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Hasselblad Focus Checks: Interesting Findings

chuckroast

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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 It was originally made for the SWC family of cameras that have no reflex viewing.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.
 
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.
 
itsdoable said:
Or maybe the infinity stop of
Click to expand...


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.
 
These focusing screens for the SWC can be vexing. I read somewhere that there are different fresnel lenses on these one for the swc and another for the flex body outfit. I have one that works with my swc, illumination falls off in corners. I've never really used it much.
 
chuckroast said:
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.
Click to expand...
I would drop Barn Owl a query about the infinity stop, it's not something that usually needs adjustment on these lenses, so they may have just done a quick out the window check during the CLA. My understanding is that the V lenses in these focal lengths had a proper infinity stop. Mine all do, but I don't have the 3.5/100 (yet).

FYI: checking focal plane (film plane to screen) is best done at a close focus distance, the deltas and differences are magnified there. Infinity can fall within the DoF of the screen.
 
chuckroast said:
On the 100mm maybe 1/32" on the 80mm less than that.
Click to expand...

Like you mention in the first post, I've also heard that on some longer lenses that infinity focus before the the infinity stop isn't unusual to accomodate for thermal differences. In recollection, it seems that mostly this was said about 35mm zooms especially. I never did that test on my Hasselblad lenses so don't know if mine perform like yours or not. Alll I know is that using infinity focus on them has resulted in fine negatives. Is this really a problem in your situation or more of an interesting intellectual pondering?
 

I am just curious at this point. My next test will be to repeat with all lenses to see if it's localized to the 100mm or something seen more widely.
 

Barn Owl has been unresponsive to emails, and others have reported much the same thing.
 
I did a bit more testing with multiple bodies and lenses today. Everything seems normal at infinity. The 100mm f/3.5 does seem to hit focus just the tiniest fraction before the infinity stop. It does agree with the ground glass, however.

HOWEVER, some of this may be difficulty in my being able to see the exact moment of "sharp" focus. My vision is generally fine, except the need for reading glasses. It is apparently this reading accommodation that is needed to see the focusing screen crisply. I don't think diopters will help this much. So have adjust around the point of focus to get as close as I can.
 
Not to be flippant. But is wide open aperture at infinity something we encounter? I can't tell without a 6x finder what's in focus at a distance. Stop down to f8, that's my process
 
mshchem said:
Not to be flippant. But is wide open aperture at infinity something we encounter? I can't tell without a 6x finder what's in focus at a distance. Stop down to f8, that's my process
Click to expand...

In practice, you're right. But I wanted to ensure proper mirror- and focusing screen tolerances and the most critical test is wide open.

Also, wide open is a thing with some lenses when shooting portraits or isolating something.
 
chuckroast said:
In practice, you're right. But I wanted to ensure proper mirror- and focusing screen tolerances and the most critical test is wide open.

Also, wide open is a thing with some lenses when shooting portraits or isolating something.
Click to expand...

Yes, I'm not doubting what you are doing. And just occurred to me that wide open is definitely a thing for portraits but not at "infinity"

My issues have been with that focusing screen attachment for swc, these are difficult for me due to the Fresnel lens. I would like a ordinary glass screen and a magnifying chimney finder. I'm not aware of a simple plain glass screen. I'm just as likely to trust the camera screen. As long as it hasn't been monkeyed with.

Not doubting your goal here.

Best Regards Mike
 

I used an 8x loupe with blacked out body - the sort of thing I use with a view camera - to see the back of the ground glass.

If i were actually checking composition not just focus, I'd have also used a dark cloth.
 
mshchem said:
Not to be flippant. But is wide open aperture at infinity something we encounter? I can't tell without a 6x finder what's in focus at a distance. Stop down to f8, that's my process
Click to expand...

With the 100mm 3.5, yes.

This was and for me still is the premier aerial photo lens and was widely known to be such in it's heyday. Astronaut Jay Apt used it for much of the work he did from space in the book "Orbit".

I also use this lens on my 102MP Hasselblad X2DII as well as my film bodies and it simply blows away every optic I have used in most formats.
 
People actually set the focus to infinity? Why?
 
...and someone in another thread wondered why one needs an autocollimator.
 
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