Calculating magnification of objective on telephoto lens

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lothman
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Calculating magnification of objective on telephoto lens

Post by lothman »

Administrators' note: This topic has been split off from How good are alternatives (clones) to Mitutoyo lenses--AdminCS
Barrelcactusaddict wrote:
Tue Oct 26, 2021 1:32 pm
.... Still, the clone looks like it does quite well; I'm kind of curious to see how the clone performs at 20x (i.e., with a 100mm telephoto lens), which is what I'd be doing in my setup. Of course, I'd be using it only to photograph insect inclusions in amber, so it wouldn't be used very often as a metallurgical-type objective.
Just a remark. It is a 10x lens, so with a 100mm tube lens you would get 5x not 20x. And I think then you would have to use APS-C Format.

On the other hand it would be interesting to edit the 60 Megapixel full frame, then shrink them down to 24 Megapixel compared to a 10x lens with 100mm tube lens at APS-C with 24 Megapixel. I assume shrinked full frame would win.

Barrelcactusaddict
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Re: How good are alternatives (clones) to Mitutoyo lenses

Post by Barrelcactusaddict »

lothman wrote:
Tue Oct 26, 2021 2:54 pm
Barrelcactusaddict wrote:
Tue Oct 26, 2021 1:32 pm
.... Still, the clone looks like it does quite well; I'm kind of curious to see how the clone performs at 20x (i.e., with a 100mm telephoto lens), which is what I'd be doing in my setup. Of course, I'd be using it only to photograph insect inclusions in amber, so it wouldn't be used very often as a metallurgical-type objective.
Just a remark. It is a 10x lens, so with a 100mm tube lens you would get 5x not 20x. And I think then you would have to use APS-C Format.

On the other hand it would be interesting to edit the 60 Megapixel full frame, then shrink them down to 24 Megapixel compared to a 10x lens with 100mm tube lens at APS-C with 24 Megapixel. I assume shrinked full frame would win.
Hm, I was under the impression that in order to roughly find the magnification factor, or actual magnification of an SLR lens, you'd need to divide the focal length by 50 (100mm ÷ 50 = 2xMF >> 2xMF X 10x objective = 20xMF). I would be using an objective on an APS-C camera, so the actual magnification would additionally appear slightly greater in a captured image.

This all seems to be the case from what I've been told from fellow user, holdinghistory on thefossilforum.com; he even recommended I step down to using a 2x objective for larger insect inclusions (anything over 10mm in length), because his 5x objective when used with the 200mm lens (SMC Pentax-A 200mm F4 manual focus lens) has too high a magnification (about 20x MF). Due to budget restraints, I mentioned I would go the route of purchasing a 100mm lens (in addition to my 200mm Pentax lens of the same model) instead of getting a 2x objective in addition to a 5x: he and I both agreed this should work in theory, but I'm waiting to see the results once my setup is complete. I should mention that he uses the 200mm lens while set to infinity, since he uses infinity-corrected objectives.

http://www.thefossilforum.com/index.php ... phy-setup/

Scarodactyl
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Re: How good are alternatives (clones) to Mitutoyo lenses

Post by Scarodactyl »

Barrelcactusaddict wrote:
Wed Oct 27, 2021 9:27 am
Hm, I was under the impression that in order to roughly find the magnification factor, or actual magnification of an SLR lens, you'd need to divide the focal length by 50 (100mm ÷ 50 = 2xMF >> 2xMF X 10x objective = 20xMF). I would be using an objective on an APS-C camera, so the actual magnification would additionally appear slightly greater in a captured image.
No, these are paired lenses and you use the ratio of focal lengths to determine magnification. A "5x" objective meant for a 200mm tube lens will have a 40mm focal length (200/40=5). Put it on a 100mm tube lens and you get 2.5x magnification. However there are limits to how far objectives can be pushed down--mitutoyos are unusually tolerant because of their enormous image circle, while most objectives will do very poorly when pushed that far out of spec.
Note the mitutoyo 2x objective is a real underperformer, particularly in chromatic aberrations towards the corners even on aps-c. That may matter less in amber where things are more monochrome.

Barrelcactusaddict
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Re: How good are alternatives (clones) to Mitutoyo lenses

Post by Barrelcactusaddict »

I have to disagree. holdinghistory takes photos with that camera-objective setup as part of his online business, and has hundreds of hours of experience (if not into the couple thousands), as he has indicated to me through personal communication on thefossilforum.com; he does not use paired lenses, reversed lenses, extension tubes, or anything of the sort: just a simple telephoto lens (at least since last I spoke with him, less than a month ago). His photos he has shared (from the previous link to his topic) have not been cropped in post-production, and are an actual representation of his setup with the 200mm Pentax lens I mentioned and a 5x infinity-corrected objective.

Something to keep in mind: insect inclusions in amber are extremely small, most only a couple mm in length, often being smaller than their extant relatives (where applicable). I routinely use a 10x BeLomo loupe (held millimeters from the eye) for inspection of my rough and finished amber, as well as during the lapidary process, and I find that average-sized viewable inclusions (2-5mm in length) are decently viewable, but anything smaller than that, and a 20x loupe is required. I'll also mention that my eyesight is quite good, 20-30.

I trust the judgement of holdinghistory, and if the technical details don't match, at least the results do. :wink:

https://www.the-digital-picture.com/Can ... Value.aspx

Here's one from B&H Photo Video:
Attachments
Screen Shot 2021-10-27 at 11.39.06 AM.png

Chris S.
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Re: How good are alternatives (clones) to Mitutoyo lenses

Post by Chris S. »

Barrelcactusaddict,

There is no disagreement between what Scarodactyl told you (correctly) and what holdinghistory has done in the example you provided. However, it appears that you’re getting tripped up by how magnification is achieved in different types of photography. The examples you cite from B&H and The-Digital-Picture are appropriate for telephoto lenses used alone with distant subjects, such as bird photography.

What holdinghistory is doing, and what Scarodactyl explained, is the use of a pair of lenses for macro or micro photography. Holdinghistory is, in fact, demonstrating a pair of lenses—the 200mm lens plus a 5x microscope objective attached in front of it. If you dig into the specifications for the microscope objective, you’ll find that its focal length is something close to 40mm. With paired lenses, the magnification on sensor equals the focal length of the rear lens divided by the focal length of the front lens: 200mm / 40mm = 5 X.
Note the mitutoyo 2x objective is a real underperformer, particularly in chromatic aberrations towards the corners even on aps-c. That may matter less in amber where things are more monochrome.
I don’t disagree with these observations, but do weigh them somewhat differently in my work. While it’s true that there are considerably better performers at 2x, and I have a few, I more often reach for the Mitutoyo 2x for its convenience. It is parfocal and parcentric with the other Mitutoyo objectives, which means I can use it without rearranging anything between 2x and 100x. Also, the particular type of chromatic aberration it exhibits (lateral, and largely limited to blue) is easy to correct in post. And as Scarodactyl said, it’s probable that very little blue light gets through amber.

--Chris S.

--edited typo

Scarodactyl
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Re: Calculating magnification of objective on telephoto lens

Post by Scarodactyl »

Yeah, don't get me wrong I use my 2x plenty, but vs the performance of their 5x, 10x and 20x it's just not in the same league. Don't want anyone to get their hopes up if they're unaware.

I do wonder if any other 95mm parfocal 2x or similar (2.5x, 3x, the other mitutoyo 2x variant) objectives perform better.

Barrelcactusaddict
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Re: Calculating magnification of objective on telephoto lens

Post by Barrelcactusaddict »

Okay, now I'm very confused.

If the tube lens (200mm telephoto) is set to infinity, and the objective is infinity-corrected, does that mean that the 200mm lens itself does absolutely nothing (could I have saved a bundle, and just have purchased an extension tube)? Also, how the heck does pairing, say, a 10x Mitutoyo objective with a parfocal length of 95mm, with that same 200mm lens equal out to just over 2.1x MF? Why should a 10x objective have less magnification than a 5x when combined with a lens already set at infinity??? This concept makes absolutely no sense to me whatsoever.

Does this mean I purchased adapters for a 100mm lens for no reason at all? I have no use for photographing inclusions at a measly 2.5x MF: that would be useless. This ruined my day, I can't believe I just wasted my money. :(

Unless... would it be possible to increase the MF if I were to go away from setting the telephoto lens to infinity, and turn the focus knob in the opposite direction (with the 5x infinity-corrected objective attached)?

Doppler9000
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Re: Calculating magnification of objective on telephoto lens

Post by Doppler9000 »

Barrelcactusaddict wrote:
Wed Oct 27, 2021 4:09 pm
Okay, now I'm very confused.

If the tube lens (200mm telephoto) is set to infinity, and the objective is infinity-corrected, does that mean that the 200mm lens itself does absolutely nothing (could I have saved a bundle, and just have purchased an extension tube)? Also, how the heck does pairing, say, a 10x Mitutoyo objective with a parfocal length of 95mm, with that same 200mm lens equal out to just over 2.1x MF? Why should a 10x objective have less magnification than a 5x when combined with a lens already set at infinity??? This concept makes absolutely no sense to me whatsoever.

Does this mean I purchased adapters for a 100mm lens for no reason at all? I have no use for photographing inclusions at a measly 2.5x MF: that would be useless. This ruined my day, I can't believe I just wasted my money. :(

Unless... would it be possible to increase the MF if I were to go away from setting the telephoto lens to infinity, and turn the focus knob in the opposite direction (with the 5x infinity-corrected objective attached)?
The Mitutoyo objectives deliver their rated magnifications with a 200mm tube lens. Ignore the parfocality numbers here.

The effective magnification scales by the ratio of the focal length of tube lens you use to 200mm. For example, if you used a 300mm tube lens with a 10x objective, you would have an effective magnification of:

300/200 = 1.5 x 10x = 15x.

A 100mm tube lens will yield a 5x magnification.

rjlittlefield
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Re: Calculating magnification of objective on telephoto lens

Post by rjlittlefield »

Adding some explanation to the points made by Doppler9000...

The microscope objective by itself makes an object that is actually small and close, look as if it's a very large object, very far away. In other words, the objective forms what's called a "virtual image at infinity". Then the tube/telephoto lens takes the light from that virtual image at infinity, and refocuses it to form a physical image back on the sensor.

If you were to remove the tube/telephoto lens, then the light reaching the sensor would be spread out all over the place, not focused to form an image. You would get something close to a totally uniform gray frame.

In this arrangement, the optical magnification of the pair of lenses (objective+tube) is determined by the ratio of focal lengths: magnification = rear focal length / front focal length.

Mitutoyo objectives are designed to give rated magnification when used with a tube lens that is 200 mm focal length. So, a 5X objective has focal length 40 mm because 5 = 200/40, and a 10X objective has focal length 20 mm because 10 = 200/20.

Because overall magnification comes from the ratio, you can adjust the overall magnification by using the same objective with different tube/telephoto lenses. Using that "5X" objective with a 100 mm tube lens will actually give 2.5X, because 2.5 = 100/40. Likewise, using the "10X" objective with a 100 mm tube lens will actually give 5X, because 5 = 100/20.

The 95 mm parfocal length has no bearing on any of this. That is just the distance between a focused subject and the shoulder of the physical mounting threads.

Does this help?

--Rik

Doppler9000
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Re: Calculating magnification of objective on telephoto lens

Post by Doppler9000 »

Some further thoughts on magnification.

The crop factor is not relevant here, the analysis is based on magnification and the sizes of the subject and the sensor.

One way to think about this is to summarize some subject size/magnification pairs that fill the frame. Your sensor is roughly 22mm x 15mm. Assuming a subject in the center 80% of the frame, the long dimension is 16mm

=

6.4mm subject @ 2.5x

3.2mm subject @ 5x

1.6mm subject @ 10x

Lou Jost
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Re: Calculating magnification of objective on telephoto lens

Post by Lou Jost »

I should also add that the quoted magnification of the telephoto lens has nothing to do with anything related to microphotography. That is just the magnification of the object on the sensor at the lens' closest focusing distance. But these lenses, when used as tube lenses, are set to infinity. The magnification of any telephoto lens focused at infinity, when used by itself without a reversed lens or objective in front of it, is essentially zero.

Barrelcactusaddict
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Re: Calculating magnification of objective on telephoto lens

Post by Barrelcactusaddict »

I've read and re-read everything very carefully, but I cannot understand any of it. My whole setup and everything I've purchased so far seems like it's now worthless for my needs. I've wasted so much of my hard-earned money, I can't believe it; over a year's worth of savings, down the drain. :cry:

I thought that an infinity-corrected objective forms an image and the light exiting the objective is a perfectly straight line. Why shouldn't that be fine in and of itself? I have an RMS thread to Canon EF-S mount adapter, that was made by RAF Camera; this thing mounts the objective directly onto the camera body, so why would he sell such a thing if it would not work? I don't get it.

Why doesn't a telephoto lens set at infinity behave the same way as an infinity-corrected objective? Why does a microscope objective hurt the magnification of a telephoto lens? Were those articles about dividing the focal length of a camera lens by "50" to find its magnification a bunch of malarkey? I don't understand what anyone is saying! :cry:

Barrelcactusaddict
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Re: Calculating magnification of objective on telephoto lens

Post by Barrelcactusaddict »

Lou Jost wrote:
Thu Oct 28, 2021 9:35 am
I should also add that the quoted magnification of the telephoto lens has nothing to do with anything related to microphotography. That is just the magnification of the object on the sensor at the lens' closest focusing distance. But these lenses, when used as tube lenses, are set to infinity. The magnification of any telephoto lens focused at infinity, when used by itself without a reversed lens or objective in front of it, is essentially zero.
So can I use a telephoto lens while set at it's closest focusing distance while using an infinity-corrected objective with it? Would this give me the magnification I had hoped for, and produce a clear image?

Lou Jost
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Re: Calculating magnification of objective on telephoto lens

Post by Lou Jost »

Why does a microscope objective hurt the magnification of a telephoto lens?
If you are confused by that, you should be. It is not true, and you must have misinterpreted something. As I said above, the magnification of a telephoto lens focused at infinity is essentially zer0. But if you put an infinity-corrected objective in front of it, the magnification is much greater than zero (and is equal to the focal length of the telephoto lens divided by the focal length of the objective).

Lou Jost
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Re: Calculating magnification of objective on telephoto lens

Post by Lou Jost »

So can I use a telephoto lens while set at it's closest focusing distance while using an infinity-corrected objective with it?
No, the telephoto should be set at infinity.

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