Zoom for Mitutoyos

rjlittlefield · Post by **rjlittlefield** » Sat Apr 28, 2018 9:46 am

Lou Jost wrote:Rik, that's not what I was referring to. I'm referring to the problem you have often pointed out, that the aperture of the system should not be in the tube lens. If the tube lens is too slow, it's aperture becomes the controlling aperture, and this will cause aberrations. The aperture should be in the objective Iif the objective is much shorter in focal length than the tube lens, as is usually the case) or between the objective and tube lens.

Understood, but my point was that adding a teleconverter is not likely to introduce that problem, which is what it sounded like you were worried about.

--Rik

Lou Jost · Post by **Lou Jost** » Sat Apr 28, 2018 11:34 am

Rik, I'm not sure why you think it is unlikely. Can you explain?

rjlittlefield · Post by **rjlittlefield** » Sat Apr 28, 2018 1:00 pm

Lou Jost wrote:Rik, I'm not sure why you think it is unlikely. Can you explain?

I thought I did, when I wrote that

it would take a quite inappropriate teleconverter to change the pupil. Teleconverters cause a larger f-number because they increase the focal length, not because they change the entrance pupil.

Here's an experiment you can do. Hold a telephoto lens facing you, with a bright surface behind it so you can see the aperture. Then mount a teleconverter behind the lens. Does the aperture seen by your eye change? (It does not, with any of the lenses I've tested.)

--Rik

Lou Jost · Post by **Lou Jost** » Sat Apr 28, 2018 1:09 pm

But the combined TC+tube lens still becomes effectively one to two stops lower than the original tube lens.

rjlittlefield · Post by **rjlittlefield** » Sat Apr 28, 2018 1:51 pm

Lou Jost wrote:But the combined TC+tube lens still becomes effectively one to two stops lower than the original tube lens.

Agreed, but I'm struggling to understand why you think that matters.

My best guess is that you're hung up paying attention to the number, when you would do better to think about the light.

The point of the experiment I suggested is that adding an appropriate TC does not change which light rays get through the system, it just spreads them out over a wider area.

When that happens, the image gets larger and dimmer but no vignetting is added.

If there's something I'm missing here, I would appreciate you explaining to me what it is.

--Rik

Lou Jost · Post by **Lou Jost** » Sat Apr 28, 2018 3:36 pm

Well, yes, I am fixing on the numbers. If a tube lens is f/11, and the front lens was f/2, then wouldn't the tube lens be the limiting aperture?

"adding an appropriate TC does not change which light rays get through the system"

Sure it does, it cuts off a large fraction of the light rays leaving the tube lens.

rjlittlefield · Post by **rjlittlefield** » Sat Apr 28, 2018 4:10 pm

Replied by PM, check there please.

--Rik

mjkzz · Post by **mjkzz** » Sun Apr 29, 2018 12:52 am

rjlittlefield wrote:
Lou Jost wrote:Rik, that's not what I was referring to. I'm referring to the problem you have often pointed out, that the aperture of the system should not be in the tube lens. If the tube lens is too slow, it's aperture becomes the controlling aperture, and this will cause aberrations. The aperture should be in the objective Iif the objective is much shorter in focal length than the tube lens, as is usually the case) or between the objective and tube lens.
Understood, but my point was that adding a teleconverter is not likely to introduce that problem, which is what it sounded like you were worried about.

--Rik

Yes, if one follows the rule to keep the tube lens wide open, this is a non-issue. I guess what Lou was trying to say is to remind people to keep "tube lens+TC faster than objective".

However, I do know people from photography world, they know the "sweet spot" of a lens, still think to get sharper image, they can just stop down one or two stops from the "sweet spot", but the focal length for tube lens requirement makes them add a TC, but since a TC does not affect the "sweet spot", they think they can still stop down a couple of stops to get sharper image.

So, say a 100mm lens with "sweet spot"of f/4, a couple of stops down make it f/8, adding a 2X TC, now the EA of tube lens as whole is f/16. I think this is what Lou is trying to remind people to avoid. Say a 4x 0.2NA objective on such tube lens system.

My post was meant to say, a TC does reduce EA of tube lens, which now it seems that is what Lou meant, too. However, I do want to correct that a 2X TC causes 2.5 stops loss, it is only 2 stops loss due to optical system, the extra 0.5 stop is due to poor glass which should NOT be included in EA calculation.

Hope this clears things up and believe me, the "sweet spot" scenario does exist in real world.