D3S focus testing

Started Feb 14, 2010 | Discussions
sens72
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Re: Dog focus
In reply to Grevture, Mar 5, 2010

Thanks for your input Grevture.

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JoeinLA
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Missed Dog Series
In reply to pgphoto_ca, Mar 5, 2010

I just took a series of my relatively small dog at a slow run from about 35 ft to about 15 ft using the 70-200 VRII, Continuous Low, AF-C, 51pt 3D Dynamix Focus, Focus Priority.

D3S didn't get a single shot in focus

That makes me sad. I'll keep playing with the 70200II and 2470.

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I'm a noob, so everything I say can be, and most likely is, wrong.

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sens72
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Re: Missed Dog Series
In reply to JoeinLA, Mar 5, 2010

JoeinLA wrote:

I just took a series of my relatively small dog at a slow run from about 35 ft to about 15 ft using the 70-200 VRII, Continuous Low, AF-C, 51pt 3D Dynamix Focus, Focus Priority.

D3S didn't get a single shot in focus

That makes me sad. I'll keep playing with the 70200II and 2470.

-- hide signature --

I'm a noob, so everything I say can be, and most likely is, wrong.

I would use single point af or 9 point af, so you can choose by your self what part of the dog has to be in focus. If you use 51 point Dynamic focus, the camera desides for you what's good. So if he likes the tail to focus on, he choose the tail.
Also better to use CH and shutter priority.
Please try it again and lets see if there are differences.

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Grevture
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Re: Missed Dog Series
In reply to JoeinLA, Mar 5, 2010

JoeinLA wrote:

I just took a series of my relatively small dog at a slow run from about 35 ft to about 15 ft using the 70-200 VRII, Continuous Low, AF-C, 51pt 3D Dynamix Focus, Focus Priority.

D3S didn't get a single shot in focus

That makes me sad. I'll keep playing with the 70200II and 2470.

Keep in mind how AF operates: it works by detecting contrast, and with a dog coming towards you the AF usually locks on to the outer contour of the dog which often translate to something like mid body focus. Or, if yoy keep the focusing point on the head of the dog, it grabs the outer edges of the head, meaning you get sharp ears, but not sharp eyes and nose.

For dogs coming at me I would not use the 3D mode, but rather use either a single point focus or dynamic area and work on keeping that selected focus point where you want it on the dog. And I almost never use the focus priority, stay with shutter priority. BTW: With dogs, I often cheat a little by setting the AF adjust to front focus a bit, since most dogs have such pointed faces.

I have shot a lot of running dogs (mostly agility training/competition) in the last three years, and when I first started I was frustrated at the low hit rate. But keep in mind that dogs are tough subjects to get focus on. Thay are fast, move irregularily (giving the predictive AF lots of trouble) and they are small so you often shoot at fairly short distances.

But it can be done.

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By the way, film is not dead.
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 Grevture's gear list:Grevture's gear list
Nikon D70s Nikon D3 Nikon D3S Nikon AF-S Nikkor 70-200mm f/2.8G ED VR II Nikon AF-S Nikkor 17-35mm f/2.8D ED-IF +7 more
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Marianne Oelund
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Focus hyperbola
In reply to Grevture, Mar 5, 2010

Grevture wrote:

Now I have sometimes shot dogs in action, and that is in my experience quite tricky.

Most lenses are, again in my experience, YMMW, distinctly slower to focus the closer you are to the subject. And for small dogs like terriers etc ... They are very small compared to a running athlete, a speeding motorcycle or any of the other stuff you shoot at high speeds. I mean if you were tracking focus on a race car at 20 feet distance ... You are in trouble

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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Grevture
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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.

-- hide signature --

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

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

 Grevture's gear list:Grevture's gear list
Nikon D70s Nikon D3 Nikon D3S Nikon AF-S Nikkor 70-200mm f/2.8G ED VR II Nikon AF-S Nikkor 17-35mm f/2.8D ED-IF +7 more
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xrdbear
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Re: Focus hyperbola
In reply to Marianne Oelund, Mar 6, 2010

Marianne (bearing in mind I know nothing), I wonder if this problem, showing up as it does in the new version of the 70-200, is compounded by the rapidly changing magnification/focal length at close distances that was discussed some time ago.
Just speculating.
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Brian
Fine Art Print sales of the Isle of Skye at:
http://www.eyeofskye.co.uk/

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