hey newbies, DOF will rock your world!!

Jason Hutchinson said:

Can you show us an example of what you're talking about with any
DOF calculator?

Click to expand...

I've found an example, focusing at 150ft with a 35mm lens on a D60. I guess it blows up at both extreme ends of the range.

Jason

Jason Hutchinson said:

Thomas Bantel said:

Correct, as long as we're talking about rather big magnifications,
or, IOW,
subject distances much smaller than the hyperfocal distance. In the
general DOF formula, Focal length and Subject distance can NOT be
replaced completely by the magnification. It's just that the "rest"
that can't be expressed by magnification gets very small for small
distances. Small enough to be ignored safely. Once subject distance
approaches the hyperfocal distance, short focal lengths feature
MUCH bigger DOF than long lenses.

Thomas Bantel

Click to expand...

The formula blows up at extremely large magnifications, not small
ones.

Can you show us an example of what you're talking about with any
DOF calculator?

Jason

Click to expand...

You can ignore focal length for large magnifications, if that's what you mean by blowing up. There will always be a term with focal length, but it gets extremely small at large magnification. Examples are easy to construct. As I'm still a 35mm guy, I'll choose a typical example for that format, showing that focal length really matters for DOF at small magnifications.

Picture 1: 20mm lens, subject (focusing) distance 2 meters, f/8. Using an acceptable circle of confusion of 0.025mm, you get DOF from 1 meter to infinity. Your subject has a magnification of 0.01.

Picture 2: 200mm lens, distance 20 meters, f/8. Same magnification of 0.01, but DOF is just from about 18.2 to 22.2 meters.

Quite a difference. Even with large magnifications, while DOF actually doesn't change with focal length, the background will look quite different, depending on focakl length. But this has already been mentioned I think.
Thomas Bantel

Thomas Bantel said:

Quite a difference. Even with large magnifications, while DOF
actually doesn't change with focal length, the background will look
quite different, depending on focakl length. But this has already
been mentioned I think.
Thomas Bantel

Click to expand...

I agree.

I guess the main difference is the use of these charts between 35mm and digicam. In the 35mm world, the most common use of these charts is to get MORE dof, which is where things fall apart in this approximation. On this forum, people with G2's and S40 etc. are trying to get the least DOF, where subject magnification is high.

Jason

Jason Hutchinson said:

Thomas Bantel said:

Quite a difference. Even with large magnifications, while DOF
actually doesn't change with focal length, the background will look
quite different, depending on focakl length. But this has already
been mentioned I think.
Thomas Bantel

Click to expand...

I agree.

I guess the main difference is the use of these charts between 35mm
and digicam. In the 35mm world, the most common use of these
charts is to get MORE dof, which is where things fall apart in this
approximation. On this forum, people with G2's and S40 etc. are
trying to get the least DOF, where subject magnification is high.

Jason

Click to expand...

Yes, the problems are quite different. With my 35mm camera and a 85/1.8 or 200/2.8 lens, it's not a problem to get a shallow DOF e.g. for a portrait shot. I will actually have to stop down a little. With the G2/G3, despite the quite fast lens, a shallow DOF with a pleasant background blur is almost non existent for portrait shots. The G3 lens, for DOF, roughly equals out to a 35-120 f/10-15 zoom. Talk about shallow DOF ... :-( But then again there's always photoshop. Still, I prefer to do it during the photographing process and not later on. Maybe (probably!) I'm just old-fashioned.
Thomas Bantel