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 Post subject: Which lense for 'flat field'
PostPosted: Thu Sep 20, 2012 3:13 pm 
Which lenses should I be looking for to attach to the canon dslr to give me a good flat field right to the corners of an image ? All my images have the star wars affect around the edges 'vignetting' is it ?? I guess this is due to the domed shape lenses that I have .... Anyway, it's annoying me now and would like a 50 mm or similar that won't cause this, any ideas ??


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 Post subject: Re: Which lense for 'flat field'
PostPosted: Thu Sep 20, 2012 9:15 pm 
Hi Dean,

Vignetting is where the edges of a telescopes field of view have a darkened effect. Or to put is slightly differently, because of the tube, i.e. T-ring or the focuser tube that the camera sensor is looking through prevents sufficient light reaches the edge of the camera sensor. This effect can be removed in post processing or by taking "flat frames". These are shots that are taken of a uniformly lit light source. You can achieve these by taking shots of the twilight sky with an exposure to allow the saturation to reach 65-75% of maximum.

I am not sure quite how you would go about fitting a field flattener between a camera and a camera lens. If you where to purchase a flattener prices start around the £100 mark and the sky is the limit - no pun intended!

Ian


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 Post subject: Re: Which lense for 'flat field'
PostPosted: Fri Sep 21, 2012 7:52 pm 
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Location: Adlington
Dean,
I assume you are talking about a camera lens. Most lenses are designed for daytime photography and shooting stars is too severe a test for them as it it easy to see when a star isn't a point. The poorer lenses need stopping down more - usually f/8 improves the sharpness into the corners. That 135mm Pentax lens that Andy had at Kelling has a good reputation but still needs to be stopped down to f/5.6. The best 50mm lens I have ever come across is the Pentax 50mm f1.4. This when stopped down to f/4 shows fantastic sharpness and the stars are still good in the corners. I got mine on ebay.


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 Post subject: Re: Which lense for 'flat field'
PostPosted: Sat Sep 22, 2012 6:59 am 
Ah ok that explains it because that was the lense I was using, 50 mm Pentax ..... Just didn't stop it down far enough ! Thanks david


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 Post subject: Re: Which lense for 'flat field'
PostPosted: Sat Sep 22, 2012 1:04 pm 
Im lucky enough to have both the 135 & 50mm pentax lenses. Kelling was the 1st time i'd ever used the 135mm unit & it yielded impressive results, i've only used the 50mm once, that was to produce the star trails - which i need to post on the gallery!!!

Im so happy with the results from both, i'm trying to find somewhere decent to leave my 'expensive' canon lenses so i can fit my bargain bucket Pentax ones in the camera carry case!!!


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