Modified Kobori Vivitar Lens Test

[David]

Earlier this week, you might have read, I fixed and then modified a Kobori-made Vivitar lens to work outside its designed parameters. due to a series of mishaps, tonight is the first night I've been able to get photos taken with it and uploaded.

Firstly: here is the lens:




To verify the make and such:





As you might remember, this lens did not fit on my K-7 at first ,but did fit on my K1000. I had to remove this pesky part:



The lens came broken and in desperate need of blade cleaning. I had to remove all the blades ,clean them individually, and then clean the housing. Not a big deal except that it took six Q-tips and three paper towels. The paper towels weren't saturated or anything, but I didn't want to use the same bit of them twice, once oiled. So with all the repairs down, here is the meat of how I modified the lens:

The aperture is controlled by a series of arms and springs, like most any aperture. The arm that controls how far the aperture stops down on this had a limiter in the form of a tiny metal plate and two screws. For fun, I took the plate and screws off to see if the aperture could stop down any further. Literally, the aperture can close entirely. However, the blades are shaped such that small holes appear on the edges of the aperture ring. I did not modify the aperture to shut that far down. However, at the modified level, small holes still appear in the outside. I couldn't capture them on the photographs.

For reference, here is the aperture at f22:



And, since I have no idea what this aperture setting really is, I'll call it f infinity:





I drew little arrows to show where the opening is since the lighting reflected in the back element a bit. To give you an idea of how much light is required to make this lens work at f infinity, a photo of the sun with what I felt was decent exposure required a 1/60th exposure. So that's pretty ridiculous.

Firstly, the logical and reasonable question is: why? Why do this at all? Curiosity. I wanted to see what would happen. But I also had a theory. I proposed, to myself, that this lens would achieve a God-like DoF. Though the DoF does seem largely uniform in many pictures, this modification had many side affects that make it an unmitigated disaster.

Here are three photos, with 100% crops, of the same shot taken at f22, f22+ and f infinity. F22+ is the setting between f22 and f infinity. It's probably around f35 or f40.

f22:


f22+


f infinity:


Here are the same photos at a 100% crop, though I suspect you can already see what's happening.





As you can see, the further the aperture diverges from the design, the blurry it becomes. The DoF is pretty much uniform throughout, but it's all a-blur. This is vrey likely because the exposure times required for these shots was ridiculous. For the f infinity shot, the exposure was 0.8 seconds in full sun.

These photos being less than stellar, I took a few more. Here are two examples with various DoFs. The focus is cleaner, but not what I would call "acceptable."






Investigating these photos, though, showed odd modification side effects. The next photos show these effects.

The sun here has a star glare, including two eleven-shaped arms coming off the diagonals. Interesting effect


And then I saw this photo, with that odd glare hash.


And this odd glare pattern is even stranger.

I suspect this is because the aperture closes down so far with the modification that there are five small holes along the outside. This is probably also why the focus is off at f infinity.

I also did some macro tests and took some color shots, and I could post and compare those, but the ultimate outcome is the same: The focus is WAY too soft, the DoF is incredibly uniform (and blurry), and the glare causes unacceptable image distortions. In short: the lens goes back to the way it was this weekend. The f22 shots reveal it's not a shabby lens, if properly assembled.

[kds315*]

Ever heard the term "diffraction"? Google it maybe ...

[PBFACTS]

[kds315*]

invited the "pinhole lens" - in 2011? That was invited when photography just begun...!!

[PBFACTS]

[RioRico]

Ah, more pinhead, er I mean pinhole photography! I use an f/181 pinhole on my APS-C camera, a Pentax K20D. Bu not very often, because the diffraction really really sucks. On such a half-frame camera, diffraction starts being noticeable to pixel-peepers at around f/11, and to normal humans at maybe f/22. Some of my DA-class (half-frame) AF zooms tell the camera that they reach f/38-45 when fully stopped-down and zoomed-out, but most stop at f/32 or less.

I think lensmakers have their reasons for placing limits on aperture stops. Like, stopping-down further makes the lens look like a bad performer. You saw the results of going past f/22. Of course the diffraction limit changes with larger formats. My favorite pinhead, er I mean pinhole platform is an oatmeal carton with an 8x10" piece of varicontrast paper as the film. Ultra-wide angle, infinite DOF, great exposure latitude, etc. And f/200 is no problem there.

[petrock]

David---I've been given a Kobori Vivitar 70-210mm f4.5 . It's a great lens. But it has a disturbing amount of mechanical wobble, between the tube that mounts stationary to the camera, and the two moving pieces considered as a unit: the zoom/focus ring and the end tube (holding the front lens element); there is no wobble between these last two.

Assuming that the lens you have taken apart here is the same or similar, can you say anything about this ? (The photos you posted about it on the MFLenses site are now unavailable.) I'm not afraid of taking the lens apart, but not if it involves specialized (or unavailable) tools, techniques, special calibration jigs, etc. In other words, I want to be able to put it back together right, whether I can fix it or not!

I've read somewhere about plastic bearing pieces wearing out. More optimistically, it also feels as if there were are a few set screws loose, as on a T-mount.

Have any advice? Thanks, Dave ! Pete Michaud, Salem, Mass.