Archived posting to the Leica Users Group, 2006/10/31

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Subject: [Leica] M8 & Depth of Fiel
From: abridge at gmail.com (Adam Bridge)
Date: Tue Oct 31 21:01:43 2006
References: <20061101034939.96782.qmail@web37703.mail.mud.yahoo.com>

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
>
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Replies: Reply from henningw at archiphoto.com (Henning Wulff) ([Leica] M8 & Depth of Fiel)
In reply to: Message from leewaileong19 at yahoo.com (Lee Wai Leong) ([Leica] M8 & Depth of Field)