URSA Mini Pro sharpening focus

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bradleyjconomy

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URSA Mini Pro sharpening focus

PostSun Aug 19, 2018 2:26 pm

Hi, not sure if this has been discussed before, but I was shooting some charts with my URSA Mini Pro and have noticed that when shooting at f2 there is a magenta cast on all sharp detail.

85mm Chart_2.0.jpg
f2 85mm Sigma Art
85mm Chart_2.0.jpg (519.12 KiB) Viewed 413 times


When I stop down to f4 it is almost gone, completely gone at 5.6 and then when at f16 it isn't there.

85mm Chart_16.0.jpg
f16 85mm Sigma Art
85mm Chart_16.0.jpg (540.6 KiB) Viewed 413 times


I suspected it might have to do with the ND filters however I reshot the charts without ND, instead adjusting the light levels to keep a constant f2 but the issue is still there.

I have tested with 18-35mm, 50mm and 85mm Sigma ART lenses.

I have been shooting with the URSA for around a year now and I haven't noticed it until shooting these charts. Is this a common issue? I can now notice it in some very detailed fabrics.

Any wisdom is appreciated.
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Uli Plank

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Re: URSA Mini Pro sharpening focus

PostSun Aug 19, 2018 3:35 pm

Color moiré, I suppose.
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Denny Smith

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Re: URSA Mini Pro sharpening focus

PostSun Aug 19, 2018 4:33 pm

Looks like moiré to me also, as it is only in the one detail area of the chart.
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Uli Plank

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Re: URSA Mini Pro sharpening focus

PostSun Aug 19, 2018 4:36 pm

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rick.lang

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URSA Mini Pro sharpening focus

PostSun Aug 19, 2018 5:52 pm

But could it also be a poorly adjusted copy of the Sigma lens showing longitudinal chromatic aberration wide open that is reduced as you stop down. I’d possibly return that lens.

Can you magnify the image to see any evidence of purple fringing colour on the solid black areas?


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Last edited by rick.lang on Sun Aug 19, 2018 6:26 pm, edited 2 times in total.
Rick Lang
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Robert Betzner

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Re: URSA Mini Pro sharpening focus

PostSun Aug 19, 2018 5:58 pm

No that's not moiré. I think that is CA. If you shoot wide open lenses are much more sensitive to CA problems. Do a search for Cromatic Aberation.

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Robert
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Aaron Green

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Re: URSA Mini Pro sharpening focus

PostSun Aug 19, 2018 6:42 pm

Do you have another EF mount camera that you can test the same lens?
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Uli Plank

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Re: URSA Mini Pro sharpening focus

PostMon Aug 20, 2018 12:04 am

Three lenses from the highly praised Sigma Art line with that much CA? I don't think so.

Try them on another camera.
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Robert Betzner

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Re: URSA Mini Pro sharpening focus

PostMon Aug 20, 2018 8:15 am

Uli Plank wrote:Three lenses from the highly praised Sigma Art line with that much CA? I don't think so.

Try them on another camera.


Could be a problem with that specific lens. Maybe a faulty one?

If you zoom into the picture you can see that the purple lines are all on one side. That is something that points clearly to a CA issue. The odd thing is that there is almost no green cast on the other side which would be the case if it is CA.

Strange

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Robert
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Uli Plank

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Re: URSA Mini Pro sharpening focus

PostMon Aug 20, 2018 10:35 am

He wrote: "I have tested with 18-35mm, 50mm and 85mm Sigma ART lenses." All three faulty?

But OTOH, moiré should get worse when stopping down (until we get into diffraction).

So, could it be the camera?
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Denny Smith

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Re: URSA Mini Pro sharpening focus

PostMon Aug 20, 2018 4:06 pm

Good point Uli, that eliminates the moiré theory...
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Denny Smith
SHA Productions

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