Why don't all lenses have infinity exactly on the infinity stop?

newarts · 01-12-2011, 12:34 PM

Originally posted by grainbelt

....
It really doesn't have to be as complicated as all the math posted above makes it out to be.

Right. You can hide the math; then it is a matter of faith in the guy behind the curtain

I was estimating how precise lens positioning must be for "in-focus" conditions which requires math (as does calculating hyperfocal conditions.)

Dave

Last edited by newarts; 01-12-2011 at 12:41 PM.

RioRico · 01-12-2011, 12:47 PM

Originally posted by sewebster

the DOF scales for the old lenses are for 24x36mm frames on film, and with the APS-C sensors you get less DOF (all else being equal...)

A handy ROT (rule of thumb): Each time sensor area shrinks by 1/2, adjust the aperture by one f-stop (plus a tad for paranoia). So, using a manual 135/FF lens of any focal length on an APS-C camera, if you want DOF to extend to infinity at f/11, focus so infinity is a bit inside f/8 on the lens' DOF scale. With that same lens on an m4/3 camera, set infinity just inside f/5.6, like around f/5.

Example: With a Tokina 21/3.8 on my K20D set to f/11, I set infinity below the f/8 mark; hyperfocal is 6 feet, and DOF is 3 feet to infinity.

Entropy · 01-13-2011, 11:53 AM - **1 Like**

Originally posted by peterh337

How to DSLRs autofocus?

I assumed they drive the focus motor while operating the sensor in video mode, and looking at the spectrum of the video coming out of the selected spot(s) on the sensor, and they drive the motor so that the high frequency components of the spectrum are maximised. Then you have the sharpest focus.

It will always involve a bit of hunting around the optimum point, but this will never be 'perfect'. At some stage the camera has to say 'this is good enough'.

That's how point-and-shoots autofocus, and DSLRs in "live view AF" mode. It's called contrast detection autofocus.

DSLRs in normal mode focus in the same manner as classic autofocus SLRs (it's called "phase-detect autofocus" - with specialized sensors in the mirror box. These sensors operate on the same principle as a split screen focus aid in the viewfinder. (Which is why the have the same aperture limitations - the situations where phase-detect AF fails are similar to the ones where splitscreens "go dark" despite a brightly lit scene.)

As to why infinity is not where the stop is - That was covered above - it is to handle thermal expansion/contraction of the lens.

Brangdon · 01-14-2011, 05:00 PM

One reason to allow the lens to focus beyond infinity, not already mentioned, is auto-focus. If you change (auto) focal distance from close-focus to infinity, the motor moves the lens fast and then tends to over-shoot the required focal distance and has to be wound back. If there was a hard, physical stop at infinity, the lens would slam up against it instead of over-shooting. That would be bad for the mechanism; it's better to let it slow down gradually, and having some play beyond infinity allows that.

So far as I know, older manual-focus lens generally won't focus beyond infinity. It's these new-fangled auto-focus lenses that have the issue.

sewebster · 01-15-2011, 08:53 PM

Originally posted by Brangdon

One reason to allow the lens to focus beyond infinity, not already mentioned, is auto-focus. If you change (auto) focal distance from close-focus to infinity, the motor moves the lens fast and then tends to over-shoot the required focal distance and has to be wound back. If there was a hard, physical stop at infinity, the lens would slam up against it instead of over-shooting. That would be bad for the mechanism; it's better to let it slow down gradually, and having some play beyond infinity allows that.

So far as I know, older manual-focus lens generally won't focus beyond infinity. It's these new-fangled auto-focus lenses that have the issue.

My FA50 does the hard BANG into the focus stop all the time

Maybe that's why I had to send it in to have the infinity stop adjusted...

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01-12-2011, 12:34 PM	#16
newarts Inactive Account Join Date: Dec 2008 Location: Ames, Iowa, USA Photos: Albums Posts: 2,965	Originally posted by grainbelt .... It really doesn't have to be as complicated as all the math posted above makes it out to be. Right. You can hide the math; then it is a matter of faith in the guy behind the curtain I was estimating how precise lens positioning must be for "in-focus" conditions which requires math (as does calculating hyperfocal conditions.) Dave Last edited by newarts; 01-12-2011 at 12:41 PM.

01-12-2011, 12:47 PM	#17
RioRico Veteran Member Join Date: Jun 2008 Location: Limbo, California Posts: 11,263	Originally posted by sewebster the DOF scales for the old lenses are for 24x36mm frames on film, and with the APS-C sensors you get less DOF (all else being equal...) A handy ROT (rule of thumb): Each time sensor area shrinks by 1/2, adjust the aperture by one f-stop (plus a tad for paranoia). So, using a manual 135/FF lens of any focal length on an APS-C camera, if you want DOF to extend to infinity at f/11, focus so infinity is a bit inside f/8 on the lens' DOF scale. With that same lens on an m4/3 camera, set infinity just inside f/5.6, like around f/5. Example: With a Tokina 21/3.8 on my K20D set to f/11, I set infinity below the f/8 mark; hyperfocal is 6 feet, and DOF is 3 feet to infinity.

01-13-2011, 11:53 AM - 1 Like	#18
Entropy Veteran Member Join Date: Jun 2007 Location: Owego, NY Posts: 976	Originally posted by peterh337 How to DSLRs autofocus? I assumed they drive the focus motor while operating the sensor in video mode, and looking at the spectrum of the video coming out of the selected spot(s) on the sensor, and they drive the motor so that the high frequency components of the spectrum are maximised. Then you have the sharpest focus. It will always involve a bit of hunting around the optimum point, but this will never be 'perfect'. At some stage the camera has to say 'this is good enough'. That's how point-and-shoots autofocus, and DSLRs in "live view AF" mode. It's called contrast detection autofocus. DSLRs in normal mode focus in the same manner as classic autofocus SLRs (it's called "phase-detect autofocus" - with specialized sensors in the mirror box. These sensors operate on the same principle as a split screen focus aid in the viewfinder. (Which is why the have the same aperture limitations - the situations where phase-detect AF fails are similar to the ones where splitscreens "go dark" despite a brightly lit scene.) As to why infinity is not where the stop is - That was covered above - it is to handle thermal expansion/contraction of the lens.

01-14-2011, 05:00 PM	#19
Brangdon Senior Member Join Date: Aug 2010 Location: Nottingham Posts: 172	One reason to allow the lens to focus beyond infinity, not already mentioned, is auto-focus. If you change (auto) focal distance from close-focus to infinity, the motor moves the lens fast and then tends to over-shoot the required focal distance and has to be wound back. If there was a hard, physical stop at infinity, the lens would slam up against it instead of over-shooting. That would be bad for the mechanism; it's better to let it slow down gradually, and having some play beyond infinity allows that. So far as I know, older manual-focus lens generally won't focus beyond infinity. It's these new-fangled auto-focus lenses that have the issue.

01-15-2011, 08:53 PM	#20
sewebster Veteran Member Join Date: Jun 2008 Location: Vancouver, BC Photos: Gallery Posts: 544	Originally posted by Brangdon One reason to allow the lens to focus beyond infinity, not already mentioned, is auto-focus. If you change (auto) focal distance from close-focus to infinity, the motor moves the lens fast and then tends to over-shoot the required focal distance and has to be wound back. If there was a hard, physical stop at infinity, the lens would slam up against it instead of over-shooting. That would be bad for the mechanism; it's better to let it slow down gradually, and having some play beyond infinity allows that. So far as I know, older manual-focus lens generally won't focus beyond infinity. It's these new-fangled auto-focus lenses that have the issue. My FA50 does the hard BANG into the focus stop all the time Maybe that's why I had to send it in to have the infinity stop adjusted...