by Chris Peterson » Thu Mar 24, 2011 2:28 pm
Whit in Chillicothe, OH wrote:The moon seems SO big. Was it taken at magnifacation after the skyline was shot? That makes it seem like a hoax.
The problem is, many people don't understand that images are translating angular scale to linear scale.
The Moon is 0.5° degrees across. It has this angular size regardless of the lens used, or where on Earth you are standing. If you shoot it with two lenses, say 100mm and 200mm focal length, the longer one will map that angular size to twice the linear dimensions. That is, the Moon will appear twice as large on the image.
Cameras do the same exact thing with terrestrial objects, of course. But what is the angular size of a terrestrial object? Unlike the Moon, it isn't fixed, but depends on how far away you are from it. Take a picture of the Moon rising over your roof from the front yard, and your chimney might subtend about 0.5°, meaning the Moon and your chimney will look the same size in the image. Take the same image from a couple kilometers away, however, and now your entire house only subtends 0.5°. The Moon and your house now appear the same size!
Note that this has nothing at all to do with magnification, the type of camera or lens used, or anything other than simple geometry. All of these moonrise images are constructed the same way: you have the position of the observer, a Moon of (effectively) invariant angular size, and intermediate terrestrial objects with angular sized determined by their distances from the observer. The trick to making the Moon look huge in an image is simply to ensure that your foreground objects are far away.
[quote="Whit in Chillicothe, OH"]The moon seems SO big. Was it taken at magnifacation after the skyline was shot? That makes it seem like a hoax.[/quote]
The problem is, many people don't understand that images are translating angular scale to linear scale.
The Moon is 0.5° degrees across. It has this angular size regardless of the lens used, or where on Earth you are standing. If you shoot it with two lenses, say 100mm and 200mm focal length, the longer one will map that angular size to twice the linear dimensions. That is, the Moon will appear twice as large on the image.
Cameras do the same exact thing with terrestrial objects, of course. But what is the angular size of a terrestrial object? Unlike the Moon, it isn't fixed, but depends on how far away you are from it. Take a picture of the Moon rising over your roof from the front yard, and your chimney might subtend about 0.5°, meaning the Moon and your chimney will look the same size in the image. Take the same image from a couple kilometers away, however, and now your entire house only subtends 0.5°. The Moon and your house now appear the same size!
Note that this has nothing at all to do with magnification, the type of camera or lens used, or anything other than simple geometry. All of these moonrise images are constructed the same way: you have the position of the observer, a Moon of (effectively) invariant angular size, and intermediate terrestrial objects with angular sized determined by their distances from the observer. The trick to making the Moon look huge in an image is simply to ensure that your foreground objects are far away.