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Forum: Pentax SLR Lens Discussion
08-11-2018, 06:07 PM
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Just for the astro aspects of this, the longer focal length glass will be better, given all the other aspects (aperture) remains the same. In that the aperture becomes larger with the longer lenses, then just following that logic the 20/f1.8 will be the best selection.
So, here are some numbers to back up the statement. I'm also assuming that the astro tracking will be for 60 seconds (however the duration really does not matter here, but I'll toss it in anyway).- 14mm/2.8=5 mm diameter, area = (pi/4)52 = 28.9 sq mm. Exposure time of 60 seconds. Light collection = 1177.5 sq mm seconds.
- 15mm/2.4=6.25 mm diameter, area = (pi/4)6.252= 30.66 sq mm. Exposure time of 60 seconds. Light collection = 1839.8 sq mm seconds.
- 20mm/1.8=11.11 mm diameter, area = (pi/4)11.112= 96.91 sq mm. Exposure time of 60 seconds. Light collection = 5814 sq mm seconds.
Then to compare the lenses...- 14mm to 15mm = 1839.8/1177.5 = 1.6 - the 15mm collects 1.6 times more light than the 14mm
- 14mm to 20mm = 5814/1177.6 = 4.9 - the 20mm collects 4.9 times more light than the 14mm
The 20mm lens collects 4.9 times more light from objects in the frame. Stars are brighter, star clouds and nebulae will be brighter.
The key in all of this is the actual physical aperture diameter (for the 14mm / f2.8 lens - that is the 14 / 2.8 = 5). Essentially, to simplify the process, just compare the physical aperture diameter of each lens, and you get the same results by - [(physical aperture long lens) / (physical aperture short lens)]2 i.e., (6.25/5)2 = 1.6x more light for the longer lens.
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Personally, I went with the 15-30/f2.8 for several reasons. First it was more general purpose for landscapes and I have found myself shooting astro at both ends - 15mm and 30mm. The 30mm end gets substantially more light, while the 15mm gets more of the overall landscape and sky combination. I've found so far, that at 15mm I get plenty of star light for a good image.
:cool:
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