Vintage lens problem

[fredjeang]

ps: Sorry for my bad english, i'm french.

I'm looking for technical clew(s) about a strange behaviour that I have never
seen before on a vintage lens. Anyone who's got an answer or serious information
about is welcome to add its experience commentaries.

I have been using vintage lens on dslr for quite some time without any problem, i thought i've seen a lot... until
now.
I've found an old K mount 24mm from Hoya and decided to give it a try on a Pentax DSLR. It's not a good lens, tipically a third party brand from the 70's, but sometimes they can be surprising. In this case, the surprise came from what I didn't expect at all.

At 2.8, the lens is sharp-enough with very little distortions and lost in the edges.
At 4 it is absolutely usable.

Then, I stop-down at 5.6 and just in the center of the frame, there is a circle out of focus,
while the rest of the frame stays sharper in perfect focus (as expected).  
this blur is clearly visible and geometrically (symetrically) perfect.
At 8, the "out-of-focused" center frame gets worse. In fact, more you stop-down more the
center frame gets blurred. I cannot say if it is out-of-focused or if it is an optical effect with dispersed light that blur the center frame.

It looks like these horrible filters fashionable in the 70's in David Hamilton's style.
I've never seen such a behaviour in the center of the frame, especially when stop-down.

On the field, this lens is only usable at 2.8 and 4, the effect start from 5.6 to 22 (totally blurred).
This phenomena happens only in the center frame, (about 15% total frame), and at all distances.
The rest of the frame is perfectly focused.
I've checked it and looks perfect to me, the six aperture blades are working well and lens is in mint condition.
All I can say is that it is not coated, but I doubt it has something to do.

Well, that could be an interesting new way to experiment different kind of picture...
To stay serious, this lens is not worth loosing time with it, but I'm curious to know the rational reason for this behaviour;
specially if such a problem has been seen and/or commented before with some vintage lens on digital slr.

In that case it would be an interesting and useful information to know
for vintage lens buyers/collectors.

Thank you very much

Fred

[Jonathan Wienke]

Post a sample image that shows the oddity.

[fredjeang]

You are right Jonathan,
Here is the first pic at max aperture 2.8, everything is OK, center frame is in focus (little soft, at 4 it's perfect) while borders are out of focus, normal at this close distance and aperture.


[attachment=19602:hoya_f4.jpg]

The second pic shows the same image at F11, same distance and focus point:  borders are now sharper than F2.8, normal;  but the center frame is
totally blured.

[attachment=19603:hoya_f11.jpg]

Any idea? ( also, the lens focused normally at infinity, but the strange thing is that with vintage lens you always keep focus confirmation, even if
there are no physical contacts with body; with this one, focus confirmation is not operative even if your are in focus )

Fred.

[Jonathan Wienke]

There is something on one of the lens elements near the center, perhaps a particle of dirt, a bit of mold, a smear of oil, or a chip in the lens itself, that is causing the blurring. With the aperture wide open, it's not noticeable because it is affecting only a small portion of the active lens area. but as you stop down, it becomes a larger percentage of the active lens area (flaw stays same size, active lens area gets smaller when you stop down) and therefore becomes more noticeable. See http://visual-vacations.com/Photography/ap..._lens_usage.htm for a demonstration of this effect. If you inspect the lens carefully you should be able to find the cause of the problem. If you're fortunate it will be on the front or rear of the lens and can be cleaned. If not, it will be inside the lens where removing it requires disassembling the lens.

[fredjeang]

Thank you very much Jonathan.
I think you are absolutly right. This also explains why focus confirmation strangely doesn't work with this lens (as setted in center) even when image is in focus. My eye can not see the "dirt" in the viewfinder but electronic yes.
I see that I am in the good forum here.
Now, I will look at it closely and try to fix it.
Thanks again for your good and fast reply.

Fred.