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  #81  
Old 09-03-2012, 08:41 PM
Martin Pugh
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Hello all - thanks for the link on how to correct tilt....I clearly have it.

I have now operated the lens at f2, f2.5, and currently f3.5. I believe there is a significant improvement particularly in the lower left/right. The existence of tilt in the upper half of the image is clearly discernible now, so I will attempt a fix on that.

If I can null that out, it will be interesting to dial back the lens to f2 to see how things may have improved there.

I will keep you posted, but here is the result so far (full size FIT - 16Mb)- so, this is the 50mm L lens, stopped down to f3.5, 30 minutes exposure, ST8300, Baader 7nm Ha filter.

http://www.martinpughastrophotograph...d%20ST8300.fit

thanks for all the pointers.

Martin

Last edited by Martin Pugh; 09-03-2012 at 09:49 PM.
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  #82  
Old 11-03-2012, 08:07 PM
Martin Pugh
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I think I have come to the end of using this lens. I got very good results finally with the lens at f4.5 with very good stars across the field.

I put the lens back on to the Canon 550D, as a trial to see what sort of results I got there, and much to my surprise the excessive off axis aberrations were back - with the lens at f4.5. So what I achieved with the ST8300 did not replicate on the DSLR.

Anyway - one lesson - I think buying anything less than a full frame DSLR for astrophotography is a waste of money and time.

cheers
Martin
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  #83  
Old 12-03-2012, 11:24 AM
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Quote:
Originally Posted by Martin Pugh View Post
I think I have come to the end of using this lens. I got very good results finally with the lens at f4.5 with very good stars across the field.

I put the lens back on to the Canon 550D, as a trial to see what sort of results I got there, and much to my surprise the excessive off axis aberrations were back - with the lens at f4.5. So what I achieved with the ST8300 did not replicate on the DSLR.

Anyway - one lesson - I think buying anything less than a full frame DSLR for astrophotography is a waste of money and time.

cheers
Martin
16MB FITs?! Some JPG examples would be nice for those of us travelling without our full suit of astro-imaging software at hand!

I don't quite follow what you think the problem is with the 550D and why that would be any less of a problem on full-frame?

Have you checked that the tilt is in the 8300 sensor and not the lens? You can be sure that the 550D is as flat/square as any sensor can be.

Phil
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  #84  
Old 12-03-2012, 01:03 PM
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I thought the same thing. I didn't follow how full frame is the conclusion from the lens not working with your APS sized 550D. The larger the sensor the more demanding it is for orthogonality by the square of the increase in size.

Greg
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  #85  
Old 12-03-2012, 01:40 PM
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Quote:
Originally Posted by Martin Pugh View Post
...., and much to my surprise the excessive off axis aberrations were back - with the lens at f4.5. So what I achieved with the ST8300 did not replicate on the DSLR.

Anyway - one lesson - I think buying anything less than a full frame DSLR for astrophotography is a waste of money and time.

cheers
Martin
No surprises here.
The tilt is not in the lens it is at ST8300 sensor.

Full frame lenses will perform better on smaller sensors. though - because of obvious reasons (smaller sensor, aberrations stay outside the corners). However, they will also be much more expensive. Which means, DSLRs with smaller sensor (not full frame !! ) will not capture those aberrations.

Last edited by bojan; 12-03-2012 at 02:09 PM.
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  #86  
Old 12-03-2012, 07:39 PM
Martin Pugh
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Hi everyone
I posted the full FITS because I assumed that if people wanted to assess the image, an uncompressed image is the way to go, rather than a JPG.

Anyway - I guess the point I am making here, is that I dont believe I would get dissimilar results with the $380 85mm lens I first bought, and that I did not expect to go these lengths with a $1600 L lens. My next trial will be just that - I will take an image with the 85mm lens, stopped down appropriately, and compare those results. Really, at this stage, I think the differences will be negligble. We shall see.

The other point I was making is that I seem to have got fairly good stars across the field at f4.5 with the ST8300, so I expected similar results by using the lens at f4.5 on the 550D. This was not the case.

Tilt? Even if it is in the ST8300 - Cant do anything about that. I tried to measure with a caliper the depth of the SBIG Canon lens adapter, and was fairly satisfied that the measurements were very close. I am not ruling the adapter out though.

One question I have though - if I gently rock the lens, I can hear the lens elements moving about slightly - is this normal?

thanks
martin
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  #87  
Old 12-03-2012, 07:58 PM
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Originally Posted by Martin Pugh View Post
One question I have though - if I gently rock the lens, I can hear the lens elements moving about slightly - is this normal?

thanks
martin
If you hear something when shaking lens, more likely it's something else (focus mechanism or iris), not the glass.
However, if it is the glass, you have to tighten the relevant retaining rings that is supposed to hold the (movable) element in place
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  #88  
Old 12-03-2012, 08:31 PM
Martin Pugh
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Thanks I will check that out.

Here is the best I could get out of the 50mm lens at f4.5 and the ST8300.

http://www.martinpughastrophotograph...4_5_ST8300.jpg

Cheers
Martin
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  #89  
Old 12-03-2012, 09:00 PM
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That doesn't look bad to me at all, mate
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  #90  
Old 13-03-2012, 06:34 AM
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Quote:
Originally Posted by Martin Pugh View Post
Hi everyone
I posted the full FITS because I assumed that if people wanted to assess the image, an uncompressed image is the way to go, rather than a JPG.

Anyway - I guess the point I am making here, is that I dont believe I would get dissimilar results with the $380 85mm lens I first bought, and that I did not expect to go these lengths with a $1600 L lens. My next trial will be just that - I will take an image with the 85mm lens, stopped down appropriately, and compare those results. Really, at this stage, I think the differences will be negligble. We shall see.

The other point I was making is that I seem to have got fairly good stars across the field at f4.5 with the ST8300, so I expected similar results by using the lens at f4.5 on the 550D. This was not the case.

Tilt? Even if it is in the ST8300 - Cant do anything about that. I tried to measure with a caliper the depth of the SBIG Canon lens adapter, and was fairly satisfied that the measurements were very close. I am not ruling the adapter out though.

One question I have though - if I gently rock the lens, I can hear the lens elements moving about slightly - is this normal?

thanks
martin
hi Martin

85mm is a little less demanding than 50mm, but broadly I would say you are right.. any high quality prime lens at f4.5 should be delivering pretty good results.

I'm with bojan.. this result is pretty impressive though. I wouldn't be able to beat that image quality at 100% scale with any combination i've used either. I'd say my 50mm f1.4 lens might be a close match but i can't use it with my mono sensor (and i know there's tilt in mine which would kill it).

Not entirely unusual to feel a small amount of movement in the lens (but it shouldn't rattle when you shake it ). Hard to say whether it is a problem or not from here.. i've got some lenses that have that slightly loose feeling but no issue as far as i can tell.

Can you send us a sample from the 550D.. f4.5 and maybe even something wider if you have it. Full res JPG more than sufficient for judging image/optical quality in this case. Perhaps a different story if you were trying to gauge noise peformance etc.

cheers
Phil
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  #91  
Old 13-03-2012, 08:39 PM
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I disagree that image is nowhere near good enough. A large amount of distracting coma and tilt.

Martin has the highest standards and I can't see him posting an image with coma that bad. You'd never accept it in a scope.

Some of these short focal length lenses both Nikon and Canon but particularly Canon often get low scores for edge sharpness.It seems to be a difficulty with these super fast lenses. That is not always a big issue for terrestial photographers because they often shoot at F11 to F16 or so or are using for portraits and the edges are often out of focus blur of the background for bokeh effects.

You are using it F4 or F5.6 or so and the lens probably doesn't start to smarten up until F8 and by then you've lost your speed.

The simple 50mm F1.8 may be the better choice as its still fast but it lacks distortion and aberrations and often reviews of any 50mm lens seems to be very good. It must be the sweet spot for lens optics or they are relatively easy to make well compared to other focal lengths. I have both Nikkor and Canon 50mm F1.8 and in my limited use of the Canon 50mm F1.8 they both seem the same to me.
They are also very cheap and around $135.

Greg.
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  #92  
Old 13-03-2012, 08:49 PM
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Yep not real flat that image Martin. Surely there is going to be a lens that is flat all the way across.
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  #93  
Old 14-03-2012, 07:56 AM
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That FIT looks awesome for a lens. Different expectations. It's not a scope. I'd be stoked with that especially 50mm.
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  #94  
Old 14-03-2012, 08:36 AM
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Admitttedly Martin has posted 100% and that will show up the warts but here is one I took at 2x2 binning with a Proline 16803 (about 16 times more demanding of squareness than an 8300 chip due to its size)
and a cheap ($250 or so) Pentax 67 55mm at F5.6:

http://www.pbase.com/gregbradley/image/142065586


There is slight tilt on the right side but minor. At 1x1 binning though the coma is gross. At 2x2 magically it is pretty precise and with the 16803 chip it still shows plenty of resolution at 2x2.


Here is one (sorry its a small image) STL11 and Nikkor 50mm F1.8 which as I recall was sharp corner to corner no coma:

http://www.pbase.com/gregbradley/image/66496862

I know you aren't interested in longer focal lengths but out of interest the Pentax 165mm F2.8 is sharp corner to corner at 1x1 binning wide open on a Proline 16803.

I conclude from this that lens makers have compromises enter in when they make short focal length faster lenses and longer focal length lenses are more likely to be useful than shorter focal lengths. And if you want shorter focal length lenses then there is no point in getting F1.2 type lenses.

There is also a Pentax 67 45mm lens. But per reviews it isn't as sharp as the longer focal length. The 75mm as I recall is considered very sharp. There is a list of Pentax 67 lenses and their ratings if you are interested.

Greg.
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  #95  
Old 14-03-2012, 07:43 PM
Martin Pugh
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Hi folks.
thanks again for all of the input. I entirely agree - and hats off to you DSLR astrophotographers. I thought it was easy, and I have experienced nothing but disappointment thus far really. Greg - you are right - I would be hard pushed to produce images that had this kind of aberration - ask Mark Bolton who witnessed my AIC talk. He will testify to that. That said, I am very keen to image the Milky Way, and see no other way to do it other than huge mosaics. I will certainly persevere with the 50mm Lens at f4.5 and now go ahead and do RGB to see what turns out. Of course - I have to implement a focus solution now.

Anyway - just for kicks, I put on my 'mistake' purchase - the Canon 85mm regular lens i.e non-L.

Below is a link to that image - 30 minutes, Ha, ST8300, at f4.5.
Look past the noise and I accept that there is something not quite right - take a look at the top left and compare it to the top right. HOWEVER! If I was on a budget, I would be buying the $385 85mm lens, and not the $1600 50mm L lens when you look at this result.

http://www.martinpughastrophotograph...4_5_ST8300.jpg

Here is the direct comparison to the same field with the 50mm lens.
http://www.martinpughastrophotograph...4_5_ST8300.jpg


Phil - you asked for a sample of the 550D. Link below, taken with the 50mm lens also at f4.5. Shockingly bad.

http://www.martinpughastrophotograph...u/IMG_0008.JPG

This was a single 30 second image at ISO1600 I think.

cheers
Martin
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  #96  
Old 24-03-2012, 08:30 AM
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hmm.. well it certainly gets complicated.

the 550D image is poor for f4.5.. i wouldn't be surprised by this at f2 but not right given how stopped down that is. definitely tilt somewhere given different shape aberrations left vs right side of frame. if you can prove it is the lens, then i think you would be able to ask to swap it for a new one. i think my 50mm f1.4 can do a little better than that.

the 50mm version on your 8300 does not look quite so bad though? and the 85mm only looks bad in one corner. are you always focussing on the centre of the frame? obviously if you focus away from centre and there is tilt then aberrations will be worse on opposite corner. looks like that in 85mm image to me?

keep at it..

Phil
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  #97  
Old 24-03-2012, 04:06 PM
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..... then there is no point in getting F1.2 type lenses.
Greg, I did a lot of testing with various lenses, and came to exactly the opposite conclusion (the same follows from Christian Buil's test results).

For example, F1.2 lens, stopped down to f4.5 will perform better than F1.8, stopped down to F4.5.

Also, lens design for full frame (24x36mm) will perform better with smaller sensor, than equivalent lens, designed to be used on that smaller sensor (1.6x smaller, used in cheaper Canon camera models).
The reasons are, lower F-ratio and bigger sensor require much more careful design of the lens system.

That is why the older, manual and today way cheaper lenses will very often perform better than new, k$-range lenses (with IS, AF and what'snot, all that mostly useless in astrophotography anyway).
As for tilt, sometimes it is a matter of lens production and/or assembly quality - I found couple of Canon lenses with un-symmetrical distortions in corners - but all this is visible on star images only. For terrestrial photography, they are (almost) perfect from corner to corner.

Astrophotography is VERY demanding on lens performance, especially in corners.
From my experience, instead of searching for perfect wide field lens, it is better to concentrate on what we have and how to squeeze maximum from that
Mosaics done with more narrow field lenses take longer time to complete, but the results may be closer to perfection, compared to single frame wide field photo (Avandonks work is a very good example of this philosophy - his (relatively) ultra-wide fields with 300mm lens are really worth looking at)
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  #98  
Old 25-03-2012, 10:30 AM
Martin Pugh
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Hi Phil
I think I am now happy with the performance of the 50mm lens at f4.5 on the ST8300. I have posted a couple of examples in the Deep Space forum.

I think I will sell of the 85mm now - it is, as you can see, perfectly usable for someone on a budget or getting started.

cheers
Martin
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  #99  
Old 25-03-2012, 11:18 AM
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[QUOTE=bojan;833958]Greg, I did a lot of testing with various lenses, and came to exactly the opposite conclusion (the same follows from Christian Buil's test results).

For example, F1.2 lens, stopped down to f4.5 will perform better than F1.8, stopped down to F4.5.


Thanks for that.

I would have thought what you say is the case originally as those lenses should be better made and with exotic glass types (they often have ED elements).

Cheers,

Greg.
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  #100  
Old 25-03-2012, 11:40 AM
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I would have thought what you say is the case originally as those lenses should be better made and with exotic glass types (they often have ED elements).

Cheers,

Greg.
All designs are a compromise.
It is VERY hard to design a F1.2 lens to perform in similar fashion like F1.8 at full aperture and in corners (the centre of the frame is relatively easy).
Stopping the lens down engages only the central parts of the glass, where the aberrations are negligible.
Because the F1.2 lens is much more carefully designed (hence more expensive) - to give acceptable performance in corners at full aperture, it will perform better than it's F1.8 counterpart at the same stop-down (because both are designed to have the same MTF at full aperture)
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