Artifacts from RF15-35mm F2.8 L IS USM

[JoshZielinski]

Hello,

For a while now, I've had bad artifacts from my RF15-35mm F2.8 L IS USM when photographing scenes in which light meets a sharp edge beyond which the scene is dark. A recent, particularly severe example is attached from underneath a recess cave. I overexposed the image so you can see how the artifact obscures the texture of the sandstone nearest to the edge. I've encountered this effect on many other similar scenes. It frequently renders the images essentially unusable due to the amount of work it would take make the exposure appear natural.

As far as I can tell, the lens is clean on the front and back, so I'm not sure what would be causing this. Any thoughts?

[JoshZielinski]

I do have a lens hood on the lens, and I am doing lens correction before merging as HDR in Adobe Lightroom. The affect is the same even without lens correction applied.

I will have to look into the Canon DPP software for ghosting correction.

[JoshZielinski]

Here is another image from the same outing. Hopefully this might help. I merged one of my brackets of five exposures and briefly edited the shadows, highlights, etc. so things appear balanced. See the irregular bright blotches along the rim of the recess cave? Exposure info:
Various shutter speeds for the five exposures between 2 seconds and 30 seconds.
f/16
ISO 100

I had a lens hood on and I believe the sun had just set by this point, so the light source is only the dim, very diffuse light visible through the cloud cover.

[JoshZielinski]

Ok! I do wonder if small aperture diffraction blur could be the problem. My understanding of diffraction blur is it more so creates a loss of detail, not a lighting/discoloration artifact, but my understanding of it could well be incomplete.

A lens hood designed specifically for 15mm is an interesting idea. That could certainly help. I regret not seeing if the affect is the same at a tighter focal length, like 35mm, so I will have to test that in the future.

[Waddizzle]

I cannot think of any valid reason to use such a small aperture or a 30 second exposure with a wide angle lens. 

 

[ebiggs1]

"I had no ND filter on the lens."

Do you have any filter attached to the lens if so remove it.

[ebiggs1]

It looks like moiré. Moiré happens when a high resolution, repetitive pattern interferes with the sensor creating the unwanted, wavy, and rainbow looking patterns. It could be as easy as changing the angle of the shot. Lightroom has a moiré slider to correct for this very easy to do.

[JoshZielinski]

No, there are no attachments on the lens besides the lens hood.

[JoshZielinski]

Ok! I could see that potentially being the issue. I wasn’t aware of a moiré slider in Lightroom, at least not in Classic. I’ll look into it. Maybe my usage of a tight aperture is expounding such an affect.

[JoshZielinski]

The small aperture is perhaps unwarranted. It is habit from shooting often with my 24mm TS-E and other lenses in which I achieve optimal sharpness throughout the image at ~f/16, or at least I perceive it that way. It’s possible the affect wouldn’t be as bad at a wider aperture. Something for me to test.

There’s certainly valid reasons for both (close foreground, astrophotography), but I agree it may be overkill with my specific scenario here.

[ebiggs1]

"Maybe my usage of a tight aperture is expounding such an affect."

I rather doubt it. 

"I cannot think of any valid reason to use such a small aperture ..."

Don't know where or why he is going with that. Perhaps he will expand on it. I would ignore it.

But small apertures can cause diffraction.  And diffraction can soften the image and reduce the IQ. I doubt it caused your issue. Diffraction is worse with shorter FL than with longer tele lenses because the aperture is smaller in shorter lenses. This is easy enough to prove shoot it again with different apertures and see which you like the most. You really need to get into the habit of doing that on shoots anyway.