Depth of Field

bob wrote:
The photographs aren't framed identically; the DSLR photograph is a cropped
version of the 35mm photograph

The DOF is identical as idicated on the barrel of the lens..

Ahem. What's marked on the lens is an entirely subjective depth of
field based on a nominal print size of 8x10 inches. If you blow up to
16x20 inches, then the DOF will be shallower ... or, if you blow up to
that size (8x10) from a smaller sensor, then the DOF will be shallower.

would be a neat
trick if you could change the depth of field just by cropping!!

Yes indeed: in the case where the cropped image is blown up to 8x10, v.
the uncropped image being blown up to 8x10.

see: http://www.nikonlinks.com/unklbil/dof.htm

Please do not top post.

Cheers,
Alan

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Bob wrote:
"Aad" wrote in message
...

schreef in bericht
ups.com...

bob wrote:
Im not talking about equvalent , if you use the same prime lens on a
SLR
and a DSLR its identical DOF
but the image is cropped on a DSLR. dont tell me its different I have
tried
this experiment
line a row of items away from the camera sit them on tape measure .. and
lo
and behold the DOF is identical
you will see the same things in focus on both SLR and DSLR

If you use two DSLRs, one with an APS sized sensor and one with a
35mm-sized sensor, and using the same f/stop and lens, then, as you
say, one image is simply a crop of the other. If you now produce two
identically-sized prints from them, the one from the APS-sized sensor
will have greater DOF (by around 1.5 times). This is because you have
to enlarge the image from the smaller sensor more (about 1.5 times). If
you change lenses to take the crop factor into account, and if you also
change the circle of confusion to take the different magnifications
required to produce a print into account, then the APS-sized sensor
will have less DOF, by again 1.5 times.

Of course, on the sensor, with the same lens and f/stop, the results
are exactly same (but cropped, in one case). But depth of field is
defined on a print (I didn't just invent this definition, it's what is
used to produce the markings on the lens that you speak of). In which
case, DOF is not independent of sensor size, for the reasons just given
(ie one needs to take into account that the recorded image must be
enlarged more, and so on).

So, if I crop the ff picture (in Photoshop) to the same size as APS
picture and print them in the same size the DOF is'nt the same?
I mean, cropping is cropping, right?
kr
Aad

it seeml ike there is magic around if you get a pair of sissors and crop a
picture you get an increase in DOF
.. crop it so much .. you will get amazing DOF and you never need to focus
.. you learn something everyday


Did you actually read what I wrote? No, guessing from your reaction.
Fine, your beliefs are your own business, even if wrong!

wrote:
Correct, I get what I can't see, but in focus and clear through my
Nikon TTL, no matter what size the sensor....as I do with my XT. At
least with the XT, I see the whole photo, or what the photo's going to
look like, the Nikon crops it.

The Rebel XT has the same 95% viewfinder coverage as a D70/80/200 and a
..80x magnification compared to the .9x 20D and .94x D80/200 but all that
has no effect on the image produced.

schreef in bericht
ps.com...

Aad wrote:
So, if I crop the ff picture (in Photoshop) to the same size as APS
picture
and print them in the same size the DOF is'nt the same?
I mean, cropping is cropping, right?
kr
Aad


No, DOF won't be the same. Think of it this way: I take a photo of
various point sources of light at various distances. These are imaged
on the sensor as a) points if they are exactly some distance from the
sensor (the distance to which I am focused), b) small circles if
they're not. Now, if I print this thing to some size, the diameters of
these circles are enlarged by as many times as the print size is bigger
than the sensor size. So, it is possible for the same circle size to be
perceived as in focus or out of focus, depending on how many times the
image was enlarged. The DOF markings on lenses are derived by assuming
that a particular size of circle on the film is in acceptable focus.

I know what the problem is: You are thinking that the DOF is fixed once
the image is recorded, while if you think about it, it's fixed only
when we print.

If you want to define DOF on the sensor (and not the print), then
that's another story (and you'll have to ignore DOF calculators and so
on, or rather, you'll have to manually set the circle of confusion to
something; if you set it equal to the pixel size, eg 6 microns or so,
you'll find out what will seem in focus if you zoom to 1:1 in photoshop
on your images).

If you search for depth of field on google, or circle of confusion,
you'll find thousands of pages of information on this.


So that means I can change depth of field in my pictures just by changing
the enlargement?
I print 10x15cm and don't like the DOF. I print 20x30cm and it's better.
Weird!
Will try it after a good night sleep.
kr
Aad

wrote:

I would like to know the way in which DOF will change across the
following cameras at same f number and focal length ..or will DOF be
same in both

An image shot with a Tamron 18-200 F3.5 - F6.3 at F4.5 100mm (29-320
on a Rebel XT)
to one taken with a Fuji S5500 5.7-57 at F4.5 24mm (37 - 370 35mm
equivalent)

You are asking equivalent vield of view... the 35mm eq. is calculated
for the Fuji but not for the Rebel. I'm assuming you figured 24mm is eq.
to 100mm but really you'd want to match 160mm on the Fuji. In any case
that's nitpicking & you may have it right.

The answer is, the Fuji will have more DOF for the same field of view
same f/stop printed at the same size.

bob wrote:
Im not talking about equvalent , if you use the same prime lens on a SLR
and a DSLR its identical DOF
but the image is cropped on a DSLR. dont tell me its different I have tried
this experiment
line a row of items away from the camera sit them on tape measure .. and lo
and behold the DOF is identical
you will see the same things in focus on both SLR and DSLR

Depth of field is dependent on print size. If you crop an image
and enlarge it to the same as uncropped, you magnify the
out-of-focus areas.

FYI:

The Depth-of-Field Myth and Digital Cameras
http://www.clarkvision.com/photoinfo/dof_myth

Roger

Aad wrote:
So that means I can change depth of field in my pictures just by changing
the enlargement?
I print 10x15cm and don't like the DOF. I print 20x30cm and it's better.
Weird!
Will try it after a good night sleep.
kr
Aad

Well, this isn't as strange as it may first sound. For example,
photographs which are slightly blurred (eg because of camera motion)
may look perfectly ok when printed at 10x15cm, but if printed at
60x90cm they won't. It's the same idea.

Or think of looking at a slide by holding it up against a light as
opposed to projecting it onto a screen (or, better, printing it at some
large size, say 120x180cm for shock value). In the second case, isn't
it true that various imperfections (such as being slightly out of
focus) become more obvious? Again, same idea. Well, at 120x180cm it'll
be rubbish anyway (if 135mm film), but never mind.

As an off-topic aside, some time ago I happened to be in Amsterdam and
went to an exhibition of award-winning journalistic photographs. It is
quite spectacular how irrelevant strong chroma noise and low resolution
become when looking at a (probably 8mp) 60x90cm print showing some poor
guy without arms being helped to get dressed by his 5 year old son (and
this in a refugee camp). Puts everything into proper perspective.

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Bob wrote:
it seeml ike there is magic around if you get a pair of sissors and crop a
picture you get an increase in DOF
.. crop it so much .. you will get amazing DOF and you never need to focus
.. you learn something everyday


Well one thing that has not been mentioned so far is the viewing
distance. That is also a factor.


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According to bob :
Sensor size has got nothing to do with DOF it is focused on the
film/sensor plane
All the sensor size does is crop the image .. sensor size does not change
the focal length of the lens
nothing can change the the actual focal length of a lens.

My 50mm lens is a 50 mm on a film camera as it is on a digital camera the
DOF is the same.

Since the DOF is sensitive to the amount of enlargement to make
the final print, and the image from the lens is cropped by the sensor,
it needs more enlargement to fill the print, thus affecting the final
DOF.

Of course -- if you crop both images to use only a part of each
the same size on the sensor or film to make the full print image, then
yes, the DOF is identical in both -- and not what would be predicted
purely from the aperture and focal length.

Enjoy,
DoN.

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schreef in bericht
ups.com...

Aad wrote:
So that means I can change depth of field in my pictures just by changing
the enlargement?
I print 10x15cm and don't like the DOF. I print 20x30cm and it's better.
Weird!
Will try it after a good night sleep.
kr
Aad

Well, this isn't as strange as it may first sound. For example,
photographs which are slightly blurred (eg because of camera motion)
may look perfectly ok when printed at 10x15cm, but if printed at
60x90cm they won't. It's the same idea.

O.k. I understand. But are'nt you forgetting something? The viewing
distance?
Bigger prints will be looked at from greater distances.
The bigger the distance, the less details you can see, the sharper the
picture looks/appears. (billboards can be printed at 70 dpi)
So don't you think that you're theorie is compensated because the viewing
distance will equalize the differences?
A 10x15 will be looked ad on tabel or lapp. A 60x90 will prob. hang on the
wall an lookded at from at least 1,5 mtr.
kr
Aad


Or think of looking at a slide by holding it up against a light as
opposed to projecting it onto a screen (or, better, printing it at some
large size, say 120x180cm for shock value). In the second case, isn't
it true that various imperfections (such as being slightly out of
focus) become more obvious? Again, same idea. Well, at 120x180cm it'll
be rubbish anyway (if 135mm film), but never mind.

As an off-topic aside, some time ago I happened to be in Amsterdam and
went to an exhibition of award-winning journalistic photographs. It is
quite spectacular how irrelevant strong chroma noise and low resolution
become when looking at a (probably 8mp) 60x90cm print showing some poor
guy without arms being helped to get dressed by his 5 year old son (and
this in a refugee camp). Puts everything into proper perspective.