[Leica] M8 & Depth of Field

[walt at waltjohnson.com (Walt Johnson)]

Yes indeed, those darn 15mm's are great for portraits too. :-P

David Rodgers wrote:

>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 
>
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>
>
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