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
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":
"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:
To complete the answer the better for you is to follow reading the
wrotniak.net's article: "Depth of field and your digital camera":
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):
You can compare it with the "Depth of field tables for 24x36 mm
cameras" (common film cameras):
For further reading see:
"Circle of confusion - Wikipedia, the free encyclopedia":
"Depth of field - Wikipedia, the free encyclopedia":
"Photo.net Depth of Field and the Digital Domain" by Bob Atkins:
"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.
Request for Answer Clarification by
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.