Archived posting to the Leica Users Group, 2006/10/31
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Wait a minute....this doesn't seem right. If you took film and cropped it to the same size as the digital sensor On 10/31/06, Lee Wai Leong <leewaileong19@yahoo.com> wrote: > There is only one plane of focus, and it is at the exact distance setting > that you put on a lens. In other words, if you focus a lens at 1 m, only > the object at exactly 1 m from the lens will be in focus. Everything else > is, strictly speaking, not in focus. > > Depth of field markings are based on the aperture and focal lengths of > the lens. Thus you can see that the DOF extends wider at smaller > apertures, and for wide-angle lenses. It is also affected by distance of > the subject. > > As mentioned, DOF is an "illusion", but more correctly, it should be > said that it is based on "acceptable" sharpness. Hence, an object at 0.99 > m or 1.01 m when the lens is focused at 1 m is not exactly 100% sharp, > but is 99% or 95% sharp and is thus of acceptable sharpness. > > The criteria for "acceptability" has to do with the degree of > enlargement of the negative and the viewing distance of the print. In > other words, what is "acceptably" sharp at 4R may be unacceptable at > 16x20, esp. if you view the 16x20 very close. > > Acceptable sharpness is closely related to the circle of confusion (CoC). > Wikipedia defines the CoC as "the largest blur circle that will still > be perceived by the human eye as a point when viewed at a distance of 25 > cm (and variations thereon)." This has generally been set at 0.2 mm. > > In other words, viewed at 25 cm (10"), a blurred circle of 0.2 mm is > perceived as a sharp point. > > Thus for an 8 x 10 print, points smaller than 0.2/8 or 0.025 mm on a 35 > mm negative will be perceived as sharp when enlarged and viewed from 10". > > The industry has generally adopted a CoC of 0.03 mm as the acceptable > sharpness limits for calculating the DOF markings on 35 mm lenses. > Depending on your visual acuity, this may or may not be good enough, but > it is useful to understand. > > So much for background. > > To your question, does DOF change with the M8, I think the answer is yes > for sure. > > Because your lens DOF markings were made with full frame 35 mm in mind, > they are definitely not appropriate for the M8. > > Simply put, when the DOF markers were calculated, they assumed the image > was on 24 x 36 mm film. Since the image is now on a digital sensor smaller > than that, this assumption is no longer valid and thus the DOF markings > are no longer correct. > > But whether the DOF change is for better or for worse is harder to say. > > Because the crop factor makes you stand further away, in all likelihood > you would not shoot the same subject from the same distance as you would > with a film camera. > > Thus, if you now shoot a portrait from 1.5 m using the M8, whereas > previously you would have shot from 1 m, then clearly DOF is increased. > > In other words, at 1 m, your DOF may range from 80 cm to 1.2 m. But at > 1.5 m, your DOF may range from 1.2 m to 1.8 m, which is larger than that > at 1 m. > > Thus even though the digital sensor is smaller than 24 x 36 mm, the > effect of you standing further away may improve the DOF for your final > print. > > So it's really hard to say which effect will be greater for a particular > lens on the M8. > > > Lee Wai Leong > > "And if you play golf, you're my friend..." > Send instant messages to your online friends http://uk.messenger.yahoo.com > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information >