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The purpose of this article is to make you rethink your decision to hack, buy and modify a lens without proper research and making informed decisions.
Don't get me wrong I'm not trying to discourage you in your decision to use some old glass on a modern camera. I'm all for it and there's some old and some extremely old glass that really deserve to to be used on modern digital systems.
Taking a first step is to research what can be done, how can it be done and if it has been done before to the lens of your choice.
How to pick a lens that you want to modify?
Many of you probably already own a lens that you think is worth enough to be modified.
As that may be true, the first thing to do is to go to a drawer, take it out and examine the beast. Assess if lens focusing ring operation is smooth, inspect the front and the rear element for scratches, shine flashlight through the lens to reveal if there is some kind of haze or fungus on the inner elements, inspect iris for traces of oil on it, and if it opens and closes correctly. Minor amount of dust particles inside a lens can be expected (especially if a lens is 40 or more years old) and it'll not pose a serious problem, some really old glass can have bubbles inside glass elements. (like some Zeiss optic dating pre WWII and up to end of 50's) If you are dealing with old SSSR (Russian) lens, it might look like there are cleaning marks on front and rear element but according some literature that's normal and was introduced during lens production. My advice: if you end up using one of these lenses, you must use a lens hood with it. A hood will reduce unexpected flares produced by micro abrasions. If your findings are negative on all major issues and lens is functioning as expected you can continue with your research. If any of major problems is present with your lens my advice is to research lens type and model to determine if it is really worth a trouble and money to restore. If you find out that you are a proud owner of some rare glass worth thousands you may consider a professional repair shop, but that will be pricy. If it is something common, search the net and find out how much it will cost to buy an exact copy but in mint condition since it is usually cheaper than to restore your original one. (professional repair shops that are willing to restore your 40 - 80 something years old lens are probably harder to find than the lens you own)
Do it yourself is not my advice, especially if you do not own special tools and the know-how (doing this before acquiring the necessary knowledge is learning it the hard way).
If you decide that cleaning and restoration is something you want to do, please be warned that it means a lot of research, experimenting, reading and starting all over again. For example, I spent more than 6 months and bought about 25 different cans of lube before I found the right one for my 40mm CZJ lens, and I had a service manual for it (in German that I do not speak, but I read it anyway). (The original lube was no longer in production)
SPECIAL WARNING: if you are lucky, and you own a rare and precious lens, do not even think about learning how to repair lenses on it. (buy some cheap lens and learn your way around on it). Another story are the tools you'll need. You'll have a hard time to find the basic set of tools and the price will be high. Most lenses will require special custom designed tools. These tools are not on open market and were built just for authorized service shops and for lens production. I was lucky with CZJ service manual which contains plans for building special tools. Not that it helped much, but you can get some basic idea what you need and what to avoid disassembling.
The next thing on the list is to determine the lens mount type and camera for which lens was originally intended. This might look like an easy task, but in reality it may be a complex and traitorous one, especially if you do not own a camera body for which the lens was designed.
If you know the exact mount type used on your lens, a simple search on ebay or on a similar site will reveal all possible options and the adapter type you need to fit lens to your camera body.
If nothing pops out in search, it might mean the following:
Lens cannot be modified to attach to your camera
Lens mount is uncommon and there is no prefabricated adapters on market
Typo or inadequate search terms
Expand search to whole www by using Google or your prefered search engine (to find out if someone already found solution to your puzzle)
If something popped out as result of your search, a further research is required to determine the best price and if all features are possible, like focus to infinity, no additional optics, AF or EMF chip preinstalled and so on. I'll try to elaborate on my example.
I'm a Canon guy and I'm using Canon EOS 7D and my search will be for M42 to EOS EF adapter. (EF is the name for Canon's lens mount used on all EOS bodies analog and digital) M42 to EOS EF is a common type of adapter and it will pop up in many different forms and prices with chip (several types AF EMF) and without. Adapter that I was looking for was for specific CZJ (carl zeiss jena) 40mm F4,5 T lens with Pentax/Praktica M42 thread, but the tricky part was that standard adapter for more modern M42 lenses will not allow screwing lens in all the way. More modern lenses with M42 thread and automatic aperture control use a slightly different adapter with special tab that pushes a pin on lens mount to allow automatic/manual aperture control. After some search refining I was able to find an appropriate adapter for my lens. Now that I found the right adapter there was one more thing to consider. I was opting for an adapter with focus confirmation chip but the right adapter was without it, so I had to find a chip with different merchant. As far as I know an AF chip can be found for Canon, Nikon and Olympus. l prefer EMF type of chip because it offers complete setup, so exif data will be somewhat correct. EMF chip allows programming focal length, maximum aperture and you can calibrate back or front focus. I will not talk about AF chips here; it is another good research topic for you.
If you were not so lucky and there is no prefabricated adapter for your lens, that does not automatically mean that it cannot be done. Now that we are certain that no comercial adapter exists we can research some more. The first thing on the list is to determine the lens register distance. The lens register distance is a magic number and it will tell us if there is enough room to build a custom mount. Nice table with about 120 lens register values can be found at Camera Mounts Sorted by Register page maintained by Stephen H. Westin. The lens register is a distance of lens mount point from film plain or sensor. For Canon EOS EF mount it is 44 mm (millimeters) for Canon R/FN/FD/FDn mount it is 42 mm so in theory Canon Early lens cannot be used on Canon EOS (that is not really true, but there is no simple solution) To hack Canon FD mount lens and modify it to fit Canon EOS EF mount you can take several different approaches. There are four ways to address the problem as you can guess there is a commercially available adapter. You can buy an adapter, but it contains a small correction lens to permit focus to infinity, this adapter also acts as a small extender and you are losing one part of F stop and gaining magnification and distortion. There is a second variation of the same adapter but without additional optics and in this case you are losing infinity focus and lens can only do macro. For me both of these adapters weren't an option. There is also a guy on ebay EdMika that makes custom mounts, but only for selected canon FD lense models usually high end ones. To use one of Ed's mounts you will need to disassemble original FD mount and install a special custom part for coupling aperture ring with iris and then install the custom EOS EF mount. This option is the only acceptable one for me, but it's too pricy. The kit for mount replacement goes for around $150 Canadian. (and that is much more than I'm ready to invest in old lens worth less than mount replacement kit and I discovered EdMika just three years too late) My first conversion was Canon FDn 200mm f2.8 non IF version first FDn incarnation. It was a painstaking process and after several attempts I finally figured out how to do it and now it is featured here in the lens database. (all Canon FDn lenses that I hacked so far were in my possession for a long time; there are some that are still waiting to be hacked)
My other hack was building a custom adapter for Altix breech lock lenses with lens register at 42.5 mm opposing to Canon EF 44mm register. It seems like something that is impossible to build but it was cheap and easy to make. The reason for this was 6 lenses donated to me by my colleagues and the absurd prices their M42 versions were sold for on ebay. The main reason was that I really like how Carl Zeiss, Jena Tessar and Meyer-Optik Görlitz Trioplan and Telefogar render picture. They may not be the sharpest and fastest tools in the shed but no modern lens can emulate charm and personality they have.
To conclude this ranting of mine.
If you decide that old glass is something that you want, do your research.
Find out what lens mount is the easiest and cheapest (prices can sometimes reach the price of modern lens with the same or better performances) to adapt to your camera system (especially if you intend to buy a lens); many lenses can be found fitted with different mounts.
If the lens is cheap, it might mean that mount register is odd and that there is no simple solution if any.
When you finally start using a manual glass, be prepared that you will lug around something with substantial weight and you'll not use it as much as you thought you would.
For those of you with AF chips on your hacked or adopted lenses read my earlier post about front or back focus calibration; you might find it useful.
As winter is near and there will be a lot of time for lens conversions, I'll try to make short visual guides on how to make Altix to EOS adapter and how to hack FDn Lens to EF mount with just simple tools.
Another word of wisdom
The best candidates for conversions are lenses that you already own especially high-end stuff of it's days because they are for free. If you want to buy an old high-end lens, be prepared to pay an absurd amount of money. They are usually pricey (read overpriced) and we can thank MSLR's for that. To pay US $600 for CZJ Flektogon 20mm f2.8 in my book is way too much and I usually giggle when I find one listed in classifieds. It is a nice lens for architecture fantastic color rendering and so on but you can get much modern lighter and equally good lens in new condition for half the price.
I just went online and this is what I have found. Both lenses were listed as used and the highest priced were considered for comparison, and both were buy now option to be fair comparison. Canon 20mm f2.8 USM with Tiffen MC 72mm SKY filter goes for US $398, and CZJ Flektogon 20mm f2.8 goes for US $579 an another $40 for adapter. So do the math and go figure this one for your selfe.