buy the one with the best star images using my eye with no astigmatism to test each barrel.

I look at in focus stars and out of focus stars each side of focus.

At the edge of the field even good binoculars often have heavily distorted star images not related to poor optical figure.

P.S.
A binocular can show good symmetrical star images, yet have poor or very poor resolution.
So besides star tests, resolution tests are needed.

This was shown with the VisionKing 5x25 that I recently got.
The stars images were quite good, yet looking at the Moon the detail in one barrel was very poor. The image was very soft.
Using both eyes this wasn't immediately obvious as the view from the good side masked the poor side.
So each barrel has to be tested separately.
And of course together, to see if the alignment is good or not.

With good quality binoculars both central star images and resolution tests are usually at least good if not excellent.
But lemons do show up even with high priced binoculars, and even £1,000 plus camera lenses.

Sections 4.3 and 12.2 of "Amateur Telescope Optics" cover both objective and eyepiece astigmatism.

https://www.telescope-optics.net/index.htm

On-axis astigmatism from the objective is a sample defect. Off-axis astigmatism from the eyepiece is more likely to be a design compromise.

Henry

Thanks Henry.

Just a couple things to add to the already excellent replies. Not sure if this is correct in an optical engineering sense, but I think of astigmatism as different magnification in different directions. What you actually see is the image getting stretched in one direction on one side of focus and then getting stretched at 90 degrees on the other side of focus. For a point source like a star, it stretches to an oval on one side of focus and then to a new oval perpendicular to the first oval on the other side of focus.

Also, like many optical systems, your eye tends to show fewer aberrations when it is stopped down, that is when your pupil is smaller. I tend to observe without glasses and star images look much better when I use an eyepiece that gives me an exit pupil of 5mm or smaller.

Alan

Here is a thread I started four years ago: https://www.birdforum.net/showthread.php?t=285031

John

Thank you all for your replies. A very helpful forum and I am learning a bit more each time I log on.

Tom