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Forum: Pentax DSLR Discussion
03-10-2023, 10:53 PM
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The Japan Hobby Tool "leather" has adhesive backing. You just cut to size.
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Forum: Pentax DSLR Discussion
02-14-2023, 11:04 AM
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On a camera lens "infinity" is just a marking on the focus ring. It's never perfect.
It's not the light years. Telescope focus works the same way as any other optical system - it doesn't have a factory-set perfect focus.
Usually there is a point where the "spot size" of the star image is as small as possible, eg it is a tiny circle at the image focal plane.
Ideally you would bring the detector (image sensor, film) to that point, by moving it mechanically.
The problem is that mechanically you can't always do that, so you move the optics closer or further from the focal plane where the detector is. That affects the spot size, and thus how sharp the star is.
With any optical system, eg a camera lens or telescope, one of the biggest factors affecting focus is temperature.
When the temperature is low, the lens assembly will contract - this includes the metal or plastic that holds the optical glass, and the glass itself.
When its warm, the assembly will expand.
So focus will shift a tiny amount, based on this expansion or contraction.
Since stars are nearly a point-source (tiniest possible spot), small errors in focus become more obvious, as the arc spanned by the star at the focal plane is close to the pixel size. (eg wide lenses, the stars will be smaller than the pixel; in long lenses, the stars will span anywhere from 1 to 8 pixels, typically).
The wavelengths of light are also a factor. e.g. blue light vs red light come to a focus at a different place, unless the optical designer has compensated for it. This is why you may see colored fringes around a star.
The atmosphere is another factor - light is refracted less when straight overhead vs going through the thicker amount of air toward the horizon.
When working with really wide lenses, if you point it straight up, and you focus on stars directly overhead, the ones further away from the zenith are going to be less sharp, due to both the atmospheric refraction, and any imperfections in the optical design.
The last thing is that the detector (sensor or film) is usually quite flat; and the optical designer has to come up with a lens design that will take the curved hemisphere of the sky and try to flatten it out as much as possible. Since every lens is a compromise (performance vs cost vs available optics), you will often find the edges of the field are less sharp than the central area.
So "infiinity" is just where the optical designer has designated the place where the lens is likely to be sharpest at far distant targets.
You'll notice most modern lenses don't have a "hard stop" at infiniity - there is a bit of play, and that is to allow the camera to adjust the lens a bit under or over the infinity marking.
I hope that helps.
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