Infinite objectives/Rated magnification

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AlexC
Posts: 17
Joined: Thu Apr 18, 2019 3:34 am
Location: Oregon

Infinite objectives/Rated magnification

Post by AlexC »

I have what I think is a rather simple question, but I can't seem to find the answer.

When having an infinite objective (let's say 5x) on a bellows, is it only possible to increase magnification past 5x by extending the bellows?

And if I'm looking for something more around the 2x-3.5x magnification mark, is it good to push an objective past it's rated magnification?

Seeing as I want less than 5x a lot of the time, would something like the Laowa 25mm 2.5-5x ultra macro lens be a good starting point rather than an expensive miti, or Nikon?

ChrisR
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Post by ChrisR »

See the faq: http://www.photomacrography.net/forum/v ... hp?t=12147 about connecting a microscope objective to your camera.

An infinite objective has to go on a "tube" lens, of 200mm to work at its rated magnification.

A shorter tube lens will give you less M, but also cover less, so it may vignette. So that depends on your sensor size. Some zoom lenses work better than others as tube lenses.

A "finite" objective can go on bellows, but works best around its designed length to the sensor, usually either 150 or 200mm, Again, you get vignetting if you go too short.

That Laowa lens isn't bad at all, though it's not as good as a Mitutoyo 5x objective.

If you do a forum search on Laowa AND 25mm and tick the "Search on Posts" option, you'll get some good results.
Chris R

AlexC
Posts: 17
Joined: Thu Apr 18, 2019 3:34 am
Location: Oregon

Post by AlexC »

I have a finite objective on a bellows currently at 150mm, but it is definitely a low end kit objective from a beginner scope and really loses sharpness around the edges. It also has quite a bit of CA. So i'm in need of an improvement for my optics.

If I purchased a miti 2.5x, would it okay to extend the bellows to get close to 5x, or would the image quality degrade by pushing it that far? I say that because im hoping to have the option of 2.5-5x without having to purchase multiple objectives.

ChrisR
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Post by ChrisR »

You would expect a finite objective to vignette and/or corner performance to degrade, if you go short - a few are better than others.
You have to be careful about which objectives to buy!

You wouldn't use a Mitutoyo (infinite objective) on bellows, it needs a "tube " lens. Again, reducing magnification, by using a shorter tube lens, will lead towards vignetting.

You can push some of the limits some of the time, and depending on how fussy you are.

I know a museum which uses a Mitty 5x on a Canon 100-300 zoom and they're happy to use it a lot. They wear cameras out in 8 months. But they don't produce monster prints. Horses and courses...
Chris R

AlexC
Posts: 17
Joined: Thu Apr 18, 2019 3:34 am
Location: Oregon

Post by AlexC »

Ok thank you for the info. I appreciate it. I was under the impression I could put something like a to raynox on the end of the bellows and use an infinite objective, is this good in practice or should I stay away from going this?

ChrisR
Site Admin
Posts: 8662
Joined: Sat Mar 14, 2009 3:58 am
Location: Near London, UK

Post by ChrisR »

You can - the Raynox acts as a "tube" lens, but it should be focused at infinity. Suggest you re-read the faq - it's all in there, though you may need to read several posts to "get it".
Chris R

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