Quote:
Originally Posted by mithrandir
Try an APS-C with 24m 3.89um pixels.
With Joe's formula if I want less than 4px of trailing, at 14mm (effective 21.6mm) I get 90° FOV and can't go beyond 16 sec.
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The crop factor changes the 600 rule to the 400 rule when going from a full frame to APS-C.
However, the crop factor does not change the focal length, it changes the FOV. When using my formula, do not multiply FL x crop factor in my formula.
Also with 24MPx, you can get away with a lot more than 4 Px of trailing. It goes without saying that these fixed tripod images usually get shown a bit smaller. Let's say you can accept 4 Px trailing at the final displayed image and you are going to scale the image to 800x1200 Px(20% scale of the 4000x6000px original).
Then you can have N, the number of pixels as
4 x 100% / 20% = 20 Px
so your max exposure is
20Px * 4µm * 14 ÷ 8 = 140s
When I made the comment about Jonathan's lens working at 12mm, I meant that the pixel smear indicated that the lens was acting more like a 12mm indicating that the manufacturer may have deliberately understated the focal length for marketing purposes. They do this all the time.
If you think about it, there is 45-55mm between the back of the lens mount and the focal plane in most brands of DSLR so any focal length shorter than about 70mm is a marketing label and not a true statement of focal length.
Joe