Hi, I would like to know something. I know there is a lot of information on the internet, but after reading some texts and watched some videos, couldn't find answer to my problem. When doing timelapse with 20sec exp, I have to set interval to more than 22sec. When I set lower time, then I'm missing the next photo. The problem is, pro timer is still counting, and not waiting till my camera save image or buffer it (I don't know what my camera eos R doing inside). So the question is, why pro timer doesn't wait for signal from my body? It is normal, or I have to set something in settings? It is normal with any intervalometer? Before pro timer, I was using MagicLantern on my eos 6d, which is waiting till my camera is not busy, so I'm a little surprised.

Interval is defined as Sum of Exposure time and Dark Time.

This means, Interval minus Exposure time is the Dark Time.
if you set 20 Seconds Exposure and 22 Seconds Interval, the Dark time will be 2 Seconds. This is the time where the camera writes the image and gets ready for the next shot.

Cameras don't provide a signal indicating that they are "ready". That's why you need to know about the minimum Dark Time that your Camera requires (usually 1-2 seconds but not longer). If you want the exact value, I've explained in my Ebook how to calculate it (see banner at the bottom of this page).

So everything is normal with your setup. Just keep in mind that Interval per definition includes the exposure and the dark time.

Thank you for your reply. Now everything is clear. I thought so it's about Dark Time. I will check your pdf!. Have a good night!