On Mar 7, 2015, at 5:50 PM, Mark C <[email protected]> wrote: > I understand what you are saying and completely agree. But you stop too > soon. (Or maybe I go one too far.) > > I should comment that what you are calling the "circle of confusion" I would > call a "defocused point" - i.e. any point that is not on the focal plan. I > use the term "circle of confusion" the describe the size at which a defocused > point loses apparent sharpness. > > A little thought experiment: Take a photo on 35mm with a 50mm lens at f8. The > physical diameter of the aperture should be 6.25mm (50mm /8). Assume there is > a point outside of the focal plan that is defocused but is within the DOF > area and so appears to be sharp. Call the point P and its diameter D. Now I > take the same shot from the same distance with a 6x7 using a 100mm lens at > f8, of course needing the longer focal length to get the equivalent frame at > the same distance. The physical diameter of the aperture is 12.5 (100mm / 8). > Since the physical size of the aperture is now larger, the defocused point P > is correspondingly larger and is 2xD. Being larger it would have less > apparent sharpness. That would seem to confirm the idea that as the format > gets bigger there is less DOF. > > But - now enlarge both those images to be 700mm wide. The 35mm exposure is > enlarged 20x. The 6x7 exposure is enlarged by 10x. In both enlargements the > diameter of point P is now 20xD. If the defocused point is now the same size > in both prints, why would its apparent sharpness differ? (I'm obviously > taking liberties by ignoring differences in aspect ratio and assuming that > 6x7 is twice the size of 35mm, which it is not exactly.) > > Guidelines for circle of confusion size for use in DOF calculations scale > with the size of the format. For example, Wikipedia's circle of confusion > page says that the circle of confusion for 35mm is 0.029mm and for 6x7 is > 0.059mm. > > That's what I don't get about the circle of confusion - it seems like all the > DOF gained in small formats would be offset by the additional enlargement of > the image to get it to viewing size. But that is not how things actually work. > > Mark > Thanks John Mark et al for your comments and links in reply to my original query. While I have not reached a state of enlightenment, I believe I have at least identified the pathway. John, thanks in particular for reminding me that f/ is a ratio, not a value. And that the physical size of the diaphragm opening does enter into all of this.
I have started, and deleted, several follow-up questions, sort of thinking aloud as I typed. I am trying to think through what would happen if, in your example Mark, you compared your 35mm to an 8x10” rather than a 60x70mm. Enlarge the 35mm image to match a contact print from the 8x10… And I realized I don’t understand enough yet to properly frame a good follow-up comment or question. I’ll go dig out my Shaw books and see if those help. stan > > 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). >> >> > > > --- > This email is free from viruses and malware because avast! Antivirus > protection is active. > http://www.avast.com > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
