Fixing back focus on J8

[Igor.Burshteyn]

From your experience - what makes for good shims if you want to fix J8 7-10cm back focus (wide open, min distance)? I opened the lens - there is the one 1mm shim inside, aluminium I think. Should I add another 1mm? How do you prepare your shims for J8?

 

[pshinkaw]

I use paper shims on the "lens mount" on the body only. If the lens needs shimming I don't use it. They make great magnifiers when reversed.

Shim the flange to film plane distance at 28.8 millimeters +/- 0.02 millimeters. If that's not good enough, the lens isn't good enough.

-Paul

 

[TVphotog]

You can make shims if you wish to collimate a lens out of just about anything. At the Kiev Survival Site, he makes his out of aluminum foil and a variety of materials. If you are going to go through all the hassle of collimating a lens, you might as well set the flange to film plane distance as well.
The collimating is a long process. Check the KSS and decide if you really want to go that far into a lens. You will also need a good set of dial calipers to perform both the collimation and the film plane distance operations.

 

[Xmas]

Igor

The J8M will work without a shim if you back of the grub screw the lens can be registered at infinity and removed and the grub used to locate in the new position. It does not need to be tightened down too much, if you remove the shim I'd keep it. Think a J8 is similar.

Registering the body is more difficult even if you have good measuring tools - unless you have an auto-collimator, - but if the body is way off you can improve it. This needs the camera reduced to large number of small parts - one weekend.

Noel

 

[Igor.Burshteyn]

thanks, but I still don't understand what thikness shims I need to add in order to fix 7cm back focus 🙁 I already checked for collimation my J9. I placed OM 1-13 focusing screen at the film plane, and inspected resulting image with loupe. J9 is perfectly focused when rangefinder is focused (inline with photographic results - j9 always was spot on). Let's suggest that body is ok with regard to flange-to-film-plane distance.

 

[Xmas]

Igor

You use the lens formula, the formula is 1/f= 1/u + 1/v
You need to assume the focal length and the 1/u for close up.
It is a lot simpler to set the lens at infinity using your focus screen, unless you only use it close up when you should set it close up. Discard the shim only use the grub screw to locate the lens.

Noel

 

[Igor.Burshteyn]

Noel,
I understand what you are proposing :> I remove grub screw, unscrew lens block from the base turn or two, and fix it with grub screw again. But what margin should I add - any idea? Btw, in order for apperture marking to line up with red dot, I will need to unscrew 2 small screws on the lens block and play a little with their final position. I have quite uncommon J8 in kiev mountg - it's serial# start with 016xx, and it has click stops.

 

[Xmas]

Igor

Sorry I get told off for cryptic posts...

If it is a Kiev you set the lock (at infinity) & with the SLR screen or ground glass or 3M matt tape across the gate... unscrew the lens in the mount body until the image is sharp, on something a long way away, and then un-bayonet the lens and tighten the grub. Then check again.

You will as you suggest then need to loosen off the aperature grubs, but you know that, dont remove them, and do this over a ice cream tub or similar deep catch device.

If you suspected that lens was not the correct focal length or the rangefinder was off (or both) then you focus if a target at normal distance for you and you focus the image sharp on the focus target.

Look after your prototype.

Noel

 

[Igor.Burshteyn]

Noel,
Thanks for very clear and "doable" instructions. Btw, why do you propose collimation with lens on infinity? I thought about min distance - 0.9m.

 

[Xmas]

Igor

You are too perceptive.
- I take most of my photos at infinity
- The simplest calibration is at infinity and min distance, but I'm always uncertain about the minium distance. I got the stars and the moon and a white cottage about 3 miles...

Brian Sweeney the doyen (sorry Brian) of lenses tends to optimise delinquents close in. If you take head and shoulders of kids use that distance.

Noel

 

[Igor.Burshteyn]

Noel,
I see. I take head&shoulders more than things on infinity. I'll stick with close focus collimation I think.

 

[alternatve]

All this talk about backfocus is scaring me. I was looking for a J3 or a J8 as a cheap, fast lens for my IIIf, but I guess it's out for me now.

 

[TVphotog]

I don't think so. I have a couple of J8s and they are great as far as I am concerned. I haven't yet noticed any focus problems. I have used both of them at wide open and have been very pleased with the results. Unfortunately, it is a luck of the draw. Go with a reputable seller(check the threads) or go to Fedka and see about a return policy. Also, the way to fix this is not anywhere near as daunting as it sounds.

 

[Xmas]

Guys

Dont panic Kiev bodies can be way off. if they have been maintained by someone who did not care, LTM is different...

Noel