On 07/03/2015 10:52 PM, P.J. Alling wrote:
That's not quite right.
It's close enough for rock and roll.
bill
On 3/7/2015 11:49 PM, J C OConnell wrote:
DOF increases when you go to a smaller format because the working
Magnification is lower.
On 3/7/2015 2:03 PM, John Francis wrote:
On Sat, Mar 07, 2015 at 11:55:07AM -0500, Mark C wrote:
The one factor I am ignoring is the "circle of confusion" and effect
that
enlarging the image itself has on the DOF of the final print.... and
that is
not something I have never been able to grok and so won't comment
on. Maybe
someone else can.
Mark
I consider that to be the most significant factor in the DOF of the
final image.
Ignore all mystical numbers like format, f stop, focal length, ...
for now.
The basic problem is you have three defining factors to consider.
They are:
1) The size of the subject you are tring to capture
2) The size of the final image you are trying to create
3) How far away from the subject you can place the sensor
You can juggle all the other numbers (how much of your sensor does
the image
cover says how much cropping and magnification you will have to do,
etc.),
but after a whole lot of maths you end up with one simple result:
The size of the "circle of confusion" is basically defined by the
actual diameter of the aperture (focal length divided by f stop);
all the other numbers cancel out.
That's why depth of field increases when you go to a smaller sensor;
it's because you generally want to keep the exposure variables (shutter
speed and f stop) around the same, and using a smaller camera system
means that you can use a physically smaller lens aperture. This reduces
the size of the circle of confusion (and thus allows for parts of the
subject that are further away from the plane of true focus to still
yield images which are perceived as being acceptably within focus;
this is another way of saying that the depth of field is increased).
--
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