Over-all-magnification = subject-size : image-size. The size of intermediate things like sensors or negatives have nothing to do with DOF. They do however affect the quality of the image. Also almost all DOF charts are calculated for an 8x10 print viewed at 10 inches. That is the standard, although many purely snapshot cameras were marked for 5x7 prints and would indicate more DOF than you would see in a 8x10. If your granddaughter is 6 inches high in the print and the aperture, not f-stop, was the same, it does not matter what the focal length of the lens or the size of the negative/sensor was the DOF will be identical.
focal length and f-stop resolve to aperture. focal length, subject distance, and enlargement factor resolve to magnification. circle of confusion is a characteristic of the human eye and if you assume an 8x10 print viewed at 10 inches it can be consider a fixed value. hence aperture-diameter and magnification are the controlling variables. It is as simple as that. For anyone still interested, I found this all out the hard way. I was trying to produce some DOF tables for my Graphic long before I joined this list. I calculated them carefully, printed them out, AND they did not work. I studied, and I researched, and still I could not figure out why. Then one day I thought, "Maybe aperture means aperture-diameter and not f-stop". So I plugged that into the formula and low and behold, the charts worked. Then I started doing simple algebra on the formulas and factoring out everything that was not necessary. That left, you guessed it, aperture-diameter and magnification. If you do not believe me, go and do it yourself, but do not leave out the important step of shooting some photos to check your results. -- graywolf http://www.graywolfphoto.com http://webpages.charter.net/graywolf "Idiot Proof" <==> "Expert Proof" ----------------------------------- John Francis wrote: > By one measure, you could actually say the shot from the small-sensor > camera actually has more "magnification", as you're magnifying a > smaller image on the sensor to fill the same sized print. But, > despite that, the small-sensor camera still ends up with more DOF. > > On Sat, Sep 16, 2006 at 04:46:53PM -0400, graywolf wrote: >> Just to clarify my other post on this, a 70mm on the digital has exactly >> the same magnification as a 105mm on on 35mm film if the images are >> framed the same and printed the same size (and they would necessarily >> have to be taken from the same distance), and thus has no effect on the >> DOF. In this particular case only the difference in aperture diameter is >> giving the 70mm a bit more DOF. >> >> -- >> graywolf >> http://www.graywolfphoto.com >> http://webpages.charter.net/graywolf >> "Idiot Proof" <==> "Expert Proof" >> ----------------------------------- >> >> >> Mark Roberts wrote: >>> John Francis wrote: >>> >>>> On Sat, Sep 16, 2006 at 01:28:26PM -0400, Mark Roberts wrote: >>>>> Ryan Brooks wrote: >>>>> >>>>>> Adam Maas wrote: >>>>>>> At f2.4, the 70 is about perfect for me. Essentially the same length >>>>>>> and >>>>>>> speed as the legendary Nikon 105 f2.5, which is a superb portrait lens. >>>>>>> >>>>>> You are getting more DOF with the 70mm though. >>>>> Not true. Not at the same subject magnification, anyway. (And that's >>>>> what you'll be doing if you compare both lenses as portrait lenses.) >>>> Actually, Mark, Ryan is right. >>>> >>>> A portrait taken with a 105mm lens at f2.8 on a full-frame camera will >>>> have a shallower depth of field than the same portrait (taken from the >>>> same spot and enlarged to the same size) >>> Right. But I specified "same subject magnification", not "taken from >>> the same spot". And "same subject magnification" is pretty much how >>> everyone does portraits: You frame as a head shot, waist-up, 3/4 or >>> full length and compare two lenses with this view. I've never heard >>> anyone saying. "Well this Lens 1 has shallower depth of field with a >>> head shot than Lens 2 does with a 3/4 shot": It's not a meaningful or >>> useful comparison. >>> >> -- >> PDML Pentax-Discuss Mail List >> [email protected] >> http://pdml.net/mailman/listinfo/pdml_pdml.net > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
