Our case is No.2, since the discussion was about having a lesser depth of field when using a 24mm lens on the *ist D, compared to using the same 24mm lens on 35mm format (from same distance and with different cropping due to different sensor size).
Dario ----- Original Message ----- From: "Robert Gonzalez" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, October 14, 2003 8:53 PM Subject: Re: I got my *ist D and I LOVE IT !!! > I way I interpreted the photo.net explanation it sounds like the > magnification factor has an effect on the DOF. Since it takes more > magnification for a smaller sensor to fill the 8x10, the DOF will be > different. In what you are saying, it sounds like M, the magnification > factor is effectively the same since you are getting the same final crop > in terms of subject sizes? Here is a summary of what that web site has > (I have substitued the *istD for his original example): > > 1. For an equivalent field of view, the *istD has at least 1.5x MORE > depth of field that a 35mm film camera would have - when the focus > distance is significantly less then the hyperfocal distance (but the > 35mm format need a lens with 1.5x the focal length to give the same view). > 2. Using the same lens on a *istD and a 35mm film body, the *istD > image has 1.5x LESS depth of field than the 35mm image would have (but > they would be different images of course since the field of view would > be different) > 3. If you use the same lens on a *istD and a 35mm film body and crop > the 35mm image to give the same view as the digital image, the depth of > field is IDENTICAL > 4. If you use the same lens on an *istD and a 35mm film body, then > shoot from different distances so that the view is the same, the *istD > image will have 1.5x MORE DOF then the film image. > 5. Close to the hyperfocal distance, the *istD has a much more than > 1.5x the DOF of a 35mm film camera. The hyperfocal distance of the *istD > is 1.5x less than that of a 35mm film camera. > > I'm not sure, but I think what you are saying is consistent with this, > correct? > > rg > > > graywolf wrote: > > DOF does not have much to do with the size of the Circle of Confusion > > (COC) on the film or sensor unless you only look at contact prints. > > Normally DOF is based on an 8x10 print viewed at 10 inches. > > > > 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 > >> > >> > > > >
