Hasselblad Focus Checks: Interesting Findings

[chuckroast]

Procedure

I got one of those Hasselblad #41025 focus screen adapters. It was originally made for the SWC family of cameras that have no reflex viewing. Basically, it is a ground glass assembly that attaches like a film back and lets you view the composition and focus at the film plane.

I wanted this to check focus accuracy of a couple bodies and several screens I own. And I did just that. I checked with several Hasselblad standard screens and an Oleson Bright Screen on a 500C/M and a 100mm f/3.5 lens. I also checked with a 501C/M body with an Acute-Matte screen with a 80mm f/2.8 lens.

"Checking" meant focusing the camera at two distances through the finder - either a waist level or chimney finder on the 500C/M or a PM5 on the 501C/M - about 30 feet and at infinity. This was done on a tripod for consistency.

At each distance, once focused through the finder, the cameras was fired on B with a locking shutter release to keep the lens open, at max aperture. I then used an 8X loupe on the ground glass of the focus screen adapter. This was to check for agreement at the plane of focus with what had been set via the reflex viewing.

Results

There were a number of interesting results:

1. No matter what screen, finder, or body I used, there was good agreement between the reflex focusing and what showed up on the ground glass. This tells me that the camera mirror positions and screens are in proper position.
   
   
2. BUT, with the 100mm lens, infinity focus was achieved just a smidge before the lens hit the infinity stop. With the 80mm lens it was almost exactly at the infinity stop, perhaps just a hair before.

2. isn't of great concern because of 1. If the viewed focus and the plane of focus are in agreement, a slight difference between true infinity focus and the lens infinity stop should be a non issue.

But I am curious as to why this occurs. A very brief bit of poking around suggests that this isn't unusual and may even be intentional to leave room for mechanical tolerances among bodies and lenses and/or thermal contraction and expansion.

If anyone has insight into this, I'd be interested to hear them.

 

[itsdoable]

Or maybe the infinity stop of the 100mm got knocked out during a service? The infinity stop may have been adjusted to match a specific body. Or digital back. There was a time when Hasselblad offered and adjustment service so the digital back matched the screen.

I have a Britescreen which back focuses, it sits just a tiny bit too high on the body screen stops. Hard to tell on the 2.8/80, but pretty clear on the 2/110.

 

[chuckroast]

Or maybe the infinity stop of

Or maybe the infinity stop of the 100mm got knocked out during a service? The infinity stop may have been adjusted to match a specific body. Or digital back. There was a time when Hasselblad offered and adjustment service so the digital back matched the screen.

I have a Britescreen which back focuses, it sits just a tiny bit too high on the body screen stops. Hard to tell on the 2.8/80, but pretty clear on the 2/110.

The 100mm was just CLAed by Barn Owl within the last year. The 80mm is factory original from about 20 years ago. If anything out it should have been that lens.

Like I said, the fact the reflex viewing agreed with the ground glass makes me believe the systems focus properly and this is just an anomaly. I may also try switching lenses to the bodies to see if the observed behavior stays with the body or moves with the lens.

 

[BrianShaw]

How far away was infinity?

 

[chuckroast]

How far away was infinity?

On the 100mm maybe 1/32" on the 80mm less than that.