Vivitar Series 1 28-90 mm f/2.8-3.5 with strong astigmatism

VLR · Joined: 05 Mar 2015 Posts: 86

Hi everyone,

so I recently acquired a very nice Vivitar S1 28-90 mm (Komine, 28xxxxxx) with minimal signs of use. The lens came with a slightly loose front element or second element, which was easy to fix by tightening three meade screws. Since I always like to check what I've bought, I shot the usual test chart. And this is what I got at 50 mm f/3.2 (it's similar at 28 and 90 mm):

Right corner

Left corner

As you can see, the lens delivers good results on the right side. What you can't see is that the center is about the same - pretty sharp, no problems. But the left side shows softness and horrible astigmatism! I double checked if the camera was perpendicular to the chart, shot it at different angles and checked if the mount was straight and solid - all good. Because I originally fastened the front element, I tried to reseat it by loosening up the screws and fastening one of them a little more than the others, but to no avail. There's little movement anyway. The front lens element seems to be centered by some other means and the screws just hold it in place. I concluded that this probably isn't the problem, but can't totally exclude it.

I'm out of ideas for now. Any of the optics gurus around here got a tip for me Question

Lloydy · Expire: 2022-01-01

By 'meade screws' are you referring to screws that are behind the name ring? I haven't had mine apart, but those screws are possibly for infinity adjustment ?

VLR · Joined: 05 Mar 2015 Posts: 86

It's probably easiest to explain with the parts list:

http://www.boggys.co.uk/download.php?filename=Vivitar_S1_28-90mm_f2.8-3.5_Komine_PL_400dpi.pdf

I'm referring to the part number 2 ("set screws") on page 2 / PDF page 6 in the parts list. Those meade screws were loose on arrival.

Lloydy · Expire: 2022-01-01

I would try loosening the screws and rotating the front lens group 180 degrees and repeating your test to see if the problem appears on the opposite side.

VLR · Joined: 05 Mar 2015 Posts: 86

OK, I tried to turn the front group (1) today, but I couldn't get it to move. Seems to me as one of the formerly loose set screws damaged the thread... That kind of narrows down my options Neutral

I still could loosen the whole front helicoid (a3), but I need a little bit more patience for this than I have today.

VLR · Joined: 05 Mar 2015 Posts: 86

OK, I got the time to play around a little, today. Turning the front group (1) by about 180� didn't change anything besides the infinity focus. I reassembled everything the way it was before and got the same images again. I then took a spanner wrench and just turned the back group (22) 90� clockwise. That made the problem way worse, but at least something changed. After trying 90� counter-clockwise from the original position and some adjustments I finally settled somewhere around 30� ccw. This is how the lens performs now:

Left corner

Right corner

Although the astigmatism is still strong, it is at least evenly distributed, now and sharpness is about equal left and right. I think it's acceptable now, but I'm also open for any ideas to make it even better!
Very Happy