# Avogadro's 16 rules of DOF

Discussion in '35mm Cameras' started by Avogadro, Sep 1, 2003.

Everything about depth of field was probably very well understood a
couple hundred years ago - which is before cameras were invented. And
yet today, DOF is poorly understood by most photographers. Why would
that be?

Well, DOF equations are not that complex, but most photographers are
not really very good mathematicians, and most would hesitate to
undertake DOF calculations. Besides, there is the difficulty of
applying the results, because they involve concepts that can be
unfamiliar, like the circle of confusion, resolution, subtended
angles, etc.

Because of these things, it is easy to make statements about DOF that
are quite confusing. For instance, what would you think about the
following:

o Wide angle lenses have more depth of field than telephoto lenses.

o All lenses have the same depth of field at the same aperture.

Both statements are quite correct when applied to many everyday
photographic situations. Notice, though, that they disagree!
Accordingly, statements like the above incite hours of discussion on
r.p.e.35mm, often liberally spiced with insult, ridicule, and
diatribe.

DOF is the zone in the picture, from near to far, that appears to be
in sharp focus. I think most of us, when presented with an actual
photo, would have trouble deciding exactly what is sharp and what
isn't. Left to ourselves, most would pick different zones of
acceptable sharpness. And it would vary by our mood too, to say
nothing of the size of the picture, the lighting, the subject matter,
and whether the photo is even critically sharp to begin with. So,
although we have equations that can calculate DOF very exactly, it is
actually a somewhat fuzzy quantity.

Although it is fuzzy, there is good justification for understanding
DOF well enough to be able to make decisions in our photography... to
answer questions like, "Will I get better DOF with 35mm or with medium
format?", "Do I have a hope of getting the front and back flower
petals in focus at the same time?", "Should I switch to a wide angle
lens?", and so on.

calculations. Or you can punch the equations into a spreadsheet and do
the calculations there. (Caution: many of the equations in books and
Web sites are simplified versions and will not always give exact or

But when we find ourselves at a shoot with a DOF issue, there's no
time for calculations. So here then are some RULES OF THUMB to guide
you in your shooting. (Photographers love rules.)

1) Wide angle lenses have more depth of field than telephoto lenses,
at EQUAL CAMERA-SUBJECT DISTANCES. (We are assuming all other factors
are the same.) In general use, wide angle lenses rarely present DOF
problems, but long teles sure do.

2) All lenses have the same depth of field at the same aperture, if
the IMAGE SIZE IS KEPT EQUAL. Thus, if you want more DOF when shooting
a portrait, changing focal lengths won't help. The only recourse is to
stop down.

3) A portion of the zone of acceptable sharpness will be on the near
side of the plane of focus, and a portion will be on the far side. The
DOF on the far side is always bigger, sometimes by a microscopic
amount, sometimes by an infinite amount. The old rule that near DOF is
1/3 and far DOF is 2/3 is ALMOST ALWAYS WRONG, but is sometimes
approached reasonably for in-between DOF situations (subject a few
feet or meters away, limited light).

4) When DOF is shallow (such as in close-up photography or with long
telephoto lenses), the near and far zones are about EQUAL in size.

5) When DOF is deep (such as when shooting scenics with wide angle or
normal lenses stopped down to f:8 or smaller), the far zone can extend
beyond the horizon. The total DOF is INFINITE (but the near zone is of
course limited).

6) You will get more DOF with smaller image formats. The 35mm format
is relatively small and is great for DOF. Large format cameras can
give very narrow DOF, and this can be effective in portraits when used
skilfully. Most film formats are larger than the retina of the human
eye, so DOF in photos often seems shallow compared to what the eye
sees.

7) Cameras with swings and tilts can place the plane of focus on quite
a slant, and the DOF follows... this can allow you to get a whole rug
in sharp focus from front to back. DOF apparently increases, but
doesn't.

8) Comparing 35mm and 120 formats (each using lenses with similar
angles of view), you will get about the SAME DOF if the film in the
larger camera is TWICE as fast (because it allows you to stop down one
stop).

9) When DOF is shallow, you can get roughly DOUBLE the total DOF by
closing down TWO f-stops. If you want to keep the shutter speed the
same, use faster film. Thus, quadrupling the ASA doubles the DOF.

10) When doing close-up or macro photography, if you want to calculate
the DOF, use the MAGNIFICATION that you are shooting at. Don't try to
use lens-subject distance (because most people won't know where the
front nodal point is). ("Magnification" is the size of the image on
the film compared to the actual subject size. Thus, in 35mm
photography, a magnification of 0.5 means the subject measures
48x72mm. Many macro lenses have a magnification scale... if not,
magnification can be estimated fairly easily.)

11) With a tele-converter, the DOF will be for the focal length of the
combined assembly.

12) Cropping an image when printing magnifies the unsharpness, so you
should have stopped down more when shooting to compensate.

13) Poor quality lenses give deeper apparent DOF... the eye is more
tolerant because of the lack of tack-sharpness. The same applies to
pictures that are unsharp because of camera movement, poor enlarging
techniques, etc.

14) When DOF is shallow, you can't just keep stopping down to get
everything in sharp focus. Macro photography is a common application
when even at f 22, things may not all be in focus, and you will have
to compromise. At small lens openings, sharpness suffers because of
diffraction (due to the wave nature of light). Diffraction becomes
noticeable at f:16-22 and gets worse as the aperture becomes smaller.

15) In actual shooting situations at a particular f stop, you can
maximize the total DOF by setting the infinity mark on the focus scale
opposite the far DOF limit (assuming these are marked on your lens).
This is the same as focusing on the hyperfocal distance. You can get

16) Some lenses have DOF preview. This is a nearly useless feature
unless you train yourself to use it... because the viewfinder image
darkens a lot, and because the eye can often see a bit past the
viewfinder screen to view the aerial image, making the DOF seem deeper
than it is.

2. ### Aaron QueenanGuest

Do you by any chance have the correct formulae for calculating DOF?

Thanks,
Aaron Queenan.

Aaron Queenan, Sep 1, 2003

3. ### Tony ParkinsonGuest

In a British autumn, depth of field is greater, because all the rain makes
the mud deeper and it goes further up your wellies

8^)

Tony Parkinson, Sep 1, 2003
4. ### David LittlewoodGuest

Avogadro, thanks for this excellent summary. I think (point 6) that the
human eye effect may have a lot to do with the ability of the eye to
accommodate rapidly to "see" different parts of the scene sharply
without the user being conscious of it - but your thought about format
size may apply too.

*6.023 x 10^23, IIRC.

David Littlewood, Sep 1, 2003
5. ### Dallas DGuest

I like mashed avo on toast with salt and pepper.

It also goes nicely with a French Salad.

It does nothing for my DOF though...

Dallas D, Sep 1, 2003
6. ### Kevin NeilsonGuest

Thank you; those rules were useful. Some, like #2, are not intuitive and
were unbeknownst to me. -Kevin

Kevin Neilson, Sep 1, 2003
7. ### Ken CashionGuest

It is actually straightforward once the acceptable circle of
confusion is established. This is just a dimension that if present on
a print, someone would call it a dot rather than a disc.
This depends on how big a print is viewed at what distance.
If it is about .001" on a negative and that negative is
enlarged to an 8x10 and viewed at a comfortable distance in our hands,
would we agree that it was a tiny little bitty dot or a blob. See,
these are rather arbitrary terms.

Ken Cashion

Ken Cashion, Sep 2, 2003
8. ### Alan BrowneGuest

What Avogadro left out was that the CofC for 35mm is 'typically' 0.035mm.

contains all of the formulae and CofC's for most common film formats.

Cheers,
Alan

Alan Browne, Sep 2, 2003
9. ### Alan BrowneGuest

OOOPPPS ...meant 0.025mm

Alan Browne, Sep 2, 2003
10. ### T PGuest

Given the very wide ownership of cameras today, it is not surprising
that such a high proportion of camera owners are profoundly ignorant.

Compare this to the time before plastic junk SLRs took hold of the
market (late 1970s, early 1980s) when a much higher proportion of
amateur photographers were drawn from the educated professional
classes ...

;-)

T P, Sep 2, 2003

Yes.

(Actually, was away.)

Suggestion: shoot them both at 75mm, if you have both. See for
yourself. Don't believe everything you hear, try it.

12. ### Michael K. DavisGuest

Hi!

[snip]

: Just remember that you won't always know the primary use that a shot
: will be put to, and that most shots will be viewed under many
: different situations.

If that's truly the case for a given shot, there's only one rational thing to do- Use a DoF scale calculated with a CoC diameter that will support the largest print possible for the format.

That makes it really difficult to shoot because it pushes your Near Sharps further away from the camera than they would be if larger CoC's were permissible. It is much "smarter" to make up your mind what size print you'll be making and what viewing distance it must suffer at your chosen print resolution before you make the exposure.

Think about it. If you produce CoC's that are so large as to be visible in the final print for ANY of the "many different situations" in which the print may be viewed, you will choose to completely avoid the unacceptable "situations" by making the print no larger than the CoC's will allow -or- you will compromise the quality of your work. It's got to be one or the other.

If we are dedicated to producing high quality images, we won't be enlarging the CoC's beyond the point where print resolution falls below our chosen minimum (5 to 8 lp/mm for a 10-inch viewing distance, or the equivalent at greater viewing distances). So why not make up our minds BEFORE we shoot? By doing a little bit of homework in advance, we can equip ourselves with the ability to determine pricesely how much DoF is necessary for each print size we're likely to produce. If the subject space demands m
ore DoF that can be had for an 11x14-inch print, then so be it - we'll make an 8x10 print and switch to the more generous DoF tables/calculations had with larger CoC's.

: Also, for many shots, conditions won't allow you
: realize your ideal goals, and compromises will be needed. Therefore
: the numbers have to be regarded as rough guides in most cases.

Compromise the enlargement factor or restrict the viewing distance (if you can), but don't compromise the image quality.

The numbers are not rough guides. They can be trusted as a rock solid truth that you can exploit to great benefit and control with amazingly little effort (read my first post). Ignoring the truth won't make it go away. The truth serves those who submit to it. Accepting poor quality in a print that could have been sharper at the desired enlargement factor isn't compromise - it's ignorance and, in my opinion, ignorance is forgivable only when it's not willfull. It's hard to summon sympathy for an "artis
t" who is disappointed on occaision with results he left to chance while chosing to ignore the principles that influence his medium. Self-expression can not precede mastery of brush and pigment.

Mike Davis

--

Michael K. Davis, Sep 22, 2003

The old "everything from here to Jupiter must be in focus" bull. As
enlargement increases so does viewing distance - at least by anyone who
isn't anal retentive.

--
http://www.chapelhillnoir.com
home of The Camera-ist's Manifesto
The Improved Links Pages are at
New email - Contact on the Menyou page.
to do- Use a DoF scale calculated with a CoC diameter that will support the
largest print possible for the format.
further away from the camera than they would be if larger CoC's were
permissible. It is much "smarter" to make up your mind what size print
you'll be making and what viewing distance it must suffer at your chosen
print resolution before you make the exposure.
in the final print for ANY of the "many different situations" in which the
print may be viewed, you will choose to completely avoid the unacceptable
"situations" by making the print no larger than the CoC's will allow -or-
you will compromise the quality of your work. It's got to be one or the
other.
enlarging the CoC's beyond the point where print resolution falls below our
chosen minimum (5 to 8 lp/mm for a 10-inch viewing distance, or the
equivalent at greater viewing distances). So why not make up our minds
BEFORE we shoot? By doing a little bit of homework in advance, we can equip
ourselves with the ability to determine pricesely how much DoF is necessary
for each print size we're likely to produce. If the subject space demands m
make an 8x10 print and switch to the more generous DoF tables/calculations
can), but don't compromise the image quality.
truth that you can exploit to great benefit and control with amazingly
little effort (read my first post). Ignoring the truth won't make it go
away. The truth serves those who submit to it. Accepting poor quality in a
print that could have been sharper at the desired enlargement factor isn't
compromise - it's ignorance and, in my opinion, ignorance is forgivable only
when it's not willfull. It's hard to summon sympathy for an "artis
chosing to ignore the principles that influence his medium. Self-expression
can not precede mastery of brush and pigment.

Yes...

With reference to Mike Davis's post, DOF is not a goal to always be
maximized... and often is intentionally reduced for effect.

DOF is just one of many tools. Just try to understand a bit how it
works and use it intelligently, balanced off against other objectives.

If image perfection is your goal, what are you doing in a 35mm
newsgroup?

16. ### Q.G. de BakkerGuest

Unless, of course, enlargment increases to get a tighter crop.

Only the analy retentive never cr*p. ;-)

Q.G. de Bakker, Sep 22, 2003
17. ### Michael K. DavisGuest

Hi Tony!

: The old "everything from here to Jupiter must be in focus" bull.

When did I insist that everything in the image be equally sharp? Selective focus is an absolutely valid choice for many subjects, but controlling selective focus to yield predictable results requires even more precision than a "here to Jupiter" shot (which can be had by stopping down further than necessary.)

: As
: enlargement increases so does viewing distance - at least by anyone who
: isn't anal retentive.

If that were true we'd all be shooting with Minox cameras.

Mike Davis
--

Michael K. Davis, Sep 22, 2003

No - because a Minox can't deliver a picture that is acceptable at minimum
viewing distance.

--
Camera-ist's Manifesto
The Improved Links Pages are at
New email - Contact on the Menyou page.
Selective focus is an absolutely valid choice for many subjects, but
controlling selective focus to yield predictable results requires even more
precision than a "here to Jupiter" shot (which can be had by stopping down
further than necessary.)

19. ### Michael K. DavisGuest

Hi!

: If image perfection is your goal, what are you doing in a 35mm
: newsgroup?

Wow! Why did you post the original article? Weren't you trying to teach?

With a just a few minutes effort, anyone interested in getting a real handle on DoF can follow the recommendations in my first post to this thread to easily work out how much they have to offset their engraved DoF scales for various print sizes. If they would prefer to use customized CoC diameters in a spreadsheet or a handheld calculator, they're welcome to do that instead. How they employ this knowledge is totally up to them - be it for selective focus or whatever. I haven't even stipulated that any p
articular print resolution must be used - I've only stated what I prefer and handed you the means to tightly control your shooting to achieve whatever degree of sharpness you prefer.

Surely there are some 35mm shooters who frequent this newsgroup who are tired of rolling the dice every time they select an aperture and set their focus.

Mike Davis
--

Michael K. Davis, Sep 22, 2003

Ever hear of a DOF preview button? Works a lot better than those idiotic
charts and calculators.

--
http://www.chapelhillnoir.com
home of The Camera-ist's Manifesto
The Improved Links Pages are at
New email - Contact on the Menyou page.
handle on DoF can follow the recommendations in my first post to this thread
to easily work out how much they have to offset their engraved DoF scales
for various print sizes. If they would prefer to use customized CoC
diameters in a spreadsheet or a handheld calculator, they're welcome to do
that instead. How they employ this knowledge is totally up to them - be it
for selective focus or whatever. I haven't even stipulated that any p
and handed you the means to tightly control your shooting to achieve
whatever degree of sharpness you prefer.
tired of rolling the dice every time they select an aperture and set their
focus.