canon 50mm f1.8 mk 2 lens problems

[ozstronomer]

Hi all

I had the chance to find 3 hours of clear sky last weekend and wanted to try out a canon 50mm lens I've had for a while but never really used.

I set up to take a wide field of Orion with my Canon 20d(modded) guided on top of a WO 110 refractor and meade dsi pro. Captured with Nebulosity, guided with PHD

After taking 10 x 3min images I noticed severe pincushioning at each corner,the centre looks ok. The lens was set at f2.8

Can anyone suggest how to adjust this lens to make it flatter. I thought these 50mm lenses were ok for Astrophotography, but I maybe mistaken.

[bojan]

That is normal behaviour of this lens (and for many other cheaper lenses..).
Try with F4 or even 5.6.

[jjjnettie]

Yep, you need to stop it down a few notches Geoff.
You'll be surprised with the difference it will make.

[ozstronomer]

Bojan, JJJ thanks for the comments. As soon as we can find clear skies I'll give it another try.

Geoff

[Astroman]

I found the same, although stopping it down does decrease the effects of it, it's still not a very good lens for Astro work...

[bojan]

That depends on what you want to do.
It is perfectly adequate for serious work, like novae and variables...

It is interesting to see the comparison of various 50mm Canon lenses.. see the link below.
http://www.astrosurf.com/~buil/50mm/test.htm
None of them is perfect at full aperture, but at f4-~5.6 are better..
F1.2 at f5.6 is the best of a lot. The same philosophy applies to other brands as well.

[barx1963]

I had fairly adequate results with this lenses unguided at f4 or thereabouts. Any wider than f4 and the stars just wouldn't focus.

Malcolm

[dannat]

what a terrific comparison -the f1.4 seems the better value to me...i wonder how the 50mm macro at f2.8 shows up on these tests -i was thinking of getting one. i think i will pass on the 50mm f1.8 -

[bojan]

I have F1.4 FD SSC version Modified to fit EOS).. it behaves very similar to F1.4 described on Buil's website. It is fantastic value! (you can get it for less than $100 on ebay.. of course, the modification is not for everyone...)

[Astroman]

Great link, some good comparisons there...

[ozstronomer]

Thanks for the link bojan, that's great information it really puts it into perspective

Geoff

[jjjnettie]

"The Astronomer" mag (UK) in October last year used an astro image taken with that lens on their front cover. So it can't be too bad for astro work. ;)

[bojan]

Of course not :)

It seems there are couple of versions of this lens out there...
Some specimens appear to be really bad.

[jjjnettie]

I still reckon the "nifty 50" is a must have in any imagers kit bag. :)

[Astroman]

I might go do some tests on mine then, although I have been happy with the lens, it hasn't given me any wow performance under the stars yet... probably been going against my own advice and others and shooting at a to wider aperture.

[Astroman]

Well seems like shooting at f/5.6 seems to be the best for this lens, I went over my other images and found I had shot them at f/2.8 also. This produced severe coma I guess on the image. I tried some tests at 5.6 and the stars look pretty good... the exposures needed to be a little longer, but it produced some pretty good stars all the way to the edges.

Thanks Bojan, JJJ, for the advice... and not giving up on the lens :) Next time I will check my settings before opening my mouth.

[jjjnettie]

:) I'm glad you stuck with it.