[Leica] M8 & Depth of Field

[drodgers at casefarms.com (David Rodgers)]

The comforting fact in all this is that plane of focus on my 15mm Heliar
at f16 is no greater than that of my Noctilux at f1. The Noctilux just
gets more confused more quickly. ;-)

I for one would like to see a tilt/shift Noctilux that would test the
mettle of every photographer!

daveR

-----Original Message-----
From: Lee Wai Leong [mailto:leewaileong19@yahoo.com] 
Sent: Tuesday, October 31, 2006 10:50 PM
To: lug@leica-users.org
Subject: [Leica] M8 & Depth of Field

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