RF50mm f1.8 - focus shift on higher f-stops?

[maboleth]

I've tried to test this lens with and without an UV filter, to see if there would be any degradation in IQ. Turned out there was none. But, I've found a much bigger problem.

Apparently my lens shifts focus at higher f-stops.

Both images are taken at f/4, R7 on a tripod, using 2 sec counter and electronic shutter.

The first one is taken with Exposure + DOF preview, which makes the aperture blades turned on. It's very sharp, as it should be.This one is using just Exposure simulation, without DOF. Focus is done without aperture. There's a visible loss of sharpness of the subject. The background is now more sharp - focus shift.

I've tested it several times and it's always there. The focus is great at 1.8, but the focal plane shifts when the aperture ring is used.

I've tested it even on a manual focus. I pre-focus at a certain spot, but as f-stop progresses (f2, 2.8, f4), the focal plane shifts to the background.

[ebiggs1]

I doubt the RF 50mm f1.8 (I don't have one) has any significant focus shift. However, all lenses show some focus shift at very close distances.

[p4pictures]

Where is the focus point? If you review the images on the back of your camera you can show the active AF points. You will find the option to shot the AF point in the playback menus, last tab I recall. It is important that it is in the same place. Probably best to turn whole area tracking off, and use one shot AF with subject to detect set to none.

[maboleth]

Well, it doesn't matter where is the focus point as this happens even on a manual focus.

So to recap, here what happens:

- Focus is on the subject. In this case, a round box with small letters. AF is disabled. Focus remains, the camera is on a tripod.
- At f1.8 the focus on the letters is super sharp.
- At f2.8 or f4, very small letters on the box can no longer be read, the focus has shifted to the back

- However, if the camera's AF is turned on, as well as Exp. Sim + DOF option, the focus will be tack sharp, regardless of the f-stop used. The reason is, camera is now focusing through the active aperture ring that is always on in this mode.

The problem - Exp. Simulation + DOF is sluggish to use, camera's EVF and LV is noticeably slower when the objects or camera move.

I tested my EF 50mm F1.4 as well as Sigma RF 18-50 f2.8 and none had this.
My experience with EF 50mm f1.4 actually made me realize this effect on the RF. Because in the EF version, the more you crank the f-stop, the sharper your subject is - at any given distance.

If anyone can test this on their RF 50 f1.8 I'd be more than grateful.

[Peter]

I can't use AF and aperture stopped down with my R6, even if I pick exp. simulation + DOF. Would be nice to get rid of the focus shift my EF50/1.2 gives me at f/2.8.

[maboleth]

Hm, strange. I'm not aware if this option is different in other R cameras. On R7, the aperture ring is basically ever-present in E. Sim + DOF. But it does come with a cost, as I pointed out. If there weren't any, I would just use that option and be done with it.

[maboleth · Accepted solution]

Ok, so I tested this lens again and there's a certain pattern of the focus-shift.

From 1.8 - 2.2 - focus is the same or getting better.
From 2.5 - 2.8 there's a slight shift and degradation in sharpness of the subject.
From 3.2 - 4 there's a visible degradation in sharpness and focus shifts towards background. This is only evident when letters and small details are focused.
From 5.6 onward, focus is restored and back to the subject.

So, if someone needs critical, pixel-peeping sharpness of the subject from f3.2 to f4, it's best to use Exp. Simulation + DOF, if the camera permits full-time Aperture ring usage, as R7 does.

[justadude]

I tested mine on my R6 Mk ll today after reading this.  I followed the same steps as you describe with both MF and AF.  I also placed some objects on a table about 6' away from the camera, and propped up a book about 8" behind the objects.   I don't see any shift after uploading the images and zooming in on each of them.  Maybe your subject was a lot closer to the camera than mine?

[maboleth]

Thanks for trying to replicate this phenomenon.

Well, distance from the objects was about 2.5m. Noticeable focus shift from f2.8 to f4 at 100% zoom.
Maybe this is more pronounced on a crop sensor, since it's extracting even more resolution from the lens.

Anyhow, Bryan from The Digital Picture confirmed my findings, commenting "Based on that comparison, my lens behaved as you experienced. Canon should be electronically adjusting focus to avoid this problem".

Also, just to be sure, the Firmware of my RF50mm lens is v1.0.

[ebiggs1]

"Maybe this is more pronounced on a crop sensor, since it's extracting even more resolution from the lens."

No that's not the problem. FF cameras generally have more resolution than crop sensor cameras