Discussion
I took some photos at the weekend and noticed some blemishes which I put down to a dusty sensor. Today, I used a blower on the sensor then popped outside to take some more photos to see if things had improved.
The wind had conveniently blown away most of the clouds to leave a nice clear blue sky - trouble was, the autofocus would not lock on the blue distant sky and the camera refused to take a picture. If I captured a bit of cloud, no problem. Is this normal, a camera issue, me, or the lens, do I have to use manual focus?
Any help would be much appreciated (this is my first photo post!). Camera is a 350D.
Bob
The wind had conveniently blown away most of the clouds to leave a nice clear blue sky - trouble was, the autofocus would not lock on the blue distant sky and the camera refused to take a picture. If I captured a bit of cloud, no problem. Is this normal, a camera issue, me, or the lens, do I have to use manual focus?
Any help would be much appreciated (this is my first photo post!). Camera is a 350D.
Bob
trackdemon said:
Doesn't AF work by bouncing an IR beam of the subject and measuring distance beam travelled using electromagic*? Hence the sky - not being a solid object - is impossible to use AF on?
If so it would work on plain surfaces like blank walls, but it doesn't. I don't know exactly how AF works but it DOES need texture/contrast to lock on to.
Sorry for the hijack...
Am I correct in assuming that autofocus doesnt work between frames on the continuous drive mode of digital SLR's?
Autofocus on my G5 is piss-poor in low light conditions. Is this the fault of the lens or the camera (just wondering how much better I can expect it to be on modern digital SLR)
I cant remember where/what I read, but I stumbled upon comments people made about autofocus not working with their cameras. Again, I assume that this only applies when one buys a sigma lens for a canon camera, for example. If you're a canon owner, what do you need to look out for when buying a lens, just the EF or EFS? All lenses except primes have autofocus, right?
Am I correct in assuming that autofocus doesnt work between frames on the continuous drive mode of digital SLR's?
Autofocus on my G5 is piss-poor in low light conditions. Is this the fault of the lens or the camera (just wondering how much better I can expect it to be on modern digital SLR)
I cant remember where/what I read, but I stumbled upon comments people made about autofocus not working with their cameras. Again, I assume that this only applies when one buys a sigma lens for a canon camera, for example. If you're a canon owner, what do you need to look out for when buying a lens, just the EF or EFS? All lenses except primes have autofocus, right?
On many compact cameras IR bouncing used to be used, not sure what the state of the art is these days as I don't have a modern compact . . .
however . . .
modern DSLR's do not use IR bouncing to range find and focus they use a system that looks at contrast - sharp things being more contrasty put very simply . . . they do this by having a sensor array (just like the imaging sensor - but not as high resolution and not linearly distributed in a simple array) which can determine this contrast from the image through the viewfinder . . . This sensor recieves it's feed of light via a semi-silvered mirror (which partially polarises the light) hence you should use a circular polariser else you may lose AF and metering (IIRS the same sensor is used for metering in many cameras).
Depends on the camera and how it's configured . . .
Cannot remember with the D70 but my D2X does perform autofocus continually during continuous drive at up to 8fps, it can be configured to only fire the shutter if in focus, or just fire it anyway - and all this relies on you selecting continuous focus in the first place . . . so there's plenty of opportunity to screw things up from an operator point of view . . . and 'claim' the camera doesn't work properly . . . not enough people RTFM . . .
Anyway - it works very well, in low light where I use a wide aperture and a shallow DOF on a long lens I use focus priority, which means only fire shutter when you have a focus lock, and you can hear the shutter cycling at a variable rate as the camera refocuses between frames . . . in brighter light I switch this off as the DOF is sufficient to mean precise focus may not be required . . . also depends on how fast things are moving . . . and how fast your lens focuses . . .
however . . .
modern DSLR's do not use IR bouncing to range find and focus they use a system that looks at contrast - sharp things being more contrasty put very simply . . . they do this by having a sensor array (just like the imaging sensor - but not as high resolution and not linearly distributed in a simple array) which can determine this contrast from the image through the viewfinder . . . This sensor recieves it's feed of light via a semi-silvered mirror (which partially polarises the light) hence you should use a circular polariser else you may lose AF and metering (IIRS the same sensor is used for metering in many cameras).
UKBob said:
Am I correct in assuming that autofocus doesnt work between frames on the continuous drive mode of digital SLR's?
Depends on the camera and how it's configured . . .
Cannot remember with the D70 but my D2X does perform autofocus continually during continuous drive at up to 8fps, it can be configured to only fire the shutter if in focus, or just fire it anyway - and all this relies on you selecting continuous focus in the first place . . . so there's plenty of opportunity to screw things up from an operator point of view . . . and 'claim' the camera doesn't work properly . . . not enough people RTFM . . .
Anyway - it works very well, in low light where I use a wide aperture and a shallow DOF on a long lens I use focus priority, which means only fire shutter when you have a focus lock, and you can hear the shutter cycling at a variable rate as the camera refocuses between frames . . . in brighter light I switch this off as the DOF is sufficient to mean precise focus may not be required . . . also depends on how fast things are moving . . . and how fast your lens focuses . . .
UKBob said:
Am I correct in assuming that autofocus doesnt work between frames on the continuous drive mode of digital SLR's?
Yep. Your focal point will be the same for as long as you hold the shutter release.
UKBob said:
Autofocus on my G5 is piss-poor in low light conditions. Is this the fault of the lens or the camera (just wondering how much better I can expect it to be on modern digital SLR)
Autofocus is as much a function of the lens as it is the camera. Some lenses have mechanical focus driven by a screw on the body of the camera, some have faster internal focusing (the naming convention is different for every manufacturer - Nikon is AF-S.)
UKBob said:
I cant remember where/what I read, but I stumbled upon comments people made about autofocus not working with their cameras. Again, I assume that this only applies when one buys a sigma lens for a canon camera, for example. If you're a canon owner, what do you need to look out for when buying a lens, just the EF or EFS? All lenses except primes have autofocus, right?
All "AF" lenses have autofocus including primes. only "MF" lenses don't have it. EF/EFS is I suspect as above - the difference between the windy screw which is slow and noisy, or the whizzy internal method which is fast and quiet... Whichever is more expensive is the clever/faster/quieter one.
[quote=fergusd]On many compact cameras IR bouncing used to be used, not sure what the state of the art is these days as I don't have a modern compact . . .
however . . .
modern DSLR's do not use IR bouncing to range find and focus they use a system that looks at contrast - sharp things being more contrasty put very simply . . .
Thankyou again, this would also explain why I can photograph an Elephant Hawk Moth in the dusk/dark, with my Sony compact, but really struggle with the DSLR
.
however . . .
modern DSLR's do not use IR bouncing to range find and focus they use a system that looks at contrast - sharp things being more contrasty put very simply . . .
Thankyou again, this would also explain why I can photograph an Elephant Hawk Moth in the dusk/dark, with my Sony compact, but really struggle with the DSLR
. simpo two said:
As I understand it, 'IF' or Internal Focusing simply means the front element doesn't rotate during focusing. Silent-wave focusing is a different aspect, be it AF-S (Nikon) USM (Canon) or HSM (Sigma). Such lenses carry the focusing motor inside so there's no mecahnical link from body to lens.
Indeed, also worth noting that the camera body (in the case of a non AFS/USM/HSM/TLA lens) plays quite a large part in how fast the focussing actually takes place . . .
simpo two said:
Yep. Your focal point will be the same for as long as you hold the shutter release.
Seems unlikely when using focus tracking (AF-C or Continuous Servo mode)
_dobbo_ said:
Some lenses have mechanical focus driven by a screw on the body of the camera, some have faster internal focusing (the naming convention is different for every manufacturer - Nikon is AF-S.)
As I understand it, 'IF' or Internal Focusing simply means the front element doesn't rotate during focusing. Silent-wave focusing is a different aspect, be it AF-S (Nikon) USM (Canon) or HSM (Sigma). Such lenses carry the focusing motor inside so there's no mecahnical link from body to lens.
Sorry, what I meant when I said internal focusing is that it's not powered by the screw on the camera body. However "internal focusing" has a specific meaning in camera nomenclature and I should have used another term!
simpo two said:
Yep. Your focal point will be the same for as long as you hold the shutter release.
Seems unlikely when using focus tracking (AF-C or Continuous Servo mode)
Now you are moving the goalposts!
Probably not, I haven't tried it, but you would doubtless sacrifice some FPS in order to use AF-C, and given when you are holding the shutter down it's usually with the view of capturing as many frames in as short a time as possible, AF-C would reduce the efficacy of this.
_dobbo_ said:
you would doubtless sacrifice some FPS in order to use AF-C
What you sacrifice (in my experience at least) is focusing accuracy - the camera will fire whether or not the target is perfectly in focus. At least, that's the case on an F70 and D70, as tested with head on shots of a running dog - the tracking couldn't keep up. This is a pretty extreme text which I'm sure better/more expensive camera/lens combos would improve on.
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