Focusing issue with only one lens? Mamiya 645 Super

[cerber0s]

I have a Mamiya 645 Super with a whole bunch of lenses. I seem to have an issue with the 55mm f2.8 where I feel like the actual focus is always closer to the camera than it should be. Other lenses seem fine. There is nothing about the 55mm that stands out otherwise, nothing seems broken. The focus can be dialed a bit past infinity, but the same goes for all the other lenses. Any ideas as to what could be going on here?

 

[MattKing]

Is the lens mount on the lens damaged, or loose, or re-assembled incorrectly?

 

[reddesert]

That is an odd problem, because you have a manual focus SLR. It's not unheard of for a lens in a rangefinder system to have a focus offset, and it can happen with some lenses in AF systems due to an interaction with the sensors, but in a manual focus SLR, you're focusing on the actual image made by the lens. If the body's groundglass or mirror were in the wrong place, it would happen with all of the lenses.

Possibilities might include that the lens is shifting a little internally as Matt suggested, or somewhat more exotically, that it has significant focus shift when stopped down to the taking aperture. Focus shift of this kind shouldn't be an issue on a modern SLR lens; if that is the case, then possibly the lens is assembled incorrectly (like incorrect element spacing). Or that there's some other issue and the link to the 55mm lens is a coincidence.

 

[koraks]

That is an odd problem, because you have a manual focus SLR.

Indeed.

The right course of action would be to:
1: determine with certainty that there actually is a problem. Take some of those tedious test shots of a ruler that runs away from the camera to see how much back/front focus you have with several lenses.
2: verify correct ground glass calibration using a piece of ground glass (or similar/equivalent) at the film gate, with the camera on a tripod and the back open so you can compare what the film 'sees' vs. what the ground glass shows.

The Mamiya 645 cameras are somewhat notorious for having the mirror go out of alignment. You notice this in the tests above as focus being spot on either in the top, middle or bottom of the frame, but not in the other two areas. This can also explain why you think you're seeing this with one lens only because you may have a habit of focusing on a particular part of the frame (bottom, middle or top) depending on what lens you use and what subject matter you have in front of you.

In any case, the first step is to verify what's really going on, objectively.

 

[cerber0s]

Is the lens mount on the lens damaged, or loose, or re-assembled incorrectly?

Not that I can see. It’s definitely not loose.

Indeed.

The right course of action would be to:
1: determine with certainty that there actually is a problem. Take some of those tedious test shots of a ruler that runs away from the camera to see how much back/front focus you have with several lenses.
2: verify correct ground glass calibration using a piece of ground glass (or similar/equivalent) at the film gate, with the camera on a tripod and the back open so you can compare what the film 'sees' vs. what the ground glass shows.

The Mamiya 645 cameras are somewhat notorious for having the mirror go out of alignment. You notice this in the tests above as focus being spot on either in the top, middle or bottom of the frame, but not in the other two areas. This can also explain why you think you're seeing this with one lens only because you may have a habit of focusing on a particular part of the frame (bottom, middle or top) depending on what lens you use and what subject matter you have in front of you.

In any case, the first step is to verify what's really going on, objectively.

Yes, I think I will have to sacrifice a few frames on a ruler.

The mirror stop wad replaced by one of two Mamiya service techs we have left here before I bought the camera. I checked it, and it’s there, but I can’t tell if it’s out of alignment of course. Not without those test shots at least.

 

[koraks]

I know the mirror on my 645 was pretty badly out of alignment when I got it. At some point I re-calibrated it; I think it's OK now. I rarely use that camera, but insofar as I do, it seems to be doing fine now.

 

[Mamiya_Repair]

I have a Mamiya 645 Super with a whole bunch of lenses. I seem to have an issue with the 55mm f2.8 where I feel like the actual focus is always closer to the camera than it should be. Other lenses seem fine. There is nothing about the 55mm that stands out otherwise, nothing seems broken. The focus can be dialed a bit past infinity, but the same goes for all the other lenses. Any ideas as to what could be going on here?

None of your lenses should be able to go past infinity. This is either a focus screen misalignment or a mirror stop adjustment (most likely). Easiest way to check is to focus on something at infinity and see if all is aligned in the finder. If not, the mirror needs to be checked for 45 degree angle. Checking lens and finder alignment is a trivial operation for a technician with the correct tools.

 

[cerber0s]

Indeed.

The right course of action would be to:
1: determine with certainty that there actually is a problem. Take some of those tedious test shots of a ruler that runs away from the camera to see how much back/front focus you have with several lenses.
2: verify correct ground glass calibration using a piece of ground glass (or similar/equivalent) at the film gate, with the camera on a tripod and the back open so you can compare what the film 'sees' vs. what the ground glass shows.

The Mamiya 645 cameras are somewhat notorious for having the mirror go out of alignment. You notice this in the tests above as focus being spot on either in the top, middle or bottom of the frame, but not in the other two areas. This can also explain why you think you're seeing this with one lens only because you may have a habit of focusing on a particular part of the frame (bottom, middle or top) depending on what lens you use and what subject matter you have in front of you.

In any case, the first step is to verify what's really going on, objectively.

I cut a piece of negative sleeve and stuck it onto the film plane, then focused the lens through the waist level viewfinder with the magnifier and checked the "ground glass" using a magnifying glass. Both the 55mm and the 80mm I compared it to focus perfectly on the film plane. The only, very horrible, conclusion I can draw is that I might need glasses. I do find the wide angle lenses pretty difficult to focus using the eye level viewfinder.

Thank you all for your help!

 

[JerseyDoug]

Which viewfinder screen are you using? I had a similar issue focusing a 50mm lens with my Hasselblad until I swapped the plain matte screen I prefer with longer lenses for the screen having a split image rangefinder spot.

 

[cerber0s]

Which viewfinder screen are you using? I had a similar issue focusing a 50mm lens with my Hasselblad until I swapped the plain matte screen I prefer with longer lenses for the screen having a split image rangefinder spot.

Mine has a split image spot.

 

[koraks]

The only, very horrible, conclusion I can draw is that I might need glasses.

Hehe, that one gets worse very fast I'm also finding out!

 

[loccdor]

I find the wide angle lenses much harder to accurately focus by eye. The wider they are, the worse it is.

The widest I have is a 14mm for 35mm format and I'm actually more accurate using the marks on the lens barrel (focus guessing) than using the SLR viewfinder.