[Leica] M8 & Depth of Fiel

[henningw at archiphoto.com (Henning Wulff)]

At 9:01 PM -0800 10/31/06, Adam Bridge wrote:
>Wait a minute....this doesn't seem right. If you took film and cropped
>it to the same size as the digital sensor

Lee is right, basically. Just think: if you cropped the film to the 
same size as the digital sensor, then made a picture (say 8x10) and 
also made a picture from the uncropped film, the uncropped image 
would need less enlargement, and thus the slightly out of focus 
elements might still appear sharp enough, while the same unsharp 
element from the cropped film would be enlarged more and might not be 
acceptable anymore. In that sense, the depth of field will change.

As mentioned by Lee and John, depth of field is mutable in that it 
assumes a certain tolerance.

With film, when I needed a very sharp photo to the horizon, I used 
the aperture markings that were three stops wider on a lens to 
calculate the hyperfocal distance; ie, if I shot at f/16, I used the 
f/5.6 markings. Normally I would use one or two stops wider, 
depending on my purpose. The actual 'depth of field' markings on 
lenses were always too generous for my taste, and resulted in 
unacceptably mushy photos.

The latter part of Lee's argument gets a bit wayward, as now he's 
changing magnification as well.

The basic thing is that as you decrease sensor size and keep the 
angle of view the same, depth of field will increase. Easy to see 
when the difference in sensor sizes is big, as in say, a P&S and any 
digital SLR, whether APS-C or FF.

The effect is there in the difference between the M8 and a film M, as 
the increase in depth of field due to using a shorter focal length 
more than compensates for the reduction in depth of field due to the 
smaller sensor (greater magnification needed for the same print size).

This is, of course, when you're talking about the same angle of view.

>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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>>
>
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