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]

Ok! I appreciate the input. I'll definitely make an effort to revisit that spot or a similar one to play around more with this lens and various apertures, ideally when I have less of a time/lighting constraint. That seems to be the best next step for troubleshooting. I have a 20mm prime lens I've also thought might be a good comparison.

[JoshZielinski]

Here are two final recent examples. The affect is less pronounced in the first one compared to the example on my original post, but I've circled where there is a bright area that appears unusual. The recess cave did not appear that way in person. In the second one, I have the camera pointed away from the diffuse light. The unfavorable affect is not present. I'm not sure if these examples might inspire any other ideas. I'll also mention I photograph similar scenes occasionally with a TS-E 17mm and have never encountered this affect.

First photo: Shutter 1/10 to 25 sec, f/10, ISO 100
Second photo: Shutter 1/5 to 30 sec, f/10, ISO 100

 

  

[Waddizzle]

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

The DOF table speaks for itself. He could have used f/8.  I picked 24mm because it’s the center of the zoom range.  At 16mm, he probably could get away with f:4.

As for the artifacts, I would want to see an image that has [not] been processed by HDR software. 

[johnrmoyer]

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@JoshZielinski wrote:

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.

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Small aperture diffraction blur is not the cause of your problem, but is a reason to favor a larger aperture.

My best guess is that the cause of your problem is flare and ghosting. With some lenses, changing aperture from F/16 to F/11 will reduce flaring. Reducing the aperture might be worth trying.

Small aperture diffraction blur is a function of F Number and spacing of photosites on the sensor. So with a higher resolution camera like the EOS R5 or with an APS-C camera, the blur will become noticeable at a smaller F Number. For a medium format sensor with the same number of pixels, the blur will not show up until the F Number is larger. Richardson/Lucy deconvolution was invented originally to deal with the problem in images from telescopes and is now used with images from microscopes as well.

I suggest that if you are using a remote trigger for the shutter, hold something to the side of the camera with the most light just out of the image to block that light from reaching the lens at an angle where it should not be visible.

https://petapixel.com/2019/06/17/canon-this-is-why-rf-lenses-are-outstanding/ 

There are also downsides to having a short back focus, though. One of the big ones is that it can introduce more flaring and ghosting as reflected light within the lens hit the sensor. Canon’s solution is its Air Sphere Coating (ASC) and Subwavelength Structure Coating (SSC) technologies to minimize reflected light (and therefore flaring and ghosting).

https://www.the-digital-picture.com/Reviews/Canon-RF-15-35mm-F2.8-L-IS-USM-Lens.aspx 

On this lens, Canon's SWC (Subwavelength Structure Coating) along with ASC (Air Sphere Coating) are utilized to avoid flare and ghosting. Our standard flare test utilizes the sun in the corner of the frame (at 15mm, it is hard to avoid the sun or other bright light in the frame at times). At f/2.8, this test produces barely perceptible amounts of flaring effects. This test generally produces stronger flare effects at narrower apertures and some effects are showing in this lens's f/16 results but the amounts remain mild.

[JoshZielinski]

An example not processed by any software would be the image included in my original post. That photo is the final in a series of five bracketed exposures. It is overexposed by four stops, thus showing the artifact more clearly than a 'properly' or underexposed image.

[JoshZielinski]

Thanks for the information, John. I'll definitely be trying larger apertures in the future when I'm at a similar location again to see I get a similar or reduced affect. I've been aware of smaller apertures causing diffraction, but my understanding is not near as in-depth as it could be, so I appreciate the additional info regarding it as well.

[Waddizzle]

“ An example not processed by any software would be the image included in my original post. That photo is the final in a series of five bracketed exposures. It is overexposed by four stops, thus showing the artifact more clearly than a 'properly' or underexposed image. “

My mistake. I thought you had indicated that it was an HDR image. 

I assume that you’re shooting RAW, not JPG.  JPG compression algorithms can do weird things. 

[JoshZielinski]

No worries. Yep, shooting RAW. I did compress the uploaded JPG file a bit, but I assure you the artifact appears virtually the same as in the RAW file.

[ebiggs1]

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

The DOF table speaks for itself. He could have used f/8."

Simple solution, try it. I doubt it will make any difference but who knows. In odd or challenging situations always try different settings.

[JoshZielinski]

Will do!

Thanks all for the ideas.