Auto-focus on digital SLRs.

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Steven Green wrote:
> How do the hi/low end cameras differ for canon?
> Is it better/worse with other brands?

I can't say much for Canon cameras being a Nikonian, but there is
definitely a difference in autofocus speed and precision depending on the
lens/camera.

For example, my D50 (yeah, baby Nikon I know) is dead in the water when I
had the opportunity to try out a D200. The D200 focuses with far more
areas and works far better in low light.

Also, my Tamron 70-300mm lens uses the camera's drive system for focus,
it takes roughly 2 seconds to get from one end of the screw to the other.
My Nikkor lens does that in less than half of a second with its internal
motor.

I've entirely missed the film era--I've used digital all my life, getting
my first dSLR just over 2 years ago. Strangely enough, I still prefer my
dad's 50mm Micro Nikkor when I have the opportunity to do some macro shots.

- --
Brendan Gillatt | GPG Key: 0xBF6A0D94
brendan {a} brendangillatt (dot) co (dot) uk
http://www.brendangillatt.co.uk
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 >> Stay informed about: Auto-focus on digital SLRs. 

When it hunts (zooms in and out), what are you photographing at the time?
An image with very little contrast? A low light shot? Is the subject
moving?

Also, what focusing mode are you using? AI Servo, AI Focus or One Shot?




"Steven Green" <steven.green30 DeleteThis @verizon.net> wrote in message
news:EdFvj.249$Hd.83@trnddc02...
>I am new to the auto-focus world, both my film cameras are manual focus.
> I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn
> some basics of the camera and see what features I really like.
>
> One thing I am developing a disliking for is auto-focus.
> On the DRXT it sometimes goes into fits zooming in and out then refuses
> to snap a picture. I find the auto-focus slow and a pain in the ass.
>
> I understand that the speed of auto-focus can be a function of the lens
> and the effectiveness on the amount of light (if I understand this
> correctly)
>
> Other than things like numbers of points and speed of lenses I don't see
> much on the effectiveness of auto-focus.
>
> How do the hi/low end cameras differ for canon?
> Is it better/worse with other brands?
>
> If this is representative of the other cameras as well I think I may add
> weight to the quality of manual-focus for my future purchase and largely
> ignore auto-focus.
>
> Thanks,
>
> Steve
 >> Stay informed about: Auto-focus on digital SLRs. 

"Steven Green" <steven.green30.TakeThisOut@verizon.net> wrote in message
news:EdFvj.249$Hd.83@trnddc02...
>I am new to the auto-focus world, both my film cameras are manual focus.
> I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn
> some basics of the camera and see what features I really like.
>
> One thing I am developing a disliking for is auto-focus.
> On the DRXT it sometimes goes into fits zooming in and out then refuses
> to snap a picture. I find the auto-focus slow and a pain in the ass.
>
> I understand that the speed of auto-focus can be a function of the lens
> and the effectiveness on the amount of light (if I understand this
> correctly)
>
> Other than things like numbers of points and speed of lenses I don't see
> much on the effectiveness of auto-focus.
>
> How do the hi/low end cameras differ for canon?
> Is it better/worse with other brands?
>
> If this is representative of the other cameras as well I think I may add
> weight to the quality of manual-focus for my future purchase and largely
> ignore auto-focus.
>
> Thanks,
>
> Steve

Remember auto-focus is a contrast detection system, if there's little or no
obvious 'edges' under the focus points, its will fail. Often the trick can
be to turn off the multi-point focus system and just use the central sensor,
point that at an obvious edge at the distance you want to focus on; half
press the shutter to let the camera focus then reframe.

Like everything, it takes a bit of getting used to.
 >> Stay informed about: Auto-focus on digital SLRs. 

In article <220220081122278921%rag@nospam.techline.com>,
"Mr. Strat" <rag.TakeThisOut@nospam.techline.com> wrote:

> In article <EdFvj.249$Hd.83@trnddc02>, Steven Green
> <steven.green30.TakeThisOut@verizon.net> wrote:
>
> > I am new to the auto-focus world, both my film cameras are manual focus.
> > I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn
> > some basics of the camera and see what features I really like.
>
> Being an old school old fart, I didn't think I'd like autofocus when I
> switched to digital. But I love it. It takes a bit of getting used to
> (and turning off all sensors except one), but it can focus a lot
> quicker than these old eyeballs.

In the old days- one knew how to pre-focus; yet oddly enough it works
with AF cameras-if one knows what the hell one is doing.

--
Reality is a picture perfected and never looking back.
 >> Stay informed about: Auto-focus on digital SLRs. 

"Steven Green" <steven.green30 DeleteThis @verizon.net> wrote in message
news:EdFvj.249$Hd.83@trnddc02...
>I am new to the auto-focus world, both my film cameras are manual focus.
> I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn
> some basics of the camera and see what features I really like.
>
> One thing I am developing a disliking for is auto-focus.
> On the DRXT it sometimes goes into fits zooming in and out then refuses
> to snap a picture. I find the auto-focus slow and a pain in the ass.
>
> I understand that the speed of auto-focus can be a function of the lens
> and the effectiveness on the amount of light (if I understand this
> correctly)
>
> Other than things like numbers of points and speed of lenses I don't see
> much on the effectiveness of auto-focus.
>
> How do the hi/low end cameras differ for canon?
> Is it better/worse with other brands?
>
> If this is representative of the other cameras as well I think I may add
> weight to the quality of manual-focus for my future purchase and largely
> ignore auto-focus.
>
> Thanks,
>
> Steve

Obviously, the more you pay, the more you get. But, when it comes to
autofocus, the digital Rebel is one of the better cameras on the market --
especially in its price range.

Given that the Rebel XT is a Canon, you can use older EF lenses on it which
have an autofocus switch; it's located near the camera body on the left
side of the lens. (I'm not sure if the newer FS lenses have that switch or
not.) You can just turn off the autofocus mechanism and focus manually.
But, there won't be a focusing scale that is of much use to you.

My wife uses that camera a lot, and her only complaint is that the
viewfinder is a bit dimmer than our old A2's viewfinder, but that is usually
only a problem in very low light conditions when manually focusing.

Also, you might want to check if the AF assist light is turned on. The AF
assist light really improves the cameras performance in low light
conditions.

Hope this helps,
Dudley
 >> Stay informed about: Auto-focus on digital SLRs. 

"Mr. Strat" <rag.DeleteThis@nospam.techline.com> wrote in message
news:220220081122278921%rag@nospam.techline.com...
> In article <EdFvj.249$Hd.83@trnddc02>, Steven Green
> <steven.green30.DeleteThis@verizon.net> wrote:
>
>> I am new to the auto-focus world, both my film cameras are manual focus.
>> I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn
>> some basics of the camera and see what features I really like.
>
> Being an old school old fart, I didn't think I'd like autofocus when I
> switched to digital. But I love it. It takes a bit of getting used to
> (and turning off all sensors except one), but it can focus a lot
> quicker than these old eyeballs.

And, when you only have about 2% of "normal" vision, it becomes
indeispensible...
 >> Stay informed about: Auto-focus on digital SLRs. 

Steven Green <steven.green30 RemoveThis @verizon.net> wrote:
>I am new to the auto-focus world, both my film cameras are manual focus.
>I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn
>some basics of the camera and see what features I really like.
>
>One thing I am developing a disliking for is auto-focus.
>On the DRXT it sometimes goes into fits zooming in and out then refuses
>to snap a picture.

This can happen if there isn't enough light or the lens isn't fast enough.
Most manufactures explicitely limit AF to lenses faster than approx. f/5.6.
> I find the auto-focus slow and a pain in the ass.

Normally autofocus on a dSLR is almost instantanious, at the order of a
fraction of a second. And normally they don't hunt either because of the
phase detetction system used, which allows a direct, non-incremental
adjustment.

>I understand that the speed of auto-focus can be a function of the lens
>and the effectiveness on the amount of light (if I understand this
>correctly)

Quite so. Unfortunately you don't tell us anything about what lenses you are
using. There is quite a difference between a 20+ year old screw-driver type
and a modern ultra-sonic AF.
Or are you by any chance using auto-focus with a closed apperture? That
would explain your problems, too.

Of course, you can always turn off AF easily enough with the flip of a
switch.

jue
 >> Stay informed about: Auto-focus on digital SLRs. 

In article <47Gvj.2472$g81.875@newsfe3-gui.ntli.net>, Anon
<abuse.DeleteThis@yahoo.com> wrote:

> Remember auto-focus is a contrast detection system,

on a dslr, it's a phase detection system (unless in live view mode on
some cameras). point&shoots use contrast detection.
 >> Stay informed about: Auto-focus on digital SLRs. 

In article <47Gvj.2472$g81.875@newsfe3-gui.ntli.net>,
"Anon" <abuse.TakeThisOut@yahoo.com> wrote:

> Remember auto-focus is a contrast detection system, if there's little or no
> obvious 'edges' under the focus points, its will fail. Often the trick can
> be to turn off the multi-point focus system and just use the central sensor,
> point that at an obvious edge at the distance you want to focus on; half
> press the shutter to let the camera focus then reframe.

You are so correct!

> Like everything, it takes a bit of getting used to.

A lot of- at first.

--
Reality is a picture perfected and never looking back.
 >> Stay informed about: Auto-focus on digital SLRs. 

In article <vabur3tcji420u9fh5hqhf8ghv74vtns5b.RemoveThis@4ax.com>,
J?rgen Exner <jurgenex.RemoveThis@hotmail.com> wrote:
>
> This can happen if there isn't enough light or the lens isn't fast enough.
> Most manufactures explicitely limit AF to lenses faster than approx. f/5.6.
> > I find the auto-focus slow and a pain in the ass.
>
> Normally autofocus on a dSLR is almost instantanious, at the order of a
> fraction of a second. And normally they don't hunt either because of the
> phase detetction system used, which allows a direct, non-incremental
> adjustment.

The seeking attribute of AF even with a slower lens, is greatly
improved by a TTL flash, I have a blast painting people with the ir grid
using the off camera cord.

--
Reality is a picture perfected and never looking back.
 >> Stay informed about: Auto-focus on digital SLRs. 

In article <3bGvj.39860$w57.28071@edtnps90>, Dudley Hanks
<hanks.dudley.DeleteThis@gmail.com> wrote:

> And, when you only have about 2% of "normal" vision, it becomes
> indeispensible...

My use of soft-focus lenses in a semi-darkened studio for many years
may have something to do with it too.
 >> Stay informed about: Auto-focus on digital SLRs. 

On Fri, 22 Feb 2008 20:11:07 +0000, Ali wrote:

> When it hunts (zooms in and out), what are you photographing at the
> time? An image with very little contrast? A low light shot? Is the
> subject moving?
>
> Also, what focusing mode are you using? AI Servo, AI Focus or One Shot?
>

I was on One shot at the time.

In this case it was a rather low light shot of my dog laying on a sofa.
Indoors with lamp lighting. I don't think he was moving at the time. I
think I tried the flash at the time and it did the strobe thing and
couldn't make up its mind.

He is a rather brown dog with a dark face. I think he has rather good
contrast including the sofa.
 >> Stay informed about: Auto-focus on digital SLRs. 

Manual focus is a wonderful tool.
Unfortunately the viewing systems in most autofocus SLRs, film and camera,
are not made for critical manual focusing. Most have at best a ground glass
surface that tends to make thinks look focused that may not be. Also because
many autofocus zooms have a maximum aperture of f4 and go down to f5.6 at
the long end the viewing image may be a bit dim. The manual focusing cams on
many autofocus lenses do not have the firmness of manual focus lenses either
but there are not many complaints about slipping out of a manually adjusted
position.
Some dSLRs, like the otherwise great D70, are severely hampered by the dim
small view they give the user.
Modern SLR autofocus under most circumstances however is probably more
accurate than manual focus overall under most conditions. Adding facial
recognition algorithms may also be an advance, although I have not
experimented much yet with my camera that has this feature.
 >> Stay informed about: Auto-focus on digital SLRs.