by Chris Peterson » Tue Jul 14, 2020 9:54 pm
johnnydeep wrote: ↑Tue Jul 14, 2020 9:37 pm
Chris Peterson wrote: ↑Tue Jul 14, 2020 7:56 pm
johnnydeep wrote: ↑Tue Jul 14, 2020 7:47 pm
Hmm. Is that because you can blow up a photograph after taking it to simulate any magnification you want?
Magnification is essentially an angular measurement, one that involves telescopic optics- that is, an optical system with both an objective and an ocular (eyepiece). A single objective (like a telephoto lens, or a "telescope" used photographically) doesn't operate the same way. You simply end up with an image of some size at the focal plane (telescopic optics are afocal- there is no focal plane).
Ok. Still trying to understand this. With a telescope there's an objective lens and an eyepiece, and dividing the focal length of the former by the focal length of the later gives you the "magnification", and you also have to "focus" the image in order to see it clearly. Yet I see articles about the "afocal method" of astrophotography that merely uses a camera in place of the human eye. But there's still focusing involved and also magnification, right? (As I said, I
really need to read more about optics, cameras, telescopes and photography...)
Telescopes are afocal. They have no focal plane- you can't hold a screen behind the eyepiece and see an image. What is coming out of the eyepiece is a fairly collimated bundle of rays. This enters your eye, and the lens in your eye brings the image to a focus on your retina. Afocal photography works the same way, with the camera lens being a required element of the optics to create a focus at the film or imager. There should be no focusing involved, except to the extent that the eye or the camera is focused at infinity- just the same focus they'd be using if you pulled the telescope out of the way. And from this, the concept of magnification makes sense. Divide the size of the object with the telescope by the size without it, and you have the magnification. You can't do that with a focal system, however, like the usual optics used for astrophotography or for ordinary photography.
FWIW: the image scale of the full-sized picture is 25.2 arcsec/pixel. Given that the E-W diameter of Stonehenge is about 35 m, this means the image was made 75 m south of the center of the circle, which is right on the big semicircular path that is around the stones. It is also consistent with a lens focal length of 150 mm.
[quote=johnnydeep post_id=304134 time=1594762669 user_id=132061]
[quote="Chris Peterson" post_id=304127 time=1594756602 user_id=117706]
[quote=johnnydeep post_id=304126 time=1594756054 user_id=132061]
Hmm. Is that because you can blow up a photograph after taking it to simulate any magnification you want?[/quote]
Magnification is essentially an angular measurement, one that involves telescopic optics- that is, an optical system with both an objective and an ocular (eyepiece). A single objective (like a telephoto lens, or a "telescope" used photographically) doesn't operate the same way. You simply end up with an image of some size at the focal plane (telescopic optics are afocal- there is no focal plane).
[/quote]
Ok. Still trying to understand this. With a telescope there's an objective lens and an eyepiece, and dividing the focal length of the former by the focal length of the later gives you the "magnification", and you also have to "focus" the image in order to see it clearly. Yet I see articles about the "afocal method" of astrophotography that merely uses a camera in place of the human eye. But there's still focusing involved and also magnification, right? (As I said, I [b][i]really [/i][/b]need to read more about optics, cameras, telescopes and photography...)
[/quote]
Telescopes are afocal. They have no focal plane- you can't hold a screen behind the eyepiece and see an image. What is coming out of the eyepiece is a fairly collimated bundle of rays. This enters your eye, and the lens in your eye brings the image to a focus on your retina. Afocal photography works the same way, with the camera lens being a required element of the optics to create a focus at the film or imager. There should be no focusing involved, except to the extent that the eye or the camera is focused at infinity- just the same focus they'd be using if you pulled the telescope out of the way. And from this, the concept of magnification makes sense. Divide the size of the object with the telescope by the size without it, and you have the magnification. You can't do that with a focal system, however, like the usual optics used for astrophotography or for ordinary photography.
FWIW: the image scale of the full-sized picture is 25.2 arcsec/pixel. Given that the E-W diameter of Stonehenge is about 35 m, this means the image was made 75 m south of the center of the circle, which is right on the big semicircular path that is around the stones. It is also consistent with a lens focal length of 150 mm.