hyperfocal distance

Dave Martindale wrote:
M Barnes wrote:

OK, but in a lens lab, don't engineers use a standard
line chart to determine the CoC. In other words, no
print is generated. Rather, the techs use the chart to
determine CoC by focusing the image on the capture
area, whether a 35x24 for film or a 66% sensor area
for digital. The question being, will this scaling, if a
print is not used, cause the HD to scale as well?

You're thinking of something else. "Circle of Confusion" is a
theoretical concept, not a measured value. Perhaps you are thinking of
resolution tests for a lens?

Yes, I was under the impression that the two were related,
in that the resolution test for a lens determined the CoC
value for that lens when computing other values, for
example, the HD for a given set of values.

In this, it would be like the "tortuosity factor" in filtration
physics, a number that doesn't exist and can't be measured,
but is essential to compute filtration efficiency in real-world
applications.

The minimum blur spot size for a lens is normally quite
a lot smaller than the CoC used in depth of field
calculations.

I'm not sure I follow this. Do you mean that the CoC is
not a base level of resolution or acuity, but only a
semi-useful construct, and that the blur spot is a more
meaningful measure of lens acuity?

The way you calculate depth of field is (more or less) this:

Pick a print size and viewing distance. Pick an angular resolution that
will look "sharp enough" for your eye. This gives you the Circle of
Confusion size on the print. A standard value is 1/1740 of the diagonal
of the print, but there are other possible choices depending on how
critical you are about sharpness.

Now scale the CoC diameter down by the reciprocal of the printing
magnification. This tells you the CoC size on the sensor that gives you
the desired CoC size on the print. If you picked diagonal/1740 as the
print CoC size, then this will be sensor_diagonal/1740.

For this discussion, I'm holding print size at 8x10, lens focal length
and all other parameters constant. The only variable will be sensor
size -- 35mm and the D100 2/3 equivalent.

From here, we use pure geometric optics.

snip a very good explanation of the theory of DOF

Finally, to determine the hyperfocal distance, just calculate the lens
focus setting that places the far limit of the DOF exactly at infinity.

(There's one lie in the above. Real DOF tables usually give distance
from the film plane, not distance from the lens front principal plane.
This makes the exact calculations messier, but there's little practical
difference between the two methods for non-macro photography).

Well, two things: I'm used to measuring distance to subjects with
a tape from the focal-plane mark on the camera body, so that
does apply to me, especially when I'm using extension tubes
and/or macro lenses; and ... the 64-dollar question -- Does all
that add up to an adjustment in the HD when sensor
size changes and all else is held constant? There seems to be
a circle of confusion regarding the answer, and I'm not smart
or experienced enough to figure it out, but you seem to be.

"Tony Spadaro" wrote in
. com:

Sorry - the intuitive answer is right.
Yes the more you enlarge the softer the image gets but it's
just
plain silly to assume any single size for the final image since as
size of the image increases, viewing distance also increases and they
cancel each other out.

The formula I gave is the formula for hyper focal distance.
It behaves exactly as I said. The hyper focal distance is
inverse proportional to the size of of the sensor, everything
else kept equal.

You may not like the definition. You may want something else;
but then it is not hyperfocal distance. You may call it Tony
Spadero distance if you want.


/Roland

"M Barnes" writes:

You're thinking of something else. "Circle of Confusion" is a
theoretical concept, not a measured value. Perhaps you are thinking of
resolution tests for a lens?

Yes, I was under the impression that the two were related,
in that the resolution test for a lens determined the CoC
value for that lens when computing other values, for
example, the HD for a given set of values.

No. CoC is not a property of the lens at all. It is a measured or
estimated tolerance of how blurred an image can be before the viewer,
looking at a print, feels it is out of focus.

The minimum blur spot size for a lens is normally quite
a lot smaller than the CoC used in depth of field
calculations.

I'm not sure I follow this. Do you mean that the CoC is
not a base level of resolution or acuity, but only a
semi-useful construct, and that the blur spot is a more
meaningful measure of lens acuity?

CoC is not a measure of lens acuity *at all*. It is a measure of how
badly out of focus something can be allowed to get before it seems
unsharp to a person viewing the image under some assumed set of
conditions. CoC is a useful construct for calculating DOF, not lens
resolution.

If you have a lens whose blur spot diameter is equal to the Circle of
Confusion for the sensor size you're working with, then *everything*
will be borderline unsharp, even what ever is in best focus. Normally,
the CoC will be several times the lens blur spot size if the CoC is
reasonable and the lens is suited to the format.

For this discussion, I'm holding print size at 8x10, lens focal length
and all other parameters constant. The only variable will be sensor
size -- 35mm and the D100 2/3 equivalent.

That's an unusual choice, because you won't get the same field of view
if you use the same lens focal length for 35 mm and for 2/3 frame. If
you do this, the short answer is that the D100 will give you only 2/3 as
much DOF as you would get with a full-frame sensor, at the same FL and
same f/number. (This is because you have to enlarge the D100 image
more).

On the other hand, if you changed the lens focal length to keep the same
field of view, so the D100 lens is 2/3 the focal length of the 35 camera
lens, then the D100 would give you 3/2 times as much DOF as the 35
camera.

(There's one lie in the above. Real DOF tables usually give distance
from the film plane, not distance from the lens front principal plane.
This makes the exact calculations messier, but there's little practical
difference between the two methods for non-macro photography).

Well, two things: I'm used to measuring distance to subjects with
a tape from the focal-plane mark on the camera body, so that
does apply to me, especially when I'm using extension tubes
and/or macro lenses;

Ok. I'd suggest you build your own DOF tables using a spreadsheet.
It's not really that hard. If you're calculating things in terms of
distances from lens front principal plane to subject, and from rear
principal plane to film/sensor, you can use classic lens formulas
directly. If you want to know subject-to-film distance, you need to add
together the lens-subject distance plus the lens-image distance *plus
the spacing between the two principal planes within the lens*. You may
have to guess at the latter number.

Also, note that some modern lenses do not focus by moving the glass as a
unit towards or away from the sensor, they move elements within the lens
like a zoom. For these lenses, the focal length may change from the
marked value when focused closer than infinity, and the usual DOF
calculations will have some error.

and ... the 64-dollar question -- Does all
that add up to an adjustment in the HD when sensor
size changes and all else is held constant? There seems to be
a circle of confusion regarding the answer, and I'm not smart
or experienced enough to figure it out, but you seem to be.

Yes, see above. If you keep the lens FL constant, thus narrowing the
FOV when sensor size decreases, the DOF also *decreases*. On the other
hand, if you make lens FL decrease along with sensor size, so FOV
remains constant, then the DOF *increases* with decreasing sensor size.

Dave

"M Barnes" wrote in news:q9-dnS6e4c3M3H7dRVn-
:

Hm. Okay, I think I get it. How is the CoC computed
for lenses then? And for film body/lens combinations?


CoC is the diagonal divided by a number. This
number can be choosen somewhat arbitrary, but it
is in the region 1500-2000 I think. One standard
value is 1740. Someone else said 1525. Both are
probably valid choices. It all depends on your
criteria for when you think a picture is sharp.


/Roland

"M Barnes" wrote in news:J4GdnT3-hYryj37dRVn-
:

'c' is going to be proportional to the size of a pixel, not the
sensor.

Well, give yourself a gold star, then. Anyway, isn't the
circle of confusion a characteristic of the lens, not the
sensor?'

CoC is neither depending upon the pixel size nor the lens.

It is a pure aestethic meassurement; how unsharp do human
beings accept out of focus parts to be before thinking they
are too unsharp. And tests have shown that the CoC is approx.
the diagonal divided by 1500-2000 for most people and most
pictures.

Also, I believe the OP was asking if it's required
to adjust the "f" in your formula due to the _apparent_ focal
length adjustments required by fractional-frame sensor form
factors, n'est-ce pas?

Yes he did.

And, yes he has to.

If the sensor has a cropping factor of 1.6, than the hyper focal
distance is increased with a factor 1.6.


/Roland

"M Barnes" wrote in news:XdednZHOAr_O1X7dRVn-
:

The best thing to do is to test. For each focal length,
for each f stop, shoot a series of images of a scene
with both near and far detail at various distance settings
on the lens starting at infinity. For each focal length/f
stop combination, record the closest distance that you
find the detail at infinity to remain acceptable.

Since it's digital, all it costs is your time. And even then,
not much of that since you don't have to scan.

Got it. I'll set up a 20-meter tape measure in my back yard
and put up letter-sized paper sheets labeled with the distance
at 1-meter intervals. That should do it. Thanks.


I hate to tell you that this test is not so interesting

The hyper focal distance cannot be meassured by photographing
rulers and tape meassures. It has really nothing to do with
actual sharpness - at least not at pixel or grain level.

Hyper focal distance is based upon an aesthetical (is it spelled
thus?) meassure; how unsharp are we ready to accept out of focus
parts to be. In practice this has shown to be approx the picture
diagonal divided by 1500-2000.


/Roland

"M Barnes" wrote in news:saKdnehg9qGnxn7dRVn-
:

You just saved me a weekend with a tape measure
in the back yard. Thanks.




/Roland

Dave Martindale wrote:
M Barnes wrote:
Dave Martindale wrote:

I was under the impression that [CoC and resolution]
were related, in that the resolution test for a lens
determined the CoC value for that lens when computing
other values, for example, the HD for a given set of values.

No. CoC is not a property of the lens at all. It is a measured or
estimated tolerance of how blurred an image can be before the
viewer, looking at a print, feels it is out of focus.

The minimum blur spot size for a lens is normally quite
a lot smaller than the CoC used in depth of field
calculations.

I'm not sure I follow this. Do you mean that the CoC is
not a base level of resolution or acuity, but only a
semi-useful construct, and that the blur spot is a more
meaningful measure of lens acuity?

CoC is not a measure of lens acuity *at all*. It is a measure of how
badly out of focus something can be allowed to get before it seems
unsharp to a person viewing the image under some assumed set of
conditions. CoC is a useful construct for calculating DOF, not lens
resolution.

OK, got it. I need to do more reading to keep up, but I'm
pretty sure I understand.

If you have a lens whose blur spot diameter is equal to the Circle of
Confusion for the sensor size you're working with, then *everything*
will be borderline unsharp, even what ever is in best focus. Normally,
the CoC will be several times the lens blur spot size if the CoC is
reasonable and the lens is suited to the format.

Ie., the lens acuity is much sharper, theoretically, than required
to produce an image that appears sharp to the viewer. This allows
for aberration, aperture, DOF, etc., etc.

For this discussion, I'm holding print size at 8x10, lens focal length
and all other parameters constant. The only variable will be sensor
size -- 35mm and the D100 2/3 equivalent.

That's an unusual choice, because you won't get the same field of view
if you use the same lens focal length for 35 mm and for 2/3 frame. If
you do this, the short answer is that the D100 will give you only 2/3 as
much DOF as you would get with a full-frame sensor, at the same FL and
same f/number. (This is because you have to enlarge the D100 image
more).

Yes, exactly. To start with, I'm looking for this artificial condition
as a learning aid, for me.

On the other hand, if you changed the lens focal length to keep the same
field of view, so the D100 lens is 2/3 the focal length of the 35 camera
lens, then the D100 would give you 3/2 times as much DOF as the 35
camera.

(There's one lie in the above. Real DOF tables usually give distance
from the film plane, not distance from the lens front principal plane.
This makes the exact calculations messier, but there's little practical
difference between the two methods for non-macro photography).

Well, two things: I'm used to measuring distance to subjects with
a tape from the focal-plane mark on the camera body, so that
does apply to me, especially when I'm using extension tubes
and/or macro lenses;

Ok. I'd suggest you build your own DOF tables using a spreadsheet.
It's not really that hard. If you're calculating things in terms of
distances from lens front principal plane to subject, and from rear
principal plane to film/sensor, you can use classic lens formulas
directly. If you want to know subject-to-film distance, you need to add
together the lens-subject distance plus the lens-image distance *plus
the spacing between the two principal planes within the lens*. You may
have to guess at the latter number.

That's what I've done with the N2020. It won't be hard to
create a new one for the D100.

and ... the 64-dollar question -- Does all
that add up to an adjustment in the HD when sensor
size changes and all else is held constant? There seems to be
a circle of confusion regarding the answer, and I'm not smart
or experienced enough to figure it out, but you seem to be.

Yes, see above. If you keep the lens FL constant, thus narrowing the
FOV when sensor size decreases, the DOF also *decreases*.
On the other hand, if you make lens FL decrease along with sensor
size, so FOV remains constant, then the DOF *increases* with
decreasing sensor size.

Ah, that's the 128-dollar question I was just about to ask. I thought
I understood, but wanted to make sure. If I create a shot with the
N2020 using hyperfocal focusing, then switch to the D100 body
and adjust -- say using a zoom -- FL to keep the shot the same,
I would need to, or could, move the near focal point outward
to compensate for the increased DOF. Thanks. This helps a lot.
I appreciate your patience and knowledge.

I pretty much figured that FL and FOV, being on opposite sides
of the dividing line in the formulae cited by others, would signify
a reciprocal relationship.

Of course, if Nikon would come out with a full-frame sensor,
I could forget all this. Until the medium-format DSLRs come out ....

Roland Karlsson wrote:
M Barnes wrote:

CoC is neither depending upon the pixel size nor the lens.

It is a pure aestethic meassurement; how unsharp do human
beings accept out of focus parts to be before thinking they
are too unsharp. And tests have shown that the CoC is approx.
the diagonal divided by 1500-2000 for most people and most
pictures.

I think I'm beginning to click into this concept from other
discussions. I've never paid much attention to it, but read
a Nikon tech paper years ago that got me off on the wrong
track. I've had to root out the synaptical assumptions and
replace them with new ones. Very painful. Not like finding
out Santa was really mom and dad, but still painful.

Also, I believe the OP was asking if it's required
to adjust the "f" in your formula due to the _apparent_ focal
length adjustments required by fractional-frame sensor form
factors, n'est-ce pas?

Yes he did.

And, yes he has to.

If the sensor has a cropping factor of 1.6, than the hyper focal
distance is increased with a factor 1.6.

Got it. Thanks.

Roland Karlsson wrote:

CoC is the diagonal divided by a number. This
number can be choosen somewhat arbitrary, but it
is in the region 1500-2000 I think. One standard
value is 1740. Someone else said 1525. Both are
probably valid choices. It all depends on your
criteria for when you think a picture is sharp.

The older I get, the more forgiving I am, and the
more lenses I need stacked up in front of me to
see any difference at all. But I do like to play
with DOF, hyperfocal focusing and macros, sometimes
combining foreground macros with infinity backgrounds,
so this is a technical point that has some interest to me.
Obviously I need to do more reading, but thanks for
the help. The two numbers you cite helps explain
why there are different values in the tables I've seen
on various Websites.