Hi guys, the bulk of response is quite a bit more than I expected, I can't entirely respond to all but lets pick out the crucial ones

Well I don't! I worked out this formula on a train ride on the way to psychology class. (I'm no mathematician!). After that I just wrote it out on paper / microsoft excel, just to confirm my results.

Someone mentioned that this is an estimate. The formula for Magnification, M = f / (d-f), f=focal length, d= subject distance. Is taken from elsewhere, its a known formula and should be accurate. Assuming that formula is accurate, my formula should be pin sharp. (unless your sensor is less than ideal and doesn't picks up the image) Or maybe shutter lag. But seriously, do you need to be that sharp?

Units aren't part of the forumla. Of course if you did math/physics in school, the units are always prescribed. But at this level, I assume people who are really interested in the formula know how to standardize units. If you decide your distance should be in meters, just make sure you convert your focal length, subject distance, displacement/second, all into meters.

Essentially the formula is more of a ratio, thus no units really either. The ratio goes like

distance travelled (in real) : distance travelled on image (in pixels) (=blur)

This is precisely why you need my formula! I never said you calculate the formula on the spot during the F1 race!!! I don't mean to offend you, but thats being a bit short-sighted. You would have done the calculations the night before as preparation. I shall build on your example, and apply the formula.

Say you're on the stand, about 50 meters from the "turn". Car is turning at 200km/h, we will use 55 m/s here (actual is 55.555556~ m/s). And you brought your 200mm F2.8 for a night race. Lets assume after you expose properly, you discover that you're at 1/640 seconds of exposure time. This results in ~54 pixels of blur! Assume you're at max ISO and aperture. 54 pixels is quite abit obviously.

What will you do?

Some won't shoot,

some would bring a shorter lens (135 F2 L?) and crop...

Some will bring a 300mm F2.8 and sit further away, this actually reduce the blur due to the mathematics, you wouldn't have otherwise realize this would you? =)

I personally will shoot underexposed and post process. Noise is better than blur for me.

Essentially given the above scenario, using the formula, you should have in mind that at this event, you don't want to shoot below 1/2500 at 50 meters. This gives 14 px of blur, live with it, you cant remove it completely.

How to improve? Sit further back, at 100m away, with the 300mm, 1/2000 will give you 13 px of blur. If you print A4, maybe the blur is only "zoomed out" to 2-3 px, assuming its 72px per cm, not observable.

I'm not sure how many times the F1 car will come around the bend for you to review or go home and find out that all your pics got blurred, and then you learn by experience. Sometimes, mathematics changes your pictures for the year or a few. Worse yet if the car crashes at the turn. =) In this case, pre-calculating and remembering your minimum exposure time you want to prepare for the shoot is a good option, agree? I'm someone who prefers using Manual mode in such instances anyway.

Note: I used a 8mp calculation for all the above, figures changes according to camera pixel density.

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