How does sensor size produce a shallower DOF?...

[Lee Jay]

Cut the center rectangle out of any print to see the effect of a
smaller sensor on what is in focus. There is none.

Hehe. This is one of my favorite examples of the difficulty of explaining or understanding how DOF changes with sensor size.

If you take a pair of scissors and cut the edges off of a negative, do you change DOF? Strangely, YES YOU DO!!! (You make it smaller).

For any given final presentation size, be it a print (8x10, for example) or a digital (say, 728 pixels wide), cropping will decrease depth of field becuase you have to enlarge more to make that final size. Here's a graphical representation of that effect. This is all the same image. All I've done is crop and resize for each of the crops shown, just like you'd do if you trim a negative and then print the result at 8x10. All of these are the same final size and, since they all came from the same image, they are obviously all at the same focal length and f-stop. As expected, the smaller effective sensor (6x crop) shows way less DOF than the full-frame image.



--
Lee Jay
(see profile for equipment)

 

[mgates]

Cut the center rectangle out of any print to see the effect of a
smaller sensor on what is in focus. There is none.

Hehe. This is one of my favorite examples of the difficulty of
explaining or understanding how DOF changes with sensor size.

If you take a pair of scissors and cut the edges off of a negative,
do you change DOF? Strangely, YES YOU DO!!! (You make it smaller).

For any given final presentation size, be it a print (8x10, for
example) or a digital (say, 728 pixels wide), cropping will
decrease depth of field becuase you have to enlarge more to make
that final size. Here's a graphical representation of that effect.
This is all the same image. All I've done is crop and resize for
each of the crops shown, just like you'd do if you trim a negative
and then print the result at 8x10. All of these are the same final
size and, since they all came from the same image, they are
obviously all at the same focal length and f-stop. As expected,
the smaller effective sensor (6x crop) shows way less DOF than the
full-frame image.

And if I enlarge a cropper's image to 10 feet by 15 feet then cut out a 4 x 6 inch print? You can't compare at different quality levels because you can get any result you want.

 

[Barugon]

Cut the center rectangle out of any print to see the effect of a
smaller sensor on what is in focus. There is none.

True if you use lenses with different focal lengths you get
different DOF effects. That is true without changing the sensor.

Does the depth of field change with focus distance? Are the circles of confusion smaller with shorter lenses? Will I have to back up or use a shorter lens to achieve the same framing using a smaller sensor?

--
Whoever said 'a picture is worth a thousand words' was a cheapskate.

http://www.pbase.com/dot_borg

 

[mgates]

Cut the center rectangle out of any print to see the effect of a
smaller sensor on what is in focus. There is none.

True if you use lenses with different focal lengths you get
different DOF effects. That is true without changing the sensor.

Does the depth of field change with focus distance? Are the circles
of confusion smaller with shorter lenses? Will I have to back up or
use a shorter lens to achieve the same framing using a smaller
sensor?

Let us say I frame a subject with a 50mm lens using 35mm film. Now I switch to APS film and change lenses to 35mm for roughly the same framing. So put the 35mm back on the full frame camera. I can crop the full frame film with scissors to achieve identical results at identical quality. I get the same DOF because the two negatives are literally identical.

 

[David J. Littleboy]

A 50mm lens on FF has 1/2 the DOF of a 24mm lens on a 4/3 camera.

And this comparison is quite valid and sensible, since the viewer
is looking at identically framed images (in prints of the same
size, of course).

Identically framed but with different apertures.

No. At the same subject distance at the same aperture, i.e. f number.

Why should you use
a larger aperture just because youre using a larger sensor? It just
doesnt make sense you should have to change your shot because of
different camera specifications. Its not a logical comparison.

???

DOF depends on field of view and aperture only.

Huh???

For the same FOV and same f number , larger formats have narrower DOF.

Are you talking about the actual physical size of the aperture, as opposed to the f number???

--
David J. Littleboy
Tokyo, Japan

 

[David J. Littleboy]

.. will make the lens look like it was shot on an f4 lens. Taking
this to the extreme - this is why there is so very little DOF
control on a P&S camera - because so little of the lens is being
used that bascially everything is in focus (unless your focus
distance is very close to the lens on a P&S)

P&S's are about a 7x crop factor.

A 2.8 lens produces the same DOF regardless of sensor size. The
properties of any lens projection is unchanged by film.

DOF isn't a property of lens projection. It's a phenomenon that occurs when a human being looks at a print at a given distance.

True you cut away some of the image. The discarded outer part of
the frame may or may not contain something in focus. It is
possible that the focal point might be located in the outer part of
the frame. Then the full frame has more in focus then the cropper.

DOF doesn't exist at all. It's a psychological phenomenon that occiors when someone looks at a photograph. (Or it's a mathematical abstraction that is claimed to model what happens when someone looks at a print.)

To "compare DOF" you have to have someone look at prints, so the usual thing (the only sensible thing) is to use prints of the same size and have the viewer stay the same distance away.

So if you keep the prints the same size, you have to enlarge the cropped image more, and the DOF goes down.

--
David J. Littleboy
Tokyo, Japan

 

[mgates]

So if you keep the prints the same size, you have to enlarge the
cropped image more, and the DOF goes down.

You should reduce the larger image to keep print size constant. Otherwise quality varies.

 

[saynomore]

The definition of DOF is drawn from looking at same sized prints from the same distance. The examples posted below by LeeJay are perfectly valid. You can't just throw around what you think DOF is. That's what these guys are pointing out.

So if you keep the prints the same size, you have to enlarge the
cropped image more, and the DOF goes down.

You should reduce the larger image to keep print size constant.
Otherwise quality varies.

 

[wrider]

You can crop all you want (look closely at those pictures, please), talk about COC (?) and claim that DOF is human perception issue (what a crock) but the fact remains that different focal lengths and f stops, used at different distances, have different DOF. Something is either in focus or it isn't. Its not an optical illusion.

Let's get real here. Sure it is hard to make a fair comparison, that is the real problem. Think of it this way, if there wasn't this (non) isuue of sensor size, you could just compare the lens results at different apertures, focal lengths and distances on one camera- and there would be defineable differences, as there always have been- the laws of physics have not changed because of sensors.

Sensors record what comes though the lens, that's it. The fact that we can't seem to separate out the variables doesn't change that. The perceived depth of field may be a net gain on a smaller sensor if that is the way you want t look at it, but it is all about the lenses used, their focal lengths, aperture size and distance from the subject (assuming they were designed for the same image size results, as opposed to P&S lenses, for example).

This is a lot like the discussion of whether crop cameras actually emlarge the image or just crop. Its an easy call until you add in pixel density. Then you have to define magnification and the subsequent relationship with small, dense sensors. More and more variables, but the lenses remain the same.

Sincerely,
--
Wendell
http://www.wendellworld.com

'Not everything that counts can be counted, not everything that can be counted counts.'
Albert Einstein

 

[Barugon]

Let us say I frame a subject with a 50mm lens using 35mm film. Now
I switch to APS film and change lenses to 35mm for roughly the same
framing. So put the 35mm back on the full frame camera. I can
crop the full frame film with scissors to achieve identical results
at identical quality. I get the same DOF because the two negatives
are literally identical.

Exactly! If you back up from your subject then you're focusing farther away and increasing your depth of field. So if you back up to fit your subject into the area of an APS-C sensor then you increase the depth of field. If you do this with a full frame camera, crop to APS-C dimensions and print the same size then you just accomplished the same thing, increased the depth of field. I believe this is precisely the information the original poster was asking for. Thanks, case closed.

--
Whoever said 'a picture is worth a thousand words' was a cheapskate.

http://www.pbase.com/dot_borg

 

[urbahn]

I understand aperture size, subject to camera distance and focal
length and I understand that all of these change compared to APC
size sensor in order to get the same FOV. But how does the size of
the sensor contribute?

Thanks

--
EOSMan (got print button?)
http://www.pbase.com/eosman

The sensor size does not change the DOF at all.

But: Longer lenses have a smaller DOF than shorter one. If your camera has a larger sensor (e.g. FF instead 1.6x crop) you need a longer lens to photograph the same Field of View at the same object distance. Therefore it appears as if the FF had a smaller DOF. But it is always the lens, not the sensor...

 

[Einstein333]

you take the same picture (exactly the same framing!) with a FF and a crop camera at the same aperture? I think there will be less DOF with the FF, right?
--
***********************************************



Visit my photo gallery: http://www.retomueller.ch
Reto Mueller

 

[mgates]

You can crop all you want (look closely at those pictures, please),
talk about COC (?) and claim that DOF is human perception issue
(what a crock) but the fact remains that different focal lengths
and f stops, used at different distances, have different DOF.
Something is either in focus or it isn't. Its not an optical
illusion.

Yes different lenses produce different DOF effects. That is the answer to a different question. Different films do not.

Let's get real here. Sure it is hard to make a fair comparison,
that is the real problem. Think of it this way, if there wasn't
this (non) isuue of sensor size, you could just compare the lens
results at different apertures, focal lengths and distances on one
camera- and there would be defineable differences, as there always
have been- the laws of physics have not changed because of sensors.
Sensors record what comes though the lens, that's it. The fact that
we can't seem to separate out the variables doesn't change that.
The perceived depth of field may be a net gain on a smaller sensor
if that is the way you want t look at it, but it is all about the
lenses used, their focal lengths, aperture size and distance from
the subject (assuming they were designed for the same image size
results, as opposed to P&S lenses, for example).
This is a lot like the discussion of whether crop cameras actually
emlarge the image or just crop. Its an easy call until you add in
pixel density. Then you have to define magnification and the
subsequent relationship with small, dense sensors. More and more
variables, but the lenses remain the same.

It is only an issue if you change the pixel density of one and not the other. That is why it is best to talk in terms of film. In terms of a quality-constant comparision DOF is never affected by film choice. It is easy to prove doctrinally since any APS image is a subset of a full frame image. To compare identical enlargements with identical quality levels you only need to snip snip.

 

[jimcreative]

How does the field of view affect DOF with lenses of various focal lengths.

The sensor adds nothing to the equation. It is the restricted angle of view that is the actual determing factor which, of course, is partly determined by the sensor size.

A 90mm Super Angulon has excellent DOF on 4x5. Not so good if you could find a way to stick it on a Minox.

--

'Hmmm, I wonder what deep thought might convince these guys I am some kind of artist...'
http://www.jimroofcreative.com

 

[mgates]

Let us say I frame a subject with a 50mm lens using 35mm film. Now
I switch to APS film and change lenses to 35mm for roughly the same
framing. So put the 35mm back on the full frame camera. I can
crop the full frame film with scissors to achieve identical results
at identical quality. I get the same DOF because the two negatives
are literally identical.

Exactly! If you back up from your subject then you're focusing
farther away and increasing your depth of field. So if you back up
to fit your subject into the area of an APS-C sensor then you
increase the depth of field. If you do this with a full frame
camera, crop to APS-C dimensions and print the same size then you
just accomplished the same thing, increased the depth of field. I
believe this is precisely the information the original poster was
asking for. Thanks, case closed.

No need to back up. You can always use the same lens at the same distance to produce an APS negative of the same quality from a full frame negative. Whether backing up or changing focal length or comparing at different quality levels might affect lens properties is a different question. The question was whether film affects lens properties.

 

[mgates]

The definition of DOF is drawn from looking at same sized prints
from the same distance. The examples posted below by LeeJay are
perfectly valid. You can't just throw around what you think DOF is.
That's what these guys are pointing out.

I am comparing the same final enlargement size at the same final quality level. They are comparing the same enlargement size at varying quality levels. If you allow changing the quality level of one print but not the other print you can produce any result you want.

 

[mgates]

you take the same picture (exactly the same framing!) with a FF and
a crop camera at the same aperture? I think there will be less DOF
with the FF, right?

Only if you change lenses. Changing lenses on one but not the other can produce any result you want on DOF.

There is no need to change lenses. Full frame film can be cropped to APS size without affecting the quality of the final enlargement or the lens.

 

[mgates]

I understand aperture size, subject to camera distance and focal
length and I understand that all of these change compared to APC
size sensor in order to get the same FOV. But how does the size of
the sensor contribute?

Thanks

--
EOSMan (got print button?)
http://www.pbase.com/eosman

The sensor size does not change the DOF at all.

Correct.

But: Longer lenses have a smaller DOF than shorter one. If your
camera has a larger sensor (e.g. FF instead 1.6x crop) you need a
longer lens

No you only need a pair of scissors.

to photograph the same Field of View at the same object
distance. Therefore it appears as if the FF had a smaller DOF. But
it is always the lens, not the sensor...

Correct. Think in terms of film and it is an easy answer to an easy question.

 

[spt_gb]

mgates wrote:
...

No need to back up. You can always use the same lens at the same
distance to produce an APS negative of the same quality from a full
frame negative. Whether backing up or changing focal length or
comparing at different quality levels might affect lens properties
is a different question. The question was whether film affects lens
properties.

Erm, no you can't. The same lens, at the same distance on APS and 35mm means that one of the two is incorrectly framed. If it's the APS camera that is incorrectly framed then you've lost the edges of the scene forever and nothing can bring them back. If it's the 35mm camera that is incorrectly framed then all you are saying is that a 35mm frame cropped to 1.6 looks just like a 1.6 crop frame (DUH!).

DOF is very simply about what appears sharp in a standard sized print (by convention this is 8x10"). If you crop a frame then you need to enlarge it more to reach the standard print size. This enlargement reduces apparent DOF. On the other hand to maintain a constant field of view (FOV) you need a shorter focal length lens, which increases DOF. This second effect is stronger than the first so crop cameras end up with more DOF for a constant FOV.

People who talk about cutting the edges off of a piece of film or a print seem to forget about the constant print size part of the equation (and sometimes even the framing). If in doubt forget the mind experements and make some actual prints. It doesn't take too long to figure out what is right providing you stick to a contant print size.

 

[spt_gb]

The definition of DOF is drawn from looking at same sized prints
from the same distance. The examples posted below by LeeJay are
perfectly valid. You can't just throw around what you think DOF is.
That's what these guys are pointing out.

I am comparing the same final enlargement size at the same final
quality level. They are comparing the same enlargement size at
varying quality levels. If you allow changing the quality level of
one print but not the other print you can produce any result you
want.

DUH again! By definition whenever you crop you lose quality. Lens MTF falls away as the frequency of detail increases. Even if you have a perfect sensor with an infinite number of pixels the cropped vesion will always have to enlarge what the lens resolved more and fine detail will be worse.

Quality isn't at issue though. You can see DOF effects without needing the sharpest, highest quality prints and the facts over how these vary have already been stated.