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  1. #1
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    Default Is There A Better Way To Focus A Wide Angle Lens For Wide Field Images?



    Hi There,

    Last night I was out at my dark site finishing my newest AP project. Part of the project entails wide field sky images. Prior to going out to take the images, I purchased a 58mm Bahtinov mask to make sure my images were focused. Well the Bahtinov works as expected for focal lengths at 55mm and above. But below about 35mm and especially between 18-24mm it was nearly impossible to see the diffraction spikes, even under high magnification. So I got the lens close based on the very tiny Bahtinov diffraction pattern. Then I moved the focus wheel in tiny increments left or right (usually left and right) until a test image showed the sharpest focus possible. These tiny movements felt like I was moving wheel in microns!

    Does anyone have a better way to focus a wide angle lens for wide field nightscape images?

    I sure hope somebody does because I wasted dozens of valuable minutes of imaging time trying to gain focus. Thanks.

    Cheers,
    JT
    Main: C-stron CPC1100 #2 Scope: 8" f/7.5 Dob mounted Newt AP Scopes: TPO 6" RC f/9 & ES 80mm f/6 APO G&G Scope: Meade 102mm f/7.8 goto Guide Scopes: 70 & 80mm fracs Mounts: C-stron AVX CGEM & GT Alt-Az, Cameras: Canon T3i 2, ASI120MC Binos: 10X502,10.5X70,15X702, 25X100 EPs: ES: 21 100°, 30 82° X-Cels: 9,12,18,25 Clubs: RCA & HAS
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  2. #2
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    Default Re: Is There A Better Way To Focus A Wide Angle Lens For Wide Field Images?

    Hi it depends a bit of what for lens is, manual focus or autofocus?
    With manual focus you could try to focus in day time and then fix it, if it got autofocus you could try the lens focusing option in APT
    I have a Samyang 14mm and I have marked the infinite fofucus position on the bodywork
    Clear Skies, Marco
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  3. #3
    Leveye's Avatar
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    Default Re: Is There A Better Way To Focus A Wide Angle Lens For Wide Field Images?

    Full manual mode. Turn autofocus off if the lens has it. LiveView and zoomed in on a star works every time for me.
    My eyes
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  4. #4
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    Default Re: Is There A Better Way To Focus A Wide Angle Lens For Wide Field Images?

    Hello !
    Bahtinov mask don't works well with short focal length, because they are far from a simple lens : for example, the focal length of my Nikon 28 mm is half the distance between the rear of the lens and the focal plane, so there are many tricks to have a short focal length. When I put a Bahtinov mask on my 28 mm, I see an un-focused image of the mask and no diffraction spikes.
    Maybe, for short focal length, you must have special masks, with many very narrow slots.
    Clear skies,
    Arsène
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  5. #5
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    Default Re: Is There A Better Way To Focus A Wide Angle Lens For Wide Field Images?

    Hi,

    I could have set up the question with a bit more info. The lens is the standard Canon kit 18-55mm lens. I have turned off auto focus so it is manual focus only. Part of the my project involves not using a computer, camera LCD only, so like I said, the Bahtinov image on the LCD is nearly unusable. I will try the daytime focus at infinity and then mark the lens casing. At least that may get me close to in focus quicker than last nights test of my patience!

    Leveye, I'm using my camera (a T1i) LCD maxed out at 10x (when checking the test images) and still can't really tell there is Bahnitov diffraction pattern with the lens set between 18-24mm. I'm curious how you are seeing one?

    Arsene, the mask is specially made for small camera lenses. It is a 58mm mask. It's like a dinky version of the one that goes on my C11!

    Cheers,
    JT
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    Default Re: Is There A Better Way To Focus A Wide Angle Lens For Wide Field Images?

    Quote Originally Posted by jaetea View Post
    Hi,

    I could have set up the question with a bit more info. The lens is the standard Canon kit 18-55mm lens. I have turned off auto focus so it is manual focus only. Part of the my project involves not using a computer, camera LCD only, so like I said, the Bahtinov image on the LCD is nearly unusable. I will try the daytime focus at infinity and then mark the lens casing. At least that may get me close to in focus quicker than last nights test of my patience!

    Leveye, I'm using my camera (a T1i) LCD maxed out at 10x (when checking the test images) and still can't really tell there is Bahnitov diffraction pattern
    JT
    You don't need the mask. Just zoom in and focus the star as small as you can. I use magnifier glasses sometimes. Sure helps. I have to be honest tele zoom kit lenses are not the best way to go. Lots of zoom creep. I'd use a prime lens myself. Try 17mm, 28mm,35mm. Tokina makes an Excellent 11-16mm that I highly recommend for a Canon cropped sensor.
    My eyes
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