But... considering what you have said: replacing a class 4 SD card by a class 10 SD card will improve the RAW+JPG writing in a SX20is by a factor of 1.1 (8.4/7.6). The "2 times faster than a class 4" is not fully usable because of the processing time and internal buffer size... Do you agree with that?
I'm not sure it would be accurate to say it is "not fully usable because of the processing time and internal buffer size". The time differences don't seem like dramatic improvements unless your requirements for a project requires you to save every second you can when taking RAW shots.
Here are just a few things that need to be considered from the test results:
1 - There was such a small number of samples used to calculate each average (only 5)
2 - Inaccuracy because of the timer used - could only use "whole" seconds
3 - Perhaps CHDK processing or script processing may skew test results
4 - The class 4 and class 10 cards were different brands (PNY vs SanDisk)
And, here is another way of looking at the data. Since we must process and save the JPGs while capturing RAW images, we should be able to subtract the JPGs processing times to determine how much the RAW processing improved:
class of card 4 10
. ---- ----
RAW+JPG times 8.4 7.6
subtract JPG times 5.6 5.8
. ---- ----
RAW diff 2.8 1.8
From this we could "assume" there was a moderate savings in the RAW processing, albeit not the 2 times mentioned previously. Others may find better or worse results during their testing.
This was just one way I decided to test my camera and cards. There may be better ways.
