When you reduce the formulas to their basics the only things that matter is COC in the final image, the size of the subject in the final image (magnification M), and the diameter of the aperture A (not f-stop). If you use a uncropped 8x10 @ 10 inches as your reference COC becomes a constant.
What that means is in that the same size subject in an 8x10 print, a given f-stop (f4.5 in the mentioned case) with a given lens (24mm NOTE: you have to specify the focal length if you use f-stop, because what is involved in DOF is the diameter of the aperture and you need the focal length of the lens to convert f-stop to aperture) has exactly the same DOF with the small sensor as it does with 35mm film. That applies whether you move closer to fill the larger film frame or crop down to match the smaller sensor as both methods give the same overall magnification.
I repeat, in the final image the DOF is exactly the same with both formats.
Dario Bonazza 2 wrote:
Good point. The 24mm should become a 36mm, while depth-of field must be conidered one stop less, hence pictures taken with the 24mm f/4.5 are
like
those taken at 36mm f/3.5, while 24mm f/11 is like 36mm f/8. However, I was expecting some more sharpness there (not more unsharp
mask!).
Can you explain your logic here? In my experience the DOF is based on the lens focal length, not the 35mm equivelent focal length.
The 24mm on the *ist D gives you the field of view of a 36mm lens but the depth of field of a 24mm lens (because that is what it is). A 36mm lens at f3.5 would have much less depth of field.
Glad you noticed that. I'll try to explain this concept (all but mine).
The depth of field is based on the concept of confusion circle: 1) Your eyes see as pinpoint each spot size below their resolution. 2) Your eyes can appreciate dimensions of each spot size above their resolution. Images look unsharp (out of focus) when the size of dots forming them are above eye resolution and look sharp (in focus) when dots forming them are below that limit.
The confusion circle is a parameter in optical design, and depth of field as indicated in DOF scales is related to it.
Since *ist D CCD sensor diagonal is 1.5 times smaller than that of 35mm film, using a 24mm designed for film on such a sensor not only gives you an angle of view equivalent to that of a 36mm lens, but in order to do that it only uses (enlarging it) a central portion of its possible image field. So, to get a print (or file as seen on PC monitor) the same size of that taken with a "true" 35mm lens on a 24x36mm sensor, you have to enlarge the image of such a 1.5 factor. Think of doing that with film: should you want to get a 35mm perspective print out from a 24mm slide or negative, you have to enlarge it 1.5 times more and then crop the print to the same size of that made with a true 35mm lens. The only difference is that the *ist D crops during shooting. You'll enlarge the image taken with the 24mm more than that shot with the true 35mm on a larger sensor (35mm film format), hence you'll push image resolution of the 24mm to a 1.5x higher extent. In other words, some dots which would stay below eye resolution when sooting on 35mm format will jump out, and a lesser part of the image will look in focus. The 1.5x ratio between the two sizes (35mm film and CCD) roughly correspond to 1 stop wider. I dindn't invent such theory. It's explained (not so well) in *ist D instruction manual (see bottom of page 137).
Not sure if I explained well in my poor English.
Dario Bonazza
-- graywolf http://graywolfphoto.com
"You might as well accept people as they are, you are not going to be able to change them anyway."
