On 3/3/03 11:22 AM, "Jez.Cunningham@xxxxxxxxxx" <Jez.Cunningham@xxxxxxxxxx>
wrote:
> Can someone explain all this to me please?
> Are we saying that with a small sensor size the DOF is inherently deeper?
> Then it should follow that with a large sensor (like 5" x 4") the DOF is
> almost non-existent?
> Thanks
I'm probably oversimplifying this but here's the basics. DOF is dependant on
the focal length of the lens. A 50mm lens (focused to the same distance and
set to the same aperture) will have the same DOF on any format. However,
what changes with the format is the field of view that lens provides. On the
4/3 system, it's equivalent to a tele (100mm). On 35mm it's a normal lens.
On medium format, it's a moderate wide angle. On large format, it's a super
wide. So, if you want to get the same field of view, you have to use a
different focal length, depending on the format. With the 4/3 system, you
would use a 25mm lens as a normal lens (with all the increased DOF inherent
in that). At 4x5, 150mm is the normal lens (which gives you much less DOF at
the same aperture).
Somebody else pointed out potential issues with diffraction. I'm not certain
this is an issue though as you won't need to stop down the shorter lenses as
much to get the same DOF as 35mm. This would be similar to how you can get
away with f32 or f64 in large format to get the DOF you want because you're
using a longer lens. I think the big issue is in getting fast short lenses
in order to get the DOF control that some people will want (with the added
benefit that focusing becomes easier due to the reduced DOF and brighter
viewfinder).
--
Andrew "Frugal" Dacey
frugal@xxxxxxxxxxxxxxx
http://www.tildefrugal.net/
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