Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Canon Lenses

Reply
 
Thread Tools Search this Thread
Old Jan 6, 2005, 6:41 AM   #1
Senior Member
 
aladyforty's Avatar
 
Join Date: Jan 2004
Posts: 6,964
Default

Just a question about what you told me in another post:-)

RE "Also the camera does the focusing and not the lens! "



I do know this but how is that that so many camera and lenssites seem to try the line of "this lens will focus faster than....." and "this lens is superior in it's focusing and locking on to the subject"

I also saw a review somewhere in which the photographer said that "the lens kept grabbing and searching like a tiger hunting":-)yet his other lens never did this. If it is all about the camera why would this happen????
aladyforty is offline   Reply With Quote
Sponsored Links
Old Jan 6, 2005, 11:02 AM   #2
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

Think about it this way:
1. How's your driving behind 'real' glasses vs kiddy plastic sunglass? - A lens can be more contrasty than another with all else being equal (ie glass lens vs lighter plastic elements)
2. A faster lens which is brighter will help the camera focus faster as well with all thing being equal (ie both with USM): a 28-70 f/2.8 will make the camera focus faster than a 28-70 f/4.5-5.6 - This effect is like putting on darker sunglass vs a lighter shade... it get worst indoor, just like seeing behind sunglass indoor
-> A wider aperture lens also has more defocus info for the camera to work too
3. The camera detects which way to focus just like your eyes and 'command' the lens to turn just like your hand would with the manual focusing ring - If you don't see well you would be 'hunting' back and forth to determine optimum focus as well!

NHL is offline   Reply With Quote
Old Jan 6, 2005, 2:16 PM   #3
Senior Member
 
Join Date: Nov 2004
Posts: 378
Default

Plus, since the actual focus mechanism is in the lens, how the motor works will also affect the speed and (most certainty) loudness, right NHL?


perdendosi is offline   Reply With Quote
Old Jan 6, 2005, 3:31 PM   #4
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

Correct!

This is why most ultrasonic lenses are quick to react to each camera command since the drive is direct which each pulse. A lens with gearings inside take a lot more 'steps' to achieve the same amount of displacement - The gearings are also what prevent the lens from being full-time AF manual overide (and where the noise come from) since you are forcing the stepper motor through the reduction gears (ie excessive multiplying force), and the AF/M switch kind of disengages it like a clutch...

There are always a few exceptions:
1. My 85 f/1.2L is probably one of the brightest lens around combined with a ring USM turn out to be one of the slowest to focus :O
2. My 28-135 IS USM is probably one of the most noisy especially when its gyro kicks in! :?
NHL is offline   Reply With Quote
Old Jan 6, 2005, 5:00 PM   #5
Senior Member
 
aladyforty's Avatar
 
Join Date: Jan 2004
Posts: 6,964
Default

Ok thanks, You seem toknow a loton the subject:-)
aladyforty is offline   Reply With Quote
Old Jan 6, 2005, 5:05 PM   #6
Senior Member
 
Mr_Saginaw's Avatar
 
Join Date: Sep 2003
Posts: 552
Default

NHL wrote:
Quote:
Think about it this way:
1. How's your driving behind 'real' glasses vs kiddy plastic sunglass?
which is why i wear (shameless plug) Oakleys.... LOL! :?
Mr_Saginaw is offline   Reply With Quote
Old Jan 6, 2005, 11:41 PM   #7
Senior Member
 
Join Date: Dec 2002
Posts: 5,803
Default

Another bit of info just so people know it.

If you're using an f2.8 lens, but are taking the picture at f5.6 the camera is focusing while at f2.8. The lens is stopped down to f5.6 just before taking the picture.

This is why it doesn't matter what fstop you are using, what matters is what the largest aperture (smallest fstop) the lens is capable of. That is what is used when focusing.

Eric
eric s is offline   Reply With Quote
Old Jan 7, 2005, 8:38 AM   #8
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

Right on Eric!

Another incentive for people owning the 20D or higher to get at least an f/2.8 lens (if they can afford it), otherwise this extra nice feature of the superior high-precision AF sensor of their camera would go to waste...

Kind of make you wonder hey? when folks blame the lens for their front/back focus problems when it's actually the camera that does the focusing :?
-> People seem to notice this issue more on an f/2.8 since the DOF is so thin as compared to lenses with smaller apertures where the extra DOF of the lens just mask the whole problem out!

FYI - http://www.a1.nl/phomepag/markerink/af_expla.htm
NHL is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 12:14 PM.