Pål Jensen wrote:
All other things equal DOF is dependent on focal lenght.
All other things equal (including focal length), DOF is dependent on the set
aperture....
All other things equal (including focal length), DOF is dependent on the
subject distance....
All other things equal (including focal length), DOF is dependent on the
optical design of the lens....
All other things equal (including focal length), DOF is dependent on the
eyesight....
All other things equal (including focal length), DOF is dependent on the
distance you look at the picture....
All other things equal (including focal length), DOF is dependent on the
magnification of the picture....
All other things equal (including focal length), DOF is affected by the
different thing among the many which affect it.
Hence, say, a Pentax 150mm lens for the 645 will give exactly the same DOF
regardless of the format of the camera it is mounted on (angle of view
though
depend on the format the lens is used on).
Yes, a given lens will always give the same DOF, provided that you fix
aperture, get the same magnification, look at the image from the same
distance, have a defined sight, and so on...
On the other hand, two lenses of the same focal length designed for
different formats can give different DOF (all other thing being equal, if
possible).
As DOF is dependent on the laws of physics it isn't really an issue for
optic design.
No, optic design is fully dependent on the laws of physics.
The circle of confusion isn't something engineered into the lenses
but a wholly subjective value calculated for DOF tabels for marking
those DOF scales on lenses (BTW Pentax 645 lenses are way off.
At least 2 stops).
Of course, you cannot find a mechanism called circle of confusion buried
into a lens. However, an acceptable circle of confusion is a design task,
dependent on the print size and the distance you are supposed to look at it,
hence dependent from film/sensor size (a smaller sensor aimed to get
comparable results needs more enlargement).
So the lens must be designed for giving a circle of confusion matching well
the largest print size you are expected to get from the picture (depending
on the film/sensor size). Otherwise, even the focused details will look
blurred (out of focus).
Demands to set THESE scales may be different from format to format but
again the DOF on a 150mm lens on a 35mm camera is the same regardless
of what format the lens was designed for.
No, no, no.
The only thing that is off may be the DOF scale on the lens but DOF on
film
will be the same. Only the plane of focus is truly in focus.
If a dot on the subject is rendered as a small dot, below a visible spot,
you perceive it as being focused. If it is rendered as a spot, you perceive
it as being out of focus. Any lens will render dots as dots even when
slightly out of the focus plane. This is called DOF.
A lens capable to give a smaller circle of confusion (hence a lens designed
for a smalller format) will render dots as dots even when farther from the
focus plane than a lens designed for a larger format can do, hence will show
a wider DOF. Why lenses for a larger format cannot be designed with the same
circle of counfusion as the lenses designed for the smaller format? It's
because designers have to correct a larger film/sensor size. If you have to
keep a larger format acceptable up to its corners, the average quality
lowers.
Dario
