High ISO noise CCD's vs CMOS

"Wolfgang Weisselberg" wrote:

I was responding to Doug McDonald: "Sometimes even with the f/2.8
super lens, wide open, there is not enough light at a shutter
speed short enough to get rid of subject movement."
(Message ID: )

I was trying to show that f/2.8 was _not_ the lower end at all.

True enough, at least for moderate focal lengths - it's tough to come by
anything below f/2.8 at 200mm. And I'd still observe that even with
2.0, 1.7, or even 1.4, there are many shooting situations where there
isn't enough light to get rid of subject movement, unless you start
looking at ISO of 1600 and above.

---------------
Marc Sabatella


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Wolfgang Weisselberg wrote:
AaronW wrote:
Wolfgang Weisselberg wrote:
Marc Sabatella wrote:
b) the DOF at apertures below 2 is so shallow that this might not yield
acceptable pictures of some scenes

True, but it WILL help focussing (manually and AF) even in the
dark.

I have a lot of AF error with 50/1.4, while 50/1.8 AF much better.

I have no AF problems with the f/1.4, even in very low light.

Under good conditions, the 50/1.4 AF fine, so the lens is not
defective.

Good conditions != wide open?

No. Sometimes the AF errors are huge that even stopped down shots are
out of focus.

Examples of bad condition: backlit, high flare, low contrast, etc. But
I use the 50/1.4 and 50/1.8 under the same conditions. With 50/1.8, I
have very few AF errors. While with 50/1.4, about half of the shots
have AF errors. But the other half look good, so I think it is not a
defective lens, but a design problem.

I think it is because AF is done at wide open, and 50/1.4
wide open at f/1.4 is softer than 50/1.8 at f/1.8.

Then you'd also find the problems with many of the other
f/1.4 and f/1.2 lenses.

I don't have any other f/1.4 or larger lens. I'd think that 85/1.2 has
the same problem. I read that some other people have this problem with
85/1.2, 50/1.4, and 50/1.0.

The softer image
makes it more difficult for AF sensors to work correctly.

Since the sensors work from the de-focussed lens image and even
work with tele lenses (where they have much less 'sharp' images to
work from if the focus is way off), I don't really buy that theory.

Maybe the small lenses in front of the AF sensors focus it for them?
Like the split images in manual focus are sharp themselves even though
the lens is out of focus.

If they really work from defocussed images, I can not believe that it
can focus accurately from vastly out of focus images. They have to get
more accurate reading when it gets close. So the lens sharpness when
wide open will affect AF accuracy when it gets close.

http://digitcamera.tripod.com/#slr

mswlogo wrote:


But I just can't get paste this ISO thing

Then buy the canon.

For me things like color saturation and overall image quality is what
matters but most gearheads seem OBCESSED with shooting at ISO 1600. Go join
their club!

--

Stacey

Marc Sabatella wrote:
"Wolfgang Weisselberg" wrote:

I was trying to show that f/2.8 was _not_ the lower end at all.

True enough, at least for moderate focal lengths - it's tough to come by
anything below f/2.8 at 200mm.

You can always try for a 200mm f/1.8. :-)

And I'd still observe that even with
2.0, 1.7, or even 1.4, there are many shooting situations where there
isn't enough light to get rid of subject movement, unless you start
looking at ISO of 1600 and above.

Yep.


-Wolfgang

AaronW wrote:
Wolfgang Weisselberg wrote:
AaronW wrote:
Wolfgang Weisselberg wrote:
Marc Sabatella wrote:
b) the DOF at apertures below 2 is so shallow that this might not yield
acceptable pictures of some scenes

True, but it WILL help focussing (manually and AF) even in the
dark.

I have a lot of AF error with 50/1.4, while 50/1.8 AF much better.

I have no AF problems with the f/1.4, even in very low light.

Under good conditions, the 50/1.4 AF fine, so the lens is not
defective.

Good conditions != wide open?

No. Sometimes the AF errors are huge that even stopped down shots are
out of focus.

Examples of bad condition: backlit, high flare, low contrast, etc.

Ok, that is hard for the AF.

But
I use the 50/1.4 and 50/1.8 under the same conditions. With 50/1.8, I
have very few AF errors. While with 50/1.4, about half of the shots
have AF errors. But the other half look good, so I think it is not a
defective lens, but a design problem.

Ok, so you are saying, with the 50/1.4 you have 50% chance of AF
error and 5% with the 1.8, right? And you say that it misfocusses
even enough to see it well stopped down.

Now, if that was a design problem, I'd see the very same with
my 50/1.4, at least when shooting in the dark w/o focus assist,
so we can probably rule out a design problem.

It could be that on your camera the AF sensors are not exactly
mounted where the focus points are. If that was the case, you'd
probably focus on the background instead of the subject.

I think it is because AF is done at wide open, and 50/1.4
wide open at f/1.4 is softer than 50/1.8 at f/1.8.

Then you'd also find the problems with many of the other
f/1.4 and f/1.2 lenses.

I don't have any other f/1.4 or larger lens. I'd think that 85/1.2 has
the same problem. I read that some other people have this problem with
85/1.2, 50/1.4, and 50/1.0.

With the minimal DOF of these lenses, it's very easy to focus on
the center of the eye --- or an eyelash --- and have the iris be
out of focus (at least on the computer screen). Tiny movements of
the photographer or the object are getting very important there.
Focus and recompose _will_ show up, as the DOF plane is slanted
and might well move completely behind the focussed-on area.

The softer image
makes it more difficult for AF sensors to work correctly.

Since the sensors work from the de-focussed lens image and even
work with tele lenses (where they have much less 'sharp' images to
work from if the focus is way off), I don't really buy that theory.

Maybe the small lenses in front of the AF sensors focus it for them?

Nope. Unless the lens has another AF (with a lens that is focussed
by yet another AF? And wouldn't that need yet another lens that
focusses?), it cannot focus in the AF sense of the word.

At best it could stop down a lot, but that would not help the AF
(if it's sharp, it's sharp, what else can the AF say?).

And would not ever confer better focus abilities for fast lenses,
yet at least for it's top of the line models Canon claims
exactly that.

Like the split images in manual focus are sharp themselves even though
the lens is out of focus.

That's because your eye _can_ compensate with it's AF. The
eye cannot compensate to a ground glass, as the glass itself
is forming the image; what's OOF there cannot be corrected
with any AF.

If they really work from defocussed images, I can not believe that it
can focus accurately from vastly out of focus images.

You may observe focus hunting when you use a long fast lens
and have the focus completely off.

They have to get
more accurate reading when it gets close. So the lens sharpness when
wide open will affect AF accuracy when it gets close.

In digital P&S cameras, it's easy: move the lens and see if
the AF areas get more or less hard borders. Continue in the
right direction until you are better than some pre-set limit.
The very short lenses (e.g. 6-18mm) necessitated by the small
sensors help thanks to their deep DOF.

In SLRs (and DSLRs), this is ... different. You cannot read
the sensor. And you want to be _much_ faster.

So what they use is conceptionally a split image, backed by a
small, 1x200 or so sensor on each side. These can, even when the
image is defocussed, usually still detect patterns (vertical or
horizontal, or both, depending on the camera and which of it's
sensors it is).

Then they can see how much said pattern is shifted on the other
sensor. That difference tells them exactly in which direction
and how much to move the lens; the lens is ordered into that
position and it is made sure that the position is, indeed, reached.
However, there is no second read from the sensor. (At least that
is how I undertand Canon works.)

If no pattern can be detected (too dark, too low contrast, long
tele very defocussed, ...), the lens may be moved through the whole
focus range in the hope of picking up something in the sensors.

Fast lenses obviously help the sensors (more light, for one).

If the lens was 'too fast' for the sensor, the AF may be off a
bit, but I still don't see how the lens would misfocus more than
a slower lens (when stopped down to be identical, at least).

A softer lens would produce more, well, softened patterns on
the sensor, still, the sensor is _used_ to softened patterns
(after all, we call that bukeh ... and usually train the AF on
unfocussed parts). Worst case: it starts hunting.

OK, the passive AF sensor is bad with repeating and weak
patterns.

-Wolfgang

Wolfgang Weisselberg wrote:
AaronW wrote:
Wolfgang Weisselberg wrote:
AaronW wrote:
Wolfgang Weisselberg wrote:
Marc Sabatella wrote:
b) the DOF at apertures below 2 is so shallow that this might not yield
acceptable pictures of some scenes

True, but it WILL help focussing (manually and AF) even in the
dark.

I have a lot of AF error with 50/1.4, while 50/1.8 AF much better.

I have no AF problems with the f/1.4, even in very low light.

Under good conditions, the 50/1.4 AF fine, so the lens is not
defective.

Good conditions != wide open?

No. Sometimes the AF errors are huge that even stopped down shots are
out of focus.

Examples of bad condition: backlit, high flare, low contrast, etc.

Ok, that is hard for the AF.

But I get much better AF with 50/1.8 under the same situations.

But
I use the 50/1.4 and 50/1.8 under the same conditions. With 50/1.8, I
have very few AF errors. While with 50/1.4, about half of the shots
have AF errors. But the other half look good, so I think it is not a
defective lens, but a design problem.

Ok, so you are saying, with the 50/1.4 you have 50% chance of AF
error and 5% with the 1.8, right? And you say that it misfocusses
even enough to see it well stopped down.

Now, if that was a design problem, I'd see the very same with
my 50/1.4, at least when shooting in the dark w/o focus assist,
so we can probably rule out a design problem.

I get about 50% good AF with 50/1.4. I don't think I can get that many
accidentally good AF with a defective lens. And I don't doubt you can
get a lot of good AF shots with your lens. And maybe your camera is
better.

It could be that on your camera the AF sensors are not exactly
mounted where the focus points are. If that was the case, you'd
probably focus on the background instead of the subject.

Many of the shots don't have anything in focus in the whole frame. It
is possible that the AF points are not accurate, e.g., when I point AF
to one eye it may actually measuring a spot next to the eye that has
low contrast. But the AF confirmation light shows AF lock. And again,
50/1.8 AF much better than 50/1.4 on the same camera, in the same
situations.

I think it is because AF is done at wide open, and 50/1.4
wide open at f/1.4 is softer than 50/1.8 at f/1.8.

Then you'd also find the problems with many of the other
f/1.4 and f/1.2 lenses.

I don't have any other f/1.4 or larger lens. I'd think that 85/1.2 has
the same problem. I read that some other people have this problem with
85/1.2, 50/1.4, and 50/1.0.

With the minimal DOF of these lenses, it's very easy to focus on
the center of the eye --- or an eyelash --- and have the iris be
out of focus (at least on the computer screen). Tiny movements of
the photographer or the object are getting very important there.
Focus and recompose _will_ show up, as the DOF plane is slanted
and might well move completely behind the focussed-on area.

The AF errors are much larger than DoF, even when stopped down.

The softer image
makes it more difficult for AF sensors to work correctly.

Since the sensors work from the de-focussed lens image and even
work with tele lenses (where they have much less 'sharp' images to
work from if the focus is way off), I don't really buy that theory.

Maybe the small lenses in front of the AF sensors focus it for them?

Nope. Unless the lens has another AF (with a lens that is focussed
by yet another AF? And wouldn't that need yet another lens that
focusses?), it cannot focus in the AF sense of the word.

At best it could stop down a lot, but that would not help the AF
(if it's sharp, it's sharp, what else can the AF say?).

And would not ever confer better focus abilities for fast lenses,
yet at least for it's top of the line models Canon claims
exactly that.

They have AF sensors spread further apart for f/2.8 lenses, in addition
to the closer f/5.6 AF sensors. So if each side has its own lens, then
the small AF sensor lenses won't affect the wider sensors being
activated by f/2.8 lenses.

http://digitcamera.tripod.com/#slr