D3S focus testing

Started Feb 14, 2010 | Discussions thread
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Thank you Marianne!
In reply to Marianne Oelund, Mar 5, 2010

Thank you for providing an explanation of a phenomena I have been able to observe empirically, but one I have yet not been able to grasp the reasons for.

I have shot some car and motorcycle racing (albeit not much) and it has to some degree surprised me how well AF work there, in spite of the high speeds of the subjects. And on the other hand I have noticed that shooting sports like volleyball or basket where athletes move at relatively (compared to racing cars) slow speeds is so much harder on the AF. But there I often end up very close to the action. (Close as in having athletes landing on you )

I distinctly remember a well known sports photographer (who's name I right now cannot recall) who made the comment that the hardest thing for him to get right in terms of focus actually was his grandchildren when they were playing around him ...

But, to get back to the dogs. This also explains another observation I made this last summer: On the first day during a two-day agility event I shot with a (borrowed) 500/4, the next day I shot with my own 300/2.8. It was bright summer and I for the most time shot at f5.6 or f8 on both days. But I got a much better yield with the 500 then with my 300. My inital thought was that there was something wrong with the AF in my 300, but I also had this nagging suspicion it might have more to do with different subject distances - getting a similar crop or field of view mean sitting closer when using the 300/2.8.

So, an advice to the signature JoeinLA: To get more consistent focus with your 70-200, make sure to have it zoomed as far towards 200 mm as possible, and rather back away a bit from your dog.

Marianne Oelund wrote:

I think this very critical point deserves some additional emphasis. Exactly how much more difficult is it to follow focus on a close subject? The answer may surprise you.

It's a result of optical principles that the focus ring position varies as the reciprocal of subject distance. In other words, move your subject in to half its distance, and the rotation required on the focus ring (starting at infinity position) doubles. [Actually, at close distances, it's even more than this, but we can neglect that for this discussion.]

Let's call the focus ring position A, measured in degrees from the infinity position. If D is subject distance, we have
Eq. 1) A = k/D (k is a proportionality constant, determined by the lens)

Now bring in a subject moving at constant speed V:
Eq. 2) D = V*t (t is time)

Combining Eq. 1) and 2):
Eq. 3) A = k/(V*t)

Let W be the rotational speed of the lens ring, required to track our subject. W is the time derivative of A:
Eq. 4) W = dA/dt = -k/(V*t^2)

Use Eq. 2 to substitute out t:
Eq. 5) W = -k*V/(D^2)

Now, it's no surprise that the focus ring speed is proportional to subject speed, V. But that 1/(D^2) relationship is Trouble in Doggieland.

Let's apply this to a practical comparison. Suppose our camera/lens is capable of successfully tracking a race car speeding toward us at 200mph, when it's as close as 200ft. [Let's also assume that we survive this encounter.] Will Fido then be trackable, when he is coming toward us at 20mph, and he's 40ft. away?

Relatively, Fido's V has dropped by a factor of 10, which may give us comfort. However, D has decreased by a factor of 5, so (1/D^2) increases by a factor of 25. Oops. Tracking Fido will require 2.5 times as much speed as our camera is capable of!

It turns out that we need to move Fido all the way out to a minimum of 63ft., before he will be trackable by AF in this example. Clearly, intimate Fido action photography will pose a serious challenge.

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I don't suffer from insanity, I enjoy every moment of it!

By the way, film is not dead.
It just smell funny

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