Depth of Field or Depth of Confusion? (SL2 vs SL3 / SL3-S)

[Stef63]

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Depth of field is not fixed for a given focal length — it depends on the Circle of Confusion (CoC) factor used, which defines what is considered “acceptably sharp”. Classic lens engravings were designed for film and typically assume a CoC around 0.03 mm.  On modern high-resolution digital bodies, this assumption is often too optimistic, so manufacturers may use a much stricter CoC for in-camera DOF indications. Leica is almost certainly aware of this and likely adapts the DOF readout accordingly.  I would like to know what CoC they are using and is this value different for different sensors. 

Experiment 1 : On my SL2, using a 35 mm lens, if I set:  f/8 and dial the far limit to 7 m the top display shows a near limit of about 5 m.

Experiment 2 : With a manual 35 mm M lens (e.g. Summilux-M 35/1.4), using the engraved DOF scale at f/8 and far = 7 m, the near limit is roughly 2.5 m, which matches the classic CoC ≈ 0.03 mm.

Conclusion : The SL2 result (≈ 5 m to 7 m with a 35 mm lens) suggests Leica is using a much stricter CoC, roughly in the 0.005 mm range, for its DOF display.

My question now is: Does this CoC changed on newer bodies or with different sensor sizes (i.e. different pixel sizes, and does Leica use a different CoC for these different sensors?  Can owners of an SL3 (60 MP) or SL3-S (24 MP) please try the same test with a 35 mm lens?  Set f/8. Set far distance to 7 m. Report the near distance shown on the top plate

I’m curious whether:

• the SL3 shows the same near limit as the SL2,

• the SL3 is more conservative due to higher resolution,

• or the SL3-S is less conservative due to lower resolution.

BTW Reason I started this experiment is that I noticed in a Leica S3 - that has the same DOF indication feature on the top display - Leica clearly uses a different CoC value for this sensor type than in the SL2. 

[jaapv]

It is the same story as motion blur, lens aberrations, non-CA purple fringing etc. The higher the pixel count, the more precisely the camera records and the smaller the tolerance margins become. 

On a sensor the CoC is a diffuse value - read up on Airy patterns, Airy disks, diffraction etc.

So yes, your question is a valid one, As DOF is a matter of perception by the viewer (and also determined by subject matter and contrast) the precision of rendering will narrow it perceptibly as pixel count increases. 

[3D-Kraft.com]

The size of the circle of confusion (or motion blur or CA etc.) is just optically determined and has nothing to do with sensor pixel pitch or film grain.

Of course, with a higher pixel pitch, more pixels are involved in recording the circle of confusion but this only becomse visible in 1:1 (100%) magnification.

The size of the CoC for a DoF calculation only depends on viewing distance and image size (print/screen size) assuming that the resolution of the human eye is also a given quantity (approx. 1-2 arcminutes).

Weird, why the SL2 calculates with a CoC of 0.005 mm...

Edited 6 hours ago by 3D-Kraft.com

[jaapv]

I always struggled with the concept of precise calculation of a subjective matter. 

[hoolyproductions]

I'll oblige.

With my SL3 and 35mm SL I get : Back 7m, Focus 4.6m, Near 3.4m

Interestingly my SL3-S gives: 7m, 3.8m, 2.7m

[LocalHero1953]

Perhaps Leica sticks to the 'classic' CoC for M lenses to avoid confusion with the DoF marks on the lenses? Which are intended to be used on both film and digital sensor.

For L-mount/AF lenses they are under no such constraint.

[Photoworks]

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3 hours ago, hoolyproductions said:

Interestingly my SL3-S gives: 7m, 3.8m, 2.7m

what is if you focus 4.6m ?

[Photoworks]

I am no expert on this, but does it not change with lens design

I have found that the SL-APO lenses have less DOF than other lenses in the same focal length, or maybe it is just more noticeable?

[hoolyproductions]

1 hour ago, Photoworks said:

what is if you focus 4.6m ?

10m, 4.6m, 3m