Some dumb questions I have been wondering about

Started Mar 28, 2013 | Discussions thread
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Joseph S Wisniewski
Joseph S Wisniewski Forum Pro • Posts: 34,114
myth, often, misconception, and another

Wellington100 wrote:

1) Why are camera sensors not square? Surely a square sensor is the most efficient way to get the best out of a lens?

It's not. That's an old myth.

Any time you shoot a square and crop to a rectangle, your corners are no longer on the image circle, so you've lost some of your lens. Since 99.99% of images look better as rectangles, "the most efficient way to get the best out of a lens" is with a rectangular sensor of the same aspect ratio as your final image.

It's also not "the most efficient way to get the best out of" a camera, at least in the decades dominated by SLRs, because the longer swinging mirror of a square format has more vibration (roughly proportional to the cube of the aspect, so it's huge) and it requires a longer clearance distance (the "back focus") between the rear lens element and the sensor, complicating lens designs and decreasing lens quality.

2) Does using a Polarising filter reduce the Dynamic Range of an image taken in sunlight with a digital camera?

Often it does, if the brightest thing in the picture happens to be something that the polarizer can darken, like the sky, or a specular reflection such as glare on water. s

3) Why do most cameras have IQ reducing AA filters when the few cameras that don't have them jump in IQ and moire is nowhere to be seen in 99.99% of the images?

That's a nonsequitar, because it requires us to agree with your assessment that "moire is nowhere to be seen in 99.99% of the images". With good technique, moire is visible in about 5% of images, in my experience with cameras lacking AA filters. Whether the moire is bad enough to be a "picture killer" is another thing, but "nowhere to be seen in 99.99%" is wild hyperbole.

The people lacking in good technique (adequate attention to shutter speed, handholding technique, tripod use, aperture choice to avoid diffraction, careful focusing, etc) will have less moire, because their technique provides enough motion blur to act as an effective AA filter. But those people won't notice an IQ change from the lack of a filter.

We're only now entering a time when cameras often have such high resolution that even excellent attention to detail still results in enough blur to take the place of an AA filter. Give things another 5 years, and AA filters will become extinct.

4) What is the optimum resolution for small camera sensors?

About 500mp for APS, 1200mp for FF. Although there are reasons to go even higher.

High resolution sensors seem to add significant file size for little discernible improvement in resolution,

You can't have it both ways. If the 20% increase in resolution from not having an AA filter is "a jump", then the 20% increase from having 40% more pixels is "a jump" too, not "little discernible improvement".

so what is the cut off for a functional and well rounded small sensor

1/2 wavelength in the green, for resonant filters. About 5 gigapixels for APS. That also works well with photon counting techniques.

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Rahon Klavanian 1912-2008.
Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.
Ciao! Joseph

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