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Infinity objective tube lens - use zoom or Raynox
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kpassaur



Joined: 24 Mar 2015
Posts: 22

PostPosted: Tue Nov 27, 2018 5:15 am    Post subject: Infinity objective tube lens - use zoom or Raynox Reply with quote

This is a simple question for those who know. From what I have read if I use a Nikon 10x Plan Objective on a Zoom Lens at 100mm I get 5x magnification and at 200mm I get 10X. What I need to do is set the rate of zoom and focus on a subject at infinity. I can't just turn the lens barrel as it will go past infinity for electronic focusing. Then add the objective and move closer or further to focus the subject. This part I understand (I hope if this is not correct please let me know).

Now if I were to use a Raynox close up lens as a tube lens the process would be the same. However, I would not be able to change the magnification. In other words if it were on a bellows it would have to be set at a given distance.

So in a nutshell with a Raynox close up lens can I change the magnification if it is on a bellows.
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santiago



Joined: 25 Sep 2018
Posts: 108
Location: Nijmegen, The Netherlands

PostPosted: Tue Nov 27, 2018 6:34 am    Post subject: Re: Infinity objective tube lens - use zoom or Raynox Reply with quote

kpassaur wrote:
So in a nutshell with a Raynox close up lens can I change the magnification if it is on a bellows.


You can to a certain degree. I own a Nikon 10x/0.25 and use it on bellows with a reversed Raynox DCR-150 (208mm) on full frame. This setup allows me to range from for example 7x through 12x without degrading the image quality badly and always covering the whole full frame. This is great for framing your subject. To get 5x from a 10x lens I think the Raynox DCR-250 (125mm) will be more suitable than the DCR-150, it will push down the Nikon 10x to 6.25X. But let's wait for other (more experienced) people to jump in...

Here is a picture of my setup (bellows + DCR-150 + Nikon 10x/0.25): https://www.photomacrography.net/forum/viewtopic.php?p=238320#238320
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kpassaur



Joined: 24 Mar 2015
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PostPosted: Thu Dec 06, 2018 4:41 am    Post subject: Thank you Reply with quote

Thank you. I have both the 250 and 150. What I am now confused about is the length of the bellows. They say set it a certain distance and you are saying there is some play in it. I'm all for the play in it. My thought was to make a ruler for the bellows and mark the magnification so I could calculate step size once and look at the bellows to see where I am at. I do that with my MPE-65, just frame, look at the scale then input the distance into my StackShot.
However, when I do the calculations they are all so close.

Mag f/stop Step Size
10x f/20 .096
9x f/18 .097
8x f/16 .098
7x f/14 .099
6x f/12 .102

Does this seem about right - a change in magnification of 1x requires a change of 1/1000 of a mm?
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santiago



Joined: 25 Sep 2018
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Location: Nijmegen, The Netherlands

PostPosted: Thu Dec 06, 2018 8:01 am    Post subject: Re: Thank you Reply with quote

kpassaur wrote:
They say set it a certain distance and you are saying there is some play in it.


For infinite objectives the tube lens must be focused at infinity, so this means that a fixed distance to the sensor must indeed be set. I advice you to try multiple distances and judge this for yourself. This is a bit personal. For example, I don't mind ranging from 8X to 12X or from 18X to 22X since the IQ remains definitely acceptable for me.

kpassaur wrote:
My thought was to make a ruler for the bellows and mark the magnification


That should work. In my tube assembly, in order to set the Nikon 10x/025 at 10x magnification, I need to set the bellows at 120mm (this distance is specific of my setup, all components (i.e. adapter rings, bellows brand) impact the total length and must be taken into account).

In case you don't know, to find this distance you can use a ruler (placed as perpendicular to the lens axis you can) as your subject to measure the actual width of your FOV. For a FF sensor, the FOV at 1x magnification will be 36mm width by 24mm height, so if you want to set your 10x objective at 10x magnification, make sure that the width of you FOV is 3.6mm (36/10). This is the actual width of the image being projected on the sensor. So for 20x the width of the FOV should be 1.8mm (36/20). Again, this is for FF sensors.

kpassaur wrote:
However, when I do the calculations they are all so close.


When using microscope objectives, step size is almost entirely determined by the numerical aperture. See Table 2-C on this page:

https://zerenesystems.com/cms/stacker/docs/tables/macromicrodof

(It does make sense, because when you slide the bellows to change the magnification (say from 9X to 11X) the distance from lens to subject doesn't change. You are just shrinking or enlarging the image projected on the sensor.)

To calculate a "safe" (no obvious OOF banding) step size in um, you can use: 0.5 / (NA^2), so for 10x: 0.5 / (0.25 * 0.25) = 8

I'm definitely not an expert on the subject, writing this reply to you is somewhat daunting considering the massive amount of expertise in this forum! Let's see if other people jump in...
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kpassaur



Joined: 24 Mar 2015
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PostPosted: Thu Dec 06, 2018 8:24 am    Post subject: Something is wrong on my end. Reply with quote

Okay, something is wrong on my end. I used the calculator on the extreme macro site to get this table.

Mag f/stop Step Size
10x f/20 .096
9x f/18 .097
8x f/16 .098
7x f/14 .099
6x f/12 .102


On the Zerene site it says for objectives use the Numerical aperture and for a .25 it is .0088. If I round up to um's? 1/thousands of a mm I have .009. I am off somewhere. But it is nice to know the step size is the same regardless of the magnification.
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santiago



Joined: 25 Sep 2018
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Location: Nijmegen, The Netherlands

PostPosted: Thu Dec 06, 2018 8:41 am    Post subject: Re: Something is wrong on my end. Reply with quote

kpassaur wrote:
I used the calculator on the extreme macro site to get this table.

Those values are correct when you use a "regular" macro lens (like the mpe65), where the f/stop is variable. I use that calculator as well.


kpassaur wrote:
On the Zerene site it says for objectives use the Numerical aperture and for a .25 it is .0088. If I round up to um's? 1/thousands of a mm I have .009. I am off somewhere.

Yes, On the Zerene site it says 0.0088 mm. Converted to um this is 8.8. When rounding a number I would personally always round to the smaller number, just to be "safe".
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kpassaur



Joined: 24 Mar 2015
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PostPosted: Thu Dec 06, 2018 9:01 am    Post subject: Converting Um's Reply with quote

Okay, it makes sense you move the decimal three places when using UM's. I'm not sure they do that on the Extreme Macro site. Or maybe its just me getting old and being American never really using the metric system.

I think now it is just getting adapters so I can move the objective and leave the tube lens in place.
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santiago



Joined: 25 Sep 2018
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Location: Nijmegen, The Netherlands

PostPosted: Thu Dec 06, 2018 10:15 am    Post subject: Re: Converting Um's Reply with quote

kpassaur wrote:
Okay, it makes sense you move the decimal three places when using UM's.


Yes, 1 milimiter is 1000 microns.

Quote:
I think now it is just getting adapters so I can move the objective and leave the tube lens in place.


I don't know if I understand you correctly but the only thing that actually moves when changing the magnification is the camera. The microscope lens and the tube lens remain at the same location.

Taking this picture as an example, to change the magnification you would need to turn the dial that is located on the upper part of the picture:



If you do that, only the camera will move and the accordion will contract or expand. The microscope lens and tube lens will remain at the same distance to the subject.
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kpassaur



Joined: 24 Mar 2015
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PostPosted: Thu Dec 06, 2018 10:35 am    Post subject: Moving the body Reply with quote

I'm confused now. If I move everything I change the focus point not the magnification. This is how I do it now with my MPE-65.

If I use a bellows and tube lens I have read that you remove the objective and focus on infinity. You then place the objective back on and move in or out to focus. If you use a Raynox 150 or 250 you will get different levels of magnification. But in each case you have the bellows set to match focus on infinity with the Raynox. So, you have two levels of magnification if you use this method.

Now If I change the distance of the objective to the tube lens does the magnification change? I know it changes if I change the distance from the sensor to the tube lens but as mentioned above you are not supposed to do that. Currently I have no real method of doing this except for putting in a balance of step up and step down rings (silly way but the only way I can think of) between the tube lens and the objective.

I have also read that the tube lens should be should be close together and in another post they should be a certain distance and finally in another you should put a couple of step up step down rings to increase the distance. This could have been to make it a certain distance it was not mentioned.

I have read on the Extreme Macro site with the Nikon Plan 10x you can get between 5x and 10x. (this is my goal) Is this being done with a zoom or by changing the distances and the tube lens magnification. Any ideas on this.
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19980
Location: Richland, Washington State, USA

PostPosted: Thu Dec 06, 2018 11:00 am    Post subject: Re: Something is wrong on my end. Reply with quote

Hi. I created the tables on the zerenesystems.com site and I'm very familiar with the calculations used by extreme-macro.co.uk.

kpassaur wrote:
Okay, something is wrong on my end. I used the calculator on the extreme macro site to get this table.

Mag f/stop Step Size
10x f/20 .096
9x f/18 .097
8x f/16 .098
7x f/14 .099
6x f/12 .102

I believe your confusion here is caused by the difference between effective aperture, nominal aperture, and numerical aperture (NA). If you don't know what those are, then please ask.

The calculator at http://extreme-macro.co.uk/focus-stacking/#calculator accepts nominal aperture as an input, and produces effective aperture as an output, using some simplifying assumptions about the optical setup.

I can reproduce your numbers by plugging in (for example) 10X f/20 and seeing .096 reported as a safe step size.

But 10X f/20 in that context essentially says that you set a lens on nominal f/20 (that is, f/20 at infinity focus), then extended that lens by 10 times its focal length to get 10X magnification. The result is an optical setup whose effective aperture is f/220, and you'll see that number printed at the bottom of the calculator as an output. That setup has a tiny aperture, would be crazy deep into diffraction territory, and the relatively large DOF (yeah, .096 mm is relatively large at 10x) reflects the small aperture.

In comparison, the zerenesystems.com tables say that with NA 0.25 a feasible step size is 0.0088 mm. It also happens that NA 0.25 at 10X gives an effective aperture of f/20. The corresponding settings in the extreme-macro.co.uk calculator would be 10x f1.8, giving an effective aperture of f/19.8 and a "safe step size" of .009 mm.

--Rik
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rjlittlefield
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PostPosted: Thu Dec 06, 2018 11:17 am    Post subject: Re: Converting Um's Reply with quote

santiago wrote:
I don't know if I understand you correctly but the only thing that actually moves when changing the magnification is the camera. The microscope lens and the tube lens remain at the same location.

Taking this picture as an example, to change the magnification you would need to turn the dial that is located on the upper part of the picture:



If you do that, only the camera will move and the accordion will contract or expand. The microscope lens and tube lens will remain at the same distance to the subject.

I see a huge opportunity for confusion here.

In the setup that is shown, the usual mode of operation is to turn the silver-colored dial/handle located at upper left in the picture. This moves the entire camera setup, including the bellows and lens, while not changing the accordion at all.

It is also possible to operate the setup by turning the small black knob on the rear standard of the bellows. If you do that, then indeed only the camera moves, while the accordion changes length and the lenses stay the same distance from the subject. However, this mode of operation is not commonly used because the movements are not as fine and are not calibrated, and the rear standard is liable to slide downwards under the weight of the camera.

--Rik
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santiago



Joined: 25 Sep 2018
Posts: 108
Location: Nijmegen, The Netherlands

PostPosted: Thu Dec 06, 2018 11:26 am    Post subject: Re: Converting Um's Reply with quote

rjlittlefield wrote:

In the setup that is shown, the usual mode of operation is to turn the silver-colored dial/handle located at upper left in the picture. This moves the entire camera setup, including the bellows and lens, while not changing the accordion at all.


Yes, this is what you do to take the sequence of pictures. Moving the whole setup forward. But when you want to frame your subject, by moving the rear knob you are changing the size of the image projected on the sensor, this is what I mean with "changing magnification" (the WD of the objective is fixed anyway). Doing this does not degrade the image, the image stays pretty much the same.

Or am I wrong? Smile
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rjlittlefield
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PostPosted: Thu Dec 06, 2018 11:27 am    Post subject: Re: Moving the body Reply with quote

kpassaur wrote:
Now If I change the distance of the objective to the tube lens does the magnification change?

If the tube lens is focused exactly at infinity, then changing the objective-to-tube-lens distance will make zero change in magnification.

If the tube lens is focused away from infinity, then changing the distance to the objective will change the magnification by a small amount, which is difficult to predict and is usually ignored.

A good model is that an infinity objective plus a tube lens is just the same as a finite objective.

Either setup is best when used at or near its design point, but you can change the magnification somewhat and still retain good image quality by changing the distance from optics to camera.

Changing the distance from optics to camera will also change the distance from optics to subject, but again that change is small and is usually ignored except for tweaking focus by eye.

--Rik
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rjlittlefield
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PostPosted: Thu Dec 06, 2018 11:41 am    Post subject: Re: Converting Um's Reply with quote

santiago wrote:
But when you want to frame your subject, by moving the rear knob you are changing the size of the image projected on the sensor, this is what I mean with "changing magnification" (the WD of the objective is fixed anyway). Doing this does not degrade the image, the image stays pretty much the same. )

I agree, because you've been careful to include the caveat that the image "stays pretty much the same".

It is the same point that I expressed in previous post as "best when used at or near its design point".

There's always some range surrounding the design point, where the image will be acceptably good. That range is wide for small NA objectives, and progressively narrower for larger NA.

In the world of finite objectives, the range is classically expressed in terms of acceptable change in tube length, summarized by the graph at http://www.science-info.net/docs/etc/Tube-Length-na.gif (original article HERE). You can see in the graph that at NA 0.25, those authors considered the acceptable change in tube length to be over 200 mm (starting from a base of 160 mm). But at NA 0.50, the acceptable change was less than 15 mm.

As a general recommendation, we say to focus the tube lens at infinity because that will work well with any objective. But yes, when working with low NA objectives, it is definitely practical to change the magnification by focusing the tube lens at other distances.

--Rik
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kpassaur



Joined: 24 Mar 2015
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PostPosted: Thu Dec 06, 2018 11:45 am    Post subject: I think I have the distance and magnificaiton bit down now Reply with quote

So, ideally I focus at infinity, add the objective and I get the best it can be. However, I can shorten or lengthen by bellows a little to change magnification. Notice I mentioned a little as it can also degrade the image. So it is what you an live with quality wise.

That being said, one more thing, (I fee like Columbo with one more thing) You mentioned changing the distance from the tube lens to the objective would not change magnification. Would it change the size of the image on the sensor. Currently I am losing a little in the corners on a FF, would changing this distance correct this or is it just how it goes? I could correct it by shortening the distance from the tube lens to the sensor but that would be a trade off as mentioned above.
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