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Old 08-07-2010, 03:29 PM
astrospotter (Mark)
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How to stop down a Canon EOS lens

In hand is a Canon 200mm f/2.8 prime focus lens that I have loved for daytime use and have used at night with a 40D camera.

Have recently bought a QSI583 for use with an imaging refractor but then picked up the proper adapters to use Canon EOS lenses as well. You maybe see where I am headed here and that is my starting to wonder if there was some way to fool a Canon EOS lens into keeping it's aperture around f/3.5 even after it is removed not only to obtain sharper stars in the corners but also to minimize vignetting through the QSI583 1.25" filter wheel small area.

Feel free to laugh and tell me it is impossible but feel even MORE free to tell of a way that this can actually be done.

Again the idea is have a stopped down Canon EOS lens mounted on the front of a CCD camera where there is no control for aperture electronic signals.

Thanks for any thoughts.
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Old 08-07-2010, 03:38 PM
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Cut a disk out of black card with a centre hole cut out.Just put it on the end of the lens. I think some of the fellas here do this.
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Old 08-07-2010, 03:47 PM
astrospotter (Mark)
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Found a way to have EOS lens hold it's f stop

No sooner did I post when a rather simple approach came to mind. I am not recommending this as I am not entirely sure that damage may come to the lens or the Canon 40D if you try this.


With the Camera turned on and f-stop set to 3.5 press in and hold the depth of field preview so the aperture reaches f 3.5. While still holding the depth of field preview, remove the lens using the normal release mechanical button and twist of the lens.

Disclaimer: Kids, don't try this at home


Darn thing holds the partially stopped down aperture.

So my next question is I seek a non speculative answer as to if this can damage the camera or the lens OR if this is in fact no-problemo.

As an electrical engineer I can easily understand that this was a foolish risk to take but I generally am curious as well so the curious side won this rather risky debate.

Thanks for your thoughts and feel free to tell me I am a fool, if you can back it up with paperwork (manual or Canon info)
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Old 08-07-2010, 03:51 PM
astrospotter (Mark)
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A much safer approach Dave

Thanks for the mask idea Dave, that I am sure would not destroy my camera as long as I avoid the easy task of not scratching the lens.

By the way, I too am a guitar player from way back as well.

(I am still curious if others know if the trick I mention is 'safe' because it would allow an easy way to getup different stops in the field)

Enjoy your guitar and your astronomy,
Mark
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Old 08-07-2010, 03:54 PM
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This is quite safe to do so. It's also mentioned on the SoCalAstro web site; the guys who sell (or, at least used to) sell an EOS EF-SBIG STL adapter.

H
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Old 08-07-2010, 04:20 PM
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I thought of this too, and wanted to try it on my standard kit 18-55 IS. I selected the largest aperture in aperture priority mode, and engaged the DOF preview button while turning off the power. After lens disengagement the iris was...well... toight. It didn't stay at the aperture selected.
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Old 08-07-2010, 04:53 PM
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I use that technique all the time when I use reverse lensing for macro on my 18-55. No problems, just watch your dust.
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Old 08-07-2010, 05:32 PM
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The DOF preview button trick is a good one, but you'll get spikey stars because the aperture is created from blades and not a perfect circle.

If you use David's method of cutting out a circle and putting that over the front, you can achieve the same thing but with get more "refractor-like" stars, if that's even a word
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Old 08-07-2010, 05:57 PM
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Troy and David,

Does this mean, if say, an objective lens (not the housing) was 100mm in diameter and was natively at f/2.8, then, to obtain f/4 you would cut out an aperture mask to allow 50mm diameter in the middle of the lens to receive light?

H
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Old 08-07-2010, 07:38 PM
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Not 50mm, that's half the diameter but you're after half the area for one full stop like f/2.8 to f/4. It'd be 100 divided by root 2 (1.4 approx). That's about 70mm. That's if my maths is right this time of night after 2 beers with dinner.
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Old 08-07-2010, 07:46 PM
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Of course. I should have known that. Duh! I just completely failed to relate an external aperture mask to being the same as aperture itself. Fail. I explain aperture/shutter speed/ISO to people who want to learn about photography all the time. I can't believe I failed to see that. For shame. For shame.

Cheers.

H
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Old 08-07-2010, 09:12 PM
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lol don't beat yourself up over it. I have, on at least 2 occassions, been trying to "help" someone sorting out their exposure problems by telling them to rotate the aperture dial 3 clicks for 3 stops etc. 3 clicks is of course only 1 stop.
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Old 08-07-2010, 09:35 PM
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Haha! OK, I don't feel so bad now.

H
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Old 18-07-2010, 06:39 AM
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Have a look here.. http://www.iceinspace.com.au/forum/s...ad.php?t=51866
The lens with external mask might perform even better than with its own iris (as discussed in abovementioned thread)

Last edited by bojan; 18-07-2010 at 08:13 AM.
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