Proper "rule" for pinpoint stars with M43

OutsideTheMatrix wrote:

Thanks for the math, I found that very illuminating!

Thanks. Usually a few people groove on the math. Otherwise, I'd stop.

The results seem to vary depending on the pixel density of the sensor.

Yes, I should have made that clearer. Pixel pitch is the relevant parameter. A 10 mpx sensor will have a pixel pitch 1.26 (i.e., sqrt 16/10) greater than a 16 mpx sensor. So, my parameter would be 40, rather than 30.  40 / f for one-pixel (max) trail.

On the 10 MP E-520 a 15 sec exposure at 14mm viewed at 100% was perfectly fine. 20-25 sec were okay, but slightly blurred. I found 30 sec objectionable. But 15 sec on that sensor seemed pinpoint to me even viewed 1:1 on a 19" 1280x1024 display.

15 secs is about 5 pixels, by my calculations.  But, just looking is probably better than computing because, as I pointed out, my calculations depend on how many pixels of length are OK to you, given your output method.

On the 16 MP E-PL6, the results are somewhat different. 15 sec at 14mm seems objectionable, because stars close to each other seem to get blurred into one.

That's sort of a 7 pixels, max (zenith), trail, by my calculations.

13 sec is about the same. 10 sec is okay,

10 secs corresponds to only around 5 pixels, again.

Funny. If somebody came up to me not the street and asked me how much of a trail would be acceptable, I would have said ="Oh, about 5 pixels." Actually, I asked myself before I calculated.

but stars are still a bit too elongated for my taste. I found 8 sec to be just right on that sensor for 1:1 viewing on a 19" 1280x1024 display. 6 sec did not seem to be significantly better so I stuck with 8 sec at 14mm.

Seems very conservative.

BTW another thing I noted is that star trails seem more objectionable with dimmer stars than the bright ones. The bright stars seem to have some bloat that actually makes the trails less apparent at 15 sec.

Yes, for sure. Bright stars seemed to be like 3-4 pixels wide in my image. So, it takes like 3-or-4-pixel trails even to make the star blob just twice as long as it is wide.

Hope I did my arithmetic right in this message. Mental math is not a strong point.