Originally posted by Kendigitize So if I want to take a 15 sec. exp every 10 seconds I would need the interval to be set at 40 seconds give or take a few?
The real killer for long exposures is the "Slow Shutter Speed NR", which is actually a second exposure, of the
same time as your
original exposure, with the shutter closed (total exposure time now is 15 + 15 = 30). This sensor noise image is then subtracted (negating sensor noise) from the original exposure. Your camera cannot take the next regular exposure until this is accomplished, as there can be no overlap of these exposure operations. Note that you should be able to turn "Slow Shutter Speed NR" OFF, but I'm not sure for the K-30 (I use K-5, where it's a Record Mode setting). Then there would be no 2nd exposure time required.
Filling the buffer is not the problem. It's having the total exposure (some hidden for NR) time being greater than the interval time, and the "Slow Shutter Speed NR" has a default of AUTO, with exposures of 1 second (K-5) adding on the 2nd sensor noise exposure.
Take 1 image at your desired exposure time, without using the interval record mode setting, and look at how long it actually takes before the camera is fully ready for another. Enabling Instant Review will show you that time, when the image pops up on the LCD. Set the interval to be a bit greater than this. Try a couple of interval exposures to ensure that there is no overlap. Then take your interval images. That's what I do for my star shots, where a minimum interval time is not required.
Other processing time (lens correction, SD card write time) can add to the problem, if the camera cannot perform these operations in parallel with the next exposures. That's why tests may be needed to ensure that the intervals don't end up overlapping. Overlap will reduce the number of shots from the expected (shots are skipped), and the actual images will not be uniformly time-spaced. Pentax doesn't give you much automatic help in avoiding this overlap.
If your camera can overlap these processing operations with the next exposure times, you can reduce the interval time, but getting to a minimum interval time will require the testing.
And try explicitly turning off "Slow Shutter Speed NR" to see if your images even need it or not. That would enable you to speed up your interval, but may increase the noisiness of your images. Your choice. Star shots may need it. Long exposures to get silky ocean waves during a sunset probably don't.