Swirl Bokeh on Nikon Fieldscope EDG85 (1 Viewer)

Hio all,

I took this picture without realizing the "Swirl Bokeh" until pointed out by friend when I uploaded it to my Facebook page.

Is it a good or bad sign of my scope ? :h?:

Digiscoped with Nikon Fieldscope EDG85 via FSA-L2 adapter on Nikon D300s



By the way, I did a star test early this morning and found out the scope has convincing good rings when infocus and defocus. I didn't take any pictures but I will try it again soon so the experts here could determined if my scope is good or not

Thanks :king:

The scope has spherical aberration outside of focus (both close and far) and gives this "Swirly Bokeh" effect. It's more pronounced at the top and bottom of the image where it's out of focus rather than the in-focus band across the middle.

I didn't realize the camera folks had a phrase for it (and even seem to like it).

As to good or bad ... it's just the design. And with a lot of spotting scopes your not so interested in optical performance out of focus is not a huge consideration for some designers. I guess they reason that you are going to focus on the target so out of focus aberrations are not a big issue. It's another point where camera optics and spotting scope optics design diverge.

I'll leave Henry (or similar) to make a more coherent comment about the "goodness" of SA out of focus but I know I find it a bit annoying in binoculars but perhaps less so in a scope.

The narrow depth of field of the 85mm scope and target choice exagerates this effect. I assume your focus point was at fairly close range?

I have a Nikor 135mm F2 DC lens that allows you control the SA of the lens to blur the background or foreground for a more pleasing bokeh. It can look very similar if I am not careful with choosing the background.

I'd never heard of this before, so naturally I Googled it. This photo from a page of examples of swirl bokeh shows that the swirl comes from vignetting, not SA, in this lens.

http://www.flickr.com/photos/17958048@N00/3183278034

There is SA present. It causes each disc of out of focus light to have a bright outer ring, but SA would not cause the radial pattern of "cats eyes" that get progressively narrower toward the edge of the field. If SA were the only aberration present each of the discs would be perfectly circular and there wouldn't be any swirl bokeh. All spotting scopes have some vignetting, just like binoculars. Off-axis astigmatism could also cause a radial pattern of out of focus ovals instead of circles, but in that case the ovals would switch their axes by 90 degrees depending on whether they are in front or behind the focal plane.

henry link said:

I'd never heard of this before, so naturally I Googled it. This photo from a page of examples of swirl bokeh shows that the swirl comes from vignetting, not SA, in this lens.

http://www.flickr.com/photos/17958048@N00/3183278034

There is SA present. It causes each disc of out of focus light to have a bright outer ring, but SA would not cause the radial pattern of "cats eyes" that get progressively narrower toward the edge of the field. If SA were the only aberration present each of the discs would be perfectly circular and there wouldn't be any swirl bokeh. All spotting scopes have some vignetting, just like binoculars. Off-axis astigmatism could also cause a radial pattern of out of focus ovals instead of circles, but in that case the ovals would switch their axes by 90 degrees depending on whether they are in front or behind the focal plane.

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Henry,
your a boffin and a very interesting one at that!
fiddler.

Thanks fiddler. I had to Google that. According to Wikipedia the closest American equivalent is "egghead". Personally, I prefer "nerd". ;-)

Henry

henry link said:

this lens

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"this lens" (if you can find one - they are a few years old and no longer manufactured) will cost you about 5000USD ... the latest version comes in at 10-12K USD ... just in case you fancied taking swirly bokeh photographs ...

RJM said:

The narrow depth of field of the 85mm scope and target choice exagerates this effect. I assume your focus point was at fairly close range?

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Yes Rick quite close.

henry link said:

There is SA present. It causes each disc of out of focus light to have a bright outer ring, but SA would not cause the radial pattern of "cats eyes" that get progressively narrower toward the edge of the field. If SA were the only aberration present each of the discs would be perfectly circular and there wouldn't be any swirl bokeh. All spotting scopes have some vignetting, just like binoculars. Off-axis astigmatism could also cause a radial pattern of out of focus ovals instead of circles, but in that case the ovals would switch their axes by 90 degrees depending on whether they are in front or behind the focal plane.

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Will it affect the result if I do star testing with the scope Henry ?

And how do you get pictures of the ring while doing star test in the evening ?

Attached is another picture showing the bokeh. This is about 80m I guessed

I don't find the bokeh formed by extended objects (like a field of grass or tree branches) very easy to use for analyzing aberrations, at least I can't tell much from the two photos. The dog photo I linked to happened to have many unfocused point sources of light, so it was more informative.

I would suggest two kinds of "star" test photos. First, two made at the highest possible magnification of a centered star. It could be an artificial star like a glitter point of sunlight from a distant small round shiny object like a water droplet, light bulb, chrome on an car, etc. Unfocus the star slightly in both directions so that you see 3-4 diffraction rings and photograph those (underexposure is usually needed to make the ring details visible). Enlarge to the size necessary to make the rings easily visible. I should mention that you may see rings in only one direction of defocus. The other side of focus may just look like a fuzzy ball. If the air is steady enough those photos will hopefully show the axial aberrations of the scope.

For the second photo type use the lowest magnification possible and place the "star" toward one corner of the photo. Unfocus the star in both directions until it is a large disc about the size of the discs in the dog photo. These photos will reveal any vignetting (cats eye shapes). Off-axis astigmatism would be shown by the same kind of photo but defocused to only 4-3 rings.

The pattern of the bokeh in your photos could be from vignetting, either from the scope prism or the photo adapter, similar to the Leica lens in the dog photo. The star test photos should confirm that, or maybe not.

Horukuru,
I think it is an effect of the FSA-L2 at lower magnification. Were the photos taken in the lower mag position? If so, try it over 750mm.
I noticed that FSA-L2 has a reduced depth of field and finer focus is very important.
I'm waiting for your test comparing your telescope to a Kowa 88...

DRodrigues said:

Horukuru,
I think it is an effect of the FSA-L2 at lower magnification. Were the photos taken in the lower mag position? If so, try it over 750mm.
I noticed that FSA-L2 has a reduced depth of field and finer focus is very important.
I'm waiting for your test comparing your telescope to a Kowa 88...

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I will do the star test soon from the tips given by henry and regarding Nikon EDG85 vs Kowa 883 which I have briefly tested last year, Kowa is a bit brighter and sharpness is similar but in regards with contrast and FOV, Nikon is better

DRodrigues said:

I think it is an effect of the FSA-L2 at lower magnification. Were the photos taken in the lower mag position? If so, try it over 750mm.

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This could be true. I noticed that Nikon warns of "vignetting" below 750mm with FX format cameras. That implies that there is no vignetting with DX format cameras, but I imagine their definition of vignetting would be completely blacked out corners of the frame. The kind of vignetting that would cause swirl bokeh wouldn't be that severe. In the dog photo with swirl bokeh made with the Leica lens the vignetting gradually reduces the effective area of the lens aperture until the brightness at the edge of the frame is down about one f-stop.

This is another picture showing the Swirl Bokeh at maximum 1750 mm (DX camera = 2625mm) on ISO 1600

Any comments ?

Large Green Pigeon

Hurukuru,

I can see many point sources of light formed by bright sky light shining through tiny openings between leaves. The points are defocused so that they appear as bright rings with "hollowed out" dark centers. That particular pattern for a defocussed point of light beyond the plane of best focus (on the bird) indicates spherical under-correction (spherical aberration). That's common in spotting scopes, although a few scopes have virtually none. Where you see the rings flatten out toward the corners (the swirl bokeh) indicates vignetting, possibly mixed with some astigmatism. The vignetting would come from an internal aperture in the scope, like the prism aperture, or the camera adapter or both.

Henry

Thanks a lot Henry