Focusing with 500mm Lens, or is it turbulence?

[oni]

Hello all! I'd like to do some deep sky work (and some moon work) so I bought a 500mm Sigma Lens with a T-Mount adapter for my Nikon D5300. Last night I took some shots of what I believe is the great nebula in Orion. I use a Vixen Polarie, aligned roughly to Polaris to help cancel the trailing. Settings were:

• ISO1600
• F8 (I think) at 500mm
• 8 Second exposure
• 80 Light frames, with 20 dark, bias and flat frames, all shot with RAW and exposure delay settings
• Default, recommended settings in DSS#

Im based in London which has pretty bad seeing most of the time. Here is the final result:

 

I suspect some weird PNG compression going on here. The TIFF is much better.

Here is the RAW/NEF as a png

 

 

You can see all the horrible light pollution we have to deal with. Nevertheless you can make out something in there

The problem I think I'm having is the focus on the lens is off. Its an old T type mount, so I use an adapter ring. The problem I feel is I want to wind the focus ring, just a little more to get things into focus, but it wont go past its maximum. During the day, if I focus on a building 500m away I get nice shots but for things at effective infinity, It doesnt seem to work.

Im aware of the trick where you go to infinity focus, and then dial it back a little. I may try this again, though Im not sure if that works. Ive had the same thing with my shots of the moon, they just dont seem to have that crispness. Any thoughts?

Cheers

 

Ben

 

 

[DorsetBlue]

I might have misread it but I think there is a link to a website in the 'Budget Canon lens' thread which says that you can only use AI(S) lenses with Nikons as they cannot reach infinity with anything else.  I am happy to be told otherwise, as I might stick with the Nikon D5000 for a little longer if this is not the case?

[oni]

Interesting... I'd not heard that. I'd best check. In the meantime, this lens looks good?

 

http://www.ebay.co.uk/itm/Cosina-100-500mm-f5-6-8-Super-Cosina-MC-Macro-lens-Nikon-Ai-fit-caps-and-case-/381548168172?hash=item58d60b03ec:g:SNsAAOSwll1WxFEN

[abhoriel]

if you can get perfect focus at a building at 500m, you should have infinity focus. But not achieving infinity focus is an issue, particularly when using lenses of other mounts with adapters on Nikon DSLRs (due to Nikon's long flange focal distance). I assume its a 500mm mirror lens you're using .. I use one as well (I use a Tamron adaptall 500mm f8 mirror lens, with a Nikon D5100), and they are very difficult to focus correctly. Use live view and zoom in on a star or the moon or something, and get the focus as perfect as you can.

Tell me the exact lens you use and I will see if I can find out anything

 

I'm really a beginner to astrophotography, but to my mind, with the vixen polarie, you should be able to get longer subs than 8s. You may need more accurate polar alignment for this. Longer subs will be the main thing that improves your images.

[happy-kat]

My 28mm Cosima has lots of CA and little ability to see stars even though it is quite fast at f2.8.

I have read before about removing infinity stops from a lens, you have to open it up a bit to do it.

[oni]

So I have two lenses I'm trying. The first is a cheap and nasty samyang 500mm prime with a T-Mount adapter. I figure this won't work on a nikon properly (using a D90 and D5300).

The second lens is the one I posted, a super cosina 100-500mm f5.6-f8 lens, which is a Nikon AI lens. The results look a bit like this.

That building is about 200-300 metres away. I've twisted the focus ring as far as it will go and you can still see a bit of a blur there.

Looking around I do hear things about adjusting screws to get the infinity focus properly but I'm debating sending this lens back instead :S

[oni]

The full picture looks a little like this:

 

[happy-kat]

Have you tested you get a better image/focus if you choose a closer object.

[oni]

I've tried objects further away (also at infinity) and the focus is poor. I've tried objects closer up (less than 30 metres or so) and the focus appears to be fine.

 

 

[happy-kat]

I was thinking take images at same time on ever closer objects so you can compare IQ to check if the lens can do better or not. Though a star sky will be best test. Daytime test on same subject might rule out grainy low light image.

[oni]

I can give that a try, cheers. I do have a 70-200 lens (or thereabouts) that I can try as a comparison on both the Nikon bodies I have. Stars appear to be much too blobby as oppose to trailing. Its odd that both the Samyang and the Cosina have this problem. If I could only just turn the focusing ring a little more. Its a pity I dont have a canon body to at least test the samyang with - I hear the flange distance is an issue.

[alacant]

On 24/02/2016 at 15:41, DorsetBlue said:

I might have misread it but I think there is a link to a website in the 'Budget Canon lens' thread which says that you can only use AI(S) lenses with Nikons as they cannot reach infinity with anything else.  I am happy to be told otherwise, as I might stick with the Nikon D5000 for a little longer if this is not the case?

I have an old 500mm f8 preset (certainly not AI anything) with a t2 adapter which works fine on the M setting of my Nikon D40. Setting the lens to infinity -when you can't turn the focus ring any more- works fine and it really is infinity. HTH.

[Thalestris24]

Hi Ben

This CN thread might be of interest http://www.cloudynights.com/topic/495238-thinnest-low-profile-nikon-t-ring/

Louise

[oni]

Thanks for the links and the tips there. The Cosina Lens is an AI fit and should therefore, focus properly, but all the shots of the stars appear too blurred.

Here is an unprocessed image, taken this evening - Cosina 500mm at 500m f8 6400ISO 14bit RAW Nikon 5300 1 8 second exposure on a polarie mount

 

 

To me it looks like the spots are all uniformly blobby and round which suggests improper focus - but I could be wrong I guess. Given this lens is an actual Nikon fit and not using any kind of adapter im pretty suprised :S

 

[happy-kat]

They don't look snappy look a bit off focus.

[AnakChan]

On 2/25/2016 at 04:09, abhoriel said:

if you can get perfect focus at a building at 500m, you should have infinity focus. But not achieving infinity focus is an issue, particularly when using lenses of other mounts with adapters on Nikon DSLRs (due to Nikon's long flange focal distance). I assume its a 500mm mirror lens you're using .. I use one as well (I use a Tamron adaptall 500mm f8 mirror lens, with a Nikon D5100), and they are very difficult to focus correctly. Use live view and zoom in on a star or the moon or something, and get the focus as perfect as you can.

Tell me the exact lens you use and I will see if I can find out anything

 

I'm really a beginner to astrophotography, but to my mind, with the vixen polarie, you should be able to get longer subs than 8s. You may need more accurate polar alignment for this. Longer subs will be the main thing that improves your images.

I actually struggle trying to get beyond 200mm with my Polarie for more than 2 min exposures. But I do have the 70-200/2.8 which is a a 1468g lens on a D800E 1000g body. I even have the Telesco-Factory setup with a counterweight but think it's a bit of an art trying to get the right balance for the Polarie's max 2kg limit. I'm actually getting a longer shaft to see if it'll help (I opted that instead of another counterweight for travel purposes).

[oni]

Yeah I think i'llbe sending this lens back and I'll try and get a genuine Nikon 70-300 instead which should be better. I'll see if the seller will take it back. Trailing I don't think is the problem as none of the images look sharp even in bright light sadly