On Sun, 15 Apr 2001 15:45:39 +1000, Tanya & Russell Mayer wrote:
>Hey everyone, just a quick question before hubby kicks me off the
>computer...
>This may seem like a totally dumb question, but please remember that
>everything I know about photography I have taught myself, so you can expect
>that I will have missed a few things here and there.
There are no dumb questions. The only way to find something out is to
ask.
>an "expert" on landscape photography, which I basically know ZILCH about.
>She claimed that his number one tip was to shoot EVERYTHING in landscape
>photography at f22 to ensure maximum depth of field. Ok, so here is my
>question, (and please forgive me if I am wwaaaaay off track here), but when
>you are shooting, say a lake, or a beach scene at 6.30 at night and you need
Before I get carried away about depth of field I will just say that
while f22 will give you more depth of field than f11, on some lenses
f11 will give you a sharper picture than f22. Pick your depth of field
according to the pictures needs.
You will find a lot of people talk about depth of field and say that
you get more depth of field with wide angle lenses (you don't but more
on that later). Most people have a simplified idea of what depth of
field really is, because to understand it properly requires a lot of
maths. The focal length of a lens only comes into the formulas for
calculating DOF because it is required for the magnification and
aperture opening part of the maths.
You get more apparent depth of field from a wide angle lens because DOF
is a relationship between magnification and aperture and usually using
a wide angle lens you are getting less magnification and hence more
depth of field. If you position your subject to have the same
magnification to the film using a 100mm and a 28mm lens both set to f8
you will get the same DOF on both.
All lenses focus perfectly on only a plain (sometimes flat sometimes
curved, it depends on the lens) in front of the lens at any aperture.
Where depth of field comes from is that for a distance in front and
behind that plain of focus the combination of the lens and the film can
only resolve well enough so that it "LOOKS" like more is in focus.
I've borrowed this text from this website
http://www.graflex.org/lenses/photographic-lenses-tutorial.html
there is lots of maths there if you want to know about optics.
"The light from a single subject point passing through the aperture is
converged by the lens into a cone with its tip at the film (if the
point is perfectly in focus) or slightly in front of or behind the film
(if the subject point is somewhat out of focus). In the out of focus
case the point is rendered as a circle where the film cuts the
converging cone or the diverging cone on the other side of the image
point. This circle is called the circle of confusion. The farther the
tip of the cone, ie the image point, is away from the film, the larger
the circle of confusion."
When you vary the aperture you vary the angle of the cone of light
coming from the back of the lens. Have a look at this site
http://www.minoxlab.com/Don_Krehbiel/mpl/dkdof.htm
the 2 pictures up the top explains it very well - I always run out of
wards at about this point as you really need to see how it works.
Sorry for the rambling.
Leon
http://www.bluering.org.au
http://www.bluering.org.au/leon
-
This message is from the Pentax-Discuss Mail List. To unsubscribe,
go to http://www.pdml.net and follow the directions. Don't forget to
visit the Pentax Users' Gallery at http://pug.komkon.org .
