There are two main reasons influencing the DOF when comparing "equivalent"
focal lengths on different formats. They fight one against the other.
1) Aperture. This does not depend on the format the lenses are designd for.
If you set the same f-stop (relative aperture), the absolute aperture (the
hole) will be smaller in the shorter lens, thus giving more DOF. For this
reason, a 50mm lens used on APS format should show more DOF than a 75mm on
35mm format (regardless on the format they are designed for and provided
that both lenses are set on the same relative aperture).
2) Circle of confusion (a lens design parameter, different for different
formats, which limits resolution). On average, this depends on the format
the lenses are designd for. Details perceived as "in focus" and "out of
focus" depend on the print size, the distance you look at the picture and
your visual acuteness. So designers have to guess the average condition and
use a circle of confusion complaint to this. Circle of confusion matches the
smallest detail the lens is designed to resolve. When comparing high-quality
lenses (allowing big blow-ups) the acceptable circle of confusion for a
smaller format is smaller than the circle for a lens designed to cover a
larger format because the lens for the smaller format is supposed to be
enlarged more. For this reason, larger formats gain resolution over a
smaller sizes, but not as much as you could expect by the difference in
their formats. So, when you use a MF lens on 35mm, you usually get smaller
DOF that when using a lens of the same focal length designed for 35mm. This
can also happen when using lenses designed for 35mm on APS cameras (either
film or digi).
This does not happen when using lenses designed for that format (compact
digicams, Olympus 4/3, Pentax DA, Nikon DX, Canon EF-S).
For this reason, a 50mm lens used on APS format should show less DOF than a
75mm on 35mm format (provided that both lenses are designed for 35mm and are
set on the same relative aperture).
To make things more complicated, there's nothing forcing designers to use
the same circle of counfusion for any lens intended for a given format. When
you say that lenses for smaller formats use smaller COF, you mean on
average. Any lens can have its own COF. Typically, macros are designed with
a smaller COF (higher resolution) than portrait lenses, hence macros suffer
less reduction in their DOF when used on smaller formats.
Factors 1 and 2 fight one against the other. However, in practical use
factor 1 tends to win over factor 2, hence 35mm lenses used on APS format
have some extra DOF. But you cannot foresee how much, any lens can be a
different case and I think that with some lenses factor 2 could well balance
factor 1, giving about the same DOF. It is also possible that in some
instances (low-end zooms?) factor 2 prevails over factor 1, giving less DOF
on the digital APS that the one on 35mm. BTW, this is what Pentax write in
their DSLR manuals, advicing you to look at one stop more open on the DOF
scale of lenses bearing it (e.g. if you set f/8, consider the DOF the scale
gives for f/5.6).
In case of lenses designed for the smaller format, factor 2 does not apply,
hence you have more DOF for sure (this is well visible on compact digicams
equipped with good lenses).
Not sure if I've been able to explain well the above concepts. If not,
please English-language folks come to the rescue.
Dario
----- Original Message -----
From: "Shel Belinkoff" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Thursday, July 14, 2005 3:49 AM
Subject: Theory of Equivalency
An 18mm lens on a Pentax DSLR should yield, within a very small degree,
the
identical result as a 28mm on a 35mm film camera. Two pics taken of the
same subject from precisely the same location should be virtually
indistinguishable wrt image size and FOV Is this correct? What, if
anything, should be different between the two pics? I ask this because
something tells me that they will not be the same in some respects.
Shel
