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Old 25-02-2009, 06:48 PM
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deadsimple (Ash)
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Off-centre coma (DSLR)

Hi. I recently made a contraption to get my Canon 450D and standard 50mm f/1.8 II lens onto an EQ6 mount by itself, so that I can dive into widefield long-exposure photography. Unfortunately after trying it out I'm getting massive amounts of coma in my shots.

The coma is off-centre too. The top (and slightly left) of each frame is the sharpest, while the bottom right is a complete comatic mess. Stopping down the aperture helps (as expected), but even at f/8.0 I still have coma in the bottom-right 30% of the frame.

I shoot at about f/4.0 but I've included an f/1.8 image with overlay below to accentuate and highlight the problem. Any ideas? This is driving me nuts. I'm going to try other lenses tonight, but I was under the impression the nifty-fifty was meant to be fine when stopped down enough.

Thanks for reading.
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Old 25-02-2009, 07:27 PM
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Almost all lenses (except perhaps those costing k$) will have coma at full aperture in corners.. It appears you also have a problem with co-axiality...

I am using modified Canon SSC FD f1.4 50mm lens. It appears to be much better than yours, even at full aperture (1.4). I was trying other lenses (for Zenit and Praktica) , but so far this one is the best in this respect (coma at corners).

See here:
http://www.iceinspace.com.au/forum/s...ad.php?t=39293

But I had problem with uneven focus as well.
I solved this with one layer of copper tape (usually used for RF shielding) placed appropriately on one side of the lens seat (to tilt it a bit) , so the lens optical axis is now aligned with the centre of the sensor frame, and I have the good focus across the whole frame (lens focal plane is parallel to sensor plane.. still a bit of tradeoff must be accepted, though.. however this is for pixel-peepers :-) ).

Last edited by bojan; 26-02-2009 at 07:08 AM.
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Old 26-02-2009, 09:14 AM
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I have checked around and you are absolutely right, 50mm f/1.8 II should be much better.
Perhaps you should return it and replaced it with another specimen, something is wrong with this one.
When playing with various lenses, I found that even the cheap & old Domiplan 50 1.8 was better performer (it uses similar lens design, Gaussian simetric).
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Old 26-02-2009, 09:49 AM
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Hi Bojan,

Thanks for the reply. Will see how quickly I can get it fixed under warranty as there's a Canon service centre just down the road

Might test my kit lenses first for comparison.
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Old 26-02-2009, 10:09 AM
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Kit lenses will also not be great performers in terms of star image quality in corners...

The best result on wide angle lenses I have was with already mentioned Canon FD 50mm f1.4 (I found it on ebay last year). It is really good..
Very sharp (in centre but some coma in corners, depends on F) is Tair-11A (135mm)
I also have 200mm Tamron and 200mm Sun (Pentax?), they both are so-so (Tamron is slightly better I think in terms of CA, and it has much better coating, so lower internal reflections), however consistent over the whole frame though.
Yes, star - test should be done before buying.. not always possible, not every supplier will be happy to accept back the number of lenses until you pick the perfect one (if such exists at all)
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Old 26-02-2009, 08:37 PM
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Yes, my nifty 50 shows a lot of coma as well.

Maybe I should have returned it but as I live so far from where I bought the lens, I decided to just put up with it. It doesn't affect daytime images, and for night time work I'm just careful with the framing.
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Old 27-02-2009, 12:23 AM
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I just put the kit 55-250mm f/4.0-5.6 IS zoom lens on.

At both 55mm/f4.0 and 250mm/f5.6 it destroys the nifty's f/8.0 coma-wise. Definitely getting the lens replaced/repaired!
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Old 27-02-2009, 06:48 AM
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So it seems there are two versions of that lens out there.. Very good (from which the "nifty fifty" got its reputation) and a bad one (maybe made in who knows where).. I can not believe the people who were so happy with their specimens did not know what they were talking about.
See also here:
http://www.astrosurf.com/buil/50mm/test_us.htm

At some stage I was wandering myself should I go for it or not, and then this FD popped out.. based on test photographs taken with similar lens designs, I have made the decision and took a risk (because at that time I was not entirely sure how the mod will go) and now I see it was a correct one.

Ash, if you can not find a good specimen, do as Jeanette.. However it should be at least as good as kit one.
And then perhaps try one of those M42 lenses on ebay.. with adapter it will cost you not much more than $50, and no or very little coma :-)
And manual mode only, of course.

Last edited by bojan; 27-02-2009 at 07:40 AM.
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Old 27-02-2009, 01:30 PM
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yes there are two incarnations of the nifty fifty, Mk 1 and Mk 2, the Mk 2 is the more commonly seen one, and is a good lens for daytime photography, although it does suffer a bit of coma, the Mk 1 was the original one that got the lens its reputation..

Other differences include the fact that the Mk1 was metal mounted, where as the Mk2 is plastic mount, (the mk2 is often referred to as "plastic fantastic")

I had a mk1 back before I got into astronomy and astro photography, and im now constantly scanning the forums/ebay for a 2nd hand Mk1.. as the Mk2 whilst it is a good lens, is not quite to the standard of the Mk1
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Old 27-02-2009, 01:37 PM
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Alex,
Christian Buil's web page refers to MKII then, see the caption under the first fig on it (f/1.8 Mark II).
What Ash showed us (and what Jeanette indicated as a slight problem) is terrible..
So, if you are right , then there are 3 version of that lens: 1) Ecellent (Mk_I), 2) not so excellent (Mk_II) and 3) a range of Mk_II crappy ones.
Also keep in mind that this (Buil's) test was done with full frame sensor.. at the left-right edges of the frame.
Here, we are comparing those lenses used with smaller frame cameras (400D, 450D, 20d .. am I right, Jeanette?), so the coma in corners should be much better (and it is, in case of my former FD lens, since I do not have to crop at all if using f3.5 or more)
Maybe Mk-I and FD F1.4 are the same lens (optically)? FD is also almost 100% metal (except couple of small plastic parts of iris mechanism inside)


Last edited by bojan; 27-02-2009 at 02:32 PM.
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Old 27-02-2009, 03:41 PM
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Yes perhaps, The Mk1 was mostly metal, the switch for AF/MF was plastic, but it was a HEAVY lens for the $160 I paid for it... Definitely heavier than the 50mm F/1.4 Nikkor lens I have for my D200... Perhaps not quite as robust as the current Canon 50/1.4..

In any case, Buil's test using a full frame sensor would definitely have shown a much more pronounced case of coma due to the 0.4x bigger sensor size.. Obviously though, not all optical elements are created equal, and the tollerances for optical mis-alignment might need reviewing..
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Old 27-02-2009, 04:49 PM
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Quote:
Originally Posted by deadsimple View Post
I just put the kit 55-250mm f/4.0-5.6 IS zoom lens on.

At both 55mm/f4.0 and 250mm/f5.6 it destroys the nifty's f/8.0 coma-wise. Definitely getting the lens replaced/repaired!
To follow up on this, I've attached a wide-open shot of the 55-250mm kit lens for comparison (2min exp):
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Old 27-02-2009, 05:09 PM
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That is looking way better than "nifty-50" (or is it? maybe it is a bad clone or forgery?) .
However, more useful would have been if you attached just a small crop from centre and corners (like I did).. that way problems would not be hidden behind pixelization :-)
But, it definitely looks way better..
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