raid
Dad Photographer
Brian has done "brain surgery" of such lenses! I love seeing such technical know-how being explained here at RFF again.
brusby
Well-known
Great info, Brian, thanks!
How are you defining "back focus"? I normally think of it as the distance the actual focus plane is behind the intended focus point. But it seems like you are using it with regard to the movement of the optical cell. Or am I misinterpreting? Thanks!
How are you defining "back focus"? I normally think of it as the distance the actual focus plane is behind the intended focus point. But it seems like you are using it with regard to the movement of the optical cell. Or am I misinterpreting? Thanks!
Back Focus is the term used in the formula for focal length, "1/B" is from the Physics book.
Not to be confused with a lens that is back-focusing. Remember the Taxi Episode with "Blue Berries" and "Blueberries". About the same.
Back-focus used in the formula is the distance that the image forms on the image plane.
A lens that is back-focusing on a Rangefinder camera is one that the actual focus is behind what the rangefinder indicates, and the lens needs to be shimmed.
Not to be confused with a lens that is back-focusing. Remember the Taxi Episode with "Blue Berries" and "Blueberries". About the same.
Back-focus used in the formula is the distance that the image forms on the image plane.
A lens that is back-focusing on a Rangefinder camera is one that the actual focus is behind what the rangefinder indicates, and the lens needs to be shimmed.
the_jim
human
Brian, thank you for sharing this information and all of the years you have put into supporting/promoting these wonderful optics.
You adjusted an early 80's J-3 of mine about 13 years ago. It was greatly improved.
Additionally, I was inspired by your above guide to adjust the helicoid to reach infinity on my particular rangefinder using an old ground 'glass' off of a Rolleiflex that I cut to fit the gate on my Zeiss Ikon ZM.
The only further 'improvement' that I made to the lens...and I don't have any before/after testing to show that it did anything (but I believe that it did) was to paint the edge of the front element black using model train paint. I was working as a lens technician at the time (working on cine equipment) and it struck me as odd that front element was unpainted on its edge. So I used the black model train paint to fix it up. I genuinely believe that it helped increase contrast.
Ken at BeckenRidge Vineyards by Jim Fischer, on Flickr
You adjusted an early 80's J-3 of mine about 13 years ago. It was greatly improved.
Additionally, I was inspired by your above guide to adjust the helicoid to reach infinity on my particular rangefinder using an old ground 'glass' off of a Rolleiflex that I cut to fit the gate on my Zeiss Ikon ZM.
The only further 'improvement' that I made to the lens...and I don't have any before/after testing to show that it did anything (but I believe that it did) was to paint the edge of the front element black using model train paint. I was working as a lens technician at the time (working on cine equipment) and it struck me as odd that front element was unpainted on its edge. So I used the black model train paint to fix it up. I genuinely believe that it helped increase contrast.
Ken at BeckenRidge Vineyards by Jim Fischer, on Flickr
David- The J-3 was made by KMZ, ZOMZ, and Valdai. The KMZ- many of these will have some level of Zeiss in them, Earlier lenses are Zeiss Sonnars assembled in Russia with a much improved focus mount, later ones- may still be cut using Schott glass. The original Zeiss focus mounts are very fragile. I've remounted Zeiss Sonnars in J-3 mounts, much more durable than the originals.
The ZOMZ- most notable is the change in the rear triplet and fixture. I've used front elements from ZOMZ lenses in a Valdai, and Valdai front elements to repair a KMZ.
In my opinion- the KMZ Jupiter-3 is the best of all J-3's, and I have 8 of them, counting the ZK. The Valdai - I went through 10 of them to find a good one. ZOMZ- I've seen 1956 and 1957 lenses that needed a lot of work to straighten out, like changing distance between the groups. 1958 and on, condition of the optics is the big variable. I have a perfect 1975 ZOMZ that is a great performer. Valdai- the one I have is as good as the perfect ZOMZ. The optics are good, the problem was the fixtures and machining of the mount. A lot of work required, buy I've set several straight. I'm retired from this particular hobby!
My Valdai J-3 shown above, wide-open on the M9:
L1007938-Edit by fiftyonepointsix, on Flickrbrusby
Well-known
The distance measured from what point to the image plane? I'm trying to visualize where this measurement would be made. The optical center of the lens? The place where shims are placed
edit: I'm suspecting the formula is using it as a differential, i.e., the amount of change in position (or back movement/back focus?) of the lens group.
Cherry-Picked Jupiter-3 lenses.
Valdai, wide-open:
1975 ZOMZ wide-open:
1956 KMZ wide-open.
1950 KMZ J-3, SN on rear showing it to be a 1943 Sonnar:
C-Sonnar Wide-Open:
The 1950 KMZ compares well against the C-Sonnar, the latter has a slight edge. The J-3's all ran ~$150, but required work- as shown in the documentation here. So easy, even a Fortran-77 programmer can do it.
Valdai, wide-open:
1975 ZOMZ wide-open:
1956 KMZ wide-open.
1950 KMZ J-3, SN on rear showing it to be a 1943 Sonnar:
C-Sonnar Wide-Open:
The 1950 KMZ compares well against the C-Sonnar, the latter has a slight edge. The J-3's all ran ~$150, but required work- as shown in the documentation here. So easy, even a Fortran-77 programmer can do it.
I try to measure from where the sensor on the camera is.
The formula IS used as a differential. The assumption is that the focal length of the lens is 52.4mm. As stated in the documentation, this can be off and the formula gives you the place to start. Final accuracy for a digital camera needs to be 0.01mm for the shim, and this almost always required rought-cut/measure with new shim/ re-adjust.
brusby
Well-known
Thanks Brian, I understand and appreciate all the wonderful info you're providing. Just trying to get a clear, precise definition of the term "back focus" as used in the formula, since it differs from the commonly accepted usage of that term. Guess it comes from my scientific background of questioning everything. I'll do some research. You probably don't remember, but many years ago we corresponded and I suggested annotations for your very informative photos to make it easier for those of us unfamiliar with the lens innards to understand your written explanations. Thanks again!
I do remember!
These are labeled figures. I fashioned this after a diagram found in a 1940s book on optics.
These go back about 7 years.
These are labeled figures. I fashioned this after a diagram found in a 1940s book on optics.
These go back about 7 years.
jc031699
Newbie
Hi Brian, thanks so much for the amazing work you have done with these lenses and for sharing your knowledge and experience. I have a Jupiter 8 bought from Ukraine on Etsy, glass all ok with light cleaning marks. The aperture ring lines up with f2 wide open, but it is assembled with the threaded locking stand-off ring and NO shims.
On my initial tests it back focuses about 10 inches. Which puts me at needing about 1.5mm of shims.
I have found an AliExpress vendor that sells custom laser cut stainless shims, ordered 10 each of 27mmx30mmx0.1mm and 27mmx30mmx0.5mm shims for about $14 USD. Waiting on the shipment now, I’ll post an update when they get here.
On my initial tests it back focuses about 10 inches. Which puts me at needing about 1.5mm of shims.
I have found an AliExpress vendor that sells custom laser cut stainless shims, ordered 10 each of 27mmx30mmx0.1mm and 27mmx30mmx0.5mm shims for about $14 USD. Waiting on the shipment now, I’ll post an update when they get here.
jc031699
Newbie
Correction- 14 inches
Thankyou for this- I would be interested in how the new shims work out, and getting a link to the company.
I've accumulated a collection of shims and set screws over 20 years of working on these lenses. Used to offer the Sonnar conversions for a small fee ($25~$50) and the excess parts. My clever plan for getting shims!
jc031699
Newbie
Here is the store
US $2.02 42％ Off | Laser Customization 304 Stainless Steel 0.1 0.2 0.3 0.5mm Ultra Thin Flat Washer Adjusting Flat Gasket Shim Din988 M2~M40 10Pcs
I also just found a circular cutter that may be good for cutting paper shims…!