Luminous Landscape Forum
Equipment & Techniques => Beginner's Questions => Topic started by: Mark Ogden on April 02, 2013, 08:20:35 pm
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If, on a 35mm "full frame" sensor DSLR, I wished to take a truly large and detailed frame-filling shot of a full moon, what sort of focal length lens would I need? I realize that the moon can have different apparent sizes during the year, but on average, what would you recommend in a lens?
Thank you very kindly!
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A telescope. :)
A reasonably decent one (http://www.celestron.com/astronomy/celestron-nexstar-4se.html) is cheaper than a big long SLR lens. £3-400 will get you one.
Make you sure you get a moon filter too. The moon is very bright. :o
Specs for image below - not mine BTW
Canon 50D with Celestron Nexstar 4 SE scope and 2x barlow. 1/60s, 2650mm, f/26, ISO1250.
And how it was taken (http://www.cameralabs.com/forum/viewtopic.php?t=10748)
(http://homepage.ntlworld.com/kawaii/cameratest/moon20081206medmono.jpg)
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If, on a 35mm "full frame" sensor DSLR, I wished to take a truly large and detailed frame-filling shot of a full moon, what sort of focal length lens would I need? I realize that the moon can have different apparent sizes during the year, but on average, what would you recommend in a lens?
Thank you very kindly!
Hi Mark,
'On average' it does vary a bit, but one could say that given the average diameter (equatorial is also a bit larger than polar diameter) of the moon, and the 'average' viewing distance, it computes to a rule-of-thumb of a bit more than 0.5 degrees (next April 25th it will be 0.54503 degrees). That means that a 100mm focal length will produce on average a 0.8727mm diameter disk, a 200mm FL will produce a 1.7453 mm diameter disk, and so on. So it would take a 2750 mm lens to fill the 24mm of the frame on average, 2523mm on April 25th, 2013.
When you divide that by the sensel pitch of your sensor array, it will give you the number of pixels it will produce.
Cheers,
Bart
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On average' it does vary a bit, but one could say that given the average diameter (equatorial is also a bit larger than polar diameter) of the moon, and the 'average' viewing distance, it computes to a rule-of-thumb of a bit more than 0.5 degrees (next April 25th it will be 0.54503 degrees). That means that a 100mm focal length will produce on average a 0.8727mm diameter disk, a 200mm FL will produce a 1.7453 mm diameter disk, and so on. So it would take a 2750 mm lens to fill the 24mm of the frame on average, 2523mm on April 25th, 2013.
When you divide that by the sensel pitch of your sensor array, it will give you the number of pixels it will produce.
Cheers,
Bart
or simply look at above shot which used a 2650mm focal length on a crop sensor. ;D
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I realize that the moon can have different apparent sizes during the year, but on average, what would you recommend in a lens?
Apparent size might wary, but the real size stays at half a degree. It can not change much as the distance stays more or less the same all the time.
A rule of thumb: 100mm lens on 135 size camera produces 24 degree diagonal angle, 200mm lens 12 degrees, 400mm 6 degrees, 800mm 3 degrees 1600mm 1.5 degrees and 3200mm .75 degrees. As you need to fit 0.5 degree object on the sensor, 2500mm seems quite right.
At those focal lengths the moon moves surprisingly fast, but fortunately it is in full sunlight and the exposure times are short. Getting it in the frame and getting the camera stable is the main problem.
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I shoot the moon to the short axis of the sensor. APS-C with 1200mm scope fits the whole moon in 1 shot. with 2x tele on the camera it is a 2 shot stitch with the diameter of the moon fairly close to the short axis of the sensor. I am not sure how much of the image radius from the average scope falls outside the APS-C format so the normal crop factor is not relevant.