[Brightcolours]

Makes me want to try this now, although a bit late at the moment! Thinking more I may have been a bit quick to blame a "bad lens", although in the use I've had so far from my 50 Art I hadn't come across anything like that. The nearest is some effect in the corners, but I've not tried with high contrast (very bright highlights) type shots so that might expose weaknesses I haven't seen yet.

 

I probably should get more sleep...

 

Edit: added the test shots I did earlier. No diamonds here, but as said these are lower contrast. And relative defocus may play a part in it?

You don't get astigmatism shapes when you are that defocussed. Test for astigmatism with in focus or nearly in focus light points at borders.

[popo]

You don't get astigmatism shapes when you are that defocussed. Test for astigmatism with in focus or nearly in focus light points at borders.

 

Sigma 50mm f/1.4 A test by Crestie Crazy, on Flickr

 

Here's one of my first tests when I got the lens. You will probably want to fight through flickr to get the full resolution to pixel peep. A starfield is one of the worst case scenarios when in focus, and there's not a lot going on even at f/1.4 until you pass roughly the APS-C circle, and even at full frame corner there isn't that much. Note I used a static mount for this test shot so a tiny amount of trailing may be seen and should be disregarded.

[JJ_SO]

Yesterday evening I went tot the sprinklers again and tried 4 lenses at f/1.4.

 

Depending on my position, direction of light, background brightness and shutterspeed, the diamond shapes were visible at each lens more or less and always only in the corners, if there was a dark background. It appears to be a connection between drop direction and shutterspeed. I wasn't trying to be more scientific than the thread-opener who remains silent about the shutterspeed he used.

 

But his conclusion about "lousy lens" and ugly diamond-shaped bokeh" says more about his qualities as "lens-tester" than that of the lens in question. Although in YouTube are hundreds of other "lens-testers" who feel the urge to steal other people's time with their "observations" and general blahblah which talk more rubbish.

 

But anyway, I need to thank joze, because now I got some nice ideas with sprinklers and also some shots with rainbows in. And drops, flying through other drops. Quite interesting this high-speed stuff

[joze]

JoJu,

 

The shutter speed of the intial posted samples was 1/1600.

I believe that the diamond shape has nothing to do with shutter speed though. The diamond orientation rotates wrt. the center of the image (approximately) and I can't imagine how this should be related to shutter speed or the speed and direction of the drops.

 

I also did some further tests with different lenses. Interestingly, I also found that a different f1.4 lens shows simliar results like the sigma lens, both in terms of diamond-shaped bokeh in the field and magenta and greenish colors of drops in the center. Unfortunately I didn't find any good explanation (in the posts of this thread) for the diamonds. I wonder if it's just the shape of heavily vignetted ouf-of-focus spots or if the refractive power of the drops has something to do with it.

 

Attached is, what I'd have expected as "usable" bokeh btw. -- 70mm f2.8 1/4000. There's a little vignetting but apart from this it's quite usable.

 



 

 

[JJ_SO]

two full f-stops slower aperture (close the Sigma to f/2.8...), +20mm FL, 2.5 times faster shutterspeed - you really think, this is comparable?

 

I wait for your picture of an f/1.4 lens which doesn't show the rhomboids in the corner - if the shutter speed remains comparatively slow at 1/1600.

 

I see you're based in Aalen, so German wikipedia might be easier to read about the Rolling-Shutter-Effekt. I'm open to new suggestions but I've the feeling this is kind of the right track.

[Scythels]

Sorry, I'm very late.

 

The diamond shaped reproduction is likely a result of pinched optics, basically a lens being held in place too tight and causing a physical change in the element. 

 

re: "You don't get astigmatism shapes when you are that defocussed"

 

Astigmatism is present at all focus depths, onion ring bokeh is one manifestation of astigmatism.

[davidmanze]

Sorry, I'm very late.

 

The diamond shaped reproduction is likely a result of pinched optics, basically a lens being held in place too tight and causing a physical change in the element. 

 

Could you please expand on this thesis?  

   

      I guess it's Sigma's new superheated steam system of AF causing stress in the lens barrel  with the different co-efficients of expansion between different materials, Sigma were obviously aware of the problem and  took many measures to alleviate it, hence the low dilatation plastics.

   As far as I'm aware they used technology from the famous Doble steam car of the late 1920s which was noted for it's rapidly heating boiler system with record-breaking  point to point speeds coupled with an  extraordinarily silent drive engine.

    In modern day times it's so easy to lose sight of our technological roots and shun some of man's greatest achievements,  it's good to see that Sigma is still paying tremendous respects to our forebears here, so it seems somewhat churlish to be "carping" about very minor shortcomings  which can stem from holding up such high traditional principles.  After all diamonds are forever!

[JJ_SO]

Interesting.

 

Joze, did you squeeze the poor Sigma? Sat on it? Stood on it, you nasty boy? I didn't, but I was going to think it could be a low tide of the drop's inner structure, caused by one of the moons of Jupiter. As explanation not much more fantastically than squeezing a glass lens.

 

I saw the effect only on the outer sides of the fountain. Therefore my guess about speed. It turned out, one of the 1/8000 shots showed the effect, too, just much less than the longer shuttertimes. Also, the longer the FL, the more likely those shapes become visible.

[Brightcolours]

Sorry, I'm very late.

 

The diamond shaped reproduction is likely a result of pinched optics, basically a lens being held in place too tight and causing a physical change in the element. 

 

re: "You don't get astigmatism shapes when you are that defocussed"

 

Astigmatism is present at all focus depths, onion ring bokeh is one manifestation of astigmatism.

Onion ring bokeh is just a sign that an aspherical element is not totally smooth... "production" marks.

And pinched optics?? Really? No.

[Klaus]

Makes me want to try this now, although a bit late at the moment! Thinking more I may have been a bit quick to blame a "bad lens", although in the use I've had so far from my 50 Art I hadn't come across anything like that. The nearest is some effect in the corners, but I've not tried with high contrast (very bright highlights) type shots so that might expose weaknesses I haven't seen yet.

 

I probably should get more sleep...

 

Edit: added the test shots I did earlier. No diamonds here, but as said these are lower contrast. And relative defocus may play a part in it?

 

I think you mixed up the image name - the greenish ones show the background highlights.

The purple ones the front highlights.