11-21-2011, 12:12 PM
[quote name='Brightcolours' timestamp='1321835967' post='13087']
Airy disks describe something that exists. Diffraction exists too. CoC does not. It is just a value to make calculations make sense, and how CoC is determined depends on the situation, or what you are trying to determine.[/quote]
I've been trying to explain that it does matter, as CoC size is directly related to DoF, because of the definition of DoF and sharpness in the DoF zone. A certain size medium at a certain print size, whether on screen or on paper, or projected for that matter, wil have a completely different DoF when the CoC is larger or smaller, and from this POV it is immaterial whether it is perceived as sharp or not. This is why a standard is used, based on the acuity of the human eye, so we can compare things, and that explains why for different recording media formats a different size CoC is required in order to get the same perceived sharpness, all according to the definition.
When the recording medium gets smaller, you inherently need smaller CoCs to get the same sharpness in the DoF zone. And although this is roughly related to crop factor and hence FL, because AoV is not linear with FL (most noticeable at large AoVs), and because DoF is not linear either, the linearity which we use for ease of calculation in the field, is not entirely correct. This is also the reason why CoCs are not entirely linear with regard to crop factor.
For "equivalent" FL calculation, only crop factor is important, yes.
It does come into play when resolution is not enough for the standard definition of DoF. It also comes into play because of changing AoVs at certain FLs. By definition, shorter FLs, independent of AoV, will have more DoF, because the area, over which the detail sharpness will fit within the point sharpness as defined by the CoC, extends drastically. This is why a crop sensor camera has more DoF at the same AoVs.
DoF is not just something that always is a given according to the DoF calculators, or the DoF scale on a lens. Those are all based, ultimately, on CoC used for the DoF calculation, which is linked to the definition of a certain print size, viewed at a certain distance, taking the acuity of the human eye into account.
As you well know, looking at a tiny web sized image, in many photographs everything looks sharp in focus. However, look at it at 100%, say 4500 by 3000 pixels, and it may well appear unsharp, with maybe nothing in focus or sharp. And at standard size and viewing distance, it may well adhere to the criteria used for calculating DoF, hence a certain CoC, for the format used of the recording medium, and look sharp.
Of course, one can choose to ignore this. For 9cm by 6 cm prints this doesn't matter much, and neither does it for small web images. It does, however, for larger prints, both CoC and perceived DoF, and resolution.
Regards, Wim
Airy disks describe something that exists. Diffraction exists too. CoC does not. It is just a value to make calculations make sense, and how CoC is determined depends on the situation, or what you are trying to determine.[/quote]
I've been trying to explain that it does matter, as CoC size is directly related to DoF, because of the definition of DoF and sharpness in the DoF zone. A certain size medium at a certain print size, whether on screen or on paper, or projected for that matter, wil have a completely different DoF when the CoC is larger or smaller, and from this POV it is immaterial whether it is perceived as sharp or not. This is why a standard is used, based on the acuity of the human eye, so we can compare things, and that explains why for different recording media formats a different size CoC is required in order to get the same perceived sharpness, all according to the definition.
When the recording medium gets smaller, you inherently need smaller CoCs to get the same sharpness in the DoF zone. And although this is roughly related to crop factor and hence FL, because AoV is not linear with FL (most noticeable at large AoVs), and because DoF is not linear either, the linearity which we use for ease of calculation in the field, is not entirely correct. This is also the reason why CoCs are not entirely linear with regard to crop factor.
Quote:In the case of DOF on APS-C and FF with equivalent lenses (and then of course we are NOT talking about different print sizes, nor different or even defined view size, let alone pixel pitch), only the crop factor to determine the equivalent lenses is of influence.
For "equivalent" FL calculation, only crop factor is important, yes.
Quote:You can assume whatever sensor resolutions, it is not important here. So leave it out of the discussion... That is just muddying it. Sensor resolution/pixel pitch only comes into play to determine how big one can print with a certain viewing distance in mind.
It does come into play when resolution is not enough for the standard definition of DoF. It also comes into play because of changing AoVs at certain FLs. By definition, shorter FLs, independent of AoV, will have more DoF, because the area, over which the detail sharpness will fit within the point sharpness as defined by the CoC, extends drastically. This is why a crop sensor camera has more DoF at the same AoVs.
DoF is not just something that always is a given according to the DoF calculators, or the DoF scale on a lens. Those are all based, ultimately, on CoC used for the DoF calculation, which is linked to the definition of a certain print size, viewed at a certain distance, taking the acuity of the human eye into account.
As you well know, looking at a tiny web sized image, in many photographs everything looks sharp in focus. However, look at it at 100%, say 4500 by 3000 pixels, and it may well appear unsharp, with maybe nothing in focus or sharp. And at standard size and viewing distance, it may well adhere to the criteria used for calculating DoF, hence a certain CoC, for the format used of the recording medium, and look sharp.
Of course, one can choose to ignore this. For 9cm by 6 cm prints this doesn't matter much, and neither does it for small web images. It does, however, for larger prints, both CoC and perceived DoF, and resolution.
Regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....