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Astrosurf

Best Lenses for DSLR Widefield

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I have a Canon EOS 40D so it’s a good camera, but the lenses are pretty rubbish, or maybe it’s a case of a bad workman. The better of the two lenses is an old Canon 70-210 EF zoom which I tried to focus on Jupiter and Venus last night. I zoomed in on live view and then focused. When the focusing went too far it became distorted and multi-coloured so I brought it back until the planet went white and even, but I still don’t think it focused perfectly.

Is it just a poor lens or is there soemthing I could do?

What do you consider are good (not too expensive!) lenses for this sort of work? I suppose they are many and varied!

Alexxx

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You will be better of with primes (fixed FL lenses) than zooms there are plenty of excellent older lenses out there from 35mm film days you will need an adaptor to use them with the 40D .. have a look for M42 mount lenses from Pentax (Super Takumars) Fuji (Fujinon) etc...

These lenses were designed to cover a 35mm frame so work very nicely with the smaller APS-C sized sensors like the one in the 40D..

Peter...

Edited by Psychobilly

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Many thanks. I will have a look around for some. It's always worrying buying second hand though. I might see if any turn up at a star party - when I get the dosh of course, if ever! :)

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My vote goes on the nifty50 as well - i picked mine up from fleabay, boxed and beautiful for 60notes a year ago - best thing i ever bought on there.

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I've got one of those as well. Shame I don't have the skies to do it justice :)

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Many thanks. I will have a look around for some. It's always worrying buying second hand though. I might see if any turn up at a star party - when I get the dosh of course, if ever! :)

Good secondhand, manual focus lenses should be easy to find on ebay, though prices seem to have sskyrocketed in the last few years due to the up urge in dslr photography. Not that long ago you couldn't give away old M42 screw fit lenses now they go for silly money.

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It can be difficult to get focus just right even with magnified live view. The standard photographic technique of stopping down the lens a bit can help with sharpness, at the cost of getting less light.

The EF 50mm f/1.8 II is a nice cheap lens, providing a nice spot in the middle but it doesn't look so great as you go towards the edges. Can't complain at the price though!

For example I used Orion as a test previously. On each line below, stars are Alnilam, Betelgeuse and Rigel, which are middle of belt, top left and bottom right (as seen from Northern hemisphere). Click here for the an example frame resized showing the positions of the stars in this test.

All are 100% crops taken on the 7D. I fixed at 4 second exposure to minimise trailing. I varied ISO to keep exposure constant, with ISO200 at f/2.0. Manual focus on Alnilam to minimise the visible spot size.

f/1.8

c18a.jpgc18b.jpgc18c.jpg

f/2.8

c28a.jpgc28b.jpgc28c.jpg

f/5.6

c56a.jpgc56b.jpgc56c.jpg

Going from f/1.8 to around f/2.8 onwards, the corners tighten up. Stopping down much further starts to make visible diffraction spikes. And for a given exposure time, turning up the ISO to compensate increases the noise levels.

As a balance of speed and quality, then budget primes will probably provide the best performance value. Specific possibilities depend on what sort of focal length you're looking for though.

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Car boot sales can often turn up some used "bargains"... laser_jock99 is right though prices on the bay are creeping up but you can still get some decent buys...

I have access to a whole range of lenses that I used on a Fuji ST705N back in the late 70s...

Peter...

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Alex, the nifty fifty is hard to bet on a cost/performance basis. I've got some old Olympus kit, and have used the 24mm (although as it was not designed for digital and doesn't have the coatings, the images suffered a bit from internal reflections from off angle light sources (buildings etc in the vicinity).

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As with terrestrial photography, you need a lens hood.

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I used one Gina, sort of (dew shield really but it does the same job), but it wasn't long enough to keep the edges completely clear of off angle light, as any longer and it got in the FOV.

Edited by jgs001

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I use a long tube of black card especially when shooting from a light polluted site. Just cut long enough to avoid vignetting. Because of the wide accpetance angle of shorter focal length lenses you need to avoid all extraneous light. Another reason why shorter focal lengths require darker skies.

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This is really good stuff. Thanks guys! :)

GlassWalker, thanks for those excellent examples. Why are the stars in the f/1.8 shot so blurred? This is exactly the colourful distortion I was experiencing. I'll try stopping down next time.

I got some coloured blobs in some of the images which must have been the internal reflections. I guessed that was the case

I think 50mm would be too wide-angle for the pics I want which are the shots that Mesky gets of nebulae etc. I think he uses a 200mm?

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I believe quite a few people are impressed with the Canon f2.8L 200mm USM. AFAIR stan26 has recently acquired one. Not exactly in the pocket change bracket though :)

James

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I believe quite a few people are impressed with the Canon f2.8L 200mm USM. AFAIR stan26 has recently acquired one. Not exactly in the pocket change bracket though :)

I'd love one of those lenses.

I have the 100-400L for aviation, but it's a f4.5 - still looking forward to going to the dark site & trying it.

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I think you might be plesantly suprised with the 100-400/f4.5-5.6L for astro... The locking zoom is handy...

The 200/2.8L is a cracker I bought one a few weeks ago and hope to give it a decent outing at SGL7...

Neither are budget lenses though...

Peter...

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I'd love one of those lenses.

Thee and me both. Unfortunately I need a tank of heating oil more. There's not a lot of difference in price, though at least there's the possibility of getting the lens second-hand.

James

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I think you might be plesantly suprised with the 100-400/f4.5-5.6L for astro... The locking zoom is handy...

Peter - yes think i'll save it till we go to Sulby again. Having a thing about the best way to mount it (as i'd still like to guide as well).

James - oh yes, you've got to stay warm :)

That shopping list, is still growing ......

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Yeah... You'd have a job to get a tank of heating oil second hand :)

And yes, I'd love one of those lenses too :)

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I'd love both of those lenses... probably starting with the 100-400...

seems to be a lot of lens envy here :)

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I'd love both of those lenses... probably starting with the 100-400...

seems to be a lot of lens envy here :)

Money envy too?! :)

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I'd love both of those lenses... probably starting with the 100-400...

seems to be a lot of lens envy here :)

I'm still paying for mine (only bought last October), moment of weakness with a credit card in hand - lol

but haven't really been able to use it yet ! Unless you count a 5 min trip to Birmingham airport .... i'll go & get my anorak ....

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