Hi all, obviously it has some valid applications that I've considered. (if not I really won't do it!) Heres some.

If you think its a hassle to calculate, I've actually done everything on an excel sheet. The formula can be simpified to

B = MsxR

where M = Magnification, R won't change if you don't change cameras.

Applications include:

Answering "do you really need that F2.8 lens or F4 is enough?!" Maybe even F5.6. If your intention is just to freeze the subject under the lights that you have. You have to decide how many pixels blur is considered 'too blur' for you.. Basically, it gives you a way to decide on your threshold shutter speed or exposure time that you want for freezing subjects. After you get the shutter speed, fill in the formulas with ISO and aperture combinations, this one varies with lighting so try and error may be needed.

Yes thanks for those who pointed out that it can be for studio use! For things like sports / dance, especially jump shots. Which in fact is one of my intended use for this formula. I will be helping my friends do a ballet rehearsal photoshoot soon.

Maybe it helps you to calculate what kind of flash sync you need too? I'm not sure, never done an actual studio shoot.

Also perhaps picture quality is implied here. Do you really want to push your ISO to 6400 to get a 1/500 shutter in a pop concert, when the subject moves less than 1m/s? Use the formula to find out, maybe 1/100 is enough.

Possibly, shooting at 150mm and cropping in, results in less blur than shooting at 200mm. =) Formula should indicate this.

Aperture wise, I did make a formula last time, but aperture is much easier to approximate, still the formula is good if you want to calculate differences harder to estimate. such as F4.5 vs F4? Less practical use because its no big deal really.

S = 2log2 ( F1 / F0)

S = step gain

F = aperture (F1 is the 'smaller' aperture ie F1 = 1.8, F0 = 1.4, S = 0.7251 ) (1.653 times faster)

Personal application, besides the dance photography part, I actually planned to use this formula for candids. I'm only using a 55-250, not very high quality lens, so if can, I have to stop down abit. I figured that if people were just walking, 1/300 should be able to stop the motion, I almost wanted to get a 70-200L lens, but no money. Now i know I don't need it.... yet. 1/200 is enough to shoot walking pedestrians with 5 pixels (100%) blur, negligible. =)

All in all, trial and error can throw my formula out the window. But really, sometimes people just need a scientific confirmation (dont you agree?). Moreover I don't have alot of finances to do trial and error.

Further, but more complicated uses of this formula probably includes measuring the effectiveness of your lens IS (using the formula in reverse). basically the formula translates actual distance over time into pixel distance over time.

Reversing the formula also tells you the exact distance you were from your subject (up to the last mm!). If you want to brag about your great candid sniping... this is the way to go.

Last of all, its really fun to do, for me at least.