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Raynox and (real ?) infinity
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Cicindal



Joined: 04 Jan 2019
Posts: 1

PostPosted: Fri Mar 22, 2019 8:09 am    Post subject: Raynox and (real ?) infinity Reply with quote

Hi,

I recently bought infinity-corrected lenses and a raynox.

I set the "infinity" on moon and did some shots : it's OK.

But something is bugging me : when we use a telephoto as bellows (like a 200 mm), it's focused at infinity (on infinite cursor), if you focus on the moon, it's not the same focus, and in a big way.

So, can not we have better results if we set up the raynox lens at the real infinity ? (don't know how, maybe at theorical focal lenght).
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enricosavazzi



Joined: 21 Nov 2009
Posts: 1183
Location: Valdemarsvik, Sweden

PostPosted: Fri Mar 22, 2019 9:23 am    Post subject: Re: Raynox and (real ?) infinity Reply with quote

Cicindal wrote:
Hi,

I recently bought infinity-corrected lenses and a raynox.

I set the "infinity" on moon and did some shots : it's OK.

But something is bugging me : when we use a telephoto as bellows (like a 200 mm), it's focused at infinity (on infinite cursor), if you focus on the moon, it's not the same focus, and in a big way.

So, can not we have better results if we set up the raynox lens at the real infinity ? (don't know how, maybe at theorical focal lenght).

The infinity mark on the distance scale of a telephoto lens does not have to be precise. Focusing on the moon (or any subject farther than 50-100 m) is far more reliable than setting infinity on the focus scale. This applies to the Raynox and other tube lenses as well (as opposed to set a given distance between lens and camera, based on calculations of focal length).

There are reasons why many camera lenses intentionally allow focusing beyond infinity (mostly to allow for changes in focusing distance caused by ambient temperature) as well as accidental reasons (impacts, improper service).

Having said that, it has become apparent recently that in some cases one may want to intentionally focus a tube lens some distance (either direction) from infinity, so infinity focus is not always the optimal solution in each and every case.
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iconoclastica



Joined: 25 Jun 2016
Posts: 219
Location: Wageningen, Gelderland

PostPosted: Fri Mar 22, 2019 9:35 am    Post subject: Reply with quote

I am not sure I understand all of your question. So perhaps I am answering the wrong point now Rolling Eyes

The raynox much behaves like a thin lens. Meaning, that when an object is in the focal point in front of the lens, the rays behind the lens are afocal, that is parallel. And reverse: an object at infinity projects an image at focal distance behind the lens.

A telephoto lens focused at infinity expects the incoming rays to cross at distance infinity. By definition, that means parallel. So with the raynox in front of that lens, the object must be the focal distance (208mm) in front of the Raynox.

When the Raynox is used as tube lens in combination with an infinity objective, the situation is reverse: the incoming rays are afocal, so it projects a sharp image at focal distance behind the Raynox. That is where the sensor should be. Even when the Raynox is reverse-mounted, the frontside back, it works nearly the same.

In theory the telephoto lens may be focused at any distance. Then the object distance in front of the Raynox increases. It does not, though, necesarily gives identical image quality.

I don't know what difference you observe between the moon and infinity. On my telephoto lens I don't notice such a difference. I never have used a lens that could focus beyond infity.

Wim
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Oscar_macro



Joined: 30 Dec 2018
Posts: 18

PostPosted: Fri Mar 22, 2019 9:53 am    Post subject: Reply with quote

...I have had and still have the same doubt, it is not the same to point to the moon that at a great distance even kilometers, I do not know which is the correct one. But reading the forum does not seem to change so much that confuses me more ... I chose to point to a micrometric rule and move the bellows until it gives the magnification that should give and so I do and it seems that everything looks sharp. This point in my case coincides with pointing very far, but not with pointing to the moon, which I usually take many pictures of. I do not know if it will help or confuse more... Rolling Eyes
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Pau
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Joined: 20 Jan 2010
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Location: Valencia, Spain

PostPosted: Fri Mar 22, 2019 9:56 am    Post subject: Reply with quote

Wim, by the reasons that Enrico has explained, many telephoto (and maybe most autofocus) lenses allow to focus beyond infinite* although the actual infinite focus is obtained when a distant subject (not necessary to be so far than the moon or sun) is in focus.

* even if the infinite mark is at its right position
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Pau
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PostPosted: Fri Mar 22, 2019 10:07 am    Post subject: Reply with quote

Oscar_macro wrote:
...I have had and still have the same doubt, it is not the same to point to the moon that at a great distance even kilometers, I do not know which is the correct one.

AFAIK with a 200mm lens it's the same, maybe it isn't with a 2000mm one. Do you see an actual focus difference with a 200mm tele or Raynox?

Oscar_macro wrote:
But reading the forum does not seem to change so much that confuses me more ... I chose to point to a micrometric rule and move the bellows until it gives the magnification that should give and so I do and it seems that everything looks sharp. This point in my case coincides with pointing very far, but not with pointing to the moon, which I usually take many pictures of. I do not know if it will help or confuse more... Rolling Eyes

Very small focus differences of the tube lens make no visible difference in sharpness, specially with objectives of NA <0.40
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Oscar_macro



Joined: 30 Dec 2018
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PostPosted: Sat Mar 23, 2019 6:12 am    Post subject: Reply with quote

Hi Pau, I'm not sure I understand your question, my English is very bad. Maybe I have not explained myself well, what I mean is that if I take the camera I put it on the tripod and point to a house about 5 kilometers and put it in focus is a measure in the bellows, according to the answers from various forums have said that it should be more than enough that distance to measure that point to infinity, I did it and it works well. At night I was curious and put back in the tripod the same camera with the bellows and the raynox, point to the moon and the point is quite different. So I do not know what exactly means to focus on infinity.
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iconoclastica



Joined: 25 Jun 2016
Posts: 219
Location: Wageningen, Gelderland

PostPosted: Sat Mar 23, 2019 8:31 am    Post subject: Reply with quote

Oscar,

Do we understand you correctly, that you only have the raynox and the bellows on your camera. That you first focused on the distant house at 5km and fixed the bellows at the proper length. That you then pointed the camera to the moon and had to set the bellows extension to a much shorter length?

From my desk I could do this test:
- I focused the bellows + raynox at the nearest chimney (about 50m) and noted the length of the bellows on its own scale;
- then, without moving the camera or anything, I focused on the furthest trees I can see (about 300m) and noted the length of the bellows on its own scale;

I did see that the trees neede refocusing. But the difference in bellows length was less than one millimeter. Following the lens formula, it should have been 0.7mm. If we will have such a beautiful moonrise tonight as we had yesterday, I may try that too.

Anyway, when I apply the distance of the house (5km) and the moon (184000km) to the lens formula, the lens-to-sensor distances should differ only 8.65 micrometer.

-------
EDIT: The moon showed up last night reliable as ever, albeit hazy behind light clouds. I therefore could not get a very reliable focus. But doing the best I could, I found it c. 25mm closer then the trees above. I admit this is intriguing...
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aidanmoore



Joined: 24 Sep 2015
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PostPosted: Tue May 28, 2019 4:51 pm    Post subject: Reply with quote

My apologies if this is redundant but I'm stumped on why my visual adjustment of a "focus at infinity" 200mm (208mm) DCR-150 Raynox is not even close to what this and other forums are saying.

Many posts here talk about optimizing the infinity focus of the DCR-150 tube lens by looking through the camera and adjusting the distance between the sensor and the Raynox until you can focus on the moon or some far away object.

I should say that I have been happily using a DCR-150 as a tube lens with a 10X and 5X Mitutoyo for some time, so my basic setup must be correct. But I cannot get anything like "infinity" focus when I look through my camera with just the Raynox DCR-150 at the end of the tube.

Here is my camera with the DCR-150 on the end.



When I look through the viewfinder, focus only occurs at a very narrow range of distances around 180mm from the end of the Raynox. Getting infinity focus is out of the question, as I cannot focus beyond the 180mm or so as I move the bellows in and out.

I must be missing something very fundamental here and I'm hoping someone can point out my misunderstanding. Thanks as always.
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abednego1995



Joined: 01 Nov 2016
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PostPosted: Wed May 29, 2019 2:46 am    Post subject: Reply with quote

The Raynox DCR-150 is 4.8diopters which equals to f=210mm. Your setup looks a bit longer than 210mm from the Raynox to the camera image plane. Taking a few extention tubes off the front would probably get you to inf.

* And, yes you still can get images from infinity objectives and tube lenses that are focused to other than infinity. Should introduce spherical aberrations to the system, sometimes it works, sometimes it doesn't.

Cheers,
John
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aidanmoore



Joined: 24 Sep 2015
Posts: 80
Location: Toronto

PostPosted: Wed May 29, 2019 3:53 am    Post subject: Reply with quote

Thanks for the tip John,

I must have misunderstood the "200mm" or "208mm" as approximate distance between the back of the Raynox and the camera image sensor plane.

When I backed off the 42mm camera extensions to a value of around 120mm from the back of the Raynox to the D800 image plane mark I can now focus out my window to 2-3km away. No moon but I assume I'm now much closer to the actual extension value, as I was previously way off the mark.

Interesting to note as stated many times here that the Raynox in forward or reverse position does not seem to have much effect on the required extension length

Thanks again!


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zed



Joined: 07 Feb 2019
Posts: 25
Location: Houston, TX

PostPosted: Wed May 29, 2019 7:45 am    Post subject: Reply with quote

For reference here is my setup with the tubes adjusted to focus on the moon. Ideally - infinity focus should be marked by focusing on an object as far away as possible. The moon is best for this because nothing on earth is farther and the moon is readily available. In addition even the best optics have at least 1% error in their focal lengths - so measuring works best for your individual setup. You can see that my tube length is at 154.74mm. Since the distance from the bayonet to the sensor for Nikon is 46.5mm - this makes my Raynox focus appropriately at 201mm.



That said - users here have noted that the Raynox can be pushed closer to the sensor with good results. See here:

http://www.photomacrography.net/forum/viewtopic.php?t=37843

The important thing to note about doing this is scaling. Your magnification will change as you move the tube lens closer to the sensor - and this will also mess with the optimal spacing between the objective and tube lens.

What I would do to find your proper tube spacing is to mount your Raynox to your bellows without any of the spacers. Get it as close as you can to the end of the bellows. Then adjust the bellows so the spacing between the end of the Raynox and the camera bayonet is approximately 155mm. Then you can focus on the moon and make micro adjustments to get it spot on.

Good luck!
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aidanmoore



Joined: 24 Sep 2015
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Location: Toronto

PostPosted: Wed May 29, 2019 12:25 pm    Post subject: Reply with quote

Thanks for your help on this. Unfortunately I can't use the bellows as even fully compressed the tube length is greater than infinity and I cannot see any distant objects in focus.

So I am using this tube length:



And you can see that the length to the image sensor is about 110mm.

And here is my setup:



When I check my 10X SLWD Nikon scale using a 20mm slide scale (0.1mm per large division) I get:



Which is quite a bit less magnified than a 10X Apo on a Nikon 90i taken through the trinocular port with the same camera:



So there is quite a bit of difference in scale; I count 19 pixels per division for the Nikon 90i vs 33 pixels per division for the DCR-150.

Anyway, quite a bit of new info for me to review, and now I just need to understand why the scale difference is so large.

Thanks again!
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rjlittlefield
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PostPosted: Wed May 29, 2019 1:57 pm    Post subject: Reply with quote

Aidan, this may be a silly question, but are you sure which model of Raynox lens you're using?

The dimensions that you're showing look about right for a Raynox DCR-250, with focal length 125 mm. But the Raynox most commonly used as tube lens is the DCR-150, focal length 208 mm.

--Rik
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aidanmoore



Joined: 24 Sep 2015
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Location: Toronto

PostPosted: Wed May 29, 2019 2:07 pm    Post subject: Reply with quote

Thanks Rik...not a silly question in fact I have had the lens in a tube for so long that I forgot the model...yes it is a DCR-250!

So now it makes sense...all the other sensor to Raynox distances mentioned here are around what I am seeing.

And I guess the 10X objective acting like a 6.5X (approximately) is due to the shorter (110mm) tube?
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Last edited by aidanmoore on Wed May 29, 2019 2:20 pm; edited 1 time in total
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