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Thread: DLA is Nonsense! It should be deleted from the reviews.

  1. #41
    Senior Member Mark Elberson's Avatar
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    Re: DLA is Nonsense! It should be deleted from the reviews.



    Quote Originally Posted by edfrometown
    I looked at the DLA before I got my 7D and all that it told me was that if I could stay below the DLA, I should try. But, fireworks, misty water, large depth of field and sunlight say use whatever f-# you need to get the image and DLA be damned

    Quote Originally Posted by neuroanatomist
    Photography, like much of life, involves compromise. Sometimes I'll choose to use a very small aperture even though I know that will result in loss of sharpness from diffraction, because I want a deeper depth of field. Sometimes I choose a high ISO even though I know that will result in elevated noise, because I want a shutter speed fast enough to freeze motion.

    Very well said

  2. #42
    Alan
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    Re: DLA is Nonsense! It should be deleted from the reviews.



    Quote Originally Posted by neuroanatomist


    Photography, like much of life, involves compromise. Sometimes I'll choose to use a very small aperture even though I know that will result in loss of sharpness from diffraction, because I want a deeper depth of field.
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    A simple solution to the deeper depth of field is to take several shots (focus on foreground, midground, background) and stack them into Photoshop, run a script, blend the layers, and voila, you have a deep depth of field. This technique works best for mostly static environments, but can be used effectively with slight movement of the subject matter. Then, one can use an f/# below the DLA of the particular camera, without compromising the sharpness of the image.


    Well worth the cost of the program, considering how much we spend on our equipment.


    My 2 cents....

  3. #43
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    Re: DLA is Nonsense! It should be deleted from the reviews.



    Quote Originally Posted by pin008
    My textbook tells Rayleigh criterion determineslimiting resolution of a telescope set by diffraction.

    Well, it is the size of the airy disc for a given wavelength lambda, given that you have a circular unobstructed aperture. It is not necessarily impossible to resolve details beyond this limit, so what your textbook says is only approximately true (though all textbooks seem to say the same thing []) For example, it is possible to resolve double stars whose angular separation is less than 1.22&lambda;/D. Maybe you'll see two discs not completely separated, but you can tell there are two stars. Also, telescopes with large central obstructions tend to spread light away from the center of the airy disc (ie, they have a low strehl ratio). This is considered bad, but such telescopes can sometimes resolve double stars that are closer than unobstructed scopes with the same clear aperture.


    Quote Originally Posted by pin008
    If you don't think DLA places hard limits on resolution, please tell me why?

    You yourself showed (with your example involving the 7D) that it does not. Your example showed that an 18mp sensor will show more detail than a 10mp sensor, even if the image is blurred enough that the blur is detectable with the 10mp sensor. Of course, if the 10mp image is very blurry, the advantage in going to 18mp will be very small, but if the 10mp image is right at the DLA, I think there will be noticeable improvement in moving to 18mp.


    Furthermore, if you have a super high resolution sensor, diffraction can be deconvolved effectively using maximum entropy or other algorithms. A 4" telescope can, in theory, produce images which show details much smaller than an arcsecond- far beyond the Rayleigh criterion.


    There is nothing magic about DLA. It is not a point beyond which no information is gained. It is simply the aperture which gives an airy disc about the size of a pixel.

  4. #44

    Re: DLA is Nonsense! It should be deleted from the reviews.



    Seems to have stopped the string!

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