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Subject: "Release priority and AF-ON vs. AF-S" Previous topic | Next topic
sophrosune Registered since 08th Jun 2006Wed 26-Jul-06 02:01 AM
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"Release priority and AF-ON vs. AF-S"


AU
          

I've been listening to the last couple of Image Doctor podcasts where they have been talking about sports photography and auto-focus settings, in particular the use of release priority and the AF-ON button for fast auto-focussing. They mentioned its applicability to the D200 and D2* cameras but I can't see why it can't be done with the D70(s) using AF-C mode and configuring the AE/AF button appropriately.

Maybe I don't understand how its being used but I'm wondering exactly what advantages the method described has over just using AF-S and relying on the half button press to lock focus. Sure, using AF-C and AF-ON will allow focus lock without actually having something in focus but I presume you need a subject that can be focussed on at some point in order to produce a sharp image. What about this am I not getting?

  

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Covey22 Moderator Expert in various fields including aviation photography Awarded for his contributions to the Resources and The Nikonian eZine Charter MemberWed 26-Jul-06 12:08 PM
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#1. "RE: Release priority and AF-ON vs. AF-S"
In response to Reply # 0


US
          

It can be done on the D70. You've correctly noted that the use Continuous Servo or AF-C allows the shutter to fire without AF lock being achieved. But what Rick and Jason are talking about is AF-activation time, not AF-lock time.

By decoupling AF-ON or AF activation from the shutter, you've reduced the latency in shutter response time. In the bad old days of manual focus and what I like to call the Mesozaic Era of AF (i.e., when it was new and not quite up to par), pre-focusing was still the dominant method of obtaining proper focus for fast-action photography. With AF-ON using a separate button, you can prefocus, say, on the corner of the track where the racecars will bunch up and press the shutter at the right time.

You cannot achieve the same effect using AF-ON on a separate button using Single Servo or AF-S. AF-S will *induce* additional delay by the camera interrogating the AF system to confirm it has indeed locked focus. Any advantage you have gained in decoupling AF activation is lost in that additional compute time. In the same breath, AF-C does not focus randomly, it does calculate a focus point, BUT it does NOT stop the shutter from firing if it thinks it has not achieved optimal lock. It's sort of a "good enough" logic.

The most optimal settings for reliable focus are clearly Single Servo (AF-S) with Single Area-AF and the center AF sensor selected and locked using the rear selector pad. Superimpose this over your target, half-press and shoot. This describes Focus Priority.

The best *compromise* between focus and shutter latency is Continuous Servo (AF-C) with Single Area AF and the center AF sensor selected and locked using the rear selector pad. If desired, decouple AF-ON from the shutter. Superimpose the sensor over your target, press AF-ON and then press the shutter to shoot. Contrary to popular belief, the CAM900's Dynamic AF is NOT very reliable in fast-action shooting. Enabling Dynamic Area AF just builds in more latency into the shutter release, even under Continuous Servo. Forcing selection and use of the center sensor takes that calculation time out of the equation.

The methods described above leads to overly centered compositions, but that cannot be helped, since the center sensor is the only cross-hair one and is the most reliable. Obviously, this technique does not apply if you're shooting with a CAM1300 or CAM2000 equipped camera.

Note also that this setting can be problematic on cameras with a combined AF-L/AE-L button - users who do not shoot in this configuration will often forget the setting is in play the next time they use their camera and will be frustrated as to why AF is not working because they are using the traditional half-press method on the shutter button. This is why a dedicated AF-ON button is a real convenience feature.

Edited for errors: replaced "rear command dial" with "rear selector pad."

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sophrosune Registered since 08th Jun 2006Thu 27-Jul-06 11:45 PM
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#2. "RE: Release priority and AF-ON vs. AF-S"
In response to Reply # 1


AU
          

>By decoupling AF-ON or AF activation from the shutter,
>you've reduced the latency in shutter response time. In the
>bad old days of manual focus and what I like to call the
>Mesozaic Era of AF (i.e., when it was new and not quite up
>to par), pre-focusing was still the dominant method of
>obtaining proper focus for fast-action photography. With
>AF-ON using a separate button, you can prefocus, say, on the
>corner of the track where the racecars will bunch up and
>press the shutter at the right time.

OK, so what's important with this approach is getting the shot at the precise moment you want it but possibly at the expense of not having the most pin-sharp image possible. Would the use of smaller apertures to increase your depth-of-field help mitigate focussing errors in this case?

>You cannot achieve the same effect using AF-ON on a separate
>button using Single Servo or AF-S. AF-S will *induce*
>additional delay by the camera interrogating the AF system
>to confirm it has indeed locked focus.

Do you mean that once AF-S has locked focus by half-pressing the shutter release, it will still reconfirm that focus when it is fully pressed and this is where the latency is? One advantage I can see with the AF-ON approach over AF-S is that even in continuous shooting mode the camera will wait until focus is locked before releasing the shutter using AF-S.

>The most optimal settings for reliable focus are clearly
>Single Servo (AF-S) with Single Area-AF and the center AF
>sensor selected and locked using the rear selector pad.
>Superimpose this over your target, half-press and shoot.
>This describes Focus Priority.
>
>The best *compromise* between focus and shutter latency is
>Continuous Servo (AF-C) with Single Area AF and the center
>AF sensor selected and locked using the rear selector pad.
>If desired, decouple AF-ON from the shutter. Superimpose the
>sensor over your target, press AF-ON and then press the
>shutter to shoot.

Thanks for the explanation, its all (slowly) becoming clear to me.

>The methods described above leads to overly centered
>compositions, but that cannot be helped, since the center
>sensor is the only cross-hair one and is the most reliable.

I guess this is why the Nikon manual talks about getting focus lock and recomposing when describing the various focus modes. Fine for static compositions, not so great for dynamic ones.

  

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Covey22 Moderator Expert in various fields including aviation photography Awarded for his contributions to the Resources and The Nikonian eZine Charter MemberFri 28-Jul-06 01:44 AM
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#3. "RE: Release priority and AF-ON vs. AF-S"
In response to Reply # 2


US
          

OK, so what's important with this approach is getting the shot at the precise moment you want it but possibly at the expense of not having the most pin-sharp image possible. Would the use of smaller apertures to increase your depth-of-field help mitigate focussing errors in this case?

Yes, smaller DOF increases your opportunity to cover *minor* focusing errors. If for some reason though, the focus locks onto something front/rear of the subject and it's out of the DOF zone, it won't matter. Also, in certain situations, such as action shooting, smaller DOF comes at the cost of keeping shutter speed high, so you'll have to strike a balance in the exposure triad to compensate; a higher ISO for example.

Do you mean that once AF-S has locked focus by half-pressing the shutter release, it will still reconfirm that focus when it is fully pressed and this is where the latency is? One advantage I can see with the AF-ON approach over AF-S is that even in continuous shooting mode the camera will wait until focus is locked before releasing the shutter using AF-S.

If I'm using half-press and AF-C, I can mash the button all the way down without half-pressing and the shutter will fire. Mashing down the button without half-press in AF-S mode will force the camera to calculate if focus is achieved. This is the latency I speak of. In your situation, if you've already half-pressed, the shutter will fire *assuming* a) focus is locked and b) the camera does not sense that the focus point has changed (i.e., Dynamic AF versus Single-Are AF). Half-pressing = AF-ON. All we've done is decoupled the half-press from the shutter.

I'm not quite sure what to make of the last sentence in your paragraph. AF-ON versus AF-S isn't really a comparison, because they are complementary settings, not opposing ones. AF-ON works the same regardless of Servo mode. AF-C will not wait for focus lock. If focus is locked, that's great, if not, it's still going to fire. Assuming you locked a focus point using AF-ON while in AF-C servo, then things will reasonably be in-focus.

I guess this is why the Nikon manual talks about getting focus lock and recomposing when describing the various focus modes. Fine for static compositions, not so great for dynamic ones.

I agree, but we should probably add - for cameras with only one cross-hair sensor. The story changes again when you're talking about a D2H with NINE cross-hair sensors. There, Dynamic AF changes characteristics; becoming a really responsive lock-on machine. Even the D200 with it's wider-area AF has a slightly more reliable Dynamic AF capability than any CAM900 equipped body.


"Toodle-loo from Covey22!"

-Armando
Nikonians Team
Nikonians News - Fresh Everyday!

The Covey Blog!

My Plan:

Get out of the car.
Get closer to the subject.
Pick the right mid-tone this time.

See My Nikonians Gallery

  

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sophrosune Registered since 08th Jun 2006Fri 28-Jul-06 02:40 AM
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#4. "RE: Release priority and AF-ON vs. AF-S"
In response to Reply # 3


AU
          


>Do you mean that once AF-S has locked focus by
>half-pressing the shutter release, it will still reconfirm
>that focus when it is fully pressed and this is where the
>latency is? One advantage I can see with the AF-ON approach
>over AF-S is that even in continuous shooting mode the
>camera will wait until focus is locked before releasing the
>shutter using AF-S.

>
>If I'm using half-press and AF-C, I can mash the button all
>the way down without half-pressing and the shutter will
>fire. Mashing down the button without half-press in AF-S
>mode will force the camera to calculate if focus is
>achieved. This is the latency I speak of. In your situation,
>if you've already half-pressed, the shutter will fire
>*assuming* a) focus is locked and b) the camera does not
>sense that the focus point has changed (i.e., Dynamic AF
>versus Single-Are AF). Half-pressing = AF-ON. All we've done
>is decoupled the half-press from the shutter.

Thanks for (patiently) clearing that up.

>I'm not quite sure what to make of the last sentence in your
>paragraph. AF-ON versus AF-S isn't really a comparison,
>because they are complementary settings, not opposing ones.
>AF-ON works the same regardless of Servo mode. AF-C will not
>wait for focus lock. If focus is locked, that's great, if
>not, it's still going to fire. Assuming you locked a focus
>point using AF-ON while in AF-C servo, then things will
>reasonably be in-focus.

I could have worded that better - as you say, it's really the AF-C setting in the AF-C/AF-ON combination that gives you the ability to trigger the release without having to wait for the camera to reacquire focus lock between shots as it does when using AF-S in continuous shooting mode.

Thanks again.

  

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