Less light means more focus? What’s going on?

Edit: See hhhyperfocus’s answer

  • sga@lemmings.worldEnglish
    1·
    1 day ago

    A bit more mathematically depth of field is roughly proportional to lambda/(NA)^2, where lambda is the wavelength (we are not doing monochromatic illumination, but it still holds regardless) and NA is the numerical aperture, in photography context you may know of something called f-number which is related but not same. basically both are ratios of size of aperture (Diameter) and distance of object, to be precise f number is the ratio, and NA is n (refractive index of medium in between, which is usually air, so 1) times sin of above ratio. long story short, basically as Diameter increases, this NA increases, and depth of field becomes shorter.

    For a explanation of why this happens is a bit harder to hand wavily explain, but i will try.

    firstly some jargon lingo - spatial frequency - consider it as frequency of something in source, the smaller the features in the source, larger as these frequency.

    also for now just take it, larger aperture helps capture larger spatial frequency (if you want to know math about it, your aperture is basically a circ function (the name called in literature) (1 inside aperture, and 0 outside, as in blocking anything outside) and mathematically it gets multiplied (or to be precise, we actulally have convolution (Assume some fancy multiplication), and for larger spatial frequency, jinc (a complimentary fucntion of circ) becomes small) - yada yada - we loose small features with small apertures)

    but above math also tells you that for larger aperture, depth of field is poorer. So Imaging is a balancing game of figuring out how to have your aperture, if you can manage with poorer depth of field, you get better “resolution” and vice versa.

  • hhhyperfocus@lemmy.worldEnglish
    4·
    27 days ago

    Light rays thru a small aperture can be close to perpendicular when they hit the imaging plane. So you can move the imagine plane forwards and backwards a bit, and the circle of confusion stays about the same size. That’s a deep depth of field.

    Light rays thru a wide aperture hit the imaging plane at a shallow angle. So if you move the imaging plane even a little bit the circle of confusion changes size dramatically.

    https://en.wikipedia.org/wiki/File:Depth_of_field_illustration.svg