K-mount lenses on istDs.... probably stupid question
Posted 12/04/2006 - 17:50
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Just to make sure that we really are off topic...
Many years ago I worked as an Assistant Film Cameraman at the BBC using both 16mm and 35mm motion picture cameras. Depth of field tables are essential in the film industry because you are 'pulling focus' all the time and it might be unnecessary to pull the focus ring from 3 feet to infinity, say, with a resultant change in angle of view when by using hyperfocal distances you might not have to 'pull' very much or at all. The drill when setting up the camera for each shot was amongst other things to always set the lens to the hyperfocal distance. However a word of caution, depth of field tables either in printed form or the slide rule type give vastly different figures depending on manufacturer and what size of circle of confusion is being used in their calculations. Best to err on the pessimistic figures I think. On manual focus lenses I've developed the habit of storing them set at the hyperfocal distance for the max. aperture so that you can very quickly grab a shot.
Ken
Many years ago I worked as an Assistant Film Cameraman at the BBC using both 16mm and 35mm motion picture cameras. Depth of field tables are essential in the film industry because you are 'pulling focus' all the time and it might be unnecessary to pull the focus ring from 3 feet to infinity, say, with a resultant change in angle of view when by using hyperfocal distances you might not have to 'pull' very much or at all. The drill when setting up the camera for each shot was amongst other things to always set the lens to the hyperfocal distance. However a word of caution, depth of field tables either in printed form or the slide rule type give vastly different figures depending on manufacturer and what size of circle of confusion is being used in their calculations. Best to err on the pessimistic figures I think. On manual focus lenses I've developed the habit of storing them set at the hyperfocal distance for the max. aperture so that you can very quickly grab a shot.
Ken
Ken
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -
Posted 12/04/2006 - 18:10
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Mannesty, I don't quite follow that hypothesis myself!
If it were true, surely it could only be guaranteed to work properly with genuine Pentax lenses and even then, it would mean that you could never actually achieve perfect focus in AF mode.
All lenses (Pentax and non-Pentax) I've tried on my DL have focussed perfectly and in the same way as I would expect with my Z1-P.
Also, there would be no need for AF sensors in the camera because it would 'know' what lens was fitted and how much focus adjustment would be needed for the chosen aperture.
As it is, the cameras use contrast levels to ascertain focus - wouldn't this be pointless if they were to automatically ignore the focal distance that resolves the highest contrast, in favour for one that delivers greater DOF?
If it were true, surely it could only be guaranteed to work properly with genuine Pentax lenses and even then, it would mean that you could never actually achieve perfect focus in AF mode.
All lenses (Pentax and non-Pentax) I've tried on my DL have focussed perfectly and in the same way as I would expect with my Z1-P.
Also, there would be no need for AF sensors in the camera because it would 'know' what lens was fitted and how much focus adjustment would be needed for the chosen aperture.
As it is, the cameras use contrast levels to ascertain focus - wouldn't this be pointless if they were to automatically ignore the focal distance that resolves the highest contrast, in favour for one that delivers greater DOF?
Die my dear doctor, that's the last thing I shall do!
Posted 12/04/2006 - 18:24
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A further thought has just occurred to me:
I'm not aware of any lens that transmits it's focusing distance to the camera (ie. how far away the subject is) so each time you focus the lens, the camera would first have to adjust it to either the near or distant limit in order to calculate the amount of rotation required in the opposite direction for it to achieve the hyperfocal distance.
If however the distance were known, then the lens would only change focus when you changed the aperture and not if the subject moved, or you would have to manually adjust the focus to shift the DOF - which is not the case!
I'm not aware of any lens that transmits it's focusing distance to the camera (ie. how far away the subject is) so each time you focus the lens, the camera would first have to adjust it to either the near or distant limit in order to calculate the amount of rotation required in the opposite direction for it to achieve the hyperfocal distance.
If however the distance were known, then the lens would only change focus when you changed the aperture and not if the subject moved, or you would have to manually adjust the focus to shift the DOF - which is not the case!
Die my dear doctor, that's the last thing I shall do!
Posted 13/04/2006 - 11:50
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just to though a bit of petrol on the fire
auto focus cameras regardless of make don't focus perfectly !!!
allow me to explain before you all jump up and down
because thay use contrast to focus the lenses arn't ground as fine as the old smc lenses which are manually focused
this is why i use old smc lenes and not auto focus lenses
yes the focus is (technically) perfect with auto focus cameras but if you look at trannys with a 10x lupi thay are not sharp
if auto focus lenes were as sharp as the old smc lenes the auto focus system in the modern camera would be unable to focus
Pentax in their wisdom use an auto focus chip that can use the older lenses and although the pentax chip may not be as fast as N C and M their cameras carnt use old lenses hence why thay all changed lens mounts
this is why PENTAX SMC lenses are the finest in the world and will never be bettered by the oposition
anyway off the soap box
tanya please dont be daunted by the technical jargon and if we can help in the future please ask and the welth of experiance on this site of your fellow users will help
cheers ali
auto focus cameras regardless of make don't focus perfectly !!!
allow me to explain before you all jump up and down
because thay use contrast to focus the lenses arn't ground as fine as the old smc lenses which are manually focused
this is why i use old smc lenes and not auto focus lenses
yes the focus is (technically) perfect with auto focus cameras but if you look at trannys with a 10x lupi thay are not sharp
if auto focus lenes were as sharp as the old smc lenes the auto focus system in the modern camera would be unable to focus
Pentax in their wisdom use an auto focus chip that can use the older lenses and although the pentax chip may not be as fast as N C and M their cameras carnt use old lenses hence why thay all changed lens mounts
this is why PENTAX SMC lenses are the finest in the world and will never be bettered by the oposition
anyway off the soap box
tanya please dont be daunted by the technical jargon and if we can help in the future please ask and the welth of experiance on this site of your fellow users will help
cheers ali
Posted 13/04/2006 - 13:40
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Getting waaay off topic here
You can tell the *ist-D focusses at the hyperfocal distance when you use a parafocal lens. The focus is always off when you zoom, and it shouldn't be. Try the lens on a film Pentax (like the Z1) and the parafocal lens is fine.
It wasn't actually me that said it, but Pete (who runs this site and ePhotozine) who was told direct by Pentax. Whether it holds true for all firmware issues, I don't know.
Generally I do trust the AF, using only centre spot focussing. I'm too short sighted to get a good lock normally, unless I have an age to get the focus right (used the SMC-A 50/1.4 and missed too many shots). But then, yes, the AF does get it wrong at times!
Matt
Quote:
the mighty Matt
ROFL! the mighty Matt
You can tell the *ist-D focusses at the hyperfocal distance when you use a parafocal lens. The focus is always off when you zoom, and it shouldn't be. Try the lens on a film Pentax (like the Z1) and the parafocal lens is fine.
It wasn't actually me that said it, but Pete (who runs this site and ePhotozine) who was told direct by Pentax. Whether it holds true for all firmware issues, I don't know.
Generally I do trust the AF, using only centre spot focussing. I'm too short sighted to get a good lock normally, unless I have an age to get the focus right (used the SMC-A 50/1.4 and missed too many shots). But then, yes, the AF does get it wrong at times!
Matt
Posted 13/04/2006 - 15:01
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Well I still can't see it but I suppose Pentax may have designed cameras with multi-point AF to select an average between the distances detected.
When the Canon EOS5 was tested, the user reported that the camera often decided to focus sharply on a point that was not the most important one and he had to manually select the one he wanted.
Perhaps Pentax decided that this was not the best solution to speeding up the process of point-and-shoot (the purpose of multi-AF) and chose instead to average out the distances and thereby maximize the zone of apparent sharpness - essentially hyper-focal focussing!
Switching to single (spot) AF, sets the camera to proper AF mode and acts like the earlier SAFOX systems, otherwise there'd be no need for the camera to have a built-in AF system at all!
As for AF lenses being deliberately designed to be less sharp than their earlier counterparts, I can't quite see that either and I don't think the MTF charts support that theory.
When the Canon EOS5 was tested, the user reported that the camera often decided to focus sharply on a point that was not the most important one and he had to manually select the one he wanted.
Perhaps Pentax decided that this was not the best solution to speeding up the process of point-and-shoot (the purpose of multi-AF) and chose instead to average out the distances and thereby maximize the zone of apparent sharpness - essentially hyper-focal focussing!
Switching to single (spot) AF, sets the camera to proper AF mode and acts like the earlier SAFOX systems, otherwise there'd be no need for the camera to have a built-in AF system at all!
As for AF lenses being deliberately designed to be less sharp than their earlier counterparts, I can't quite see that either and I don't think the MTF charts support that theory.
Die my dear doctor, that's the last thing I shall do!
Posted 13/04/2006 - 15:45
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Multi-point autofocus is one of the most amazingly usless inventions in the history of technology. The camera doesn't know what you're trying to focus on!
Stick witgh single point focus or manual, you'll save yourslf a lot of grief.
Stick witgh single point focus or manual, you'll save yourslf a lot of grief.
42 
Posted 13/04/2006 - 16:36
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It's not down to multipoint AF. It's a question of how the SAFOX focusses. You'll notice that the *ist-D "knows" what lens is connected. Each lens has a 2-byte ID. In theory, it is possible for the camera to compensate for the given focal length/lens/aperture combination and focus on the HF point.
Maybe that's why some third party lenses misfocus? (Their ID reads as a Pentax lens, and the body focusses according to its table of values). Just a hunch, of course
Matt
Maybe that's why some third party lenses misfocus? (Their ID reads as a Pentax lens, and the body focusses according to its table of values). Just a hunch, of course
Matt
Posted 13/04/2006 - 17:02
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Quote:
It's not down to multipoint AF. It's a question of how the SAFOX focusses.
Matt
Brings me back to my point - why bother with a contrast detecting AF system if it'll be over-ruled by an algorithm that give precedence to the lens information?It's not down to multipoint AF. It's a question of how the SAFOX focusses.
Matt
Pentax design their cameras for use primarily with their own lenses and the *D, being pro-spec., is hardly a 'point and shoot' model.
I can see that being biased toward HF focussing could be a useful optional mode (like the Z1s MTF function) but having a top-line auto-focus camera that is set that way by default, must surely be a clear sign of 'shooting one's self in the foot'
Die my dear doctor, that's the last thing I shall do!
Posted 13/04/2006 - 17:05
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kimbo wrote
As for AF lenses being deliberately designed to be less sharp than their earlier counterparts, I can't quite see that either and I don't think the MTF charts support that theory.
all of the photographic press i.e practical photography amater photography digital photography the list goes on and on have at some point said "that autofocus lenses dont focus as sharp as manual focus lenses due to the fact that thay use contrast
maybe pete could shed some light on this as he used to be with PP
As for AF lenses being deliberately designed to be less sharp than their earlier counterparts, I can't quite see that either and I don't think the MTF charts support that theory.
all of the photographic press i.e practical photography amater photography digital photography the list goes on and on have at some point said "that autofocus lenses dont focus as sharp as manual focus lenses due to the fact that thay use contrast
maybe pete could shed some light on this as he used to be with PP
Posted 13/04/2006 - 17:08
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Quote:
I can see that being biased toward HF focussing could be a useful optional mode (like the Z1s MTF function)
I quite agree Kimbo! But then I haven't done any further tests lately, and am not even sure if I have any parafocal zooms left.I can see that being biased toward HF focussing could be a useful optional mode (like the Z1s MTF function)
Perhaps there's some test that could be setup to try this out?
Matt
Posted 13/04/2006 - 18:25
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Quote:
all of the photographic press i.e practical photography amater photography digital photography the list goes on and on have at some point said "that autofocus lenses dont focus as sharp as manual focus lenses due to the fact that thay use contrast
maybe pete could shed some light on this as he used to be with PP
I must confess that most of my mags. are from the late '80s to mid '90s but I don't recall any articles to that effect. all of the photographic press i.e practical photography amater photography digital photography the list goes on and on have at some point said "that autofocus lenses dont focus as sharp as manual focus lenses due to the fact that thay use contrast
maybe pete could shed some light on this as he used to be with PP
Some early AF lenses felt sloppy because of the new light-weight polycarbonate materials and the internal focussing mechanisms meant that they didn't feel as 'tight' but optically, they were pretty much on a par with equivalent MF lenses.
Pentax certainly haven't compromised with their primes and why have such good AF lenses if the camera is not going to focus them precisely?
I'm not saying that AF cameras are perfect and no matter how good the AF system is, it may be possible to improve the result with a little manual adjustment but I'm sure that we wouldn't be getting such good results if a) the lens was deliberately unsharp and b) the camera chose hyper-focussing.
I'd certainly be interested to see the results of a proper test
Die my dear doctor, that's the last thing I shall do!
Posted 13/04/2006 - 19:20
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Quote:
Multi-point autofocus is one of the most amazingly usless inventions in the history of technology. The camera doesn't know what you're trying to focus on!
Stick witgh single point focus or manual, you'll save yourslf a lot of grief.
These are true words of wisdom. AF is OK in a happy snap situation but why would you let something so crucial as focus be left to the camera (and a lot of very knowledgable people on this forum are unclear or are in disagreement about what the camera is actually doing). You wouldn't let it choose your exposure, would you???Multi-point autofocus is one of the most amazingly usless inventions in the history of technology. The camera doesn't know what you're trying to focus on!
Stick witgh single point focus or manual, you'll save yourslf a lot of grief.
Ken
Ken
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -
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7650 posts
21 years
Totana,
Spain.
In the post at https://www.pentaxuser.com/forum/viewtopic.php?t=1555&highlight=hyperfocal+d... mattmatic wrote that he thought the *ist-D focussed at the hyperfocal distance. Can that be true?