Archived posting to the Leica Users Group, 2003/04/24
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Clive said this better than me. When you crop a given percentage of a photo taken with a 50mm photo, nothing changes in terms of depth of field, or the relation of the subject to the background. The photo will look the same except that it is cropped. Moving away from the subject to get the same FOV as an uncropped image will change the DOF. To get the same FOV while maintaining the same distance to subject, you will need a wide angle lens. While you now have the same FOV and distance to subject, the photo will look like one taken with a wide angle. I started thinking about this during during my documentary film class, in which we are using Sony PD-150s, which use 1/3" chips. In confined spaces, we have a difficult time getting the backgrounds out of focus. With this camera, even a very wide apertures, you have to position the subject quite far from the background, the camera set to a long focal length in order to throw the background out of focus. We had a news cameraman doing some demos for us with the PD150, and he was surprised at this, until he realized that he normally shoots with larger ENG cameras with 2/3" chips. Larger chips require longer focal lengths, so with these cameras, they can achieve much shallower depth of field than we can at the same given aperture and distance to subject. Sounds like we're going to need an experiment to learn the truth here. I did some cursory tests similar to what Clive describes, which bear out my opinion on this. I will try to whip up something tomorrow to post for review. A digital camera with a smaller than 24x36 frame does *not* change the lens' focal length. It merely crops the image projected by that lens. DOF and perspective are the same as they would be on a full-frame camera, but the image is cropped. - --Jim - ----- Original Message ----- From: "Clive Moss" <chmphoto@sbcglobal.net> To: <leica-users@mejac.palo-alto.ca.us> Sent: Thursday, April 24, 2003 9:08 PM Subject: Re: [Leica] Digital Aesthetic > On Fri, 25 Apr 2003 10:10:43 +0800, Red Dawn <reddawn@singnet.com.sg> > wrote: > > .. > > > > u got me wrong. Wat i was comparing was using a Canon 50mm f1.4 lens on a > > digital SLR like the EOS 10D, where the effective focal length would be > > 50 x > > 1.6 = 80mm, versus usng a true 80mm lens like the Summilux R 80 f1.4 on a > > Canon EOS 1v film body, or a R8 for that matter. > > > > BOTH configurations will have the same angle of view, and will be able to > > frame the same subject with the same composition and magnification, and > > produce pictures with the same DOF. > > > > Hence this thing about digital not being able to get enough shallow DOF > > is > > simply a fallacy. With digital SLRs that are not full frame, you got to > > change your mindset - a 50mm lens is NO LONGER a 50mm lens and does not > > give > > a field of view of a normal 50mm lens anymore!!!!!!... > > Forget about digital, for a moment. The concept of "effective focal length" > is kinda bogus. The focal length of a 50mm lens is 50mm. Period (to be > repetitive). Take a picture with a 50mm lens on a regular 35mm camera with > a 24x36 image frame, and crop it down to the field of a digital sensor, > what you have is a cropped image from a 50mm lens. Cropping does not change > the focal length, effective or otherwise. Now, do not move the camera > (assume it is on a tripod). Remove the 50mm lens and put on the 80mm lens > (or use a zoom lens :-)). You get an image with the same composition, 1.6x > (or so) larger -- but with reduced depth of field, based on my memory and > experience. > > I have in the last few minutes tried an approximation to this experiment > using a Canon G3, and zooming instead of changing lenses. Guess what -- the > results validate the theory. Without moving the camera, the depth of field > at the longer focal length was dramatically less then that at the shorter > length, after adjusting the images to the same apparent size. Gee -- why am > I surprised? I was a mathematician! > > Try it yourself using your own camera and lenses. You will be convinced. > > -- > Clive > http://clive.moss.net > -- > To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html