A question on optics and light ratios for astro photography.

dave100

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My knowledge of optics and light transmission are not adequate enough to solve this problem so am asking for help from a more knowledgeable photographer.

Which is going to give me the best light gathering given the same ISO setting.
1 A 17mm f 2.8 lens set at shutter speed 30 secs (500 rule) ( Sigma 17- 35 mm f 2.8)
2 A 24 mm f 1.8 lens set at shutter speed 20 secs (500 rule) ( Sigma 24 mm f 1.4 Art) stopped down to reduce coma.
I am aware there are better lenses for this type of photography but these are what I have.
Thanks in advance.
 
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dave100

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I have probably answered my own question. I have done a rough and ready comparison in daylight by putting a 10 stop filter on the lenses, pointing them at a white sheet and shooting both in turn at the speed and aperture settings in my first post and then comparing the frames on the histogram. The 24mm at f 1.8 wins by just over 1 stop. I am quite surprised that the extra 10 secs exposure didn't cancel out the wider aperture setting. I will do a more controlled comparison tonight if the sky is clear.
 

jsecordphoto

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There are more factors to shooting Astro than light transmission. Shooting at f1.8 might let more light in, but that lens may have comatic aberration or be soft in the corners at 1.8. The quality of the lens matters just as much as the widest aperture.

Also, the rule of 500 is a little outdated and tends to be overly generous on your maximum exposure time at a given focal length, especially with higher pixel density sensors. Even on my d750 with "only" 24mp, I don't go beyond 15" at 15mm to get truly pinpoint stars.
 

jsecordphoto

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There are more factors to shooting Astro than light transmission. Shooting at f1.8 might let more light in, but that lens may have comatic aberration or be soft in the corners at 1.8. The quality of the lens matters just as much as the widest aperture.

Also, the rule of 500 is a little outdated and tends to be overly generous on your maximum
 

petrochemist

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The extra 10s is only half a stop more.
2.8 to 1.8 is about 1.5 stops, depending on how the values on the lenses have been rounded (they are rarely accurate).
 
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dave100

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Thanks for your reply jescordphoto. I am aware of the factors that you have kindly pointed out. I have shot astro with both of these lenses and am aware of the aberrations of both, but that was not the question I asked. Thanks for pointing out that the 500 rule I am using is outdated, can you suggest a better one?
 
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jsecordphoto

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Here is a good article my buddy Aaron put together NPF Rule for Sharp Stars

Aaron knows more about the technical side of photography than anyone I know.
 
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dave100

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Sorry jescordphoto, I didn't see the second part of your reply, there are two replies from you. The reason I was asking the question is that although the Sigma 24 f 1.4 is a better lens for astro if stopped down a touch, I was toying with the idea of using the other lens because of its shorter focal length. My interest is night landscapes with milky way and a wider field of view is always handy. The camera I am using is a Canon 5D mk3
 
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TCampbell

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The "500 rule" assumes a full-frame camera. If using a camera with an APS-C sensor you can multiply the focal length of the lens by the crop-factor of the sensor before applying the rule.

But setting that aside for the moment to get to your question. You can simplify this by ignoring the crop factor and the focal ratio temporarily.

500 ÷ 24 = 20.8 seconds
500 ÷ 17 = 29.4 seconds

So you gain just about 9 seconds by using the 17mm focal length.

The next question is... how much light will you gather per second?

f/1.8 is 1.33 stops faster than f/2.8. Since a stop is effectively a "doubling" then to multiply by "1 stop" you'd really multiply by 2x. That is to say, the difference in the amount gathered by the f/1.8 aperture in just 1 second is equivalent to what an f/2.8 lens would gather in 2.67 seconds.

So 20.8 seconds x 2.67 = 55.5. In other words, the 24mm stopped down to f/1.8 but only exposing for 20.8 seconds will collect roughly the same amount of light that your 17mm f/2.8 lens would collect in 55.5 seconds. But since the 17mm is limited to a 29.4 second exposure your best option is to use the 24mm which will collect roughly 43% more photons.

The reason I ignored crop factor is because I don't know which camera you plan to use. But assuming it's the same camera for both lenses, then the crop factor would be the same and that ratio of collecting 43% more light would also be the same.

All of this ignores the suitability of the angular field of view across the sky. I.e. depending on what the subject is that you'd like to compose... a wider or narrower lens might be selected. I'm more into shooting deep-sky objects (usually no landscape in my astro photos) and that means I'm using a telescope mount or tracking head ... capture time becomes less important if you can ignore the "500 Rule".
 

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