Always best to underexpose?

[Wanderers]

As a wedding photographer I would generally aim to underexpose in order to ensure I definitely didn't overexpose. The reason being that raw data is always retained in an u/exposed shot and can be recovered in lightroom but overexposed detail is lost forever.  

Does the same rule apply in ap?

[vlaiv]

No.

In AP we use stacking - so many individual exposures.

Rules that apply are:

- SNR of stack depends on total integration time - more time you spend - better the image

- Individual sub duration is primarily determined by level of read noise of your camera compared to other noise sources (I say primarily, because you might choose based on some other criteria like - ability of your mount to track precisely, storage space availability, likelihood of individual subs being ruined for your setup due to wind or some other effects ...)

- Over exposed parts of the image (mostly star cores, but sometimes bright parts of targets as well - like galaxy cores or very bright nebula parts) are handled by using set of shorter exposures that capture data for these parts only and you blend those in in processing to replace over exposed parts in regular stack.

[Wanderers]

Thanks for such a comprehensive answer, Vlaiv

Really useful. 

[MalcolmP]

2 hours ago, Wanderers said:

 generally

overexpose.

 lost forever.  

Does the same rule apply in ap?

Because you specify "generally" "rule" etc I would answer Yes !

One particular case springs to mind : if you are checking out a variable star (photometry) then data will be lost if the star is overexposed in any of the sequence. Sometimes special measures can be put in place to partially recover the situation, never-the-less it is not ideal,  stricto sensu, information has been lost.

Usually eg. stacking for pretty and/or special effect pictures, it is not a worry.

So another answer would have been "it depends"
 

Edited February 24 by MalcolmP

[vlaiv]

47 minutes ago, MalcolmP said:

So another answer would have been "it depends"

You are quite right about being careful not to over expose in some cases, but in principle - it is never ok to underexpose.

We should really say that one should always strive to do as much exposure as "sensible" or needed.

In photometry that you mentioned - under exposing will produce less reliable results because SNR suffers. There is even technique to avoid over exposing star core and to still capture as much signal as possible. Star is defocused a bit so that star profile is no longer sharp in center but rather spread in doughnut. That allows for longer exposure and accumulation of signal and it avoids over exposed parts.

Similarly - in planetary / lucky imaging. One can argue that subs are really under exposed - but they are not in general sense. They are not needlessly shorter than they need to be. In order to freeze the seeing one is indeed using very short exposures - like 5 ms or less - and that can look like very dark / under exposed single frame - but max exposure is governed by seeing and no one would go lower than is actually needed to get the sharp image.

On the other hand - for say Lunar imaging - one might go lower than what is needed to freeze the seeing - but that is because signal is so strong (moon is very bright target) that shot noise swamps the read noise and shortening the sub duration does not have as much impact on final SNR, but shorter exposures allow for even more stable subs with respect to seeing.

[wimvb]

8 hours ago, Wanderers said:

Does the same rule apply in ap?

Basically, yes. In my experience (which is limited to deep sky photography), if you want to keep star colour, you can't have them blown out. Modern cmos cameras have enough dynamic range to allow you to keep the stars under control, while also capturing the fainter details. But, unlike daytime photography, the tatget's brightness range is also very large. To get to the really faint parts of a target, you need to capture enough data, ie a long total integration time. In each single frame, you need enough exposure so that you can't see the read pattern (bands or lines in the single frame).

So, expose for the highlights and develop for the areas in "shade".

[900SL]

17 hours ago, Wanderers said:

As a wedding photographer I would generally aim to underexpose in order to ensure I definitely didn't overexpose. The reason being that raw data is always retained in an u/exposed shot and can be recovered in lightroom but overexposed detail is lost forever.  

Does the same rule apply in ap?

Depends on what you are shooting. For lunar and planetary targets yes, you don't want blown highlights. 

For dim DSO you need to expose long enough to get the fainter nebulosity signal above the noise floor. My simple understanding is that this may result in some blown out stars in some instances, which can be addressed by bracketing exposures