# Please enhance my knowledge of dof scales

Discussion in '35mm Cameras' started by Monica Schulz, Jun 2, 2007.

1. ### Monica SchulzGuest

Hi,

Trying to understand the theory behind the dof scales of my Nikon
lenses I wonder what the distance between two f-stop marks stands for.
I read in Merklingers " The Ins and Outs of FOCUS" that the dof scale
is a ruler tha measures distances in the unit of the max allowable
circle of confusion. But the distance between the f-stop marks is not
the diameter of the circle of confusion, which is 0,03 mm in the case
of Nikon. If I divide the distance between the focus mark and the mark
for f/22 on my f/2,8 24 mm its 11 mm and 11 mm/22= 0,5 mm. Maybe the
distance is coc*the pitch of the helicoid of the lens barrel?! Does
someone know better?

Best regards!
Monica

Monica Schulz, Jun 2, 2007

2. ### Alan BrowneGuest

Funny.

similar nature from "different people" at

83.135.226.213 (Bielefeld)
83.135.221.172 (Herne) http://tinyurl.com/yqejjj

Cities less than 150km apart in Germany.

Surely there is a Crop Circle of Confusion in Nordhein-Westfalen !!

(Why do I get the feeling I know exactly who the poster is?)

Alan Browne, Jun 2, 2007

3. ### Nicholas O. LindanGuest

Yes, you need the distance from the film plane. How much you turn
the lens is irrelevant and is dependant on the pitch of the
helicoid.

For 35mm work the relationship of lens movement to CoC is
of little to no practical interest - just go by the scale on
the lens. If you want 1/2 the mfg's CoC use 1/2 of the dof
span marked on the lens.

The acceptable CoC depends on the photographers desire for
sharpness, the enlargement ratio and the viewing distance.

Note that the CoC calculations assume a perfect ray bundle and this
is not the case in real life. The rays don't come together at
a point but form a 'sploge' at the film plane at the point of sharpest
focus. The size of the splodge before and after the film plane
is not as assumed in the DOF calculations.

See the various aberrations here:

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aberrcon.html#c1

Nicholas O. Lindan, Jun 2, 2007
4. ### Monica SchulzGuest

Well done Alan but there is no mystery: Marc is my brother and as we
share the same interest in photography we take part in posting to the
usenet what we can´t solve on our own. Anyway, I wonder if there is a
list out there that connects those ip numbers to the right cities?!

Best!
Monica

Monica Schulz, Jun 3, 2007
5. ### Monica SchulzGuest

I´m sorry but that´s not clear to me. Do you mean the distance between
the f-stop marks equals or corresponds to the distance from the film
plane?

Monica

Monica Schulz, Jun 3, 2007
6. ### bob.kirkpatrickGuest

On the off chance that this is straight: The DOF marks indicate the
distance
limits for acceptable DOF. Say you focus at 5 meters and you are
shooting at
f/22. If one of the f/22 marks is now at 3 meters and the other is at
9 meters then
the Depth Of Field or range of "acceptable" focus extends from 3
meters to 9 meters
when focused at 5 meters at f/22 with that particular lens.

bob.kirkpatrick, Jun 5, 2007
7. ### DracoGuest

Bob has the right answer for you. The marks on the lens are to show
the range of in focus with a certain f/stop. They really have not much
to do with CoC or distance to the film plain to the lens opening. They
allow you to "preset" your lens at a distance and with the proper f/
stop you can shot and run and still get an acceptable image.
The f/stop is in direct relation to the opening size to the lens
focal length. So a 50mm f/2 lens will measure 25mm across the opening.
Okay? Nothing to do with the marks on the barrel.

There isn't any stupid questions. Just some silly answers.

Draco

Getting even isn't good enough.

Doing better does.

Draco, Jun 5, 2007
8. ### Scott WGuest

But as Nicholas pointed out what is "in focus" is a bit subjective.
And this will also depend on the camera that is being used. I find
with either the 20D or the 350D that I have to use about 1 f/stop
wider on the scale to get what I consider in focus. So for example if
I am shooting at f/16 I use the f/11 marks on the lens. If I was
shooting with a FF camera then I would likely find the f/16 marks to
work much better.

And if you are using a film camera then it will also depend on what
film you are using since for example a ISO 800 print film will never
be close to as sharp as say a 100 ISO slide film.

Scott

Scott W, Jun 5, 2007
9. ### Monica SchulzGuest

Thank you Bob and Draco, it´s perfectly clear to me what the dof
scales are beeing used for.
What I´m interested in is how the position of the f-stop marks are
beeing derived. I know that they are based on the max allowable circle
of confusion that the manufacturer thinks is appropriate. You can
calculate that value for any lens using the formula that Norman Koren
has on his website (http://www.normankoren.com/Tutorials/
MTF6.html#DOFscale):

coc = f^2 / (Nmax(s-f)

For most lenses it´s around 0,03 mm. But if I take a lens and measure
the distances between the marks I find they are larger than this
value. That was what made me ask if the pitch of the helicoid is
involved in any case. - Nicholas said "Yes, you need the distance from
the film plane." I still wonder if this means "Yes it is".

Best regards!
Monica

Monica Schulz, Jun 5, 2007
10. ### Monica SchulzGuest

As far as I know about the theory diffraction and pixel size might
play a role in this case.
At f/16 a lens produces a diffraction disk of 0,0217 mm diameter, at f/
11 it´s diameter is only 0,0149 mm.
If the 20D or 350D have pixel spacings much smaller than this the
large diffraction disk will blur the image noticeably.
And this is really the case. You can see it in a wonderful interactive
sample on http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
("VISUAL EXAMPLE: APERTURE VS. PIXEL SIZE", middle of the page).

Best!
Monica

Monica Schulz, Jun 5, 2007