Vignetage with relay lens

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soldevilla
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Vignetage with relay lens

Post by soldevilla »

Hi. As I have explained in previous posts, I am using finite microscope lenses as if they were infinite, mounted in front of a Vivitar zoom telephoto 100-200. Do not ask me the reason, but I get much better quality with this method. With the 4x and 10x, which are normal and cheap lenses, not LWD, I am more than happy with the result. The 20x is LWD, otherwise there is no way to illuminate. I have 3 20x; a Nikon, a Leitz and a JMI and all offer me with this system a field with little distortion but with enough chromatic aberration. Today I thought that maybe it is good not to force so much the increase and I have tried (I repeated an image taken with the 20x leitz, in fact) the ELWD x40; and I've zoomed out from my telephoto lens by 200 to 120 . The chromatic aberrations have practically disappeared, and that's very good, but when I reduce the focal length of my zoom, an unacceptable vignetting appears. I have tried with a telephoto of 135 and the vignetting is similar.
Is there a trick I do not know to lower the focal of the relay lens without appearing vignetted?

This is complicated. One is solving a problem and two problems appear as a consequence.

Image

This is an image with the 10x. No words...

Image

And this is the image with the 40x. No chromatic aberrations, but vignetage... It is only the Zerene image output, I have a lot of work before this become a picture to show.

harisA
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Post by harisA »

From what I understand your are using your finite objectives as infinites in combination with a telephoto lens as tube lens. This is possible with low magnification objectives since are much more tolerant than high magnification ones. When you are using the finite as infinite you are practically work with a different working distance (other than the one the objective was originally designed) forcing it to produce a parallel light beam instead of converging one. Optical aberrations will certainly arise especially with objectives of high mag.
When you are using an infite objective with a lower focal length tube lens vignetting is always a problem especially with objectives with small image circles so there is no cure for that other than using an objective with a big coverage.

rjlittlefield
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Post by rjlittlefield »

harisA wrote:When you are using an infite objective with a lower focal length tube lens vignetting is always a problem especially with objectives with small image circles so there is no cure for that other than using an objective with a big coverage.
To clarify, the vignetting that soldevilla sees is caused by the aperture in the lens fighting with the aperture in the objective. Light rays coming from the objective and headed for the edges of the sensor are blocked by the tube lens aperture, which is too small and too far back to allow those rays to pass. This problem is particularly common with zoom lenses that are set to be shorter than their maximum focal length.

Using an objective with a bigger coverage will not help this problem. The bigger coverage objective will just pass a wider range of light rays to get blocked by the lens aperture.

Using an objective with a wider aperture (larger NA) can help a little. The larger diameter bundle of light rays will include some that can get past the lens aperture at slightly greater angles.

A better approach is to use a short prime lens as tube, since its aperture will be larger and closer to the front of the lens. The best reduction in vignetting will be provided by something like a Raynox 250, mounted close to the objective. In that case no light rays will be blocked by the tube lens. Vignetting can still occur, solely due to the design of the objective, and in that case using a wider coverage objective definitely can help.

--Rik

soldevilla
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Post by soldevilla »

harisA, I agree, I am not using the objectives exactly as they were designed, but my tests offer me better results. Maybe, I do not know, I lose a bit of resolution in the center and in return I get a flatter image and with fewer aberrations in an image circle to fill the chip of my APS camera.

Rik, thanks for the note. I do not have a Raynox and I have found little information about its optical system, only it has 3 lenses and it has 8 diopters (125mm focal). But the Raynox looks like an improved version of the positive lens that is screwed to a camera lens to turn it into a macro. I have built some better than the simple lens using achromatic objectives from binoculars. I have just visually tested all my microscope objectives in front of an achromatic 120mm focal length and I do not have vignetage with any. I see spherical aberration in the 4x, but that lens is the one that worries me the least, because it works perfect with the zoom.

I now have a little work with my lathe to build the couplings, but I'm going to try this achromatic lens like Relay lens this week. And of course, I will explain results.

Thank you!

Lou Jost
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Post by Lou Jost »

Soldevilla, there are several kinds of Raynox, including one with 208mm focal length, perfect for a tube lens. As you know, many people on this forum use them with great results. They are also easy to couple to objectives.

soldevilla
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Post by soldevilla »

Well, first test done with my homemade "Raynox". Very happy with the result, the image is taken with the 40x and the FOV is 0.8mm. only. I like the definition and no hint of vignetting. I think I will look for binocular objectives from several focal points and will do some more testing.

the mineral on quartz is pending analysis.

Image

soldevilla
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Post by soldevilla »

Another test with the binocular objective as relay lens. This time with the JMIx20. FOV 2mm.
The colors are much more saturated (originally, they have no additional processing) and the haloes are much less obvious. The formula seems less optics produces sharper images

Image

concon
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Post by concon »

Opening this back up for clarification.

I'm using a vintage 200mm as a relay lens: https://www.pentaxforums.com/userreview ... mount.html

I have some massive vignetting that is similar to the original photo (if not a bit more). I shoot in aspc mode which negates this, but I'd like to wrap my head around it all and fix the problem.

I have an adapter which does push the lens out- is this contributing to the vignetting? I've played with the Raynox setup, but I don't quite enjoy certain aspects of it; I like how solid a telephoto relay lens is.

I don't mind investing into a newer 200mm with a lower F stop. I'm using the Mitutoyo BD objectives which I know have a different NA than the Ms. Do I need a lower stop to counter this?

(Also- I do have a telephoto lens that goes to 240mm- not sure if that'll be good to use or if I should just pick up one of the 200mm Canon primes on ebay)


Appreciate everyone's time!

rjlittlefield
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Post by rjlittlefield »

concon wrote:I have an adapter which does push the lens out- is this contributing to the vignetting?
Yes. When using a telephoto as tube lens, vignetting is always made worse by mounting the objective farther from the telephoto.

As a quick test of the potential to reduce vignetting, you can cut an 11 mm diameter hole in a piece of black paper and hold it against the front of the telephoto. There's no objective involved in this test. The hole in the paper mimics the rear aperture of the objective in its closest possible mounting position.

--Rik

concon
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Post by concon »

Thank you for confirming my suspicions :) I'll try playing with a few of the zoom lenses I have and will run some tests to see which may work best. Thank you Rik!

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