From: Wilba on
Ofnuts wrote:
> Wilba wrote:
>>
>> The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
>> shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2)
>> erratic focus under difficult conditions (low light, shallow DOF).
>>
>> Many people claim that 2) is a result of the crudeness of the cheap
>> focussing motor and electronics in the lens, that those components are
>> not able to provide the required accuracy and control of motion of the
>> focus ring.
>>
>> But I wonder if 2) is actually a result of 1) - if the AF sensors have
>> fuzzy images to work with, how /could/ the system nail the focus in
>> difficult conditions?
>
> AF sensors work quite well with other lenses that are not that sharp, so I
> doubt that the "softness" of the lens at f/1.8 is really a culprit.
>
>> It would be interesting to see what happens when the AF sensors have
>> sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses
>> to AF when the DOF preview button is pressed, so I can't test that.
>> External aperture perhaps?
>
> Read this excellent explanation of the DSLR phase-detection AF system:
>
> <http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf>

It's a good document which has influenced my thinking.

> To make it short, for AF sensors:
> - they are designed to work with a minimum aperture (usually f/5.6 or
> better)(and don't benefit from a bigger one)
> - the more accurate you want the sensor, the wider the design aperture has
> to be.
>
> IIRC in the 450D most AF sensors require f/5.6 minimum, and the central
> one is doubled with a "bigger" one that requires f/2.8 and is put in
> action when the mounted lens reports that it has a maximum aperture of
> f/2.8 or better. This allows a more accurate focus with these lenses,
> which is required since the aperture of the lens can lead to very shallow
> DoF (some entry level DSLR haven't got that second AF sensor and cannot be
> efficiently used with lenses opening at f/2.8 or better).
>
>> Any ideas for how these competing hypotheses could be tested? Is there
>> a consequence of either hypothesis that could be disproved empirically?
>
> The 450D is an entry-level camera, so don't expect miracles. With the
> 50/1.8 the accuracy of its AF system may be a bit pushed to its limits.
> And make sure that you are using the central sensor for the the AF.

Thanks for sharing your thoughts. Unfortunately, they don't answer my
question.


From: Chris Malcolm on
Wilba <usenet(a)cutthisimago.com.au> wrote:
> Chris Malcolm wrote:
>> Wilba wrote:
>>>
>>> The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
>>> shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2)
>>> erratic focus under difficult conditions (low light, shallow DOF).
>>>
>>> Many people claim that 2) is a result of the crudeness of the cheap
>>> focussing motor and electronics in the lens, that those components are
>>> not able to provide the required accuracy and control of motion of the
>>> focus ring.
>>
>>> But I wonder if 2) is actually a result of 1) - if the AF sensors have
>>> fuzzy
>>> images to work with, how /could/ the system nail the focus in difficult
>>> conditions?
>>
>>> It would be interesting to see what happens when the AF sensors have
>>> sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses
>>> to AF when the DOF preview button is pressed, so I can't test that.
>>> External aperture perhaps?
>>>
>>> Any ideas for how these competing hypotheses could be tested? Is there
>>> a consequence of either hypothesis that could be disproved empirically?
>>
>> The AF sensors pay no attention to the aperture at which you're going
>> to take the picture. They do their work before the lens is stopped
>> down. Their construction gives them an effective aperture of their
>> own. Often this is around f6. That means that when the largest
>> aperture of a lens is smaller than that they can't get enough light to
>> work properly. That's why generally speaking you can't make reflex
>> lenses autofocus, because for technical reasons their best compromise
>> aperture is often smaller than that, e.g. 500mm f8.
>>
>> More expensive DSLRs will also have larger aperture AF sensors at the
>> central position, e.g. around f3, with which they'll be able to get
>> focus in lower light with lenses which with max apertures which open
>> that far. It also improves the focus on very fast lenses with
>> spherical aberration and corresponding aperture related focus drift,
>> such as the old spherical type of 50mm f1.4 lenses.
>>
>> Since the DOF gets very thin indeed at wide apertures and close
>> portrait type distances, which is often what is going on in a dimly
>> lit interior, the slightest error in AF will leave the image blurred
>> at the point you wished to focus on, and sharp nearby. For example in
>> a portrait you might have focused on the eyes, and find that the eyes
>> aren't in focus, but the tip of the nose, or the ears, are. The reason
>> for that is often that when DoF gets so sharp it becomes smaller than
>> the small residual error in the AF of your camera, i.e. your camera
>> has a slight front or back focus in the AF sensor plane calibration
>> which is larger than the DoF at these wide apertures.
>>
>> If you find a systematic error of this type in your camera than you
>> either must switch to manual focus, or compensate yourself, e.g. by
>> holding down focus on the eyes and then simply moving your head back
>> or forwards a few cm to take up the systematic error.
>>
>> Usually the more expensive DSLRs have better AF sensors so they can
>> focus better in lower light. The wider aperture AF sensors are also
>> able to get a tighter focus for wide aperture low light work because
>> the AF sensor itself has effectively a shallower DoF. That will also
>> rein in some of the aperture related focus drift of wide aperture
>> spherical lenses.
>>
>> The more expensive DSLRs are also sometimes able to read lens-specific
>> focus compensation factors from the lens, and use that to trim out
>> systematic errors in autofocus for that specific lens.
>>
>> The most expensive DSLRs go one better than that. They have user
>> trimmable tables of focus compensation for specific lenses in order to
>> get better focus with the more awkward lenses in the more awkward
>> situation, in which the AF will have slight lens-specific systematic
>> focus errors.

> Thanks for your effort. Unfortunately, there is no answer to my question
> within it. :- )

Your qestion doesn't make sense because you don't understand enough
about what might be going on with your specific camera and your
specific focus difficulties. Your questions need to be revised in the
light of an improved understanding, part of which must come from doing
some experiments of your own to discover exactly what with your
specific camera and focus problems the most important problems
are. For example, your problems could be simply not enough light for
your camera's AF sensors to work properly. If so there's nothing you
can do with the way you use your lens to improve that. But there are
several ways of helping your AF to work better in poor light. On the
other hand your difficulties could be due to a small AF calibration
error which starts to matter under those circumstances with your
lens. If so there are a number of things you can do about that.

But it's not worth going into all the details of all the possibilities
until you have found out more about which particular limitation of the
several possibilities your camera and lens are coming up against.

--
Chris Malcolm
From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> Chris Malcolm wrote:
>>> Wilba wrote:
>>>>
>>>> The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
>>>> shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2)
>>>> erratic focus under difficult conditions (low light, shallow DOF).
>>>>
>>>> Many people claim that 2) is a result of the crudeness of the cheap
>>>> focussing motor and electronics in the lens, that those components are
>>>> not able to provide the required accuracy and control of motion of the
>>>> focus ring.
>>>>
>>>> But I wonder if 2) is actually a result of 1) - if the AF sensors have
>>>> fuzzy images to work with, how /could/ the system nail the focus in
>>>> difficult conditions?
>>>>
>>>> It would be interesting to see what happens when the AF sensors have
>>>> sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses
>>>> to AF when the DOF preview button is pressed, so I can't test that.
>>>> External aperture perhaps?
>>>>
>>>> Any ideas for how these competing hypotheses could be tested? Is there
>>>> a consequence of either hypothesis that could be disproved empirically?
>>>
>>> The AF sensors pay no attention to the aperture at which you're going
>>> to take the picture. They do their work before the lens is stopped
>>> down. Their construction gives them an effective aperture of their
>>> own. Often this is around f6. That means that when the largest
>>> aperture of a lens is smaller than that they can't get enough light to
>>> work properly. That's why generally speaking you can't make reflex
>>> lenses autofocus, because for technical reasons their best compromise
>>> aperture is often smaller than that, e.g. 500mm f8.
>>>
>>> More expensive DSLRs will also have larger aperture AF sensors at the
>>> central position, e.g. around f3, with which they'll be able to get
>>> focus in lower light with lenses which with max apertures which open
>>> that far. It also improves the focus on very fast lenses with
>>> spherical aberration and corresponding aperture related focus drift,
>>> such as the old spherical type of 50mm f1.4 lenses.
>>>
>>> Since the DOF gets very thin indeed at wide apertures and close
>>> portrait type distances, which is often what is going on in a dimly
>>> lit interior, the slightest error in AF will leave the image blurred
>>> at the point you wished to focus on, and sharp nearby. For example in
>>> a portrait you might have focused on the eyes, and find that the eyes
>>> aren't in focus, but the tip of the nose, or the ears, are. The reason
>>> for that is often that when DoF gets so sharp it becomes smaller than
>>> the small residual error in the AF of your camera, i.e. your camera
>>> has a slight front or back focus in the AF sensor plane calibration
>>> which is larger than the DoF at these wide apertures.
>>>
>>> If you find a systematic error of this type in your camera than you
>>> either must switch to manual focus, or compensate yourself, e.g. by
>>> holding down focus on the eyes and then simply moving your head back
>>> or forwards a few cm to take up the systematic error.
>>>
>>> Usually the more expensive DSLRs have better AF sensors so they can
>>> focus better in lower light. The wider aperture AF sensors are also
>>> able to get a tighter focus for wide aperture low light work because
>>> the AF sensor itself has effectively a shallower DoF. That will also
>>> rein in some of the aperture related focus drift of wide aperture
>>> spherical lenses.
>>>
>>> The more expensive DSLRs are also sometimes able to read lens-specific
>>> focus compensation factors from the lens, and use that to trim out
>>> systematic errors in autofocus for that specific lens.
>>>
>>> The most expensive DSLRs go one better than that. They have user
>>> trimmable tables of focus compensation for specific lenses in order to
>>> get better focus with the more awkward lenses in the more awkward
>>> situation, in which the AF will have slight lens-specific systematic
>>> focus errors.
>>
>> Thanks for your effort. Unfortunately, there is no answer to my question
>> within it. :- )
>
> Your qestion doesn't make sense because you don't understand enough
> about what might be going on with your specific camera and your
> specific focus difficulties. Your questions need to be revised in the
> light of an improved understanding, part of which must come from doing
> some experiments of your own to discover exactly what with your
> specific camera and focus problems the most important problems
> are. For example, your problems could be simply not enough light for
> your camera's AF sensors to work properly. If so there's nothing you
> can do with the way you use your lens to improve that. But there are
> several ways of helping your AF to work better in poor light. On the
> other hand your difficulties could be due to a small AF calibration
> error which starts to matter under those circumstances with your
> lens. If so there are a number of things you can do about that.
>
> But it's not worth going into all the details of all the possibilities
> until you have found out more about which particular limitation of the
> several possibilities your camera and lens are coming up against.

I'm not having focus difficulties. The question is stated in the 5th
paragraph. All the rest is preamble.


From: rwalker on
On Mon, 21 Dec 2009 14:04:56 +0800, "Wilba"
<usenet(a)CUTTHISimago.com.au> wrote:

snip

>
>Yeah, we're not all called Bruce.
>
Only the philosophers.
From: Paul Furman on
Wilba wrote:
> The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two
> shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic
> focus under difficult conditions (low light, shallow DOF).
>
> Many people claim that 2) is a result of the crudeness of the cheap
> focussing motor and electronics in the lens, that those components are not
> able to provide the required accuracy and control of motion of the focus
> ring.
>
> But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy
> images to work with, how /could/ the system nail the focus in difficult
> conditions?
>
> It would be interesting to see what happens when the AF sensors have sharper
> images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when
> the DOF preview button is pressed, so I can't test that. External aperture
> perhaps?
>
> Any ideas for how these competing hypotheses could be tested? Is there a
> consequence of either hypothesis that could be disproved empirically?

If you are using the center AF point, sharpness shouldn't be a problem.
In fact the shallower DOF should improve accuracy.

One factor could be the phenomenon where focus shifts when stopping
down, but I suspect they've included corrections for that in the
firmware. There could also be issues with curvature of field, if you are
concerned about something off to the side, etc.

--
Paul Furman
www.edgehill.net
www.baynatives.com

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