VisualUniverse wrote in post #13858196
Allow me to clarify: if you care only about the center of the FOV (e.g. a planetary nebula) at a very short exposure where the lens is at ambient temperature and the temperature will not change over time, then concern about DOF is unnecessary. But if you want the entire FOV to contain tight stars and account for focus shift from temp changes, then yes...DOF is a concern.
TKerr was initially attacking my assessment about wide-open lens shooting. Sorry for distracting with the DOF reference.
No I was initially asking about your statement that "it is Normal Practice" To stop down a bit from wide open. Which could be a misleading statement for someone just getting started if not clarified.
It might be a practice that some people do stop down, but it is not a practice that everyone does. therefore it is not Normal Practice to stop down the aperture.
And then you replied:
VisualUniverse wrote in post #13858196
Yes, but when you open up the lens all the way, star distortion occurs (spherical distortion), especially toward the edge of the field of view, along with chromatic aberration on brighter stars.
By giving a little bit of depth of field, these issues are minimized. So for f/1.4 lens, I do 2.0 to 2.5. For f/2.8 lens, I'll do f/3.2 to f/3.5.
I replied that DOF isn't the issue, and I will still argue that it isn't. Because it is not.. DOF has nothing to do with those distortions or aberrations.
There is no DOF at those distances against the dark night sky..
DOF is a non-issue for astroimaging.
What you seem to be concerned with however is an "Even Field" or Flat Field as well as other issues that cause aberrations such as CA, Coma and Astigmatism. Focal ratio is only one, which by increasing can help decrease the effects of some of those problems that are inherent to the lens design and build.
Focus shift is another issue all together. Unless you have a rapid and drastic temperature change, as long as you allow your camera lens to acclimate properly, the amount of focus shift on a small camera lens due to the expansion and contraction of the glass elements is very insignificant if there is any. And the amount of time for a small camera lens to acclimate is very short.
Those things are more of an issue with larger telescopes that are more prone to temperature variances. Otherwise focus shift is a mechanical issue which should be a non issues once set.
Mechanical Focus shift is also a problem using a camera on a telescope when the focuser isn't or doesn't lock down tight enough. When that happens it will gradually start slipping out of focus as the telescope increases its angle while tracking.
Otherwise as long as you have your focus set and locked in you should be fine.
Still Nothing to do with DOF. There is no circle of confusion, nor area of acceptable sharpness.
At those distances, "provided you have a flat field", all the stars are sharp and in focus or they are all out of focus. And they will stay that way throughout your total exposure, or the night if you're taking multiple exposures for stacking.
Even by stopping down, when using a short fast lens you probably won't totally reduce the distortions all the way out to the edge of the field. But you can significantly reduce the effects and more so the amount of CA which will help make your stars appear much sharper and less bloated.