Raynox DCR-150 tube assembly with flocking

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Post by rjlittlefield »

mawyatt wrote:Is it that important to get this distance just right from an IQ standpoint, rather than an exact magnification standpoint?
It's not at all important at low NA, using objectives at say 10X and below. But the scaling goes as NA^4, so it gets more important quickly for higher magnification objectives.

My standard reference for this is the set of graphs at http://www.science-info.net/docs/etc/Tube-Length-na.gif , which I'll show as an inline display here for convenience:

Image

Long ago I shelled out for the full text of an original article that contains this figure and explains how it was constructed .

Quick summary is that the curves come from experimental observation, using data from a couple of observers who were very strict. Quoting their words: "The star test is so critical that data obtained with it represent the least possible image degradation, one that probably could not be detected in most images."

The observers noted that at NA 0.25, their nominal 160 TL objective could tolerate over 200 mm of additional tube length before even their star test showed a problem. Their NA 0.50 tolerated almost 15 mm, and even at NA 0.65, tolerance was 5 mm. Interpolating their results, it would require NA 0.55 to barely detect a difference with +-3.5 mm of tube length, which is the accuracy that be achieved with standard extension tube sets.

The experiments reported in the article were concentrating on spherical aberration near the center of the frame. Other aberrations can kick in farther from center, and they won't necessarily scale the same way. But even so, the rule of thumb is that small changes are fine, especially at low NA.

Perhaps the biggest problem I face in trying to provide guidance in this area is that what's OK depends so strongly on NA. The tolerance goes inversely as NA^4, so a delta that has no significant effect at NA 0.25 can become a big deal at NA 0.50, where NA^4 is 16 times larger. Even a delta of +-3.5 mm could become relevant if somebody mounts up an NA 0.90 or 0.95 objective.

--Rik

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Post by Lou Jost »

Rik, this is for finite objectives, right? Is it really relevant to the question of how much extension to give a tube lens? Seems like that is a different question.

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Post by rjlittlefield »

Lou Jost wrote:Rik, this is for finite objectives, right? Is it really relevant to the question of how much extension to give a tube lens? Seems like that is a different question.
Ah, sorry, I should have addressed that question in my post. I agree that it seems like a different question. But really it's not.

You can think of an infinity objective plus tube lens, in combination, as being a finite objective. The light doesn't care. It just comes in as a wide cone at the front of the lens, makes its way through the glass, and exits as a much narrower cone that focuses some distance back.

If I take away your prior knowledge that the light rays happen to form parallel bundles at some place inside the assembly, then you can't tell whether you're dealing with an infinity combo or an objective that was designed to be finite from the beginning. In fact some finite objectives have infinity sections in their middles anyway, as part of their overall finite design.

In either case, the effect of changing back focus distance ("extension", if you like) is to alter front focus distance by a complementary amount. Making this change ends up altering the optical path lengths by different amounts that depend on how far each ray is from the optical axis, and that's what introduces spherical aberration.

--Rik

Ultima_Gaina
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Post by Ultima_Gaina »

rjlittlefield wrote:
Ultima_Gaina wrote:This is valuable information and a good starting point for the experiment.

I am using the tubes from Wemacro. It means that I am starting from 160mm + the adaptor thickness. I just ordered an M42 helicoid and I'll do some tests.
Glad to hear.

Note that my measurements are from the front end of the lens. Extension length to the back of the lens will be smaller by the thickness of the lens, about 15.5 mm for the one that I have.

So, given that Canon EF flange focal distance is 44.0 mm, the labels on my lens imply that extension from sensor plane, with the lens in reversed orientation, calculates to be 175.9 - 15.5 + 44.0 = 204.4 mm. This is pretty close to the 208.3 mm that one might guess based on the nominal 4.8 diopter strength of the lens. For practical purposes, either value would probably work fine, but again, if you want it dead on there's no substitute for focusing on a distant object.

--Rik
Ok, the helicoid has arrived and I did some infinity focusing tests (well, near infinity, I focused with live view zoomed 10x, on a license plate, 564 feet away)

As you know, WeMacro tubes are made of 3 x 50mm segments and 1 x 10mm segment.

Image

I replaced the middle 50mm segment with the helicoid, and here is what I found.

Image

It means that my current setup is 50mm - 41.2mm = 8.8mm too long. The good news is that the system has this 10mm tube, which seems to be unnecessary for infinity focusing.

Image

By removing it, I will end-up 1.2mm too short and that should be better than 8.8mm too long.
I don't think I can land better than this unless I keep the helicoid in place.

As reference, I measured the total length of the tubes + the reverse Raynox adaptor, but without the lens (my caliper is not big enough).
That's 155.1mm in total.

Image

This confirms that, without the 10mm tube, given a few tenths of mm measurement error, the achieved focal length is 155.1mm+44mm = 199.1mm
That's virtually spot-on for nominal magnification and best IQ.

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Post by rjlittlefield »

Ultima_Gaina wrote:the helicoid has arrived and I did some infinity focusing tests (well, near infinity, I focused with live view zoomed 10x, on a license plate, 564 feet away)
Excellent, now we can run a crosscheck.

I see your setup as 10 + 50 + 41.2 + 50 mm of tubes, plus roughly 6 mm for some reversing adapter in front of the lens. Including the 15.5 mm of lens thickness, that puts total extension to the front shoulder of the Raynox at 172.7 mm for you, versus the 175.9 mm that I measure with my DCR-150 on Canon mount. If your target were at infinity your extension would be a little shorter yet, about 0.25 mm difference by my calculation.

So in total there's about 3.4 mm of difference between your setup and mine.

Some of that is undoubtedly due to individual variation between the Raynox lenses.

Some of it could also be that the measurement is best done with the Raynox stopped down to 11 mm diameter like a Mitutoyo objective will do. This will avoid influence of spherical aberration in the Raynox when used wide open. My 175.9 mm is with the stop, versus 174.7 mm without a stop (which is not nearly as crisp).

--Rik

Ultima_Gaina
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Post by Ultima_Gaina »

rjlittlefield wrote:
Ultima_Gaina wrote:the helicoid has arrived and I did some infinity focusing tests (well, near infinity, I focused with live view zoomed 10x, on a license plate, 564 feet away)
Excellent, now we can run a crosscheck.

I see your setup as 10 + 50 + 41.2 + 50 mm of tubes, plus roughly 6 mm for some reversing adapter in front of the lens. Including the 15.5 mm of lens thickness, that puts total extension to the front shoulder of the Raynox at 172.7 mm for you, versus the 175.9 mm that I measure with my DCR-150 on Canon mount. If your target were at infinity your extension would be a little shorter yet, about 0.25 mm difference by my calculation.

So in total there's about 3.4 mm of difference between your setup and mine.

Some of that is undoubtedly due to individual variation between the Raynox lenses.

Some of it could also be that the measurement is best done with the Raynox stopped down to 11 mm diameter like a Mitutoyo objective will do. This will avoid influence of spherical aberration in the Raynox when used wide open. My 175.9 mm is with the stop, versus 174.7 mm without a stop (which is not nearly as crisp).

--Rik
I took your hint and I repeated the test, focusing on a telecom tower 5070 feet (1.55km) away. I removed the 10mm extension tube and the helicoid gave me 50.7mm (instead of the default 50mm), at maximum sharpness:

Image

The 42M reverse adaptor for the Raynox adds exactly 5mm. Maybe there is roughly one more mm between the lens glass and the edge of the tube.

So, the total focal length for a Canon camera is roughly 44 + 150.7 + 5 + 1 = 200.7mm

I removed the helicoid and put back the default 50mm tube.
With this permanent configuration, and a 50x objective, I got this, on a 22.3mm APS-C sensor:

Image

1 unit is 10 microns. It translates in an effective magnification of x52.5, instead of the nominal x50.

Now, I suppose this is enough evidence to advise Wemacro tube users to remove the 10mm tube, even if the website is depicting it as part of the system:

Image

Maybe the purpose of this 10mm tube is to use it for 210mm objectives, but this is not evident.

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Post by rjlittlefield »

Ultima_Gaina, it looks to me like your numbers do not list the M42 adapter on the camera.

Can you measure how much extension that adapter adds?

(A "standard" adapter would be 1.46 mm, that being the difference of flange focal distances between Canon 44 mm and Praktica-Pentax 45.46 mm.)

--Rik

Ultima_Gaina
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Post by Ultima_Gaina »

rjlittlefield wrote:Ultima_Gaina, it looks to me like your numbers do not list the M42 adapter on the camera.

Can you measure how much extension that adapter adds?

(A "standard" adapter would be 1.46 mm, that being the difference of flange focal distances between Canon 44 mm and Praktica-Pentax 45.46 mm.)

--Rik
You are right Rik; the adapter has exactly 1.46mm:

Image

The total 44+1.46+150+6 = 201.46mm is still not enough to explain the practical x52.5 magnification, but it's one step closer.

Anyway, what seems clear is that the 10mm segment is not needed.

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Post by rjlittlefield »

Ultima_Gaina wrote:Anyway, what seems clear is that the 10mm segment is not needed.
Yes, I agree.

I'm just using your careful measurements, combined with mine, to get a little feel for how manufacturing tolerances in the lenses might affect people's setups. Two data points is not enough to get a reliable indication of variance, but two is infinitely better than one in this regard!

So now, I have your extension from camera flange to shoulder of lens, at infinity focus, as being 1.46+50+50.7+50+5 ~= 157.2 .

In comparison, I have just now rechecked my setup, and the corresponding number is 158.9 .

I'm still wondering, did you happen to stop down the Raynox when taking the measurement? If not, then your measurement is likely to be a little bit too short still. This is due to spherical aberration in the Raynox, which causes its outer regions to focus a little closer than the center.

In any case, we're now down to only about 1.7 mm difference between your measurement and mine.
The total 44+1.46+150+6 = 201.46mm is still not enough to explain the practical x52.5 magnification, but it's one step closer.
Earlier, you wrote
This confirms that, without the 10mm tube, given a few tenths of mm measurement error, the achieved focal length is 155.1mm+44mm = 199.1mm
But that's not correct. You've overlooked that the total extension from sensor to glass is not the same as the lens focal length. Focal length is measured with respect to special places called "principal planes". There are two of those, one for each orientation of the lens. With some lenses, such as telephotos or very wide angle lenses, the relevant principal planes can be quite far outside the lens. With the Raynox, both planes are probably someplace inside the lens body, but there's no reason at all to expect that they'll be located at any lens surface.

When the rear lens is focused at infinity, all that counts is its focal length. Physical extension just whatever it needs to be to reach infinity focus.

Given that the Raynox is nominal 4.8 diopter = 208.33 mm focal length, and assuming that you're using a 50X objective designed for 200 mm tube lens, then the expected magnification would be calculated as 50*208.33/200 = 52.1X .

Seems to me that you're quite close to that!

--Rik

Ultima_Gaina
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Post by Ultima_Gaina »

rjlittlefield wrote:
Ultima_Gaina wrote:Anyway, what seems clear is that the 10mm segment is not needed.
Yes, I agree.

I'm just using your careful measurements, combined with mine, to get a little feel for how manufacturing tolerances in the lenses might affect people's setups. Two data points is not enough to get a reliable indication of variance, but two is infinitely better than one in this regard!

So now, I have your extension from camera flange to shoulder of lens, at infinity focus, as being 1.46+50+50.7+50+5 ~= 157.2 .

In comparison, I have just now rechecked my setup, and the corresponding number is 158.9 .

I'm still wondering, did you happen to stop down the Raynox when taking the measurement? If not, then your measurement is likely to be a little bit too short still. This is due to spherical aberration in the Raynox, which causes its outer regions to focus a little closer than the center.

In any case, we're now down to only about 1.7 mm difference between your measurement and mine.
The total 44+1.46+150+6 = 201.46mm is still not enough to explain the practical x52.5 magnification, but it's one step closer.
Earlier, you wrote
This confirms that, without the 10mm tube, given a few tenths of mm measurement error, the achieved focal length is 155.1mm+44mm = 199.1mm
But that's not correct. You've overlooked that the total extension from sensor to glass is not the same as the lens focal length. Focal length is measured with respect to special places called "principal planes". There are two of those, one for each orientation of the lens. With some lenses, such as telephotos or very wide angle lenses, the relevant principal planes can be quite far outside the lens. With the Raynox, both planes are probably someplace inside the lens body, but there's no reason at all to expect that they'll be located at any lens surface.

When the rear lens is focused at infinity, all that counts is its focal length. Physical extension just whatever it needs to be to reach infinity focus.

Given that the Raynox is nominal 4.8 diopter = 208.33 mm focal length, and assuming that you're using a 50X objective designed for 200 mm tube lens, then the expected magnification would be calculated as 50*208.33/200 = 52.1X .

Seems to me that you're quite close to that!

--Rik
Yesss. That makes sense. I was indeed assuming that the measurement must start from the glass surface. But your calculation explains everything. Thank you!

Otherwise, the difference between my two measurements comes from the 1.46mm of the M42 adapter indicated by you, and the length of the reverse lens adapter. It is 5.1mm when I measure the external ring, but it looked to me that the surface of the glass is a little deeper inside the lens, and I used 6mm in my second calculation, matching your initial assumption. To compare our measurements, we should stick to 5.1mm. So we are down to 1.6mm difference.

I didn't stop down the lens. The telecom tower I focused on was in the middle of the frame, so I don't think that potential spherical aberrations mattered too much.

P.S. since the system has two clamps, I realized that I can unscrew one end of the middle segment just by 0.7mm, while everything is still firmly kept in place. Maybe I will insert a 0.7mm wire around that small gap and properly tighten everything back.
Last edited by Ultima_Gaina on Sat Feb 01, 2020 11:54 pm, edited 1 time in total.

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Post by rjlittlefield »

Ultima_Gaina wrote:I didn't stop down the lens. The telecom tower I focused on was in the middle of the frame, so I don't think that potential spherical aberrations mattered too much.
It's not a huge amount, but yes, spherical aberration matters even in the center of the frame for this sort of measurement. What happens is that without the stop, something over 3/4 of the light that forms the image is coming through the outer section of the aperture, which (because of SA) focuses closer than light coming through the center section. Essentially you're setting infinity focus on a part of the lens that you'll never actually use, at least in the center of the frame.

Checking my post from earlier in this thread, I see that I said
My 175.9 mm is with the stop, versus 174.7 mm without a stop (which is not nearly as crisp).
So, almost a 1.2 mm change, with the stop versus without.

If it works the same with yours, then our difference would be down to only 0.5 mm.

--Rik

Ultima_Gaina
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Post by Ultima_Gaina »

rjlittlefield wrote:
Ultima_Gaina wrote:Anyway, what seems clear is that the 10mm segment is not needed.
Yes, I agree.
I think I understand now why the 10mm segment is included in the Wemacro kit.

Without it, I get some vignetting on a FF camera: on a 5D Mk3, I have to crop the image from 22.3Mpix, down to 19-20Mpix. A little disappointing :(

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Post by rjlittlefield »

Without [the 10mm segment], I get some vignetting on a FF camera
Hhmm... That's odd. With this sort of setup, vignetting is usually caused by small diameter tubes where they enter the camera. In that case, if the tube inner diameter does not change, then the vignetting is made slightly worse by adding extension. That's because the added extension makes the corner rays slightly more parallel to the optical axis, which gives them less chance to spread out after they exit the tube at the camera.

Does your 10 mm segment have thinner walls or thinner flocking than the next tube in line?

--Rik

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Post by Ultima_Gaina »

rjlittlefield wrote:
Without [the 10mm segment], I get some vignetting on a FF camera
Hhmm... That's odd. With this sort of setup, vignetting is usually caused by small diameter tubes where they enter the camera. In that case, if the tube inner diameter does not change, then the vignetting is made slightly worse by adding extension. That's because the added extension makes the corner rays slightly more parallel to the optical axis, which gives them less chance to spread out after they exit the tube at the camera.

Does your 10 mm segment have thinner walls or thinner flocking than the next tube in line?

--Rik
The 10mm segment has no flocking and the same diameter as the other tubes. I remember having tried the FF camera before (with the 10mm segment included) and there was no vignetting.
I'll do some more testing.

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Post by Ultima_Gaina »

rjlittlefield wrote:
Without [the 10mm segment], I get some vignetting on a FF camera
Hhmm... That's odd. With this sort of setup, vignetting is usually caused by small diameter tubes where they enter the camera. In that case, if the tube inner diameter does not change, then the vignetting is made slightly worse by adding extension. That's because the added extension makes the corner rays slightly more parallel to the optical axis, which gives them less chance to spread out after they exit the tube at the camera.

Does your 10 mm segment have thinner walls or thinner flocking than the next tube in line?

--Rik
I did an additional test with my FF 5D Mk3 (vertical sensor size 24mm), using a Nikon CFI Plan Fluor 20x/0.45 ELWD
Here are the results (1div = 0.1mm)

1. without the 10mm segment, I get ~1.15mm or x20.8 magnification and visible vignetting.
Image

2. with the 10mm segment, I get ~1.12mm or x21.4 magnification and no vignetting.
Image

3. I installed the 10mm segment, in the middle between two 50mm tubes.
And I get the vignetting! Therefore, it's not about the additional magnification, as I thought.

You were right, again, Rik!

The vignetting comes from the flocking of the nearest 50mm tube! Maybe a could shave off a little, without affecting its purpose.

Thanks a bunch!
Last edited by Ultima_Gaina on Thu Feb 06, 2020 6:59 pm, edited 3 times in total.

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