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Quite an unusual Minolta lens
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PostPosted: Sun Jan 28, 2018 3:41 pm    Post subject: Quite an unusual Minolta lens Reply with quote

Out of curiosity I've got a couple of Minolta lenses which were part of a microfilm printer.

From outside they look almost like any projection lens, but with a dented ring around.




And when you look inside... there is a prism which is put over the front lens.







The lenses are x29 and x33. They only focus very close macro mode and have a very short distance to sensor. I could make "freelens" shots fitting the lens directly to the mount ring of my Nex-5N.




The bokeh is smooth but it seems to me I never arrive to get the appropriate pin sharpnes which any reproduction lens should have.



And as you see, the lens gives a mirror inverted Alice in Wonderland image thanks to the prism.




Maybe you have an experience with such lenses nad got better results than I could have? I presume, the alignment of the lens to the sensor should be much stricter, and the quick result I've got is compromised with quite an approximate orientation to the sensor.

As for the lens itself, I tried to count bulb reflexions. As seen, all elements are coated. But I am confused, is it 3 or 5 reflexions? I mean those two fader ones are just ghosts, the ones from cemented elements or "normal" ones from another two elements?



I ask myself what if I unscrew the prism, aside the mirror effect disappear, could I have a better distance to focus and easier way to get the sharpness?

Overall, a cute item, isn't it?


PostPosted: Sun Jan 28, 2018 8:56 pm    Post subject: Reply with quote

I think unless somebody else knows you will try removing it and show us!


PostPosted: Mon Jan 29, 2018 3:00 am    Post subject: Reply with quote

You are right! I will wait some time if the expert knowledge comes from the forum members. If not, will set myself to disassemblying.

Meanwhile I thought over the mode the lens was used initially and figured out that I might wrongly oriented it for the first test shots. I put it with the "mount" side to the camera mount and stretched the prism towards the object. But then I said: maybe in printing microfilms that should be the contrary, the prism side towards the film, and the lens side is projecting the image. Maybe?

So I turned the lens bottom up and here are results, mostly from the 33x which seems to be sensibly brigther, by the way, than the 29x. Like one or two stops.

#1


So, the image comes much closer now, like in real macro. These are not crops but full images resized. And I have no way to get a wider angle. That makes me ask how the heck they were used in microfilm printig, with such close focusing distance from the both sides.

#2


The images also become better focused (if I may say it like that), at least in a very tiny zone. But the issue remains the same, I don't arrive to have a shot which seems a "normal" one, with a selected sharpness point. All the shots give an impression of my hands shaking hard or, at best, of a special OOF effect.

#3


So, I take another attempt and shift from keyboard buttons to a more conventional object which is a piece of coin. This one is with 33x and the next is with 29x.

#4


Is not that strange? Well, the keyboard button #2 even pleases me quite a bit (its top left corner), as it has an aspect being made of moving sunlight spots. But there must be another way to use this lenses to print A4 paper sheets from microfilms, right?

The next shot is just to give you a proof of normality. I took a normal Hexanon, put a 100mm projector lens on its front lens and that gives a kind of macro. As you see, even handheld it is finely handled.

#5


Hmm...


PostPosted: Mon Jan 29, 2018 6:54 am    Post subject: Reply with quote

These are lenses from a microfiche reader with x29 and x33 being the magnification factors
(29x, 33x). The built in prisim is to erect the image. Not really that useful those lenses these
days.


PostPosted: Tue Jan 30, 2018 3:09 am    Post subject: Reply with quote

I easily unscrewed the lens "hood".




Just to discover that the screws fixing the prism plate to the barrel are glued.




So I have to find a solvent and a stronger (longer) screwdriver than my tiny watch ones. The further disassembly is delayed.

What is interesting, the prism is tall. It seems to arrive up to the front lens.




Looking from outside I had an idea that the prism was short, with a large air space between it and the lens.


PostPosted: Thu Feb 15, 2018 3:39 am    Post subject: Reply with quote

I dismantled the prism. Finally, that was not difficult at all, the screws only seemed to be glued. Here is the lens:

#1


And here is the prism pulled out.

#2


What result that gives? A nice macro-lens. Without the prism the lens is in focus at several centimetres, with quite a pleasant OOF rendering.

#3


The colours are natural (no post-processing).

#4


And shots are sharp (a 100%-crop, unprocessed).

#5


Here is a demonstration of the sharpness it gives.

#6


A 100%-crop of the previous, unprecessed. Please note the paper texture in focus.

#7


The same subject, with the lens reversed.

#8


Another type of surface, straight view, unprocessed.

#9


The same, with the reversed lens, unprocessed.

#10


Not bad at all. What would you say about it, if taken as a dedicated macro-lens?


PostPosted: Thu Feb 15, 2018 5:01 am    Post subject: Reply with quote

The results look much more usable.