Old Vivitar Zoom -- which way does this interior element go?

cooltouch · Joined: 15 Jan 2009 Posts: 9097 Location: Houston, Texas

OK, I picked up this old Vivitar lens for very cheap -- mostly because it was made by Tokina, so I'm figuring it might just be a decent lens. It's a two-ring 75-260mm f/4.5 in Nikon pre-AI mount.

The lens has issues. It's been apart before, and was dismantled by an amateur. A spring used to keep tension on the aperture iris was never hooked back up and the little detent ball used to produce the clicks with the aperture ring is also missing. As I dismantled the lens, I found a very small spring that had become trapped under the aperture ring. But it wasn't meant for that, and it's too small to be used to keep tension on the aperture iris. So I have no idea what this little spring was used for. Maybe I'll figure it out yet.

So there's nothing I can do about the detent ball. I don't have any that small. Hmmm -- maybe a drop of solder. I can produce some spherical drops by melting it, then tossing it in the air? It'll dry to droplets on the way down to the floor or table. Just a thought. I have a spring assortment, so I found one that I was able to bend into shape and make work for the aperture iris.

Okay, but as I was handling the mechanism that contains the iris, I inverted it and a single lens element fell out, hit the floor and rolled around some. Dang! And the element is not symmetrical. One side is flat and one side convex.

It is located toward the rear of the lens. There s a rearmost element group, consisting of five lenses in one cylinder. The rearmost lens in the group is convex facing out. The front most element in this group is slightly concave. The single element whose orientation I don't know is positioned immediately in front of the front element in this group. It rests against the rear of the iris mechanism, and the 5-element cylinder thread into the iris mechanism. When threaded all the way down, it holds this single loose element in place.

So, I'm calling on those of you who know a thing or two about optics. Which way do you think this single element should be oriented? With the convex side facing the iris, or the flat side facing the iris? On the other side of the iris are the zoom group(s) and, beyond that, the front group or groups.

I've noticed this much. If I hold the element against the front of this rear group with the convex side facing out, it focuses on a subject about 4 or 5 inches away and things are in focus from edge to edge. If I flip it around, it still focuses on a subject about 4 or 5 inches away, but it is not in focus from edge to edge. It's in focus in the center, but blurry on the edges.

So just based on this quick little test, I'm inclined to think the convex side should face the iris. Does this make sense to you?

I don't want to put the lens back together only to find I've positioned this one lens opposite from how it should be, and have to take it apart again. It was easy to disassemble, but still . . .

Here are a few pics.

A shot of the front of the lens, showing the Tokina serial number:

Here is the lens in pieces. If you look closely at the iris mechanism (the piece with a couple of levers sticking up), you can see the spring I added to it so the iris will be in tension.

And here's a side view of the element in question:

visualopsins · Expire: 2025-04-11

That seems correct, concave facing aperture, based on design diagrams I've seen -- the element other side of aperture should also have concave face toward aperture, i.e. toward lens rear -- two concave surfaces face each other, with aperture in-between.

Good luck!

cooltouch · Joined: 15 Jan 2009 Posts: 9097 Location: Houston, Texas

But whichever way I orient that one element, there will be no "concave facing aperture." The element is flat on one surface and convex on the other. Not sure about the group on the other side of the aperture -- I haven't really paid much attention to it.

But with the groups following the aperture, I have the choice of a flat surface facing the aperture or a convex surface facing the aperture. As I mentioned above, I'm leaning toward the convex orientation since that provides the clearer image.

visualopsins · Expire: 2025-04-11

cooltouch · Joined: 15 Jan 2009 Posts: 9097 Location: Houston, Texas

I just took a look at the rear element in the front group (in front of the iris) and it is very slightly convex.

So I'm gonna go with the 'convex toward the iris' configuration for that loose element.

visualopsins · Expire: 2025-04-11

Looks like a winner!

image belongs to http://kmp.bdimitrov.de/lenses/

cooltouch · Joined: 15 Jan 2009 Posts: 9097 Location: Houston, Texas

Thanks, looks good to me.

Reassembly has been kinda tricky so far. Figures.