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01-29-2012, 12:57 AM   #1
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confused with infinity focus on a mod project.

So I am confused on infinity focus with a project I am working on. I am getting drastically different results with different lenses. I'm starting to think one of the lenses is bad but I don't have that many canon lenses to test with.

So the project is a vivitar 2x teleconverter in canon fd mount. I am cutting back the lens and putting a k mount on the rear so it acts as a canon fd lens to k mount adapter, hopefully with better glass than the cheap ebay ones. If I can get all the bugs worked out, I have a kiron 1.5x teleconverter in fd mount that I can convert.

So the main lens I have been using to test is a vivitar 28-70mm (the obscure one that has self contained auto focus and batteries). at 28mm, the lens will only focus to about 10 feet and at 70mm it will focus to about 3 feet (I might have those 2 reversed). Even as I cut large amounts off the lens, I didn't seem to gain much focus distance. In frustration, I thoroughly cleaned the converter and tried it on a vivitar 90mm 2.5 (the main lens I want to use it with but not a lens I would use for constant testing). When it was set to infinity, it was in focus at about 30 feet. I then started to post but remembered another lens to test with. I have a five star 500mm and a similar 300mm (preset and basically just a tube with glass), both of which are t mount. I use the 300mm, put a k mount adapter on it, focused at about 40 feet, then with focus untouched, switched to a canon mount and the teleconverter, and it was in focus at nearly the same point (about 40 feet). the 300mm still had a lot of rotation for focus towards infinity (too dark out to tell right now if it could actually focus to infinity).

So why so much variation on the lenses? Is the vivitar 28-70mm defective since it is so far off from the others? I can only remove about 1/4mm more and maintain accuracy/flatness of the new mount. I'm happy with 30-40 feet for a first try but am I going to get that with every lens (I have an idea how I might be able to go further on another converter but its too late for this one)?

Maybe I'm not putting 2 and 2 together but I can not understand the variation between lenses?

01-29-2012, 10:34 AM   #2
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I would tend to think you would to look at maintaining the element to focal plane / sensor geometry or distance of a Pentax system but the element to mount (for the lens mount ) of a canon system. Maybe you can make some minor adjustments by moving the TC group either forward or backward in the TC
01-29-2012, 11:06 AM   #3
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That is what I was trying to do minus adjusting where the elements sat in the body. The inner metal body that housed all the glass only attached to the outer body at the very rear. That rear portion that attached the two is the exact area that I had to cut away to set the lens back. I took all the extra parts off the lens so I just had the outer shell, and the inner shell with all the glass attached together properly. All the rings and levers and pins to operate things had been removed. That should have had the glass aligned properly and the spacing properly for a canon lens as it was in its original configuration without modification. I then filled the cavity of the lens with about 1/2-2/3" in height epoxy. At this point it is still basically as it originally was with all the extra parts removed. I then started cutting back the rear of the lens to set it back as close as possible to pentax flange focal distance. The area where the two parts attached is gone, but the epoxy is holding the inner assemble in place to the outer shell in the same position it originally was. Basically I jest kept it as is but set the converter back closer to the camera. I don't have time to clean it up and test before work but I'll check it tonight. It is cut as far as it can go while keeping the mount surface level. I will be attaching an m42-pk adapter for the mount. For now I have been attaching the adapter to my camera, and then I have hand held the canon lens and modified converter locked as a unit, to the camera. It has a nice clean smooth level surface so it wasn't hart to keep it sitting right by hand (holding it and focusing was a bit more challenging to try).
01-29-2012, 11:11 AM   #4
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Just a dumb idea here but can yoou pull the elements as a unit? Maybe set up a focusing screen, out of tissue paper and look at the distance between the center elements, the focusing screen and lens. Set up with lens at infinity and move things around until you get focused. This is the relationship you need to maintain when all is mounted up. I can see doing this in a paper tube, with the lenses attached to a stick, and the "focusing screen" stretched across the end of the tube. All you need to do is get the distances right.

01-29-2012, 11:27 AM   #5
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I can not pull them as a unit any more. the inner housing that holds the glass is epoxied to the outer body. I guess I'll see how it behaves tonight.
01-29-2012, 12:44 PM   #6
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QuoteOriginally posted by ripit Quote
I can not pull them as a unit any more. the inner housing that holds the glass is epoxied to the outer body. I guess I'll see how it behaves tonight.
Can the whole TC group unscrew? That is often how it works.

As I said, it would be easier to figure this out with the elements out of the housing
01-29-2012, 02:51 PM   #7
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Just a small observation before you get carried along too far...

For macro photography you have to use longer tubes in order to get sam magnification when using lenses with longer focal length. If I am correct the length of the tube should be as long as the focal length of the lens itself so that you gain magnification 1:1 when the lens is focused to the infinity (or very close to magnification 1:1). Now the actual formula could be more complex but that is a good rule to start with.

So what has this to do with the covnerter you are trying to make? Well quite a lot. For instance you could add fairly thick macro tube to your 500mm lens and still be able to focus to infinity (or very close to infinity), whereas at the wide angle the focus would be way off. Now with wide angle (and both 28mm and 70mm are rather wide compared to 500mm) your focus would be way off.

With this in mind you are getting rather consistent results: all your lense do not focus to infinity (at least what you can judge) and your wide lenses are more off then your teles. So it seems that you have to loose some more material at the mount side. Bare in mind that the precisson is more critical with wider lenses than with tele-lenses, but in this case I assume that you would still use this combination on the tele side. If you have a chance to compare the result of your zoom with some other lens of same focal length (ideally with some primes) it would give you more reliable result as of wheter you did gain the infinity focus or you still have to go.

Anyway good luck with your project!
01-29-2012, 06:45 PM   #8
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@Lowell Goudge, This is how it was constructed. There is a single piece tube with a flange at the back. It is one solid piece. Inside it are the glass elements. It looks to have steps and rings inside like there are loose elements held at various levels. We will call it tube A. I assume that is what you are talking about. It is a self contained unit with all the glass in it. There were rings and ball bearings around tube A with all the levers but they have been removed. On the outer body, there is a thin ring at the front with the mount for the lens. from there on back including all of the mount except the lock ring, it is one solid piece. Well call that body B. There is a recess in the rear mount portion of body B that the flange at the rear of tube A slides into. Screws then go into the side of body B and into the flange on tube A to hold the 2 together. Tube A just floats and doesn't contact body B anywhere except at the back flange.

I assume that taking tube A out body B is what you are talking about. It is a self contained unit with the glass. Tube A is now permanently attached to body B with epoxy on this one now though. I basically partially filled the lens with epoxy to glue them together. In order to cut the back of the lens to set it closer to the camera, the flange on tube A, and the corresponding pocket on body B were completely ground and filed away. The only thing holding tube A to Body B now is epoxy. See the grey epoxy in the picture. As far as I can tell, if I start unscrewing rings on the back, individual elements would come out.

Of course your suggestion might be of some help with the second one (the kiron 1.5x that I plan on latter converting).

@stanislav, that is actually very helpfull info. I didn't think it would make so much difference but apparently it does. The mail lens I want to use with it gets about 30' plus so its good enough and I know I can use it with longer but not shorter lenses.

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Last edited by ripit; 01-29-2012 at 06:54 PM.
01-30-2012, 02:23 PM   #9
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@stanislav, do you know what effect using a 1.5x converter would have as opposed to a 2x? Assuming the focal distances were the same (same distance to the camera from the teleconverter and same distance to the lens). Would I gain or loose long range focus?

anyway, here are a couple of pics. I may start a new thread for opinions when I get further and or et some better pictures but would appreciate any feedback on image quality any one would like to give..

Imageshack - imgp9093.jpg
Imageshack - imgp9106d.jpg
01-30-2012, 02:51 PM   #10
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I'm not sure that it matters but the kiron 1.5x is supposedly matched to their 28-85mm and 70-150 lenses. I would think that would be a good thing for a 90mm at least for image quality but I'm not sure if it would effect infinity focus at all.
01-31-2012, 01:37 AM   #11
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QuoteOriginally posted by ripit Quote
@stanislav, do you know what effect using a 1.5x converter would have as opposed to a 2x? Assuming the focal distances were the same (same distance to the camera from the teleconverter and same distance to the lens). Would I gain or loose long range focus?

anyway, here are a couple of pics. I may start a new thread for opinions when I get further and or et some better pictures but would appreciate any feedback on image quality any one would like to give..

Imageshack - imgp9093.jpg
Imageshack - imgp9106d.jpg
That is a hard question!

Well first I have to say that I never did such adaptation nor did I try. My experience is based on a Soligor 400mm f6.3 in Canon FD mount which I have adapted to the Pentax K and some experimentation with macro tubes as well as macro focusing teleconverter.

Now I'll try to answer your question: With 2x teleconverter you have effectively double the focal range which means that the combination (lens with attached teleconverter) is half as sensitive to thikness on the mount side. What happens with 1.5x teleconverter compared with 2x teleconverter is basically that you compare effectively lenses with two different focal lengths. So lets assume that you start with 100mm f2.8, with 2x tleconverter you'll gett effectivly 200mm f5.6 whereas with 1.5x you'll get about 150mm f4.5 (or something like that). So this is the combination that gives you both the longest focusing distance and even the DOF.

Provided that you TC's are good on the mount they are designed for (Canon FD) - meaning that you can gain the infinite focus and that lenses are alligned (no tilit/shift effect) than you should try not to fiddle with the position of the lenses in your TC. It should be however possible to slightly move the lens groups towards the lens to gain infinity focus, but then you risk to missalighn it so that the result beign something as fixed tilt/shift converter spoiling all your images.

So finally to your original question: assuming that you have removed exactly the same ammount of material from the "camera" side of your TC you would get "further" focus with your 2X TC compared to your 1.5X TC. My suggestion is that you check whether you can remove about 5mm on the mount in which case it may be worht doing the conversion (you'll still need about 1-1.5mm for the mount). Now in that assesment you have to take into account both the "thickness" of the mount and the position of the rear element. In other words if the rear element is protruding behind the mount, there is a risk that the mirror could hit it.

Well I really hope that you'll sort these things out.

PS feel free to contact me privatly as well (then I'll be able to answe faster)

cheers, and good luck!
01-31-2012, 07:51 AM   #12
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Thanks a lot for the help. I have actually converted a couple of canon lenses but in those cases, they were after market lenses, and just removing the mount removed plenty of material. This is the first time I have actually cut the lens back to try and set it back. Right now, there is no issue on the 2x of misalignment and tilt shift. If you look at the rear of the lens you will see a small black ring on the back face around the outside. The original outermost surface is still intact. There is no reason that I could not cut the back of the lens away further, other than I may develop an untrue surface and wind up with a tilt shift effect. It is too bad I don't have access to a lathe. I could cut it back as far as I needed in 5 minutes and have a true surface. doing it by hand is another story (though I may decide to try if I can not find a better solution). I still have some metal working tools (lathe bits etc) from when I went to school. I was almost thinking of rigging some sort of jig that would use the rear of the lens as reference, and cut into the side. If I could get it to cut deep enough that I had a true reference surface to mount to/cut down to, removing all the inner material shouldn't be an issue. Its back to the drawing board I guess. I may try to cut the 2x back further by hand since it is too late to do anything else with it. On the 1.5x, I will look into setting the center elements back before I epoxy it but with out it screwed in place, getting it straight would be an issue like you said. Fyi I did take the mount into account. I attached a m42-pk adapter to the camera and then hand held the teleconverter and lens up to it for testing. I will use a diffrent m42-pk adapter for the actual mount that is actually .010" thinner (about 1/4mm) but I don't think thats enough to be much help.

Thanks a bunch for your help. It has been quite valuable. I think I'm going to take my time with this one and explore my options. I got the kiron 1.5x from b&h for about 9$ shipped, but looking around its not very common, and getting another 1.4 or 1.5x teleconverter cheap is a big long shot. If nothing else, I may try to cut the 2x back by hand and see how it goes. I'm still thinking using a jig and cutting into the side of the 1.5x might be the best approach though.
QuoteOriginally posted by stanislav Quote
That is a hard question!

Well first I have to say that I never did such adaptation nor did I try. My experience is based on a Soligor 400mm f6.3 in Canon FD mount which I have adapted to the Pentax K and some experimentation with macro tubes as well as macro focusing teleconverter.

Now I'll try to answer your question: With 2x teleconverter you have effectively double the focal range which means that the combination (lens with attached teleconverter) is half as sensitive to thikness on the mount side. What happens with 1.5x teleconverter compared with 2x teleconverter is basically that you compare effectively lenses with two different focal lengths. So lets assume that you start with 100mm f2.8, with 2x tleconverter you'll gett effectivly 200mm f5.6 whereas with 1.5x you'll get about 150mm f4.5 (or something like that). So this is the combination that gives you both the longest focusing distance and even the DOF.

Provided that you TC's are good on the mount they are designed for (Canon FD) - meaning that you can gain the infinite focus and that lenses are alligned (no tilit/shift effect) than you should try not to fiddle with the position of the lenses in your TC. It should be however possible to slightly move the lens groups towards the lens to gain infinity focus, but then you risk to missalighn it so that the result beign something as fixed tilt/shift converter spoiling all your images.

So finally to your original question: assuming that you have removed exactly the same ammount of material from the "camera" side of your TC you would get "further" focus with your 2X TC compared to your 1.5X TC. My suggestion is that you check whether you can remove about 5mm on the mount in which case it may be worht doing the conversion (you'll still need about 1-1.5mm for the mount). Now in that assesment you have to take into account both the "thickness" of the mount and the position of the rear element. In other words if the rear element is protruding behind the mount, there is a risk that the mirror could hit it.

Well I really hope that you'll sort these things out.

PS feel free to contact me privatly as well (then I'll be able to answe faster)

cheers, and good luck!
02-02-2012, 12:28 PM   #13
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I'm still a little dumb founded on this. I hogged another 2mm of the mount plus a little more for clean up. The shorter focal length lens is still not near infinity though the 90mm now focuses out to about 100 yards (rough estimate using a golf scope to judge distance).

In theory, I should only have to remove 3.5mm off any of them to get infinity with any lens, right? It seems I have already removed more than that?
02-02-2012, 11:10 PM   #14
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Well, you are right, so now I am puzzled with your rezults as well.

Just out of curiosity: did you have the infinity focus with teleconverters on native Canon FD body? One explanation could be that there has been some issue with TC even before you started the conversion. That could explain why you have to go longer than expected. Otherwise... well I don't have a clue. On the other hand, if you can focus to 100 yards, you are getting closer, maybe another milimeter would give you the infinity.
02-03-2012, 02:20 AM   #15
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It seems there are issues with teleconverters and infinity focus. I tried a quantaray AF 2x teleconverter with the 18-55mm kit lens (both native k mount, not modified), and it would focus to infinity at 18mm but would only focus to maybe 25 feet when set to 55mm. I tried it with a vivitar MF 2x teleconverter and got the same results. I tried a sigma AF 28-200mm though, and it would focus to infinity at 28mm as well as 200mm. I recall from a couple of vivitar super zooms that I converted from minolta mount, that they had kind of a curved focus being similar at either end and at a different focal point in the middle. It could be that it is too dark out right now, but the sigma didn't seem to be able to reach infinity in the middle of the range. I know teleconverters are sometimes matched to a certain lens. Canon makes 2 2x teleconverters, one for under 300mm and one for over 300mm. It would seem that there are issues with teleconverters though I don't know or understand the particulars yet. With the 90mm lens, 100 yards is plenty (not sure I would use that lens for landscapes but who knows). I'm more concerned with what the 1.5x will do (assuming it turns out to have decent glass). I do know (or read one place but have not confirmed) that the kiron 1.5x I have is matched to their 28-85mm and 70-150mm lenses? I don't know if that means that it is most optimized for the middle of that range which would be right about 90mm (it would be sweet if it were perfectly matched to the 90mm lens), or if it depends as much on how the lens is designed as its focal length?

I didn't even think about it before but I do have a canon fd body (broken but the viewfinder works) to test how a particular teleconverter works with particular lenses before modification. I also have a minolta body (those are the main 2 I wouldn't mind having adapters for) but I have no idea if the view finder on the minolta works? There doesn't seem to be any issue with the 90mm and the 1.5x on the canon body. I'll have to play with it a little more tomorrow when there is daylight.
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