I think your explanation, and your English, are excellent. Very understandable considering how complex an issue it is. Thanks for taking the time to write it all out.
Don > -----Original Message----- > From: Dario Bonazza [mailto:[EMAIL PROTECTED] > Sent: Thursday, July 14, 2005 2:36 AM > To: [email protected] > Subject: Re: Theory of Equivalency > > > 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
