Exposure Time and Blow-out

[hatflyer]

I have an 85mm f/1.4 lens on my Nikon D5500. I set it to ISO 100, f/4. But long exposure come out way too bright. Does that limit the time of exposure, or can I change a setting to be able to take long exposures?

Thanks.

[KathyNS]

The only reason to shoot long exposures is to collect enough photons. If you are getting too many, cut back on the exposure times.

Terrible light pollution might cause over-exposed images. Misjudging the brightness of the Moon could also do it. (Hint: the Moon is a rock in bright sunlight. The correct exposure is a daytime setting no matter what the camera's meter says.)

Some targets are hard not to over-expose. M42, the Great Orion Nebula, is notorious for this. An exposure long enough to capture the dim outer fringes of the nebula will be too long for the very bright inner core. Conversely, the correct exposure for the core will under-expose the outer regions. Special high dynamic range (HDR) techniques are necessary to capture it all. (That is beyond the scope of Beginner Astrophotography.)

[hatflyer]

The only reason to shoot long exposures is to collect enough photons. If you are getting too many, cut back on the exposure times.

Terrible light pollution might cause over-exposed images. Misjudging the brightness of the Moon could also do it. (Hint: the Moon is a rock in bright sunlight. The correct exposure is a daytime setting no matter what the camera's meter says.)

Some targets are hard not to over-expose. M42, the Great Orion Nebula, is notorious for this. An exposure long enough to capture the dim outer fringes of the nebula will be too long for the very bright inner core. Conversely, the correct exposure for the core will under-expose the outer regions. Special high dynamic range (HDR) techniques are necessary to capture it all. (That is beyond the scope of Beginner Astrophotography.)

So does stacking 50 images add photons? Or does it only allow to reduce noise to signal?

Btw, my camera has an HDR feature. Is it as simple as using that?

[KathyNS]

So does stacking 50 images add photons? Or does it only allow to reduce noise to signal?

Btw, my camera has an HDR feature. Is it as simple as using that?

What matters in this case is the number of photons per frame. 50x 10-sec frames and 10x 50-sec frames will capture the same number of total photons. But the 10-sec frames are less likely to be over-exposed than the 50-sec frames. The two combinations will give you the same total exposure time (500s) and pretty much the same SNR. The difference is that, if the longer frames have saturated pixels, data has been lost.

So, if you are finding saturated pixels, you should reduce the exposure times of individual frames, and increase the number of frames to compensate.

I have no idea what you camera's HDR feature does. You should consult the manual for specifics. It might help, or it might not.

[hatflyer]

So does stacking 50 images add photons? Or does it only allow to reduce noise to signal?

Btw, my camera has an HDR feature. Is it as simple as using that?

What matters in this case is the number of photons per frame. 50x 10-sec frames and 10x 50-sec frames will capture the same number of total photons. But the 10-sec frames are less likely to be over-exposed than the 50-sec frames. The two combinations will give you the same total exposure time (500s) and pretty much the same SNR. The difference is that, if the longer frames have saturated pixels, data has been lost.

So, if you are finding saturated pixels, you should reduce the exposure times of individual frames, and increase the number of frames to compensate.

I have no idea what you camera's HDR feature does. You should consult the manual for specifics. It might help, or it might not.

So what is the benefit of tracking if 50 frames of 10 sec is the same as 10 frames at 50 secs, if there are no star trails at 10 second exposures? You would probably have to move the camera a bit if no tracking tho?

[JayTee]

Before you start your imaging, it is advisable to take several different images of a single object using increasingly longer and longer exposure times or higher and higher ISO values. What you are going for is a histogram that looks similar to the ones described in this article.
https://www.astropix.com/html/astrophot ... raphy.html

Cheers

[KathyNS]

So what is the benefit of tracking if 50 frames of 10 sec is the same as 10 frames at 50 secs, if there are no star trails at 10 second exposures? You would probably have to move the camera a bit if no tracking tho?

I wasn't suggesting 10 seconds as an actual exposure time. It was just an example to put some numbers on it. You will need to determine the proper exposure time for yourself. Start with what you have and adjust it.

Even if the exposure time were only 10 seconds, you wold see movement with any reasonable focal length.

[Baurice]

It would depend on what you are trying to photograph.

I would also say that an undriven exposure of about 5 seconds is the maximum at the celestial equator but you can use longer exposures as you get nearer the pole.

For the moon and planets, you need a much shorter exposure but also a longer focal length. I attach my DSLR to my telescope to get focal lengths of 1.5 metres and 4.5 metres. I do a lot of shots at 300mm, though.