If shooting in RAW mode decreases the burst rate to 2 frames per second (on Canon 1100D), then would using a lower resolution JPG setting increase the frame rate?
Please tell me if I have mis-understood the concept. Thanks.
Generally, a DSLR has a fixed maximum frame rate, and it will take images at that rate until the memory buffer is full. After that the rate drops depending on how fast the images can be converted and written to the memory card.
The Canon 1100D has two different maximum frame rates, 2 fps for RAW format and 3 fps for JPEG format:
"Continuous: 2 fps up to 5 RAW frames / 3 fps up to 830 JPEG"
So, whatever resolution you pick, it won't go faster than 3 fps.
A lower resolution will let you take more images before the memory buffer fills up and the rate drops, so that would increase the 830, but you would not normally take that long bursts anyway...
The amount of data to save to the memory card is one of several factors which affect framerate. As you've noted, big RAW files can drag things down. Saving smaller files — lower resolution or higher compression — can work around this limitation.
But at some point, other limitations, like physical shutter speed, come into effect. If you've hit one of those, smaller files won't help.
On DSLRs, 99% of times, the frame rate stays the same regardless of resolution.
Yours is one exception but not the only one. The Pentax K20D for example shot at 20 FPS at 1 MP.
From capture to storage there are a chain of events happening and the frame rate is limited by the slowest step: AF, Metering, Exposure (Shutter), Mirror, Reading sensor, converting sensor data (JPEG only), write to buffer and write to memory card. These are more or less the steps involved.
It is most likely the 1100D has a limited throughout somewhere and cannot fill its buffer fast enough for 3 FPS of RAW data. Some other cameras quote a different speed when AF is locked at the first frame versus when it is continuously adjusted. You can guess which part does not keep up. So the camera achieves more speed by skipping the AF step. The K20D skips the shutter (using an electronic one) and the mirror (keeping the viewfinder blank during the burst) in order to reach 20 FPS.