Just a little showcase

Are you talking about the Sigma 10-20mm which has a maximum aperture of f4 ? If you are 12 feet from the brick wall, at 20mm focal length you have a DOF that extends from 6 feet to 44 feet! How were you able to tell the difference in focus ?

Not the 10-20 but the kit lens and yes throuigh pixel peeping I felt I could see a difference. I am no longer a believer in DOF if I ever was. All my geometric instincts tell me that there is only one plane in sharp focus and DOF is saying well it looks sharp enough really even though it's out of focus.

That said it was a photograph taken with the 10-20 that led me to wonder about the focus. It was taken down the length of a log and the fungus I wanted in sharp focus wasn't because I had used the centre spot for focus and the fungus was 2/3 down the length of the log (which wasn't all that long). I thought 10mm? DOF will sort it out. Well it didn't and so now I am far more fussy.

Mike

Well certainly the definition of "acceptably sharp" on which depth of field relies, is open to debate. More so these days, with sensors of different sizes and types, and an ability that we never had before to examine so easily at pixel level.

But the 10-20 on the K10D is a funny beast. I've used it a lot over the last few months, and it takes a bit of practice. Sometimes if the AF sensors think it's in focus, it doesn't refocus, and the result can indeed by an unexpected softness.

If I need to be sure it's going to focus on the spot I want, I'll often make it start from one end of the focus range or other, to ensure that it starts, well out of focus. Then it will focus fine. As you said, the clue is in when you don't hear it refocus when you thought it should.

I think what it is, is that if the thing you're pointing at is within the DOF given by the previous point of focus, it won't necessarily refocus. The consequence being that if your DOF expectation was based on your aiming point, your expected DOF may be outside the previous one, with unpredictable results.

I had some soft pictures to start with, but I'm very pleased with the results from it now.

Chris your experince mirrors mine but we seem to have different solutions to the problem although I too will sometimes bully the lens to refocus.

The phrase 'acceptably sharp' has to be totally subjective and as you say I don't think the same rules apply with digital.

Mike

You are certainly right that digital enables us to examine our photos in a lot more detail than we ever did with film.

If I ever get round to scanning all my old negatives i am sure a huge amount of "good" pictures will be dismissed by me as out of focus !

Hmm do you think so or is digital a less forgiving technology and things tend to be sharp or not rather more quickly?

Mike

I'm not sure I've understood this question right, but if you mean:

"Is the actual subject range, with digital, over which the circle of confusion is a given size or smaller, smaller than it was with film?"

.. then no, it isn't.

The light rays heading in through the front, getting focused and then chopped off by the iris, have no idea whether they're heading for film or a sensor, so the amount they'll be spread out by the time they hit it will be the same, all other things being equal.

However, there is the issue of magnification.

Using the same lens, if you want the whole frame, and enlarge to the same size print, then the amount of enlargement an APS-C 'negative' will get, is larger than the amount of enlargement a 35mm negative will get, so any circles of confusion will be bigger on the print. So you might think there's less depth of field.

But the resulting pictures are not the same. To truly compare 35mm film with digital, you have to crop the inner APS-C-sized portion out of the 35mm negative, and enlarge that to the same size as you did with the digital image.

Then of course, there's no difference.

I suppose, if you used different lenses that were nevertheless hypothetically identically sharp (whatever that means), say a 300mm on digital, and a 450mm on film, and took identical pictures, then the digital negative will need more enlargement than the film negative to get a print the same size.

I have no idea whether the differences are visible though - I never had good enough lenses, and I never enlarged things that much in my film days.

Just a quick note to agree and confirm with pschlute, I use both the viewfinder magnifier O-ME53 and for the last week or so a Katz-Eye focus screen.

I'm a very happy bunny, it makes a huge difference for me with my manual lenses, cos I'm an old fart with the eyesight going off.