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Old 30-07-2021, 01:34 AM
Martin Pugh
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Should a camera lens focus at infinity or close to?

Hi there.
Interesting observation. I am testing out a new lens which has a backfocus of 55mm. First I attached a QHY600 and all the necessary adapters to achieve 55mm, and the lens came to focus at infinity or very close to.

Changed the camera to a ZWO ASI6200, and all of the adapters necessary to achieve 55mm, and the lens came to focus 'significantly' away from the infinity mark on the lens.

The stars are not great, at all, even on axis. So I am wondering whether I should move the lens to infinity and then adjust the adapter length until the lens comes to focus at or near that point. In the hope that the star shapes come good.

Anyone experienced this?
thanks in advance
Martin
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Old 30-07-2021, 07:40 AM
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gregbradley
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What sort of lens?

Fast lenses are very likely to have significant coma, like Sigma Art F1.4 lenses except for the 40mm. Wide angle lenses are also tough to get to work, like a 14mm. The longer the focal length of the lens the easier it should be.

It sounds a bit like the same issue with quad TCC and an AP refractor with these IMX455 cameras. The smaller pixels are showing up optical aberrations larger pixelled cameras don't show up. Roland Christen did a post recently on the AP users group explaining this.

Short answer, I would experiment with spacing just like you dial in a flattener or a reducer and the resulting ideal point may be a surprisingly large spacing difference.

My Quad TCC seems best at 113mm yet manufacturers recommendation is about 104 including the filters difference.

Speaking of that did you allow to add the difference for the filters and cover glass and chamber glass to the backfocus?

Greg.
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Old 30-07-2021, 07:57 AM
Stefan Buda
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Quote:
Originally Posted by Martin Pugh View Post
The stars are not great, at all, even on axis. So I am wondering whether I should move the lens to infinity and then adjust the adapter length until the lens comes to focus at or near that point. In the hope that the star shapes come good.

Anyone experienced this?
thanks in advance
Martin
I would do exactly as you suggest.
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Old 30-07-2021, 08:08 AM
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rustigsmed (Russell)
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Hi Martin,

My understanding is that most modern lenses donít focus exactly at infinity - particularly if it is not a fixed focal length design.

That sounds strange as theyíre both the same chip although you have different distances to space correctly. Interested to hear the results Iíve got a 2600mc on the way with EF adapters so it sounds a little alarming - the zwo options looked like they had their adapters in order more so than qhy (probably due to more room to play with in the spacing).

Cheers

Russell
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Old 30-07-2021, 08:36 AM
AdamJL
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Following. I've had success with using my Canon 100 f/2.8 L IS on a 2600MC with standard spacing. Not perfect, and now that you bring up the subject, the edge performance could be down to spacing issues (I thought it was a lens thing)

https://www.iceinspace.com.au/forum/...d.php?t=191832

But I plan on using a lot of lenses in future so this would be an important thing to know
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Old 30-07-2021, 08:58 AM
glend (Glen)
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I believe it is Depth of Field issue, in that lenses focused at infinity deliver the best Depth of Field. I would play with the adaptor, I prefer to use variable adjustable spacers, allowing customisation of the length for the situation.
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Old 30-07-2021, 11:33 AM
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Ryderscope (Rodney)
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I'm working on a similar configuration Martin in that I am setting up my ASI6200 with a Canon EF200mm f2.8 L USM lens. As Greg alluded to, I am finding that this camera is very unforgiving of lens aberrations so it is proving difficult to get the back focus and tilt adjustments dialed in. I did have to stop down the lens to f5.6 before I got anything like reasonable stars in the corners of the image so it is important to do this before proceeding further.

For what it is worth, here are the details of my configuration thus far to see if this helps.

I have purchased the Astromechanics adapter that they have designed for the Canon lens. These adapters have the inbuilt controllers to adjust the f stop and focus of the lens. They also have an inbuilt tilt adjuster which also provides the option of adjusting the back spacing. Having software control of the aperture and focus mechanism is a real bonus and I have found that the auto focus routine works well with SGP Pro. In my case, the lens comes to focus near the left hand edge of the infinity mark.

The Astromechanics adapter that I am using is designed to give the correct flange to sensor distance of 44mm for the Canon lens. The components in the image train are; adapter 11.5mm, ZWO 2" EFW 20mm, ASI 6200 camera 12.5mm. As indicated above, the adapter can be adjusted to increase the spacing and adjust tilt. I have wound out the tilt adjuster to give me an extra 1mm spacing to allow for my Astronomik 50mm filters which have an optical thickness of 1mm. I am not entirely happy with winding the tilt adjuster out this much and am thinking of having a spacer made to pack out the adapter by 1mm.

My testing last night is promising though it is quite apparent that there is much tilt to be adjusted which I will approach very carefully as it is very easy to make things much worse very quickly. Here is a link to a sample sub from last nights testing if you want to have a peek. I have an additional problem with star trailing as well but this should be unrelated to getting the back spacing and tilt correct.

Your experience with the difference between the QHY600 and the ASI6200 is interesting in terms of not being able to reach focus with the correct back spacing distance. What is different between the QHY and ASI6200 set up? Are the filters different and therefore a different optical thickness? I see that the back focus consumed by the QHY600 is 18mm compared to 12.5mm for the ASI6200 so I'm assuming that your adapters in the image train allowed for this? I would also be interested to know which lens you are using that has a back spacing requirement of 55mm.

Clear skies,
Rodney
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Old 30-07-2021, 12:00 PM
AdamJL
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I’ve got that Astromechanics adapter as well, Rodney.
How are you finding getting AF to work? I couldn’t figure it out (I use NINA)
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Old 30-07-2021, 12:24 PM
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Quote:
Originally Posted by Martin Pugh View Post
Should a camera lens focus at infinity or close to?
Yes the lens should focus at the infinity mark for an infinity target, if the Lens flange to sensor focal plane distance is correct, provided that no-one has adjusted the sometimes present hard infinity stop on the lens (typically on manual lenses).

Quote:
Originally Posted by Martin Pugh View Post
Interesting observation. I am testing out a new lens which has a backfocus of 55mm. First I attached a QHY600 and all the necessary adapters to achieve 55mm, and the lens came to focus at infinity or very close to.

Changed the camera to a ZWO ASI6200, and all of the adapters necessary to achieve 55mm, and the lens came to focus 'significantly' away from the infinity mark on the lens.
It seems very odd that there should a significant difference in where the lens focuses (the lens focus mark position) with one camera compared to the other. It suggests that the Flange-focal plane distance is somehow different between both cameras, either via differences in adapters in use (if they indeed differ between one camera or the the other in any way) or perhaps there is a small difference the sensor position (via possibly differently adjusted tilt adjustment screws) between the 2 cameras.

Of course I don't know your brand of lens, but one thing that seems unusual (to me) is the back focus distance of 55mm. Are you sure it is correct for your make of lens? The likes of Canon, Nikon, Sony etc.. are typically in the mid 40mm range for DSLRs, with mirrorless of course much lower. Of course lenses designed for larger formats are another story and have larger flange focal distances.

Perhaps just to confirm for your lens, have a look at the following Flange Focal Distance Table, just to make sure and good luck in your quest ....
https://en.wikipedia.org/wiki/Flange_focal_distance

Last edited by JA; 30-07-2021 at 12:36 PM.
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Old 30-07-2021, 10:21 PM
Martin Pugh
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Hi all. Thanks for the replies.

So the lens is an ASKAR 200mm f4.....allegedly designed for astrophotography with a backfocus of 55mm, M48 output thread.

I have of course adjusted the optical line up to compensate for the differences in the 2 cameras. I was initially using a Cyclops Optics UV/IR filter and that put the focus position significantly past the infinity mark. When I then put in a thicker Antlia 3nm Ha filter, the focus position moved closer to the infinity mark as you would expect (given that filters extend, or push out the backfocus), but still a good distance away.

Today I put on a much smaller sensor - ASI1600.....no filters, just a direct connection with M48 spacers to exactly 55mm. The lens came to focus very, very close to the infinity mark. So there is something very wrong with the specs on the ZWO camera...buyer beware.

Thinking that tilt and whatnot was the cause of the rather poor stars I am seeing, I used the smaller sensor to eliminate this. Now interestingly, when I mounted the QHY600 and ZWO6200, I aligned the camera to north. The stars, even on axis, have a teardrop shape. I didnt bother to align the ASI1600 today, and the first image off the camera following focus shows the teardrop shaped stars have rotated....indeed, when I rotated the lens/camera assembly to match the others, the teardrop shaped stars matched.

So if the 'optical defect' rotates with the lens, it has to be an optical defect and...it cant be a tracking problem (all of the test images are 10 sec unguided).

cheers
Martin

Last edited by Martin Pugh; 30-07-2021 at 11:33 PM.
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Old 31-07-2021, 08:12 AM
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After reading the manufacturers specs on this lens it would appear your copy is a dud. You seem to be matching what they say.

A lens element is probably decentred. That's the usual issue with poorly performing camera lenses.

Greg.
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Old 31-07-2021, 04:54 PM
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Sunfish (Ray)
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I have pulled apart an old Pentax 67 400 lense which did not quite focus to infinity in the way I expected and had no head room.

I discovered that the lense could be assembled in slightly different ways , depending on the helical thread start, which would achieve slightly different infinity head room and also different near focus.

My thought was that someone had reassembled this lense in error or deliberately to achieve different near focus. It has good infinity focus near the mark now with a little head room after some fiddling. I am sure more modern lenses could not be altered in this way but an old school helical focus lense could have this error.
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