Focus shift with 40mm, 60mm + 90mm, and how to compensate for it

[epforever]

A helpful note for any Alpa users working with HPF rings on the Rodenstock 40mm f/4 HR, the Schneider 60mm f/5.6 XL and/or the Schneider 90mm f/4.5 Apo-Digitar: I got curious about whether these lenses exhibit focus shift. The short answer is, 40mm and 60mm, yes; 90mm, no.

I shot some tests at a distance of roughly 1.8m to the LensAlign chart, in order to see small differences in focus placement. (The minute adjustments I suggest below would probably make very little difference when focusing at great distances.) Here's what I found and what I'm doing about it:

40mm:
Focus moves slightly farther away as you stop down from f/4 > f/5.6 > f/8. Since I figure I'm shooting at f/8 whenever possible with this lens, I adjusted my HPF so that focus is dead-on at that aperture (using a Leica Disto D5 for measuring distance to the LensAlign). Therefore at f/5.6 and f/4, I'll be slightly front-focused.
Compensation determined through brief trial and error:
To shoot at f/5.6, focus 1 degree farther on the HPF ring than your Disto reading. To shoot at f/4, focus 1.5 degrees farther than your Disto reading.

60mm:
Again, focus moves slightly farther away as you stop down from f/5.6 > f/8 > f/11. Since I figure I'm shooting at f/11 whenever possible with this lens, I adjusted my HPF so that focus is dead-on at that aperture (using a Leica Disto D5 for measuring distance to the LensAlign). Therefore at f/8 and f/5.6, I'll be slightly front-focused.
Compensation determined through brief trial and error:
To shoot at f/8, focus 0.5 degrees farther on the HPF ring than your Disto reading. To shoot at f/5.6, focus 1 degree farther than your Disto reading.

90mm:
No focus shift.

 

[vjbelle]

Testing for focus shift is extremely easy with a camera that has peaking as the peaking will move along with any shift in focus.

I haven't used my disto for photography in years and I don't use HPF rings as I focus on the LCD with a loupe which is very accurate since the 3100.

I have tested every one of my Schneider lenses for focus shift (35, 60, 72, 100, 120, 150, 180) and they don't budge when focusing on a LensAlign. I've also found that focus shift is inherently a lens design issue so I am curious about your findings regarding the 60mm.

The only lens I own that has shift is the Rody 90 W/SW. The only real way to work around it is to focus at working aperture unless the shift is minimal enough as to not negatively influence the image.

Since you have found shifting with your Rody lenses I suspect that all of the Rody lenses have some shift issues due to their design.

Victor

 

[epforever]

Testing for focus shift is extremely easy with a camera that has peaking as the peaking will move along with any shift in focus.

I haven't used my disto for photography in years and I don't use HPF rings as I focus on the LCD with a loupe which is very accurate since the 3100.

Yep -- I'm aware that focusing and managing focus shift is easier with a 3100, an H6D-100c, etc. My fuel costs would also be lower if just simply bought a Porsche Cayenne hybrid instead of my current SUV. :grin:

That being said, I find focusing with the Disto and HPF rings to be very fast and accurate.

Regarding my findings with the SK 60mm, as I mentioned, the focus shift is subtle. Take a look at the three attached images, each a 100% crop of my recent focus-shift test. The first is at f/5.6, the second at f/8, the third at f/11. I didn't change the focus in any of these. In the first two, you can see that the point of focus is slightly in front of the "0" markings. For example, the foreground "20" in the middle column of numbers is sharper than the background "20." Again, the focus shift is very slight, but it's there.

 

[vjbelle]

Regarding my findings with the SK 60mm, as I mentioned, the focus shift is subtle. Take a look at the three attached images, each a 100% crop of my recent focus-shift test. The first is at f/5.6, the second at f/8, the third at f/11. I didn't change the focus in any of these. In the first two, you can see that the point of focus is slightly in front of the "0" markings. For example, the foreground "20" in the middle column of numbers is sharper than the background "20." Again, the focus shift is very slight, but it's there.

It's just negligeable from MPOV. I would never concern myself with that subtle/almost indistinguishable focus shift.

Now the Rody 90 is a very different story. It 'Really' does shift and any owner of that lens should be aware of that characteristic. I highly recommend using the 'Working' aperture for focusing or at least one stop lower than 'Working Aperture' for focus and then shoot at working aperture.

FWIW when I focus with the 90mm at f5.6 and then stop down to f11 the focus target doesn't suffer..... it's just that the foreground vs. background changes significantly with the foreground shifting to the far almost to the edge of the target. Sometimes it's not all that important but any user should be aware of this.

[Regardless of this anomaly the Rody 90 S/WS lens is an outstanding lens and should be on the short list of any potential purchaser]

Regards......

Victor

 

[dchew]

I am going to split the difference and agree with both of you. When testing lenses, squareness, alignment, etc., I think focus shift comes into play. If you are sending images to Jim Kasson, you better pay attention!
:toocool:

But, when outside I just don't worry about it, whether using the HPF rings or focusing wide open then stopping down to shoot.

Dave