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How Do I Stop Down Lenses?

astrophotography dslr equipment
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#26 xiga

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Posted 21 February 2019 - 11:32 AM

Hi Ciarán,

 

That's very impressive. A couple of question pls....

1. What is the model number of the Tamron 135/28, I'm look for something similar.

2. What was you final effective aperture before you got rid of the come?

 

Thanks,
Rudy

Hi Rudy

 

This was the lens itself:

https://www.ebay.co....872.m2749.l2649

 

Wide open the aperture is 48mm (simply based on 135/2.8).

With the 37mm ring it was operating at F3.65 (135/37).

The homemade ring was, I believe, 24mm, which gives about F5.6.

So not a lot of aperture compared to something like the Samyang 135 (which at F2.8 would have 48mm of clear aperture). But with an EQ mount I should hopefully have no probs just shooting longer subs to compensate (albeit with reduced resolution, but hey for £20 I can hardly complain!).

I still need to use the lens properly though, so I can't say how it perform at this F-ratio just yet.

 

For giggles, if you want to see what the homemade ring (made in a mad rush as it was already late and I was wasting clear skies) then have a look here lol.gif

https://stargazerslo...a8386852281.jpg



#27 xiga

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Posted 21 February 2019 - 11:36 AM

ps - I'm also hoping that when I shoot Ha with the lens, I will shoot it wide open at F2.8. Reason being, I plan on removing the stars from the Ha subs anyway, and just using them to Lighten the reds in the RGB subs, so hopefully the ugly CA won't be an issue for them.



#28 Samir Kharusi

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Posted 28 February 2019 - 10:29 AM

Pssst. You cannot have chromatic aberration with a single (Ha) color... Shoot Ha wide open and be happy.



#29 whwang

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Posted 28 February 2019 - 11:55 AM

Chromatic aberration and spherical aberration are often coupled with each other. Imaging with an Ha filter avoids the chromatic aberration but cannot reduce the spherical abrrration if it exists. It’s possible that stopping the lens down gives you sharper images even with an Ha filter, but you will need to do some experiments to find out.

#30 Jim Waters

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Posted 28 February 2019 - 12:04 PM

Thanks all for your inputs.




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