I know how to take a time-lapse on my Nikon D7100, but if I want to take a time-lapse of, say 3,000 pictures at 1 frame per second, do I need an external timer?
Based on what is in the manual for the D7100, it does not appear that an external interval timer is needed for a time-lapse with several thousands frames. According to page 145 (titled "Interval Timer Photography"), once you are at "interval timer shooting" in the camera menu there are three variables you need to set for interval shooting to create the time-lapse: the interval length, the number of intervals, and the number of frames per interval.
- the interval length is the duration the intervals are spaced apart from each other, though not necessarily the time between each frame, as there can be multiple frames in an interval
- the number of intervals, which each occur successively after the given interval length, when combined with the number of frames per interval gives you control over the number of the frames all together; when # of intervals is multiplied by the # of frames per interval, the result is the total # of frames all together
For example, if you had the number of intervals set to 18 with an interval length of 5 sec, and there were 2 frames per interval, then overall there would be 36 images (18x2) taken over the course of 90 seconds (18x5).
How to get to over a thousand frames: according to the manual, though the maximum number of intervals is 999, you can have multiple frames per interval, thus allowing for more than 999 frames all together.
For example, if you had 800 intervals with 4 frames per interval, then all together you would have 3,200 frames. For the interval length, you would need to set it to be longer than it will take for the 4 frames to be captured. For more detailed info on exposure time relative to the # of frames, see this Wikipedia article.
The result of the interval shooting will be individual image files for each frame on your SD card, which to make into a time-lapse, you need to combine them in post-production. There are numerous methods and software you can do this with, though this is a good tutorial on how to do it in Premiere Pro.