optics for low distortion for 0.5x-1x-objective/macro/te

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rjlittlefield
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Post by rjlittlefield »

Macro_Cosmos wrote:Image

Image

Image
Lou Jost wrote:Interesting test! I wonder why it's not 1x though. Maybe it has to do with them not being infinity-corrected optics?
The main reason it's not 1X is because the rear conjugate is way too long.

To get 1:1 with infinity objectives, the rear objective has to be positioned so that the sensor is located at that objective's normal parfocal point. For example, if the objective is rated at 15 mm WD, then the sensor has to be only 15 mm from the end of the objective. With finite objectives, the distance would be even a little shorter, so there's still infinity space between the objectives.

From the photos I can't see exactly where the rear objective is, but I gather that it's deep inside that Thorlabs tube, probably out beyond the end of the helicoid, which will place it something like 100 mm in front of the sensor.

The increased distance is also why this setup is giving full sensor coverage of a 35 mm frame. It's like taking a normal fullframe lens, putting it on 10 inches of extension, and observing that it will then cover 4x5.

--Rik

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Post by Lou Jost »

Also, shouldn't these finite objectives be spaced 300mm apart?

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Post by rjlittlefield »

Lou Jost wrote:Also, shouldn't these finite objectives be spaced 300mm apart?
That would give the as-designed focus relationship.

However, I'm pretty sure that if you did that, you'd find that the system vignettes horribly. The reason is that most of the light rays leaving the front objective would be too far off-axis to enter the second objective. That problem could be rectified by sticking a 150 mm FL "field lens" between the two objectives, to match the pupils without affecting the image focus, but that is needless complexity.

Infinity objectives, mounted close together, should be the best approach.

BTW, the reason I suggested 4X objectives is because they're commonly available, have good working distance, and the high quality field corresponds to the size of sensor that jimduk is talking about. Using lower magnification objectives will probably cut into the working distances, and higher magnification objectives will risk not giving enough coverage. If the 4X objectives give not enough DOF, then he could add a Waterhouse stop between the two objectives.

--Rik

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dolmadis
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Post by dolmadis »

rjlittlefield wrote:
One approach that comes to mind is to place two infinity objectives back-to-back, with an infinity section in the middle. For example, placing two 4X NA 0.10 objectives back-to-back will give you a 1:1 system with the same field and sensor sizes that the objectives would normally have, so roughly 6-10 mm diagonal.

--Rik
Rik - please explain "with an infinity section in the middle" to me in greater detail. Thanks. What this is for in the concept of the optical design. Do you mean that there needs to be a gap between the two objectives which will need to be determined experimentally?

Thanks


John

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Post by rjlittlefield »

dolmadis wrote:
rjlittlefield wrote:
One approach that comes to mind is to place two infinity objectives back-to-back, with an infinity section in the middle. For example, placing two 4X NA 0.10 objectives back-to-back will give you a 1:1 system with the same field and sensor sizes that the objectives would normally have, so roughly 6-10 mm diagonal.

--Rik
Rik - please explain "with an infinity section in the middle" to me in greater detail. Thanks. What this is for in the concept of the optical design. Do you mean that there needs to be a gap between the two objectives which will need to be determined experimentally?

Thanks


John
Here's a ray-trace of the concept, showing two objectives with 50mm FL and 5mm radius (NA 0.1), placed back-to-back with 40mm of separation and a 2.5mm radius Waterhouse stop to take the system down to NA 0.05.

Image

My phrase "infinity section in the middle" refers to the area between the two lenses, where each point on the subject is represented by a bundle of parallel rays. Different points on the subject are represented by bundles of rays that are parallel within each bundle, but at different angles for different bundles. This is just like looking at the night sky, "infinity".

I've set up this model to use a 4 mm object radius, so 8 mm diagonal. Notice that there's no vignetting in the model, because the stop and the distance between lenses are sized so that any ray aimed to go through the stop will also be able to make it through the glass.

There's nothing very particular about distance between the objectives. The shorter it is, the more you're using the central area of each lens, which will probably minimize aberrations.

The system can also be made telecentric by extending it far enough that the stop is be placed at the rear focus distance for the front lens. That forces the edge of the field to use more peripheral sections of the lens, which probably cuts into the image quality. The system may also have to be stopped down farther to avoid vignetting.

In this second model, I've made the system telecentric, kept the 8mm diameter field, and reduced the NA to 0.03 so as to eliminate most but not all of the vignetting. Notice that the ray fan for the edge of the field is narrower than for the center (so, corner darkening), and also that the edge of the field uses only the edge of the lens.

Image

I hope this helps.

--Rik

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Post by dolmadis »

Thank you very much Rik.

This will take me time to understand and appreciate but the content and style of presentation will make that so much easier.

BR


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Post by jimduk »

Hi all, just wanted to thank everyone in this thread, really helpful, and I've got to some decent results

So I took the back to back lens idea (thanks Rik/Enrico/Lou), bought the thorlabs tubes ( thanks Macro_cosmos) and some 4x Luminoptic infinity corrected plan achromat objectives (£50 each).

Sensor used was a monochrome global shutter 4.8 micron Onsemi 1MP - 1280x1024 slightly bigger than target 3.75 micron but easiest thing to hand.

Job 1 was to find focal lengths of the objectives - thought from the internet it was 4mm ish but turned out to be 40mm ish from front lens to target.

Job 2 was to get one lens in the tube focussed on the sensor - did this by using it to take focussed pictures at a distance (20 metres - see attached pic slightly cropped
takes a fairly nice picture) Image

Job 3 is then to put in the other lens, mount the system on a manual stage and get an LED screen picture ( attached) - as you can see the planarity of the picture is pretty good and the focus looks good enough. Depth of field fairly tight but not a problem with a micrometer screw stage, (didn't put much effort into ensuring alignment, also system on a wooden table which wobbles)
Image

Thanks so much - Job 4 is now to test, and then later to work out if we could get similarish quality results (maybe at a single illumination) using the same principle with simpler back to back lens, or whether this is good enough.

Thanks again - this (macro/micro) is a really interesting area and obviously goes deep

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Post by rjlittlefield »

jimduk, nice job!
Job 1 was to find focal lengths of the objectives - thought from the internet it was 4mm ish but turned out to be 40mm ish from front lens to target.
The focal length of an infinity objective will always be tubeLensFL/objectiveMagnification. I don't know the Luminoptics in particular, but typical tube lens FL is either 180 mm or 200 mm, so 4X objective will be either 45mm or 50mm FL.

But I think what you really need to know is the parfocal distance, not the focal length. All objectives intended to be used together will have the same distance from mounting shoulder to focused subject. If that distance is DIN standard 45mm (common for inexpensive objectives), then 45mm is the proper distance from mounting shoulder to sensor. That gives you an infinity section between the two lenses.

With the rear lens focused at infinity, as you've done beautifully, the net magnification will be the ratio of objective FL's, or equivalently, the ratio of nominal magnifications. So, pairing a 4X with a 4X will give you 1:1, and so will pairing a 10X with a 10X. The difference is that the first pair will be at NA 0.10 (or whatever else the 4X objectives are rated for), while the second pair will be NA 0.25. The first pair will have significantly less resolution, but will cover a larger area.
Depth of field fairly tight but not a problem with a micrometer screw stage
If the DOF you have is not enough for your application, you can put a "Waterhouse stop" between the two objectives. That stop is just a flat plate with a hole in it. The simplest material is something like black paper, which can be punched without leaving reflective edges on the hole. If you need some particular NA, the radius of the hole is just NA*objectiveFL.

Please keep us informed as you move forward.

--Rik

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Post by dolmadis »

rjlittlefield wrote: The system can also be made telecentric by extending it far enough that the stop is be placed at the rear focus distance for the front lens. That forces the edge of the field to use more peripheral sections of the lens, which probably cuts into the image quality. The system may also have to be stopped down farther to avoid vignetting.

In this second model, I've made the system telecentric, kept the 8mm diameter field, and reduced the NA to 0.03 so as to eliminate most but not all of the vignetting. Notice that the ray fan for the edge of the field is narrower than for the center (so, corner darkening), and also that the edge of the field uses only the edge of the lens.--Rik
Hi Rik

Is there any mileage please in considering this telecentric arrangement of two back to back objectives for an S&S macro photographic rig?

Is there anything which would need to be changed or upgraded?

Thanks


John

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Post by rjlittlefield »

dolmadis wrote:Is there any mileage please in considering this telecentric arrangement of two back to back objectives for an S&S macro photographic rig?
In most cases this would not be the best approach. The basic problem is that in any telecentric system, the telecentric field can be no larger than the width of the front element, minus the width of the entrance cone where it enters the front element. Microscope objectives typically have small diameter lenses, so the telecentric field would be small also. I mentioned the possibility for jimduk's system because of his special needs with small subject, small sensor.

For most applications, it would be better to use a larger sensor and larger lenses, giving a larger telecentric field if that's even needed. Typically some highly asymmetric system like the DIY combos described in https://www.photomacrography.net/forum/ ... hp?t=18323 would be a better approach, assuming that you don't have the budget and inclination to go with a commercial telecentric lens.

--Rik

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Post by dolmadis »

Thanks Rik for the explanation.

John

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