Digital Rebel Disappointment

"acl" wrote in message
...

As I said, there was a long and tedious thread about some time ago.

Briefly: You have a smaller f (by a factor A, where A the crop factor bet
the two cameras). This gives a larger DOF for the same f-stop. But you can
stop down so that you have the exact DOF. But then you have a slower
shutter speed. So youn increase the ISO to compensate. It's not hard to
show that you will get no worse noise (assuming only photon noise) this
way.

I think you're either reading a lot into what I'm saying, or we're talking
about two different things. Simply put, can a 7mm lens and a 19mm lens at
the same aperture offer the same depth of field?

I understand that you can stop down on the SLR, but if you want to read a
ton of variables in to the situation, you'll never be able to answer the
question.



Maybe I should repeat it again: You will have to stop down to do this, and
increase ISO to keep the shutter speed constant. This is what I said from
the beginning.

"J. Clarke" wrote in message
...
On Wed, 06 Dec 2006 12:26:56 -0500, Kinon O'Cann wrote:


Isn't DOF also a function of the focal length of the lens, as well as the
aperture?

It's also a function of the resolution of the sensor.

How?



http://clarkvision.com/photoinfo/dof_myth/index.html

David J. Littleboy
Tokyo, Japan




--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

"Kinon O'Cann" writes:

"acl" wrote in message
...

As I said, there was a long and tedious thread about some time ago.

Briefly: You have a smaller f (by a factor A, where A the crop factor bet
the two cameras). This gives a larger DOF for the same f-stop. But you can
stop down so that you have the exact DOF. But then you have a slower
shutter speed. So youn increase the ISO to compensate. It's not hard to
show that you will get no worse noise (assuming only photon noise) this
way.

I think you're either reading a lot into what I'm saying, or we're talking
about two different things. Simply put, can a 7mm lens and a 19mm lens at
the same aperture offer the same depth of field?

If the camera with the 19mm lens has a much larger circle of
confusion, then yes. The circle of confusion depends on the pixel
size of the sensor (or grain size of film) and the intended size of
prints. With real-world sensors and prints, the DoF with the 19mm
lens is usually smaller.

--
Måns Rullgård

Kinon O'Cann wrote:
"acl" wrote in message
...

As I said, there was a long and tedious thread about some time ago.

Briefly: You have a smaller f (by a factor A, where A the crop factor bet
the two cameras). This gives a larger DOF for the same f-stop. But you can
stop down so that you have the exact DOF. But then you have a slower
shutter speed. So youn increase the ISO to compensate. It's not hard to
show that you will get no worse noise (assuming only photon noise) this
way.

I think you're either reading a lot into what I'm saying, or we're talking
about two different things. Simply put, can a 7mm lens and a 19mm lens at
the same aperture offer the same depth of field?

I understand that you can stop down on the SLR, but if you want to read a
ton of variables in to the situation, you'll never be able to answer the
question.


OK. I said both times "if you stop down". And you can certainly answer
the question if you allow these variables; a very definite statement
can be made. It has been discussed to death here. If you choose to fix
the aperture then no, the DOF will not be the same (assuming the circle
of confusion is appropriately scaled and the DOF is measured at the
print), but, for reasons I can explain, it is meaningful to allow a
change of aperture.

If you want, you can a) search google groups for the discussion, or b)
ask, I'm more than happy to explain if you want to know. If, instead,
you prefer to add qualifications to a statements until you end up
right, then I concede up front to save our time: You're right and I am
completely wrong. (Note that I am not claiming this is what you are
doing, just trying to avoid another long, tedious and pointless
"discussion" like last time this came up).

Cheers,
Achilleas

Kinon O'Cann wrote:
"J. Clarke" wrote in message
...
It's also a function of the resolution of the sensor.

How?


It's a function of the circle of confusion, which is bounded below by
the resolution of the sensor. That's the closest this statement gets to
reality.

David J. Littleboy wrote:
"Kinon O'Cann" wrote:
wrote:

This is the last time I'll post about my absolute underwhelm-ment with
the 400D. I thought I'd think I died and went to heaven with this
camera. I've taken your comments about my "BUSY" problem and believe
I've gotten a handle on it, but I just feel so disappointed with this
item.

Last question: Am I incorrect in thinking that the 18-55mm lens gives
*less* depth of field than my beloved PowerShot A620? Do I have to buy
another $500 lens in order to get precise photographs (I mean without
suturing a tripod to my arm)?
At the same angle of view, the A620 will have more depth of field due to a
much shorter focal length of the lens. Not your imagination.

We've been through this before here recently, but...

(1) DOF increases as you stop down the lens. (Yes, I know you know that.)
(2) Diffraction limits sharpness if you stop down too much. (Yes, I know you
know that, too.)

But that means that if you have a sharpness requirement, then any camera has
a maximum DOF.

Are we OK so far?

Here's the punch line: that means that for cameras of the same pixel count,
_regardless of the format_, the maximum DOF is the same.

So the laws of optics and physics make it impossible for the A620 to have
"more DOF".

There's another factor that makes dof quite subjective, and that is the
maximum sharpness in the print. A print with very sharp rendition at
the focused distance will apparently have less dof that one where the
maximum sharpness is not as sharp. That's because the point at which
the eye judges the image as unsharp depends on comparison with the
sharpest area. A print that is soft will appear to have a much greater
dof, since the comparison between sharp and unsharp is less obvious.

Colin D.

--
Posted via a free Usenet account from http://www.teranews.com

Kinon O'Cann wrote:
"J. Clarke" wrote in message
...
On Wed, 06 Dec 2006 12:26:56 -0500, Kinon O'Cann wrote:


Isn't DOF also a function of the focal length of the lens, as well as the
aperture?

It's also a function of the resolution of the sensor.

How?

A higher resolution sensor has two effects, first and most obvious is
that the circle of confusion can be smaller and so this limits the DOF,
but in addition to make use of the higher resolution sensor you need to
open the aperture more, a sensor with 6 um pixels might do ok at f/11
but one with 3 um pixels can't get full resolution with a lens
stopped down below around f/5.6.

Ultimately as you push the resolution of an image up you have to loose
DOF and the relationship between resolution and DOF is independent of
the camera format.

Scott

On Thu, 7 Dec 2006 00:08:56 +0900, "David J. Littleboy"
wrote:


"Kinon O'Cann" wrote:
wrote:

This is the last time I'll post about my absolute underwhelm-ment with
the 400D. I thought I'd think I died and went to heaven with this
camera. I've taken your comments about my "BUSY" problem and believe
I've gotten a handle on it, but I just feel so disappointed with this
item.

Last question: Am I incorrect in thinking that the 18-55mm lens gives
*less* depth of field than my beloved PowerShot A620? Do I have to buy
another $500 lens in order to get precise photographs (I mean without
suturing a tripod to my arm)?

At the same angle of view, the A620 will have more depth of field due to a
much shorter focal length of the lens. Not your imagination.

We've been through this before here recently, but...

(1) DOF increases as you stop down the lens. (Yes, I know you know that.)
(2) Diffraction limits sharpness if you stop down too much. (Yes, I know you
know that, too.)

But that means that if you have a sharpness requirement, then any camera has
a maximum DOF.

Are we OK so far?

Here's the punch line: that means that for cameras of the same pixel count,
_regardless of the format_, the maximum DOF is the same.

So the laws of optics and physics make it impossible for the A620 to have
"more DOF".

http://clarkvision.com/photoinfo/dof_myth/index.html

David J. Littleboy
Tokyo, Japan


But you (mistakenly) believe that the OP has at least a passing
understanding of the fundamentals of photography.
His past posts have demonstrated that he knows little about
photography, and less about the tool he has chosen.
Digitalrube should get a book on photography and devote some time and
practice to learning the fundamentals.
Or, possibly, a class in a local community college.
Trying to bypass this basic learning leads to, as in his case, an
underwhelming feeling towards a very decent camera.
--
Bill Funk
replace "g" with "a"

wrote in message
ups.com...

Last question: Am I incorrect in thinking that the 18-55mm lens gives
*less* depth of field than my beloved PowerShot A620? Do I have to buy
another $500 lens in order to get precise photographs (I mean without
suturing a tripod to my arm)?


After taking hundreds of pictures with my digital P&S I came across a
picture taken with a 35mm P&S. It was a funny head shot of a friend, and I
suddenly realized how beautifully blurred the background was. This picture
would not be the same with a digital P&S.

But, you want the opposite. High ISO and aperature numbers are what you
need. Enjoy!

S

On Wed, 06 Dec 2006 13:20:52 -0800, acl wrote:

Kinon O'Cann wrote:
"J. Clarke" wrote in message
...
It's also a function of the resolution of the sensor.

How?


It's a function of the circle of confusion, which is bounded below by
the resolution of the sensor. That's the closest this statement gets to
reality.

Well that is precisely it. How large is a pixel on a 10 megapixel point
and shoot? How about on a 10 megapixel APS-C SLR?

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)