For a common aperture, a one second exposure time at ISO 6400 and a one minute exposure at ISO 100 are equivalent. Use this to approximate time for a long exposure in the field. For example if your camera at ISO 6400 would make a proper exposure at 1.5 seconds, then for the same aperture at ISO 100, you would keep the shutter open for 90 seconds (1.5 minutes). Another example, a shutter speed of 1/30 second at ISO 6400, is equivalent to a 20 second exposure at ISO 100.
The trick is to flip the camera from Bulb mode and ISO 100 to ISO 6400 and get the shutter speed without changing the aperture. And avoid tying yourself in knots or knocking your camera, messing up your composition and (mental) focus.
The Sony A7R III (and later) has a feature that makes it possible. In particular it is possible to define a group of camera settings in a Custom Hold 1. When assigned to a button on the camera you can toggle between the current settings and those in the custom hold 2. If you have the Sony Bluetooth remote, there is a button on the remote linked to the C1 button on the camera. So by assigning the Custom Hold to C1, you can remotely toggle the camera to ISO 6400, get the exposure and quickly calculate equivalent exposure time for ISO 100.
In the Camera-2 page 8/9 menu, go to Custom Key and set Custom Button 1 to “Recall Custom hold 2” 3
When you click on “Recall Custom hold” you enter another menu where you can define and toggle the settings to recall. For this trick use the following.
|☑️||Soot Mode||Aperture||Keeps consistent for the clac|
|☑️||Drive Mode||Single Shooting||It is a long exposure, right|
|☑️||Exposure Comp||+0.0||Adjust after calc|
|☑️||ISO||ISO 64000||Baseline for calc.|
|☑️||Focus Mode||DMF||Allows you to manually refocus while holding button|
|☑️||Focus Area||Wide||Avoid diddling with area|
|☑️||AF On||On||This will do a refocus when the button is pressed, so maybe Off|
This trick came to me from video by Daniel Ockeloen