Originally posted by keithlester You dont focus on the hyperfocal distance itself. You put your infinity mark on one of your aperture marks and are in focus down to the other mark. ...
Hi Keith
Were you speaking with the assumption of using the lens on a 35mm film body? I didn't state it clearly but my question relates to using the lens on a digital body. While what you say is true for a P-FA lens used on a 35mm film camera body, it is not true for the same lens used on a digital SLR with APS-C sensor.
The circle-of-confusion is different for the two, and this results in different depth-of-field for the same lens.
Please don't ask me to explain circle-of-confusion (I think it is
very well named!) but there is a good article on Wikipedia that is quite readable, at least the first half. The article does make it clear that the C-of-C figure for 35mm film is different from the C-of-C figure for an APS-C sensor, and it explains why.
There is a good table at
Circles of Confusion for Digital Cameras that quotes 0.030mm as the reference figure for 35mm film, and shows 0.020mm as the figure for the Pentax APS-C digitals. These figures are consistent with the range of figures discussed in the Wikipedia article.
If I prepare another table (as above) with the 0.030mm C-of-C figure, it gives me different hyperfocal distance figures compared to the table with the 0.020mm C-of-C figure for digital. With the 31 at f/16 the hyperfocal distance is a meter closer to the camera, and the close-focus point is half a meter closer to the camera, with DoF now extending from 1.02m to infinity (1.52m to infinity for digital). And these new figures match with the DoF marking scale on my lenses. Fine with 35mm film.
However, the DoF scale markings are wrong for digital. And my real-life testing demonstrates this too. To use hyperfocal distance focusing for landscape work, I need a better method than relying on the scale on the lens.
Thanks to all - I've learned a lot in the course of this thread!