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Full frame camera options for \$750USD used?

Started 2 months ago | Questions thread
Re: Full frame camera options for \$750USD used?

Dominic Ryan wrote:

1. Wouldn't longer focal lengths shorten the exposure time I can use to prevent trails? I'm only using the NPF calculator in PhotoPills, however my 7.5mm Laowa has a max recommended exposure time of 21 secs where my Sigma 16mm F/1.4 has a max recommended of 9 secs. I may have muffed my math here, however my Sigma is going to have about 1/3 of a stop less light gathering from the reduced exposure to my Laowa.

Yep, you muffed the math a bit, but most AP newcomers do. You can't make light-gathering comparisons based just on the varying f-ratios of lenses with different focal lengths, which I think describes what you've done.

The allowable shutter duration to avoid trailing changes in inverse proportion to the focal length, so yes, it will be shorter with a longer lens. But the ratio of light passing through two apertures is proportional to the squares of the diameters. A two-inch circle has four times the area of a one-inch circle. If two lenses have the same focal ratio, but one has twice the focal length, the aperture diameter of the longer lens is twice that of the shorter, but its aperture area admits four times as much light. Even when halving the shutter duration to avoid trailing, it still collects twice as much light as the shorter lens.

The longer-lens advantage is even greater when comparing the lenses you've mentioned. The 7.5mm f/2 lens has a 3.75mm aperture, while 16mm f/1.4 has 11.4mm. The 16mm wide open admits more than 9 times as much light : (11.4x11.4)/(3.7x3.75.)=9.24.  In a 9-sec exposure, it still admits almost 4 times as much light as the 7.5mm does in 21 seconds: 9/21x9.24=3.96. So basically, instead of losing 1/3 of a stop, you gain 2.

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