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Prime Focus Lens and 'Extenders'

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#1 Dave Lee

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Posted 26 May 2013 - 01:11 PM

Canon sells 'Extenders' (similar to Barlow's - at least in function). They currently have a 1.4x and 2x model.

Does anyone have any AP experience with these devices? Thanks.

dave

#2 mmalik

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Posted 26 May 2013 - 01:14 PM

Do you have a link?

#3 Dave Lee

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Posted 26 May 2013 - 01:26 PM

Down toward the bottom of this page

http://www.usa.canon.../ef_lens_lineup

are links to both of Canon's offerings here.

dave

#4 DrGolds

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Posted 26 May 2013 - 02:29 PM

The 2x extender definitely degrades images, I have seen comparisons before. But if you're doing long exposure AP, chances are your seeing impacts your results too, so losing some sharpness with an extender may not mean much.

#5 mmalik

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Posted 26 May 2013 - 03:11 PM

Additional magnification is generally used for planetary imaging (NOT DSOs). Here... is an example of widely used magnification for planetary/lunar imaging.

DSOs are a different ball game, mostly the domain of reducer/flatteners/LPS, etc. An example here....


With that said, what are your goals, DSO or planetary imaging? What kind of camera/scope/lens you have? May be that will be the right way to look at your question/needs. Regards

#6 Dave Lee

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Posted 26 May 2013 - 03:34 PM

I've got a 50mm (f1.8) and a 200mm (f2.8) prime focus lens. My interest is mostly DSO stuff and even 200mm is pretty darned short for many DSO objects (obviously not all of them). Both lenses are attached directly to a 1100D Canon DSLR.

dave

#7 DrGolds

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Posted 26 May 2013 - 04:08 PM

Even 400mm is pretty short for DSOs. I've attached an old result from my 400mm f/5.6 @ 6.3 on my modified 40D. This is downsampled a lot for web posting.

Attached Files



#8 HeyJP

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Posted 26 May 2013 - 04:09 PM

They work fine as defined and intended... But you trade off lens speed for increased focal length. You have to verify lens compatibility... They don't work with all lenses.

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#9 DrGolds

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Posted 26 May 2013 - 04:10 PM

The extenders will work fine with all lenses, what won't always work is autofocus. That's not a problem for AP.

#10 Dave Lee

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Posted 26 May 2013 - 04:18 PM

VERY nice, Dr. G.

dave

#11 Dave Lee

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Posted 26 May 2013 - 04:19 PM

The extenders will work fine with all lenses, what won't always work is autofocus. That's not a problem for AP.


For completeness, here is a list of the compatible lenses.

http://gdlp01.c-wss....f-2x-iii-en.pdf

Note that this is the 3rd generation for this device. ebay buyers need to be careful here.

dave

#12 DrGolds

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Posted 26 May 2013 - 04:25 PM

That doesn't seem like an exhaustive list. Since we don't care about autofocus, I suspect the list is much larger. Aperture control *may* be an issue, but I don't see why off-hand, since the extender should just have pass-through pins for aperture control.

#13 pfile

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Posted 26 May 2013 - 06:25 PM

i've done 200mm + 2x and the chromatic aberration was quite bad. since then canon has new versions of the TCs which may be better.

there are a lot of good targets for 200mm lenses: north america nebula, sadr area, extended rosette area, the area between orion's belt and M42, the heart nebula, the tadpoles+flaming star nebula...

rob

#14 Dave Lee

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Posted 26 May 2013 - 07:00 PM

i've done 200mm + 2x and the chromatic aberration was quite bad. since then canon has new versions of the TCs which may be better.

there are a lot of good targets for 200mm lenses: north america nebula, sadr area, extended rosette area, the area between orion's belt and M42, the heart nebula, the tadpoles+flaming star nebula...

rob


By your comment I am assuming that this means that the CA with the extender was noticeably worse than without it (assuming no change in aperture).

Thx for the comment. This was the kind of feedback I was looking for.

dave

#15 Dave Lee

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Posted 27 May 2013 - 05:27 AM

An interesting quote from the marketing verbiage regarding the latest (ver. III) of the 2x extender.

"High optical performance with improved correction of chromatic aberration and is optimized to reduce ghosting and flare"

dave

#16 dasams

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Posted 27 May 2013 - 09:23 AM

The extenders will work fine with all lenses

Just to clarify, the design of some lenses does not allow extenders because the rear glass element is too close to the flange. If you try to add the extender, you'll find the extender impacting the lens :foreheadslap:

#17 Samir Kharusi

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Posted 27 May 2013 - 10:32 AM

I have done quite extensive testing of Canon tele-extenders on Canon super-teles and reported the results here. In brief, do NOT buy any extender. It is much more cost effective to purchase a new DSLR, if necessary, with small pixels (the current Rebels have 4.3 micron pixel pitch) and simply crop. Most camera lenses cannot withstand the combination of tiny pixels and tele-extenders, but I must admit that I have NOT tested the latest generation of Canon super teles like the 600mm/4.0L IS II. I suppose if I paid $12k for a lens I would willingly pay a few hundred $ for an extender that may be border-line useful. Nevertheless there are times when we wish to use Barlows, e.g. for planetary imaging, and a 2x extender does make a very high quality substitute. It's also reasonable to expect that a lens delivers better on-axis performance than off-axis, and all planetary imaging is on-axis. Consequently planetary imaging could benefit more from tele-extenders than wider images.

#18 pfile

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Posted 27 May 2013 - 12:47 PM

i've done 200mm + 2x and the chromatic aberration was quite bad. since then canon has new versions of the TCs which may be better.

there are a lot of good targets for 200mm lenses: north america nebula, sadr area, extended rosette area, the area between orion's belt and M42, the heart nebula, the tadpoles+flaming star nebula...

rob


By your comment I am assuming that this means that the CA with the extender was noticeably worse than without it (assuming no change in aperture).

Thx for the comment. This was the kind of feedback I was looking for.

dave


yes - relative to the lens without the TC the CA was noticeably worse. in fact, the red and blue images required re-registration to the green image, which really surprised me. i'm used to seeing one of the planes not focused simultaneously with the other (red usually being soft when g/b are in focus) but the images being offset from one another was really a first.

#19 Dave Lee

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Posted 27 May 2013 - 03:32 PM

I have done quite extensive testing of Canon tele-extenders on Canon super-teles and reported the results here. In brief, do NOT buy any extender. It is much more cost effective to purchase a new DSLR, if necessary, with small pixels (the current Rebels have 4.3 micron pixel pitch) and simply crop. Most camera lenses cannot withstand the combination of tiny pixels and tele-extenders, but I must admit that I have NOT tested the latest generation of Canon super teles like the 600mm/4.0L IS II. I suppose if I paid $12k for a lens I would willingly pay a few hundred $ for an extender that may be border-line useful. Nevertheless there are times when we wish to use Barlows, e.g. for planetary imaging, and a 2x extender does make a very high quality substitute. It's also reasonable to expect that a lens delivers better on-axis performance than off-axis, and all planetary imaging is on-axis. Consequently planetary imaging could benefit more from tele-extenders than wider images.


Thanks - VERY helpful information.

dave

#20 DrGolds

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Posted 27 May 2013 - 04:20 PM

The extenders will work fine with all lenses

Just to clarify, the design of some lenses does not allow extenders because the rear glass element is too close to the flange. If you try to add the extender, you'll find the extender impacting the lens :foreheadslap:

Ah that's something I hadn't considered. I don't have such issues with the lenses I own. Thanks for clarifying.






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