Apparent Depth of field with Canon 20D

W wrote:
Very interesting. I am not sure I understand the explanation. How do
uninterrupted rays get focused in the eye?

Without the focusing screen, looking thru the viewfinder would be like
looking thru a monocular of 1:1 magnification. For a 50mm objective and
50mm ocular your eyes will always see an image formed by f10 lens
(50mm/5mm iris opening). That's the limitation of our iris - the same
width the iris opening is, the same width of lens aperture is used. The
rays outside this aperture fall ON the iris and are wasted. So even if
you had a 50mm 0.5 lens, the image would be no brighter and DOF the
same as at f10.
It's like looking thru a magnifying glass, which is exactly what the
viewfinder is, only a very small one. Image forming rays of one detail
pass only thru a small portion of the glass that has the same width as
the opening of our iris, so a very big part of the magnifying glass is
not used for that detail.
"Uninterrupted" rays are just like that, only a lot dimmer, since most
of the rays form an image on the screen.

With the help of the focusing screen you can observe all the rays,
because the image is actualy formed as if it was printed there. Idealy
that would be so, but in reality it isn't.

It's just a guess, I don't see any other explanation for it.


Bhogi wrote:
W wrote:
Folks,

I recently did a shot where I bracketed f-stops to tailor depth of
field. The shot was of eye glasses sitting on a page of print. I was
surprised to see that the viewfinder image appears to have way more
depth of field than the actual photographed image (20D with 50mm f
1.4). With the lens wide open (f/1.4), I could almost read the print on
the page (focus was above the plane of the page on the eyeglasses) when
looking through the viewfinder. On the photographed image, the page was
a total blur. It seems the viewfinder appeared to have significantly
greater DOF than the actual image. Any ideas why this would be?

I observed this with the 50mm 1.8 on 20d. I think the reason might be
that the focusing screen lets some of the image forming rays pass thru
uninterrupted.

To approximate, the image of the 50mm in the viewfinder of a DSLR is
magnifyed 1:1 so the viewfinder also has 50mm focal length. This means
a very limited actual aperture of say 5mm (taking the iris in account),
if there was no focusing screen. This gives an actual f10, not f1.8!
I tryed that on some trees in the distance while focusing on 0.45m.
There were no trees in the photo at 1.8 at all, while at f10, the shape
resembled what I saw in the viewfinder only not in such detail.

This means it's a combination of the two "images" we are observing in
the viewfinder, so it should appear sharper than the real image formed,
and it realy does.

That makes sense, the majority of light rays form the image on the
screen, but the ones that pass are still enough to increase the DOF.

W wrote:
The focusing screen is the standard 20D screen (not removable). Not
sure exactly the type but it does not appear to be 'pure' ground glass
screen.
Taking a picture through the viewfinder that's too hard . But I can
assure you I can read the text through the viewfinder and not on the
captured image.

I think it's called "laser matte" screen and it's supposed to be
brighter than ordinary ground glass. I think that means it redirects
more light in the general direction of the viewfinder. Ground glass
perhaps casts light in all directions and so wastes brightness.


Dave Martindale wrote:
"W" writes:
Okay, I repeated this experiment using careful focusing, mirror lockup,
and cable release. The effect I saw was indeed real. I shot at f/1.4
with a 50mm f1.4 lens. The shot was of a pair of folded glasses sitting
on a page of small text. The focus was on the glasses probably about an
inch to inch and a half above the page. The camera was 2 to 3 feet away
from the subject. I could read the text when looking through the
viewfinder but it was out of focus and unreadable in the captured
image.

What kind of focusing screen does the camera have? If it's not a ground
glass, you may not be seeing the effect of all of the light that comes
through the lens. If light from the outer edges of the lens doesn't
actually make it into your eye, the effect is as if the lens had a
smaller aperture (and more DOF). So what you see doesn't match what
the film sees.

I am not sure how to post the image here. Also, that would be of little
use because I cannot post what I actually see through the viewfinder.

You *could* take a picture of what's seen in the viewfinder by using a
second camera to shoot into the viewfinder.

Dave

Take a look here
http://www.mir.com.my/rb/photography...ns/screens.htm
If the focusing screen is realy shaped like that "honeycomb", it's not
difficult to see it passes uninterrupted rays at regular intervals. But
I think the situation is much more complex than that.


Bhogi wrote:
W wrote:
Very interesting. I am not sure I understand the explanation. How do
uninterrupted rays get focused in the eye?

Without the focusing screen, looking thru the viewfinder would be like
looking thru a monocular of 1:1 magnification. For a 50mm objective and
50mm ocular your eyes will always see an image formed by f10 lens
(50mm/5mm iris opening). That's the limitation of our iris - the same
width the iris opening is, the same width of lens aperture is used. The
rays outside this aperture fall ON the iris and are wasted. So even if
you had a 50mm 0.5 lens, the image would be no brighter and DOF the
same as at f10.
It's like looking thru a magnifying glass, which is exactly what the
viewfinder is, only a very small one. Image forming rays of one detail
pass only thru a small portion of the glass that has the same width as
the opening of our iris, so a very big part of the magnifying glass is
not used for that detail.
"Uninterrupted" rays are just like that, only a lot dimmer, since most
of the rays form an image on the screen.

With the help of the focusing screen you can observe all the rays,
because the image is actualy formed as if it was printed there. Idealy
that would be so, but in reality it isn't.

It's just a guess, I don't see any other explanation for it.


Bhogi wrote:
W wrote:
Folks,

I recently did a shot where I bracketed f-stops to tailor depth of
field. The shot was of eye glasses sitting on a page of print. I was
surprised to see that the viewfinder image appears to have way more
depth of field than the actual photographed image (20D with 50mm f
1.4). With the lens wide open (f/1.4), I could almost read the print on
the page (focus was above the plane of the page on the eyeglasses) when
looking through the viewfinder. On the photographed image, the page was
a total blur. It seems the viewfinder appeared to have significantly
greater DOF than the actual image. Any ideas why this would be?

I observed this with the 50mm 1.8 on 20d. I think the reason might be
that the focusing screen lets some of the image forming rays pass thru
uninterrupted.

To approximate, the image of the 50mm in the viewfinder of a DSLR is
magnifyed 1:1 so the viewfinder also has 50mm focal length. This means
a very limited actual aperture of say 5mm (taking the iris in account),
if there was no focusing screen. This gives an actual f10, not f1.8!
I tryed that on some trees in the distance while focusing on 0.45m.
There were no trees in the photo at 1.8 at all, while at f10, the shape
resembled what I saw in the viewfinder only not in such detail.

This means it's a combination of the two "images" we are observing in
the viewfinder, so it should appear sharper than the real image formed,
and it realy does.

That makes sense, the majority of light rays form the image on the
screen, but the ones that pass are still enough to increase the DOF.

"W" wrote in message
oups.com...
The focusing screen is the standard 20D screen (not removable). Not
sure exactly the type but it does not appear to be 'pure' ground glass
screen.
Taking a picture through the viewfinder that's too hard . But I can
assure you I can read the text through the viewfinder and not on the
captured image.

Dave Martindale wrote:

FYI the screen is perfectly removeable, it's just a bit fiddly because the
screen release tab doesn't project so far out that you can see it. The tab
is still there, though, you can get at it with a flat screwdriver.

Third-party focusing screens with various manual-focusing aides are
available for the 20D. The "focus pop" varies, depending on the type of
screen... the 20D is designed for autofocus so it actually makes a bit of
sense for Canon to sell it with a focusing screen that doesn't blur things
too much. Easier to use for the newbies and so on.

Interesting, I was not aware that you could remove it and that there
were alternatives. Personally, I would like to have a split image
"focusing aid" in the center as I tend to manually focus for "tripod"
shots. Also, I find it interesting that the stock screen significantly
misleads the user in terms of depth of field. Isn't the point of an SLR
that you see the image you will capture?

Ståle Sannerud wrote:
"W" wrote in message
oups.com...
The focusing screen is the standard 20D screen (not removable). Not
sure exactly the type but it does not appear to be 'pure' ground glass
screen.
Taking a picture through the viewfinder that's too hard . But I can
assure you I can read the text through the viewfinder and not on the
captured image.

Dave Martindale wrote:

FYI the screen is perfectly removeable, it's just a bit fiddly because the
screen release tab doesn't project so far out that you can see it. The tab
is still there, though, you can get at it with a flat screwdriver.

Third-party focusing screens with various manual-focusing aides are
available for the 20D. The "focus pop" varies, depending on the type of
screen... the 20D is designed for autofocus so it actually makes a bit of
sense for Canon to sell it with a focusing screen that doesn't blur things
too much. Easier to use for the newbies and so on.

Thanks for the info. Very interesting. I will from now on beware that
what I see in the viewfinder can vary significantly in terms of depth
of field from what the camera will end up capturing ( ).

Bhogi wrote:
W wrote:
Very interesting. I am not sure I understand the explanation. How do
uninterrupted rays get focused in the eye?

Without the focusing screen, looking thru the viewfinder would be like
looking thru a monocular of 1:1 magnification. For a 50mm objective and
50mm ocular your eyes will always see an image formed by f10 lens
(50mm/5mm iris opening). That's the limitation of our iris - the same
width the iris opening is, the same width of lens aperture is used. The
rays outside this aperture fall ON the iris and are wasted. So even if
you had a 50mm 0.5 lens, the image would be no brighter and DOF the
same as at f10.
It's like looking thru a magnifying glass, which is exactly what the
viewfinder is, only a very small one. Image forming rays of one detail
pass only thru a small portion of the glass that has the same width as
the opening of our iris, so a very big part of the magnifying glass is
not used for that detail.
"Uninterrupted" rays are just like that, only a lot dimmer, since most
of the rays form an image on the screen.

With the help of the focusing screen you can observe all the rays,
because the image is actualy formed as if it was printed there. Idealy
that would be so, but in reality it isn't.

It's just a guess, I don't see any other explanation for it.


Bhogi wrote:
W wrote:
Folks,

I recently did a shot where I bracketed f-stops to tailor depth of
field. The shot was of eye glasses sitting on a page of print. I was
surprised to see that the viewfinder image appears to have way more
depth of field than the actual photographed image (20D with 50mm f
1.4). With the lens wide open (f/1.4), I could almost read the print on
the page (focus was above the plane of the page on the eyeglasses) when
looking through the viewfinder. On the photographed image, the page was
a total blur. It seems the viewfinder appeared to have significantly
greater DOF than the actual image. Any ideas why this would be?

I observed this with the 50mm 1.8 on 20d. I think the reason might be
that the focusing screen lets some of the image forming rays pass thru
uninterrupted.

To approximate, the image of the 50mm in the viewfinder of a DSLR is
magnifyed 1:1 so the viewfinder also has 50mm focal length. This means
a very limited actual aperture of say 5mm (taking the iris in account),
if there was no focusing screen. This gives an actual f10, not f1.8!
I tryed that on some trees in the distance while focusing on 0.45m.
There were no trees in the photo at 1.8 at all, while at f10, the shape
resembled what I saw in the viewfinder only not in such detail.

This means it's a combination of the two "images" we are observing in
the viewfinder, so it should appear sharper than the real image formed,
and it realy does.

That makes sense, the majority of light rays form the image on the
screen, but the ones that pass are still enough to increase the DOF.

W wrote:
Interesting, I was not aware that you could remove it and that there
were alternatives. Personally, I would like to have a split image
"focusing aid" in the center as I tend to manually focus for "tripod"
shots. Also, I find it interesting that the stock screen significantly
misleads the user in terms of depth of field. Isn't the point of an SLR
that you see the image you will capture?

Katz Eye Optics http://www.katzeyeoptics.com/ make focusing screens
for most DSLR cameras.
I use one in my Pentax *ist-Ds and it's pretty bright, clear and the
split prism and collar make manual focusing so much easier.

In article . com, W
writes
Folks,

I recently did a shot where I bracketed f-stops to tailor depth of
field. The shot was of eye glasses sitting on a page of print. I was
surprised to see that the viewfinder image appears to have way more
depth of field than the actual photographed image (20D with 50mm f
1.4). With the lens wide open (f/1.4), I could almost read the print on
the page (focus was above the plane of the page on the eyeglasses) when
looking through the viewfinder. On the photographed image, the page was
a total blur. It seems the viewfinder appeared to have significantly
greater DOF than the actual image. Any ideas why this would be?


That is a very common, but rarely discussed, disadvantage of "bright"
focussing screens which prevents the SLR from actually achieving one of
its main objectives - showing the viewfinder image "as is".

An ideal focus screen would accept all of the rays forming the image
from the lens and project them equally into the eyepiece so that you
could see the image exactly as it was formed on the sensor.
Unfortunately it is only possible to achieve a rather crude
approximation of that ideal where the focus screen scatters the image
forming rays equally in all directions. Some of these rays then reach
the eyepiece and hence the eye. Without such scattering, rays from the
periphery of the lens would miss the eyepiece and hence would not form
any of the image that you see in the viewfinder. Only rays coming from
the central portion of the lens would reach the eyepiece lens and hence
form the viewfinder image. Since the screen has scattered all of the
image forming rays, you see a relatively faithful reproduction of the
image from all parts of the objective lens - you see light from the
peripheral parts of the lens just as much as light from the central
parts and hence get a view of the depth of field of image. The downside
of this is, of course, that the highly scattering focus screen means
that most of the light doesn't reach the eyepiece and hence although the
image is reasonably accurate, it is fairly dark.

Brighter focus screens scatter less of the image, so that more of the
light reaches the eyepiece directly, however this is at the expense of
more of that light coming from the centre of the lens and less coming
from edges, which must be deflected to reach the eyepiece. The downside
is that the image in the viewfinder, despite being brighter, actually
corresponds more to the image from the stopped down lens than it does to
the wide open aperture that the lens uses in normal viewing. You will
still see a darkening of the screen when the lens is stopped down,
because some of those peripheral rays are still scattered towards the
eyepiece, but the darkening is much less than would be produced from an
older, darker, conventional ground glass screen.

If you remove the focus screen entirely, you get a very bright image
indeed, but completely lose the ability to assess DOF in the image. In
addition, the image will not darken as you stop the lens down until you
reach very high f/#s.

The image in the viewfinder of the SLR is only an approximation of what
the sensor sees. How good an approximation depends - advantages in some
aspects can be traded off against disadvantages in others. People
generally value the brighter viewfinder at the expense of DOF accuracy
for AF cameras. However these focus screens are almost useless for
manual focus lenses because they only show a "stopped down" DOF. Hence,
for many of their interchangeable SLR cameras, Canon also offer manual
focus screens (which scatter more of the peripheral rays into the
viewfinder hence producing an image more representative of the open
aperture). These manual focus screens usually have the caveat that the
viewfinder image is darker than the standard screen.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

Kennedy McEwen wrote:
In article . com, W
writes
Folks,

I recently did a shot where I bracketed f-stops to tailor depth of
field. The shot was of eye glasses sitting on a page of print. I was
surprised to see that the viewfinder image appears to have way more
depth of field than the actual photographed image (20D with 50mm f
1.4). With the lens wide open (f/1.4), I could almost read the print on
the page (focus was above the plane of the page on the eyeglasses) when
looking through the viewfinder. On the photographed image, the page was
a total blur. It seems the viewfinder appeared to have significantly
greater DOF than the actual image. Any ideas why this would be?


That is a very common, but rarely discussed, disadvantage of "bright"
focussing screens which prevents the SLR from actually achieving one of
its main objectives - showing the viewfinder image "as is".

An ideal focus screen would accept all of the rays forming the image
from the lens and project them equally into the eyepiece so that you
could see the image exactly as it was formed on the sensor.
Unfortunately it is only possible to achieve a rather crude
approximation of that ideal where the focus screen scatters the image
forming rays equally in all directions. Some of these rays then reach
the eyepiece and hence the eye. Without such scattering, rays from the
periphery of the lens would miss the eyepiece and hence would not form
any of the image that you see in the viewfinder. Only rays coming from
the central portion of the lens would reach the eyepiece lens and hence
form the viewfinder image. Since the screen has scattered all of the
image forming rays, you see a relatively faithful reproduction of the
image from all parts of the objective lens - you see light from the
peripheral parts of the lens just as much as light from the central
parts and hence get a view of the depth of field of image. The downside
of this is, of course, that the highly scattering focus screen means
that most of the light doesn't reach the eyepiece and hence although the
image is reasonably accurate, it is fairly dark.

Brighter focus screens scatter less of the image, so that more of the
light reaches the eyepiece directly, however this is at the expense of
more of that light coming from the centre of the lens and less coming
from edges, which must be deflected to reach the eyepiece. The downside
is that the image in the viewfinder, despite being brighter, actually
corresponds more to the image from the stopped down lens than it does to
the wide open aperture that the lens uses in normal viewing. You will
still see a darkening of the screen when the lens is stopped down,
because some of those peripheral rays are still scattered towards the
eyepiece, but the darkening is much less than would be produced from an
older, darker, conventional ground glass screen.

If you remove the focus screen entirely, you get a very bright image
indeed, but completely lose the ability to assess DOF in the image. In
addition, the image will not darken as you stop the lens down until you
reach very high f/#s.

The image in the viewfinder of the SLR is only an approximation of what
the sensor sees. How good an approximation depends - advantages in some
aspects can be traded off against disadvantages in others. People
generally value the brighter viewfinder at the expense of DOF accuracy
for AF cameras. However these focus screens are almost useless for
manual focus lenses because they only show a "stopped down" DOF. Hence,
for many of their interchangeable SLR cameras, Canon also offer manual
focus screens (which scatter more of the peripheral rays into the
viewfinder hence producing an image more representative of the open
aperture). These manual focus screens usually have the caveat that the
viewfinder image is darker than the standard screen.

Wow. And thank you. I think I now understand both the question and answer.

But I am not agreeing to sitting for an exam here, though!

--
John McWilliams

Interesting and thanks. I had always thought that when I used my lens
wide or close to wide open that the depth of field in the captured
image seemed less than what I percieved when I shot the image. I had
always attributed that to the fact that I would examine the image at
larger magnification and with more scrutiny than when I shot it (let's
say quickly for a street shot). When I shot the image I described
(glasses above text), I did so very carefully. It was then that I
realized that there was more than meets the eye (literally in this
case!).
Thanks for the enlighting explanation(s)!

Kennedy McEwen wrote:
In article . com, W
writes
Folks,

I recently did a shot where I bracketed f-stops to tailor depth of
field. The shot was of eye glasses sitting on a page of print. I was
surprised to see that the viewfinder image appears to have way more
depth of field than the actual photographed image (20D with 50mm f
1.4). With the lens wide open (f/1.4), I could almost read the print on
the page (focus was above the plane of the page on the eyeglasses) when
looking through the viewfinder. On the photographed image, the page was
a total blur. It seems the viewfinder appeared to have significantly
greater DOF than the actual image. Any ideas why this would be?


That is a very common, but rarely discussed, disadvantage of "bright"
focussing screens which prevents the SLR from actually achieving one of
its main objectives - showing the viewfinder image "as is".

An ideal focus screen would accept all of the rays forming the image
from the lens and project them equally into the eyepiece so that you
could see the image exactly as it was formed on the sensor.
Unfortunately it is only possible to achieve a rather crude
approximation of that ideal where the focus screen scatters the image
forming rays equally in all directions. Some of these rays then reach
the eyepiece and hence the eye. Without such scattering, rays from the
periphery of the lens would miss the eyepiece and hence would not form
any of the image that you see in the viewfinder. Only rays coming from
the central portion of the lens would reach the eyepiece lens and hence
form the viewfinder image. Since the screen has scattered all of the
image forming rays, you see a relatively faithful reproduction of the
image from all parts of the objective lens - you see light from the
peripheral parts of the lens just as much as light from the central
parts and hence get a view of the depth of field of image. The downside
of this is, of course, that the highly scattering focus screen means
that most of the light doesn't reach the eyepiece and hence although the
image is reasonably accurate, it is fairly dark.

Brighter focus screens scatter less of the image, so that more of the
light reaches the eyepiece directly, however this is at the expense of
more of that light coming from the centre of the lens and less coming
from edges, which must be deflected to reach the eyepiece. The downside
is that the image in the viewfinder, despite being brighter, actually
corresponds more to the image from the stopped down lens than it does to
the wide open aperture that the lens uses in normal viewing. You will
still see a darkening of the screen when the lens is stopped down,
because some of those peripheral rays are still scattered towards the
eyepiece, but the darkening is much less than would be produced from an
older, darker, conventional ground glass screen.

If you remove the focus screen entirely, you get a very bright image
indeed, but completely lose the ability to assess DOF in the image. In
addition, the image will not darken as you stop the lens down until you
reach very high f/#s.

The image in the viewfinder of the SLR is only an approximation of what
the sensor sees. How good an approximation depends - advantages in some
aspects can be traded off against disadvantages in others. People
generally value the brighter viewfinder at the expense of DOF accuracy
for AF cameras. However these focus screens are almost useless for
manual focus lenses because they only show a "stopped down" DOF. Hence,
for many of their interchangeable SLR cameras, Canon also offer manual
focus screens (which scatter more of the peripheral rays into the
viewfinder hence producing an image more representative of the open
aperture). These manual focus screens usually have the caveat that the
viewfinder image is darker than the standard screen.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)