#1  
Old 30-04-2011, 07:18 PM
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hotspur (Chris)
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Using a step down adaptor for 70-200 F 2.8

A while ago,I asked if my 70-200 F 2.8 could be used for astrophotography,as I had made a few attempts,but with so much glass in the lens I got reflections.Some of you suggested 'step-down adaptors' to eliminate this problem and thus be able to use this fine lens.

Here is a few of the ones I found on ebay,there is plenty similar.

http://cgi.ebay.com.au/77mm-52mm-77-...tn%3D4%26po%3D

and

http://cgi.ebay.com.au/77-72-67-62-5...I%26otn%3D4%26

Some even suggested a piece of cardboard with a hole in it.I think I might like to get a step down,has anyone here used one similar to these ebay offerings.One goes straight from 77-to-52 mm,the other has a range,do I need such a range for what I plan to do?As astronomy photography is something I seem to be less and less off,just wondering which item I should get for the task.

Any pointers?thanks Chris
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Old 01-05-2011, 12:29 AM
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Waxing_Gibbous (Peter)
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I think your 60mm Macro (or just about any prime lens) would be a better choice for a-p.
Even top-notch 'L' zooms often give soft or blurry results on starfields.
Some results can be seen in the book, "The Backyard Astronomer's Guide" (p.279 ), by Dickinson & Dyer.
Quite a useful chapter on a-p actually!
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Old 01-05-2011, 08:20 AM
space oddity
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zooms and astro

Generally speaking, the more elements a lens has, the more internal reflections it will suffer. Zooms always have more elements, therefore more internal reflections. Optical performance will always be inferior to fixed focal length for the same speed lens.In all my days of testing lenses, I have NEVER seen a zoom that gives sharp images wide open-they need to be stopped down a good 2 stops to get good performance, except the ridiculously slow kit lenses which are way too slow for astro. Same applies to fixed focal lengths, although some of the more expensive fixed focal length lenses perform quite admirably wide open. Internal reflections are best reduced with a lens hood-it is most likely built in to an f/2.8 lens although it is protected just to the 80mm focal length. Make sure all dust has been blown off the front element.
Good luck
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Old 01-05-2011, 08:48 AM
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Cloudyagain (Neale)
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Hi Chris,

I have attached a picture I took with the 70-200 F2.8 Canon lens at 200mm FL. The aperture was stopped down to 45mm diameter by a piece of card with a hole in it stuck to a 77-72mm stepdown ring. I also used a custom light shield (tube of insulating foam) extending 15cm in front of the lens. The stars in the corners are not round and I have now made another mask 39mm diameter which I haven't tried yet. As you can see reasonable results are possible.

Neale.
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Old 01-05-2011, 11:21 PM
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Waxing_Gibbous (Peter)
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I hate to rain on a parade but I've kinda been scratching my head here and thinking "Why?"
I mean, I like the "twofer" idea but, just about any decent 80mm - 90mm scope will give you better results at 1/2 the price.

My 'L' glass will get me. ;D
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Old 02-05-2011, 05:43 AM
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Octane (Humayun)
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The 70-200mm f/2.8L II USM is an exception to the norm, it is pin sharp at f/2.8, even with the new 1.4x and 2x extenders.

You gets what ya pays for.

H
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Old 13-05-2011, 06:12 PM
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hotspur (Chris)
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Thanks Neale.

I found a cardboard tube just wide enough to slip over the 70-200,on the end I stuck apiece of cardboard over the end I cut the hole in the end about 46 mm Diameter,I then slide it carefully over the body.I have cut the tube just long enough so it slides down and the other end touches the tube ring steam,so as the end of cardboard does not touch the front lens.Not sure if this will work.

I'll post an image of this.Chris
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Old 13-05-2011, 07:17 PM
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hotspur (Chris)
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re lens

Here is image of tube with smaller hole in end,not sure if this will work.Any ideas??
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Old 13-05-2011, 08:55 PM
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dugnsuz (Doug)
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Hi Chris - I've been using the ebay step down rings to stop down both my 70-200 f4 and 400mm lenses. Seem to work well, although I haven't done any thorough tests with and without the rings. I reckon they're the easiest way to achieve what you want at very little cost.
Doug
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Old 14-05-2011, 08:51 AM
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Cloudyagain (Neale)
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Chris, if you position the end of your tube further out than the front of the lens you will lessen the chance of off axis light entering and causing reflection artefacts. I had bad reflections until I extended the tube about 6 inches in front of the lens.

Also I have read that the front aperture should be perfectly round without dags as this can cause diffraction artefacts, I have no personal experience regarding this latter phenomena though.

Neale.
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Old 15-05-2011, 08:02 AM
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hotspur (Chris)
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re light

Yes,thanks Neale and Doug.

Doug-yes-the cheap 77-52 mm step down for $5 from HK will work fine,I just have not got around to ordering it,and usually can take up to 3 weeks to get here,The reason for the attempt I posted in image,was I thought I may need it for conjunctions,But yesterday I used 70-200 F 2.8 and had no reflection issues.

Yes,Neale,I agree with your thoughts,but I think its too much bother to get this thing I made (that looks like a toliet roll,and I'm not too fussed having it on my most expensive lens),all perfectly round etc.I'll get that adaptor so I can use lens on winter sky objects,It is so sharp this lens.

cheers Chris
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