Blackmagic Forum

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.



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



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.

Color moiré, I suppose.

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

Have a look at this: http://rawlite.com/olpf-for-blackmagic- ... -pro-4-6k/

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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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.

Cheers

Robert

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

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

Try them on another camera.

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

Cheers

Robert

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?

Good point Uli, that eliminates the moiré theory...
Cheers