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dof calculations vs SL2-s


aj55
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Dear all,

 

I compared several online dof calculators vs my SL2-s with SIGMA lenses. Focused on 3 meters and aperture at F/16.

I wanted to get a grip on dof when shooting landscapes. 

I am puzzled by the differences. Anyone any idea why the Leica is so different?

DOF calculations.pdfDOF calculations.pdf

 

Cheers, Arnold

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Arnold,

Depth-of-field calculators use an arbitrary value called "circle of confusion" to determine what is acceptably sharp and what is not.

There are several issues with this.

First, traditional CoC numbers are based on 6x9cm prints. That's not a very demanding standard, although it could apply to small smartphone screens. For this reason, many people use tighter CoC values, although the actual size isn't agreed-upon. That's probably why Leica's DoF estimates are a lot more stringent than the online calculators': they expect that you will make large prints.

Second, DoF calculators assume that all lenses have the same distribution of sharpness in-front-of and behind the plane of focus. That's not the case. The actual distribution of sharpness varies by lens and by focusing distance. I don't know if L-Mount lenses communicate this data to cameras (and if they use it), but online calculators definitely do not.

To show you how much difference lenses make, you used to be able to buy guides that listed depth of field for cinema lenses. Cinema has tighter DoF requirements, because of the "big screen", and because several characters need to be in focus at the same time. This was so important that you couldn't just rely on one chart for every 50mm lens (for instance). You needed one for Zeiss, one for Cooke, one for Panavision, etc., plus the 50mm position of cinema zooms. The last thing you want is for an important character to be blurry, forcing you to re-shoot an entire scene at huge expense!

 

In your case, the best thing to do is experiment. Take some test pictures with distance measurements, and try to figure-out which DoF calculator is the most accurate for you.

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Posted (edited)

It is even worse still. Take a wall mural, that is sharp at 5 meters and go with your reading glass at 0.5 meters.

At the plane of focus, that is the distance on which the lens is set, the optimal sharpness of the lens is reached. In the back and front of this distance the sharpness gradually degrades. I remember, Leica had the 1/30 mm rule. That means, if a point light grows to 1/30 mm on the negative, it was a point still (per definition). With  this definition the DOF distances were/are calculated.

The SIGMA data are remarkable. Where did you find these data?

Edited by jankap
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Posted (edited)

As mentioned above, DoF is dependent on circle of confusion acceptance thresholds, the smaller the acceptance threshold, the narrower the depth of field. So it largely depends on what CoC values are used in the calculations.  It should also be noted that CoC threshold depends on what contrast for CoC diameter is considered acceptable. It’s conceivable that a given sensor resolution is used to define the CoC diameter but different calculations use different contrast thresholds to define what is acceptable or not. According to Peter Karbe, 50% contrast will be perceived as ‘sharp’. So it wouldn’t surprise me if the SL2-S uses 50% contrast as a parameter to define CoC thresholds. On the other hand, it’s not uncommon for DoF calculators and camera manufacturers to use less stringent contrast parameters like 10% to define acceptable CoC thresholds. In which case,  you can have the same lens, sensor, etc… but one definition of acceptable CoC and contrast says that the DoF goes out to infinity while a more stringent definition of adaptable CoC and contrast says something very different.

Edited by beewee
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There are times when this forum is a highly valuable source. Finally, I learned to what CoC in stills photography is related.

I can’t add/emphasise enough how different lenses can be in terms of CoC although everything is equal and only the manufacturer and version/build year change.

As a side note to CoC, There’s a huge difference in perceived and measured sharpness, being the first one by far the more important one. Perceived sharpness can be determined by the size of the projection, the distance to the projection or image, and the kind of image. Eg, sharp long shot, panorama shot vs portrait, close up with blurry backgrounds, and anything between. And of course contrast, both luminance and colours (warm/cold), add to the perceived sharpness too.

In the end it’s highly subjective. Sharpness to some extent even depends of how much we like the picture. 

In terms of sharpness in landscape photography, misty, bluish mountains in the back or a gentle mist hanging over a lake can add with their softness considerably to the perceived sharpness of the foreground. 

And lastly, the aperture one chooses can add to that too, of course. At f 16, however every lens I know, even the incredible Zeiss Masterprimes, lose visible sharpness due to diffraction. 

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@Jankap,

The data were either read of the top screen of my SL2-s or as calculated by the online programs whose links are at the bottom of the pdf.

All in meters btw......

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