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#331
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EF 50/1.8 AF Experiment?
MikeWhy wrote:
I swear by gumb. 40x0.1mm should be cause enough to wrap you in padding and committed. I can't empathize, but I'm pleased to see an active curiousity live on in the this world. I had a spare 1/2 hour and gear that would do it, so why not? :- ) For a short time, depending on how the forum moderators feel about appropriateness, a no effort nifty-fifty shot is at: http://photo.net/no-words-forum/00W8Os. Nothing special; this quality of focus is routine. It was just happened to be up in LR as your last post popped into my newsreader. Anyone can do that at f/4. :-D |
#332
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EF 50/1.8 AF Experiment?
Wilba wrote:
Chris Malcolm wrote: Wilba wrote: Chris Malcolm wrote: Wilba wrote: The central "enhanced precision" PD AF sensor is sufficiently confused by the fuzziness caused by spherical aberation for it to confirm focus over a range of camera to subject distances of the order of the minimum DOF (widest aperture, closest focus distance). The fuzziness caused by spherical aberration is caused by the fact the perfect lens shape is only nearly spherical. So in a spherical lens the rays from the lens edge focus at a different point than those from the centre. But this won't cause any fuzziness in a PDAF sensor because that works by taking two small samples of rays from two spaced apart little apertures equally spaced about the centre. The distance between them is in effect like the baseline of a rangefinder. That distance apart also corresponds to an aperture of that diameter. So the PDAF can't work with lenses of smaller aperture than the width apart of the AF sensor pair. Therefore in a spherical lens the AF sensor finds exact focus at the point where the rays from that particular part of the spherical lens surface focuses. But the image is taken at the specified aperture. So if it is stopped down from the effective AF aperture the focus will drift one way. That probably won't matter because typically the DoF is increased a lot more than the drift. If on the other hand the image taking aperture is larger than the effective AF aperture, as it can be with wide aperture lenses, then the focus drifts the other way, while at the same time the DoF is narrowing. In this case the resultant focus drift can take the point of focus outside the DoF. Yes, that's as I've understood it since before I began this thread. Can you use the above to explain why an initial gross front-focus results in optimal focus, and an initial gross back-focus results in front-focus (with my calibrated 450D and 50/1.8)? Everything is identical in both cases except for the spatial relationship of the subject and the plane of focus prior to PD AF being invoked. It doesn't matter whether I use f/1.8 (the widest aperture of the lens), or f/2.8 (the rangefinder base aperture of the PD AF sensor), throughout an individual set of front and back-focussed test exposures, I still get the same pattern of optimal focus or front-focus. How does aperture-related focus shift explain that phenomenon? That baffles me. It doesn't happen with my 50mm f1.4 lens. Which bit? _That_ it happens with the 50/1.8, or _why_ it happens? Both. -- Chris Malcolm |
#333
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EF 50/1.8 AF Experiment?
Chris Malcolm wrote:
Wilba wrote: Chris Malcolm wrote: Wilba wrote: Chris Malcolm wrote: Wilba wrote: The central "enhanced precision" PD AF sensor is sufficiently confused by the fuzziness caused by spherical aberation for it to confirm focus over a range of camera to subject distances of the order of the minimum DOF (widest aperture, closest focus distance). The fuzziness caused by spherical aberration is caused by the fact the perfect lens shape is only nearly spherical. So in a spherical lens the rays from the lens edge focus at a different point than those from the centre. But this won't cause any fuzziness in a PDAF sensor because that works by taking two small samples of rays from two spaced apart little apertures equally spaced about the centre. The distance between them is in effect like the baseline of a rangefinder. That distance apart also corresponds to an aperture of that diameter. So the PDAF can't work with lenses of smaller aperture than the width apart of the AF sensor pair. Therefore in a spherical lens the AF sensor finds exact focus at the point where the rays from that particular part of the spherical lens surface focuses. But the image is taken at the specified aperture. So if it is stopped down from the effective AF aperture the focus will drift one way. That probably won't matter because typically the DoF is increased a lot more than the drift. If on the other hand the image taking aperture is larger than the effective AF aperture, as it can be with wide aperture lenses, then the focus drifts the other way, while at the same time the DoF is narrowing. In this case the resultant focus drift can take the point of focus outside the DoF. Yes, that's as I've understood it since before I began this thread. Can you use the above to explain why an initial gross front-focus results in optimal focus, and an initial gross back-focus results in front-focus (with my calibrated 450D and 50/1.8)? Everything is identical in both cases except for the spatial relationship of the subject and the plane of focus prior to PD AF being invoked. It doesn't matter whether I use f/1.8 (the widest aperture of the lens), or f/2.8 (the rangefinder base aperture of the PD AF sensor), throughout an individual set of front and back-focussed test exposures, I still get the same pattern of optimal focus or front-focus. How does aperture-related focus shift explain that phenomenon? That baffles me. It doesn't happen with my 50mm f1.4 lens. Which bit? _That_ it happens with the 50/1.8, or _why_ it happens? Both. And that's what it's all about. :- ) I think you hinted at an explanation a couple of posts ago - "Therefore in a spherical lens the AF sensor finds exact focus at the point where the rays from that particular part of the spherical lens surface focuses." If the rays from that particular part of the lens surface _cannot_be_ exactly focused, then how likely is it that exact focus in the centre of the image can be achieved using those rays? I haven't seen a better explanation. |
#334
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EF 50/1.8 AF Experiment?
"Wilba" wrote in message
... MikeWhy wrote: I swear by gumb. 40x0.1mm should be cause enough to wrap you in padding and committed. I can't empathize, but I'm pleased to see an active curiousity live on in the this world. I had a spare 1/2 hour and gear that would do it, so why not? :- ) That was 4 months ago. For a short time, depending on how the forum moderators feel about appropriateness, a no effort nifty-fifty shot is at: http://photo.net/no-words-forum/00W8Os. Nothing special; this quality of focus is routine. It was just happened to be up in LR as your last post popped into my newsreader. Anyone can do that at f/4. :-D My point exactly. |
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