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Old 02-07-2016, 10:55 AM
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Atmos (Colin)
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Poor Focus or OSC?

I have recently started playing around with my Nikon D700 and some astrophotography work. I had used it a little in the past with a LX200 10" F/10 (pixel scale of ~0.65"/pixel) which gave what appeared to be reasonably sharp results although nothing fantastic with 6s exposures!

I have recently put it on my 130mm F/5 astrograph (~2.59"/pixel) and everything appeared soft, almost out of focus. I used as bahtinov mask to focus although the PA at the time still left some trailing at 30s so it could very well have been that that was the issue.

Monday night I did some wide field with my 24-120 Nikkor lens, zoomed to 120mm, digitally zoomed in as much as possible and then eyeballed the focus (smallest star possible) before going back to 24mm. This gives an image scale of ~72.58"/pixel, so, VERY under sampled.

This is what I got.

To me it looks like the stars are quite bloated, if I was using a mono sensor I would say that the focus is way off. My question to anyone has has used OSC (this is my only OSC that I have used), is it the fact that the D700 is a OSC with 8.445 micron pixels that gives this bloated look or is it my focus?
I am thinking I may need to get a bahtinov mask for this lens if focus is the issue.
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Old 02-07-2016, 01:21 PM
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osc?
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Old 02-07-2016, 01:23 PM
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Atmos (Colin)
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One Shot Colour
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Old 02-07-2016, 01:37 PM
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billdan (Bill)
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Hi Colin,

Not that I have much experience with zoom lenses, but are you sure focusing at the 120mm setting is going to maintain the same focus at the 24mm setting.

How difficult is it to focus at the 24mm setting?

Lovely image by the way, its only when you pixel peep do you see the bloated stars.

Bill
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Old 02-07-2016, 02:12 PM
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Originally Posted by billdan View Post
Hi Colin,

Not that I have much experience with zoom lenses, but are you sure focusing at the 120mm setting is going to maintain the same focus at the 24mm setting.

How difficult is it to focus at the 24mm setting?

Lovely image by the way, its only when you pixel peep do you see the bloated stars.

Bill
What Bill said - it's very common for zoom lenses to have a different infinity focus point depending on whether you are zoomed in or not. Ony use digital zoom for checking focus. Also make sure the camera is actually in manual mode and doesn't do any sneaky 'searching' for focus when the shutter button is half-pressed.
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Old 02-07-2016, 02:14 PM
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Atmos (Colin)
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Originally Posted by billdan View Post
Hi Colin,

Not that I have much experience with zoom lenses, but are you sure focusing at the 120mm setting is going to maintain the same focus at the 24mm setting.

How difficult is it to focus at the 24mm setting?

Lovely image by the way, its only when you pixel peep do you see the bloated stars.

Bill
It seems to work fine when I am doing terrestrial photography but in many ways it is less demanding than stars pin point light sources.
It isn't noticeable until zooming on though.
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Old 02-07-2016, 03:31 PM
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Always have to stop down wide angle lenses to get better focus.
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Old 02-07-2016, 07:16 PM
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Always have to stop down wide angle lenses to get better focus.
I don't quite understand how stopping down a lens has anything to do with focus?
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Old 03-07-2016, 01:27 AM
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I don't quite understand how stopping down a lens has anything to do with focus?
Photographically speaking, most lenses perform best in the middle of their aperture range, say between f8 and f4. But this shouldn't affect you being able to achieve focus outside of those ranges.
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Old 03-07-2016, 03:01 AM
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Maybe movement

Hi, first post here.
If I can give a small opinion from an outside view.
My first reaction to the photo was just great, many would love to take a photo like this.
Deeper look at it, it appears you were enhancing contrast of surrounding gasses.
Looking at some stars on the edges I noticed some were elongated maybe there is some mechanical movement or wind/breeze.

But overall I really like it, wish I could take photos like that.
Getting set up and someday here soon.
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Old 03-07-2016, 07:48 AM
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Originally Posted by Stonius View Post
Photographically speaking, most lenses perform best in the middle of their aperture range, say between f8 and f4. But this shouldn't affect you being able to achieve focus outside of those ranges.
Totally agree, it performs nicer at F/7.1 than at F/4. I am wanting to cut exposure time however and don't mind cropping out the corners Stopping down the lens decreases the light cone increasing the critical focus zone but that is about the only thing I can think of relation to focus.

Quote:
Originally Posted by adam_mizer View Post
Hi, first post here.
If I can give a small opinion from an outside view.
My first reaction to the photo was just great, many would love to take a photo like this.
Deeper look at it, it appears you were enhancing contrast of surrounding gasses.
Looking at some stars on the edges I noticed some were elongated maybe there is some mechanical movement or wind/breeze.

But overall I really like it, wish I could take photos like that.
Getting set up and someday here soon.
You are correct John, I did do some enhancements to make it pop out a bit more Not a lot but it did help with the galactic bulge. At this stage it is just a single frame because of perpetual cloud lingering in Melbourne.

The star elongation in the corners is caused by field curvature. I have a feeling I may have to put it on 35mm and not the bottom of the range at 24mm. OR just get a nice 24mm and not a telescopic lens which probably perform at their best in their mid range (75mm).

All of my experience to date in astrophotography has been with mono cameras and if the stars looked like that in a mono camera I would say it is out of focus. With a DSLR (or any OSC) you have four pixels creating colour so I know it reduces resolution but I am curious as to whether the apparent bloat is caused by near 9 micron pixels with a bayer matrix. I do have a couple of other theories that I have been wanting to test for the past week but cloud and rain have stopped me.
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Old 03-07-2016, 08:05 PM
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I don't quite understand how stopping down a lens has anything to do with focus?
Lens wide open, unless top notch super expensive, will perform much better stopped down.
As a photographer it's a common thing to stop down a lens in order to achieve a sharper image. (also adds depth of field)
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Old 03-07-2016, 08:28 PM
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Lens wide open, unless top notch super expensive, will perform much better stopped down.
As a photographer it's a common thing to stop down a lens in order to achieve a sharper image. (also adds depth of field)
Totally agree, it works much better at F/9 than F/4 but I still don't see how it helps with focus except by increasing the critical focus zone. Doesn't actually effect focus, just makes it easier to achieve by increasing the depot of field.
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Old 03-07-2016, 09:00 PM
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Hi Colin,

It's a good photo I think you're probably right though, it does look a little soft.

I use my Canon 600D (un-modified) and generally I've been very happy with the focus (with and without a lens). Previously when I've used the 18-55 and 55-250mm lens I've always focused at the focal length I plan on using (I use the x5 or x10 live-view zoom feature, usually at max ISO so I can find a relevant star - easier said than done mind you ).

Suggest you try again but this time try focusing at the 24mm instead?

Cheers, Evan
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Old 04-07-2016, 09:50 AM
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Totally agree, it works much better at F/9 than F/4 but I still don't see how it helps with focus except by increasing the critical focus zone. Doesn't actually effect focus, just makes it easier to achieve by increasing the depth of field.
There you've got it ...
By reducing the angle of incidence to the sensor, ie narrower cone from the lens you reduce chromatic aberration for a starter, ie reduced halos. And don't tell me your lens is perfect and hasn't got some dispersive elements in it's multiglass make up. Why do you think Pro Photogs prefer primary lenses, minimal glass.

The focal point becomes slightly less critical and in astro imaging even the tiniest deviation from the critical point will introduce some halo or 'blobbing' . You may think you have achieved perfect focus but your camera knows better. It's the one reason I bought a Canon DSLR (or two) just for astro imaging with BYEOS. I'm a SONY man by preference but there is no software for SONY gear that allows image focusing at the PC.

And focussing then shifting the zoom just isn't going to cut it. All Lenses I have ever used have shown some small amount of focal shift when zooming.

Yep, astro-imaging is quite demanding ...
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Old 04-07-2016, 09:56 AM
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^^what he said ^^ a zoom lens wide open will always produce soft images unless super expensive etc.
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Old 04-07-2016, 10:35 AM
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There you've got it ...
By reducing the angle of incidence to the sensor, ie narrower cone from the lens you reduce chromatic aberration for a starter, ie reduced halos. And don't tell me your lens is perfect and hasn't got some dispersive elements in it's multiglass make up. Why do you think Pro Photogs prefer primary lenses, minimal glass.

The focal point becomes slightly less critical and in astro imaging even the tiniest deviation from the critical point will introduce some halo or 'blobbing' . You may think you have achieved perfect focus but your camera knows better. It's the one reason I bought a Canon DSLR (or two) just for astro imaging with BYEOS. I'm a SONY man by preference but there is no software for SONY gear that allows image focusing at the PC.

And focussing then shifting the zoom just isn't going to cut it. All Lenses I have ever used have shown some small amount of focal shift when zooming.

Yep, astro-imaging is quite demanding ...
Sigh...

First off, I have never said this lens is great for astro work as demonstrated in the opening of the following two threads.
http://www.iceinspace.com.au/forum/s...d.php?t=146564
http://www.iceinspace.com.au/forum/s...d.php?t=146994


All I have asked is whether the apparent star bloating has been caused by focus or whether it is a consequence of a Bayer matrix on a large pixel camera because I have no real experience with OSC cameras and astro work (only used it for day time photography).

What you say is correct and I have already agreed with you in earlier posts. Increasing the critical focus zone makes it easier to focus but has zero effect if it is in focus. It may decrease star bloat because it makes blue focus less demanding but the blue halos I have are likely caused by it being out of focus anyway.

It is good to know that there is focus shift with zooming, I had wondered by haven't been able to get any conclusive test results.
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Old 04-07-2016, 10:44 AM
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It is caused by the lens not being stopped down. Instead of being defensive try it and come back to us.
If we are wrong then at least that is the easiest thing to eliminate. As someone who is in the photography business the photo is soft because the lens was not stopped down.
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Old 04-07-2016, 11:35 AM
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Atmos (Colin)
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It is caused by the lens not being stopped down. Instead of being defensive try it and come back to us.
If we are wrong then at least that is the easiest thing to eliminate. As someone who is in the photography business the photo is soft because the lens was not stopped down.
Just frustrated today for reasons outside of IIS but all I have really wanted is an answer to my original question.
Quote:
Originally Posted by Atmos
is it the fact that the D700 is a OSC with 8.445 micron pixels that gives this bloated look or is it my focus?
I have a bunch of things I want to test but haven't had much in the way of clear skies for two weeks and it's not looking good any time in the future.
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Old 04-07-2016, 12:44 PM
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Fair enough but process of elimination of the easiest things first then go from there. Also frustrated with lack of skies currently
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