It is necessary to find the right exposure (as always in photography you’ll say), except that here, on the one hand, we have time to set it with total care, and, on the other hand, if we use an image stacking software, this exposure should be constant from one photo to another.
First point: select “manual exposure” mode. Thus, once the settings made, they’ll not be able to change.
If you have a choice between spot, multi-zone or weighted metering, choose one of the latter two. In any case, the measure will only give a starting point. Adjust the diaphragm around f/8 (see depth of field) and the shutter speed to the value indicated by the camera. Take a picture. It is likely to be under-exposure. Indeed, the white background (if you have one planned), preponderant, deceives the metering device. The camera corrects the brightness globally too high by choosing a too short exposure time.
Do not rely on the picture appearance on the screen of your camera: it is set very bright to remain readable in the sun, and gives a false impression of clarity to the image.
The only appropriate way to judge exposure is the histogram. This is a kind of graphic that gives the number of pixels that have been exposed to a certain light. The weak luminosities are on the left (one says “the shadows”), and the strong ones are on the right.
In the above example, the right part, i.e. the strong lights, is practically empty: it is the proof of the under-exposure.
It will therefore be necessary to carry out successive tests, increasing the shutter speed by a third or a half “speed”, and display each time the histogram, until the curve comes to touch the rightmost point. Not more! Otherwise, we’re going to overexposure this time! Here we went from 1/350th to 1/125th s.
We see that a part of the histogram that was cut to the left becomes visible. Clearly, this means that there will be more details in the darker parts of the picture.
Note: the pictures of this page are of poor quality, because they are screen shots. Moreover, there are some reflections due to parasitic lights — on the first photo, we can even see the reflection of the camera taking the picture! But the important thing is to understand that by “pushing” the histogram to the right, with no excess, we will obtain more detailed shadows, and consequently an overall richer image.