[Leica] M8 & Depth of Field

[leewaileong19 at yahoo.com (Lee Wai Leong)]

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than that, this assumption is no longer valid and thus the DOF markings are 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 

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