Originally posted by Dartmoor Dave I also find myself in the unfortunate position of not entirely agreeing with @Photoptimist, because I think he's oversimplifying how downsampling works in his example. But having said that, I know from experience that disagreeing with him can end up with me having egg on my face, because he actually knows a heck of a lot more than I do.
I do have to point out to Photoptimist though: The Windows 7 computer that I do my photo editing and viewing on has got no idea what physical screen size I'm using, but only the pixel resolution of the monitor attached. Nor does it have any idea what physical size sensor was used to take any particular photo, but just the pixel resolution. So any relationship between the physical sensor size and the physical viewing size is incidental rather than direct, caused only by the fact that each pixel on the monitor is far bigger than the pixels on the camera sensors (and the pixels in the monitor image are the result of downsampling too). And when we're talking about prints, I think we're better off sticking to non-interpolated prints of the full sensor resolution, because resampling algorithms are a factor better left out of a discussion like this.
LOL!
First, regarding downsampling: Yes, downsampling affects image sharpness and it affects the distances between image features and sizes of features if those distances are measured in pixels. A 2:1 downsampling will reduce pixel distances and sizes by half.
Second, regarding Windows and screens: You are probably right that neither Windows nor its apps know the physical pixel pitch of the monitor. It's possible that some clever apps do know the physical sensor size by reading the EXIF data, looking up the camera model #, and adjusting their automagical algorithms based on sensor size, lens, aperture, etc. But for purposes of DoF, Windows does not need to know this.
The bigger issue is not to lose sight of what DoF is really about.
DoF is about a person looking at a flat 2-D image (print, monitor, or projected slide) reproduced from a flat 2-D photosensitive system (a refracting lens in front of some film or silicon array) of a 3-D scene and then judging whether subject, foreground, and background are equally sharp. DoF is not about absolute sharpness but about the perceptual variation in sharpness with variation in subject distance. DoF is about how the system renders a 3-D world into a 2-D image and how the viewer judges the relative sharpness of different foreground and background objects relative to the subject in the plane of focus.
So, we have 4 key parts to the chain from the photographed scene to the photograph viewer:
1)
The 3-D scene: A bunch of objects at various distances to the lens and camera. Certain objects are the subject and others might range from the foreground in front of the subject to the background behind the subject.
2)
The 2-D photosensitive imaging system: A lens of some focal length and aperture that refracts and projects rays of light back to a 2-D photosensitive medium (film or silicon) of some overall physical size and grain size or pixel pitch. Rays of light from the subject at the plane of focus tend to converge almost to a point on the film or sensor. Rays of light from the foreground objects tend to converge almost to a point BEHIND the film or sensor which means those rays of light are a blurry circle where they hit the film or sensor. Rays of light from the background objects tend to converge almost to a point IN FRONT of the film or sensor and then diverge which means those rays of light are a blurry circle where they hit the film or sensor. For various messy mathematical reasons, the physical distances, physical focal length, and physical aperture all affect the physical size of the blur circle which can be measured in millimeters or microns or whatever.
3)
The 2-D image output device: A chain of processes that converts the captured camera image into viewable output of some physical size. The output could be a print, computer screen, TV, projected image, billboard, or whatever. The process may include scanning, resampling, interpolation, optical magnification, and output pixel pitch. Regardless of all the intermediate details, the overall point is that there is a scale relationship between physical distance in the final output and physical distance in the original film/sensor input. That is, 1 physical millimeter on the film or sensor comes out as N physical millimeters in the viewable output.
4)
The person: The visual acuity of the person and their distance to the output image determine how they judge the different parts of the image to be more or less sharp.
To get the right answer for DoF, the photographer really needs to start with the person viewing the output image. That viewer might be pixel peeper at close range on a computer screen, an art gallery customer browsing prints on the wall, or they might be driving by a billboard showing a beautiful breakfast of eggs, bacon, toast, coffee, for a restaurant in the next town. Those viewing conditions will determine the acceptable CoC for the 2-D output image. In turn, the 1:N relationship between the camera and the output will determine the CoC on the film or sensor. Next, it's up to the photographer to decide what they want "in focus" versus "out-of-focus." Once the photographer decides the desired range of in-focus foreground-to-background distances (with all else being out-of-focus), they then need to use a DoF calculator and the estimated CoC to crunch the math and get the best aperture and subject distance setting that renders the 3-D scene into the desired 2-D output.
For most "normal" print viewing conditions and most modern high-resolution cameras, the sensor pixel size doesn't really affect CoC. Obviously, the pixel peeping scenario is different. There, the CoC might be only a pixel or two especially for pixel shift or super-resolution images viewed by the most persnickety pixel peepers.
Originally posted by Dartmoor Dave Note to Photoptimist: I'll take the eggs on my face fried sunny side up, with nice crispy brown edges please. No doubt you'll be serving them up to me soon, with some math on the side.
I can only hope that both the near and far edges of this big egg of a post are crisp. But that will depend on your perception, your viewing distance from the post, and how the internet and Windows resamples or magnifies these words.