A common photographic technique is to isolate a focused subject
against a blurred background by using a wide aperture to reduce the
depth of field.

This is much more difficult to do with a digital camera, and I would
like to understand why.

I believe the answer will have something to do with factors such as
sensor size, lens size, focal length, distance between lens and
sensor, etc.  I would like to understand, but without being
overwhelmed by technical terms.

Thank you

Hi!!


Let me start recalling that depth of field (DoF), in photography, is
the distance in front of and behind the subject which appears to be in
focus.
The problem with DoF in digital cameras is due the usually small size
of the sensors (chips).
To start dealing with this problem we need to know about the called
"Circle of Confusion":
This concept is related to the "human eyes resolution", that is the
smallest size of a point before it looks like a small disk rather than
a point. On an 8" x 10" print viewed from a "lecture distance" of
about one feet, assuming perfect vision and ideal light conditions,
the size of such point is 0.25mm or 250 microns (1/100").
To get a point of this size on the 8" x 10" print we need to magnify
our original image (negative or digital sensor surface).
On a 35-mm film camera the frame size is 24x36mm, a typical good but
non-SLR digital camera has a sensor of 2/3" (6.6×8.8mm), that is four
times smaller. Therefore, to get the print, the magnification ratio
for a digital camera with 2/3" sensor is about four times greater than
the 35-mm film camera.

How this affects the depth of field? Just see the following paragraph,
I cannot copy it here due copyrights restrictions. Basically the
problem lyes in since the small size of the sensors: they need, for a
same field of view, a proportionally smaller lens focal length.
"Basic Facts" & "What's so special about digital cameras?" at
wrotniak.net "Depth of field and your digital camera":
http://www.wrotniak.net/photo/tech/dof.html#BASIC


Read this:
"What About Digital? 
A common complaint about digital cameras is that when using one it's
not possible to get nice out-of-focus backgrounds. Why therefore do
digital cameras have greater Depth of Field? The reason for this is
that the imaging chips on most consumer digitals is very small, around
the size of ones smallest finger nail. This means that a normal lens
for a format that small is as short as 15mm. A 15mm lens at f/5.6 has
Depth Of Field from about 2.5 feet to infinity. Not too much
opportunity for selective focus, is there?
This is one of the unspoken drawbacks of low-end digital cameras. Only
expensive SLRs like the Nikon D1x, Canon 1D and their ilk have chips
close to the size of a 35mm frame, and therefore offer enough DOF to
allow creative control over out-of-focus backgrounds."
From "Understanding Depth Of Field" at The Luminous Landscape:
http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml


To complete the answer the better for you is to follow reading the
wrotniak.net's article: "Depth of field and your digital camera":
http://www.wrotniak.net/photo/tech/dof.html

It has all concepts that you will need to completely understand this
topic. In the case that you need further help with this just request
for a clarification.

At the mentioned article there are also links to tables that will be
useful to use for estimate the depth-of-field. The tables are
depth-of-field tables for four groups of Olympus digital cameras with
different sensor sizes and they are also applicable to other cameras
of similar sensor sizes. For example see the "Depth of field tables
for the Olympus E-10/E-20 cameras"(2/3" sensor):
http://www.wrotniak.net/photo/tech/dof-e10.html

You can compare it with the "Depth of field tables for 24x36 mm
cameras" (common film cameras):
http://www.wrotniak.net/photo/tech/dof-35mm.html


For further reading see:
"Circle of confusion - Wikipedia, the free encyclopedia":
http://en.wikipedia.org/wiki/Circle_of_confusion

"Depth of field - Wikipedia, the free encyclopedia":
http://en.wikipedia.org/wiki/Depth_of_field

"Photo.net Depth of Field and the Digital Domain" by Bob Atkins:
http://www.photo.net/learn/optics/dofdigital/


Search strategy:
"Depth of field" digital
"circle of confusion" digital


I hope this helps you. Remember to not hesitate to use the
clarification feature if something is missed in this answer. I will be
glad to offer you further assitance on this topic if you need it.


Best regards,
livioflores-ga

---

Request for Answer Clarification by nosredla-ga on 20 Nov 2006 16:09 PST

Thank you for your efforts. However I must tell you, with great
respect, that this is not what I had wanted.

When I was a boy, I asked my parents "why does water boil". They
replied that it boiled because heat was put under it. I did not find
this to be an explanation because of course I could tell this from
observation. I wanted to understand why the application of heat would
have a specific observable effect on the water. I am having similar
disatisfaction with explanations about the difficulty that digital
cameras have in obtaining a shallow depth of field.

I own several text books on digital photography, and have done a good
deal of internet research through which I have already discovered the
Luminous Landscape and Wikipedia sources that you referred me to. I
already understand what depth of field is, and I already understand
the concept of the Circle of Confusion. I will plough through the very
lengthy wrotniak source you gave me, but at first glance it still
seems to say only "digital cameras have difficulty allowing shallow
depth of field because they have small sensors" (equivalent to "water
boils because heat is applied")

Perhaps my inability to understand is my fault, but I would really
appreciate if you could try, in your own words, to give me in two
short paragraphs of layman's language a simple explanation of why the
small sensor of a digital camera makes it difficult to obtain a
shallow depth of field.

Please forgive any obtuseness on my part.

Many thanks

---

Clarification of Answer by livioflores-ga on 20 Nov 2006 21:36 PST

Hi!!

I am so sorry because my answer does not satisfy you; take into
account that I am only a researcher not a photograph expert. So please
give some time to prepare the requested explanation. I guess that some
additional concepts must be included, a good explanation on this could
take more than two short paragraphs.

Regards,
livioflores-ga

---

Request for Answer Clarification by nosredla-ga on 21 Nov 2006 10:04 PST

livioflores-ga

I am sorry if my expectations were incorrect. I am usually quite
effective myself at research, but had been unable to understand this
issue to my satisfaction from the sources I found. I turned to Google
Answers because I thought my question would go to a subject matter
expert.

I now feel it would be unfair to ask you to do a lot more work on
this, so I am happy to thank you for what you have done, which
certainly was very good research, and absolve you from further
responsibility.

Best wishes

---

Clarification of Answer by livioflores-ga on 21 Nov 2006 20:33 PST

Hi again!!

Well, you did it. I hope that at least some smaill part of my work
have been helpful. You relieved me from a huge research because I
started to deal with some theoretical optics stuff.

In order to satisfy my own need to bring something useful to you I
want to add the following pages as additional sources:
"Digital Depth of Field":
This page gives some basic explanations and tells us that on some
special circumstances "we can say that over the range of focus
distances which aren't in the macro range (where D is close to F) and
which aren't close to the hyperfocal distance (where D = F*F/fn*c) you
can "guesstimate" that the depth of field ratio between two lenses
used at the same aperture and focused at the same distance by assuming
it's proportional to the size of the circle of confusion and inversely
proportional to the square of the focal length.
http://www.bobatkins.com/photography/technical/digitaldof.html


The above confirms the fact that the focal length of the lens has the
most dramatic effect on the depth of field.

The other page is "Digital SLRs and Depth of Field":
At this page an experiment is taken using the same lens on a 35mm-SLR
and a Digital-SLR camera. Here is clear how the field of view change
from one camera to another.
http://www.dofmaster.com/dof_dslr.html


Joining both above results your final conclusion follows. 

The following two related pages could add some additional background
on this subject:
"depth-of-field explanation":
http://www.aesthetic-endeavors.com/photo/dof/

"depth-of-field example":
http://www.aesthetic-endeavors.com/photo/dof/dof_eg.htm


Again, I hope this could be on some help to you. 

Thank you for your understanding.

Best regards, 
livioflores-ga

Depth of Field is affected primarily by three factors: a) the aperture
setting of the lens where wider apertures result in less depth of
field, b) the distance from the subject, where the greater the
distance the greater the depth of field, and c) the focal length of
the lens where a shorter focal length results in greater depth of
field. Of these three factors, it is c) that has the most dramatic
effect.

Because digital cameras have a smaller sensor (than 35mm film), they
require a wider angle lens to give the same field of view. This means
a shorter focal length, which results in a significant increase in
depth of field, and consequent difficulty in obtaining a shallow depth
of field.

There - two paragraphs!

I rate this answer three stars because despite livioflores-ga's best
intentions, I had to answer my question myself from my own research.

Hi,

To achieve a small DoF (which creates this blur effect), you have to
combine wide aperture and long focale length.

The problem with compact digital cameras is that the aperture is often
small, and the angle wide. Great DoF effects appear when opening at
f2.8 and below.

You can also try to zoom because DoF is more perceptible when the
focale length is greater. You will also have to use the manual mode of
your camera, so you can try to set the lowest value as possible for
the aperture. Try this indoor, because outdoor, you will get too much
light for wide apertures (compact cameras also don't shut as
lightspeed, generaly 1/1000 sec, while you would need 1/2000, 1/4000
or even 1/8000 to combine outdoor sjot + wide aperture). And don't
fire the flash ;-)

Exercise with objects very close to you, and far background. You have
a macro-mode, try it, DoF in macro is very small.

And well, if none of this helps, you'll have to consider buying a SLR
camera with a great lens (if you love small DoF, you've got to try
some f1.4 50mm or 85mm ;-))

Regards