What good is AF-S anyway?

Started Apr 12, 2007 | Discussions
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Tom Christiansen Senior Member • Posts: 2,239
What good is AF-S anyway?

Why would one prefer single-servo AF instead of continuous-servo?

With all the carping about the D200's C/S/M focus server slipping out of
its position by itself on many people, I wonder whether they're using their
camera differently from the way I am. I also wonder whether perhaps I'm
missing out on something, because this accident of theirs just doesn't
happen to me. I suppose it may have happened to me one or twice, but if
so, it's so infrequent I can't remember it ever happening after 70k shots.
I believe this is because I've very nearly always got the lever at the C
position; this is a much more stable position than the S position.

I have a serious question about this. What good purpose do so
many people seem to find with Single-Servo AF compared with
Continuous Servo AF?

Now that I hear it it being asked, it sounds like a "Well, duh!" kind of
thing, like it should be so obvious that everybody knows it. But no
lightbulb is going on in my head. Maybe I'm just stuck in a rut and am
missing out on fabulous opportunities by having my lever at the
nicely stable C position instead of in the (possibly dodgy) S.

Why would one prefer AF-S over AF-C? I've come up with just one
semi-practical reason, plus maybe a few more that are super minor.

That possibly ok reason may be:

  • Because you need to use AF Assist beacon to help focus, and this

only comes on in AF-S mode, not AF-C mode. I believe that a
speedlight's red/near-IR grid may only come in AF-S mode, too,
although this is separately controlled.

These are reasons that don't really seem to matter that much:

  • Because you find the BEEP of a focus-lock reassuring, knowing that

if you don't hear yet but can still take the picture, you are no
longer in AF-S Focus Priority mode, so you should check your lever.
But if you're in a hig-noise situation you wouldn't notice, and in a
low-noise situation, this might risk annoying you or others. And
would you really pay attention to this if you aren't even bothering
to pay attention to whether you've got something in focus before
tripping the shutter release?!

  • Because you want to hand the camera to someone who's, um, camera-

incompetent ("waiter mode"), so you quickflick the body lever over
to AF-S with Focus Priority, and also probably flick the other body
lever to Dynamic Area Closest Focus

  • Because you think that AF-S will drain your battery more than AF-C

and you desperately need to conserve power.

  • You want to be Focus Priority, but it's too hard to reach the AF-L

button for you (or you've reassigned it?) and you want single-button
shooting.

I find I nearly never use AF-S. With AF-C, you don't run the risk of
shooting something that's moving (or you are) and the camera not
compensating, and you seem to have full-time everything--at least with AF-S
(argh) lenses. I use AF-On for focusing, and usually have the AF-L/AE-L
button set to AE-L only; if I am put it in Focus Prioirity (but still AF-C),
then I'll put the AE-L back to also being AF-L, but that's not as frequent.

But from all the complaints about people knocking the lever by accident
from its middle S position, a lot of people must be using it there.
Why? What do they know that I don't know? What am I missing out on?

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tom

virtuamike Veteran Member • Posts: 3,569
Composition

I don't like composing based on where the AF sensors are.

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James Kei Senior Member • Posts: 2,156
continuous focus like a DV CAM

$$$ talking.

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ewelch Senior Member • Posts: 1,022
Re: What good is AF-S anyway?

AF-S is actually the type of lens that has the focusing motor in the lens in Nikon nomenclature. For example, the D40 and D40X only autofocus with AF-S lenses.

Continuous mode is for photographing moving subjects (sports, birds, cars, kids) where the subject will move out of the focus plane while focus in the single mode would not follow.

Thus the single mode is for normal everyday shooting where you don't need follow focus. You lock the focus when you touch the shutter release and then while it's locked you recompose and press the shutter release the rest of the way to get the photo. It's kind of the way you work with a Leica rangefinder. Focus with the spot in the middle and then recompose. Obviously not the way to shoot football. For example, with portraits it works way better than continuous mode where the lens will hunt in and out looking for better focus.

Both serve their purpose and neither is a cure-all. It's not either/or. And don't forget, manual focus works great too. I was shooting a hummingbird the other day in a dense bush and if I had relied on AF I would not have gotten any photos in focus. With manual focus I got lots of photos of the little buggers gathering nectar.

It's pretty simple really when you consider the logic.

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Eric

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Tom Christiansen OP Senior Member • Posts: 2,239
Re: Composition

virtuamike wrote:

I don't like composing based on where the AF sensors are.

So what does putting the AF lever into S buy you for that
scenerio that putting it into C does not?

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tom

Dave_Lawrance
Dave_Lawrance Regular Member • Posts: 380
I've NEVER used AF-C!

I've never used AF-C as the things I shoot don't move.

Also.....

I never use other than the central focus sensor.
I've never used a shutter speed faster than 1/500.
I've never used higher than ISO 400.
I hardly ever have the metering off the spot setting.
I always shoot in manual exposure mode.
I never use continuous "drive".
I would manual focus if the viewfinder was good enough.

Mind you I've also....

Never used a noise reduction plug-in/application.
Never used a sharpening plug-in.*
Never used a plug-in/application to turn an image mono.*
That wasn't part of Photoshop.

Am I mad?
--
Dave
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Tom Christiansen OP Senior Member • Posts: 2,239
Re: What good is AF-S anyway?

ewelch wrote:

AF-S is actually the type of lens that has the focusing motor in
the lens in Nikon nomenclature. For example, the D40 and D40X only
autofocus with AF-S lenses.

I'm fully aware of that: hence my "argh" exclamation at the overloading
of the term. Single-Servo-AF is also often called AF-S. Continuous-
Servo-AF is AF-C. We have two things that AF-S means. That
didn't have anything (much) to do with the question. AF-S pertaining
to lenses is different from AF-S pertaining to cameras. But one
can spell out the servo thing if people need cluing in--I guess.

Continuous mode is for photographing moving subjects (sports,
birds, cars, kids) where the subject will move out of the focus
plane while focus in the single mode would not follow.

In AF-Single, a moving subject is still tracked if was in motion when
the lock was made. AF-Continuous will catch motion that starts up
after the initial focus is made that wasn't there initially. It keeps
reapprising the situation. If you are using one of the 3/4 AF modes
that permit tracking to a different sensor than the initial one used,
then if the subject moves so far/fast that it can use another sensor,
it will. Focus tracking with AF-S works fine if you AF a moving subject.
With AF-C, if the subject isn't moving at when you first engage the
AF system, but then later starts moving, then that is also tracked.

Thus the single mode is for normal everyday shooting where you
don't need follow focus.

Have you noticed the difference in getting in-focus shots that are
extreme close-ups between AF-S and AF-C? Perhaps it's just me,
but I find I do better with AF-C. For example, the 105VR; use
that and get closer than I dunno, 1:4, 1:5 or so, and you're bobbing
around a fair bit. With AF-C, I get more keepers.

You lock the focus when you touch the
shutter release and then while it's locked you recompose and press
the shutter release the rest of the way to get the photo.

What does "lock the focus" even mean, really? How is this any
differerent than leaving your camera on AF-C, tapping your AF-On
button to focus once--or holding it to focus continuously until
you like it--then releasing your focusing button and recomposing,
then shooting? You can focus and recompose set to AF-C just as easily as set
to AF-S; more so, maybe.

It's kind
of the way you work with a Leica rangefinder. Focus with the spot
in the middle and then recompose. Obviously not the way to shoot
football. For example, with portraits it works way better than
continuous mode where the lens will hunt in and out looking for
better focus.

It does seem to, doesn't it? But the subject's eye isn't really moving,
is it? I use the 105VR for portraits a bit, and I don't keep the AF
system constantly engaged for that reason. But why would I have
to be in AF-S mode to do that? How is this any different that tapping
the AF-On, touching up the focus by hand if you want to and can
manage to see, then releasing the shutter?

Both serve their purpose and neither is a cure-all. It's not
either/or. And don't forget, manual focus works great too.

Great is overstatement. If I had to rely on my eye (which is 20/20)
I wouldn't get many, especially in low light. I've tried shooting
the Ai-S 105/2.5 at f/2.8 in indoor, nighttime lighting, and it is quite
tough to nail the eye. About the only time I override is for fence
shots where you're shooting through a grating, or for real macro
work (1:1 or thereabouts). Otherwise my idea of when it's in
good enough focus is often not what the green dot thinks.

I was
shooting a hummingbird the other day in a dense bush and if I had
relied on AF I would not have gotten any photos in focus. With
manual focus I got lots of photos of the little buggers gathering
nectar.

Why? Was the background tricking the AF system into being more
attractive than the birds? Were they too small for the get good
coverage of the AF sensors? Did you know where they were going
to be so you prefocused in the general vicinity and relied upon an
aperture that was small enough to include them?

I know I couldn't possibly focus fast enough and accurately enough
to follow hummingbirds in the general case. It would take a lot of
set-up and observation. Consider a 300mm lens at 30' away. At f/8,
you have something like a 1' depth of field of so. But if you were
closer than that, or with a wider aperture, it's worse. Half the subject
distance to 15' and open up a stop to f/5.6, and you now have a DoF
that's under 2". That's asking a lot for eyeballing--at least for me.
I wish otherwise, but that's just the way it is. With a 2" target zone
on a subject that small, I need any help I can get. Running AF-S
lenses in AF-C mode certainly helps. Hard to get it settle down though
on little targets, but it still does much better than I can.

It's pretty simple really when you consider the logic.

I'm afraid I'm not there yet then. I haven't figured out what AF-S
buys you over just tapping the AF-On key and letting go of it and
being in AF-C mode.

And if you operate that way, you stop cursing the AF selector lever,
because if it's AF-C, it doesn't flip off of its setting. I guess the people
using it set to S keep messing up. There's another thread where they
curse it a lot. This doesn't happen to me, because if you leave
it in AF-C mode, you all three without trouble. Grab the lens and
twist if you want to MF sometime or other. Hold the AF-On button
for continuous focusing. Tap it for one-time--and roll over to AF-L
if in focus priority not shutter prioirty. With the camera set to AF-C,
you get all those. With AF-S, well, you don't. There must be something
I'm not seeing about what makes AF-S a desirable lever setting.

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tom

John W.
John W. Senior Member • Posts: 2,187
This is why !

AF-C (Continuous Servo AF Mode)

Since AF-C mode never “locks” the focus, it's always ready to take a picture. It will focus on the subject as long as you hold the button down, but even small camera or subject movements will make it refocus over and over. You'll hear the lens chatter as the focus stays on your subject, and constantly makes small adjustments. When you press the shutter button fully, the picture is taken in whatever the last focus position was, which may not be correct.
--
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The only normal people are the ones you don't know very well....!!!

williams-pics Contributing Member • Posts: 874
Re: This is why !

Most of my work is PR, that means a lot of certificate and trophy presentations with usually only two people in the picture, so by using "S" I can focus on one of the people recompose and with the focus locked at that distance I am able to take several shots without moving the camera but in "C" the camera would focus on the background between the people and they would be out of focus.
Regards,
Bruce.

ldo Forum Member • Posts: 87
Re: What good is AF-S anyway?

I was wondering of the same thing a couple years ago when I first got hold of the F5. Which I believe is the first Nikon body to have these two modes (but I could be wrong).

Anyways, for my experience, with AF-S, you have more control on where the camera will focus. You select the focus point and the camera will only focus on that selected bracket, even if the subject moves (but not outside the selected bracket).

With AF-C, you can select the focus area, but as the subject moves to other focus points within the viewfinder, the camera will switch to the appropriate focus points to track your subject. The only problem is that it doesn't really tell you which one it has switched to. Focus tracking ability of modern Nikon camera is very good, but not fool proof, it might lose your intended subject and focuses on something else as the camera switches focus points. This could be because the AF points are spaced too far (?). It would be cool if the camera can highlight the focus point it selected, though I figure it would also give me a headache, not to mention draining battery power when trying to focus on an eratic moving object.

Also, with AF-C, you get to choose to trip the shutter with release priority or focus priority. With AF-S, I believe you only get focus priority (don't quote me on this, I'm too lazy to open up the manual).

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Gordon Chau Regular Member • Posts: 247
AF-On button

A lot of AF-C users prefer to dissociate focussing from the shutter button and use AF-On button to initiate focus. That way, by lifting off your thumb from the AF-On button, one effectively achieves what AF-S does.

Grzzl
Grzzl Senior Member • Posts: 2,967
if you are...

...i am too.

I shoot the same way as you, except 400 isio. I easely go for 1600 (but not much)
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S.A. Veteran Member • Posts: 4,072
Re: What good is AF-S anyway?

I think the reason so many people primarily use AF-S is because the majority haven't discovered the greatness of the AF-ON button.
--
Scott A.

drmrbrewer Senior Member • Posts: 1,175
Re: It's just a choice

As far as I am aware, the only way you can lock focus when on AF-C (to recompose, for example) is by using AF-ON (lift off to lock) or AF-L (press to lock) -- both assuming you have the buttons set up correctly to do this. This style of shooting would not suit many people, who would simply prefer an automatic lock after focus is achieved to allow them to recompose. I use both styles, depending on the situation, and I have my Banks set up for a quick change. If I know the subject is more likely to be moving than static, I use the AF-C / AF-ON method. If I know the subject is more likely to be static than moving, and perhaps I'll be flipping from portrait to landscape orientation, and perhaps moving around myself, and doing lots of focus/recomposing, I find the AF-S method easier. If in doubt, I use the AF-C / AF-ON method because it does cater more readily for whatever eventuality arises.

Mike

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Rakesh Dhareshwar Contributing Member • Posts: 535
Re: AF-On button

Gordon Chau wrote:

A lot of AF-C users prefer to dissociate focussing from the shutter
button and use AF-On button to initiate focus. That way, by lifting
off your thumb from the AF-On button, one effectively achieves what
AF-S does.

Perfect...I use this 100% of the time and it works beautifully for my subjects - birds.
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digitall Senior Member • Posts: 2,223
Re: What good is AF-S anyway?

If you use AF-C the lens will re-focus if you move the camera so that the focus area moves to an object at a different distance, so it's not suitable for most photography where you would want to hold the focus with a half depression of the shutter release. Try it by focusing on a near object then half-depress the shutter and move the camera to point at a much nearer or further object. You'll see the lens re-focus. I'm surprised you haven't discovered this by accident. As a matter of interest have you found many of your pictures to be out of focus? Perhaps you are not in the habit of recomposing after focusing but I'd have thought you must do so occasionally. Hope this helps.
--

1bradman Regular Member • Posts: 159
Re: What good is AF-S anyway?

Digitall, you might want to read his post, he makes it clear that he uses the AF-ON button to focus, not the shutter release button!
Matt.

MacBrian Contributing Member • Posts: 587
Totally agree...

SA -

I totally agree with you... I used AF-S forever until recently discovered the power of seperating the focus off the shutter button and using the AF-ON button instead. I learned this tip from Mike Colon

It's really powerful once you learn how to use it... also in AF-C mode you still get a lock indicator in the viewfinder

Dave_Lawrance
Dave_Lawrance Regular Member • Posts: 380
A slight correction...

Dave_Lawrance wrote:

I've never used AF-C as the things I shoot don't move.

Actually I have the AF switch in the "C" position all the time!

Clarification: As I use the AF-ON button to focus, (or at least the AE-L/AF-L button on my Fuji S2 set to behave as an AF-ON button), I have to have the AF switch set to AF-C, otherwise the camera would not let me take a shot if it thought it wasn't in focus.
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digitall Senior Member • Posts: 2,223
Re: What good is AF-S anyway?

Ah yes, that accounts for it. Thanks for pointing that out. I read his post but skimmed toward the end.

1bradman wrote:

Digitall, you might want to read his post, he makes it clear that
he uses the AF-ON button to focus, not the shutter release button!
Matt.

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