Potential dead horse: how bad is FF's deep DoF disadvantage?

Started 9 months ago | Discussions thread
Mike Davis
Contributing MemberPosts: 690
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Larger sensor = more f-Numbers from which to choose...
In reply to sportyaccordy, 9 months ago

A larger sensor = more f-Numbers from which to choose without concern for diffraction inhibiting a desired print resolution at an anticipated enlargement factor.

Many tiny-sensor cameras have lenses offering f-Numbers that would allow diffraction to inhibit a desired print resolution of 5 lp/mm at the enlargement factors encouraged by the number of megapixels they deliver. In other words, with many tiny-sensor cameras, you have to shoot wide open (or nearly so) to actually exploit the resolution one might anticipated with a given number of megapixels.

With equivalent Pixel Count, DoF, Diffraction, Print Size, and Viewing Distance...

Small Sensors can give us the same DoF and diffraction as larger sensors, but with faster shutter speeds at smaller f-Numbers, and thus, fewer "diffraction-free" f-Numbers from which to choose (unless the lens is much faster than the lenses available with most small-sensor cameras).

Large Sensors can give us the same DoF and diffraction as smaller sensors, but with slower shutter speeds at larger f-Numbers, and thus, more "diffraction-free" f-Numbers from which to choose.

It's the higher enlargement factor required by small sensors having the same pixel count as larger sensors that forces the use of smaller apertures (smaller Airy disks at the sensor) before magnification, to produce like-sized, like-resolution prints.

The cameras with tiny sensors suffer greater enlargement factors to achieve a given print size, thus, both the Circles of Confusion (defocus) and Airy disks (diffraction) must be smaller at the sensor, in order to be the same size after magnification in the print.

So, aside from the better signal-to-noise ratio had with larger sensors, especially at higher ISO settings, the only difference between shooting with a small sensor vs. a large sensor is finding yourself with fewer diffraction-free stops from which to choose - with the smaller sensor forcing the diffraction-savvy photographer to shoot wide open or nearly so, while the larger sensor allows a choice of several f-Numbers that will not inhibit the same desired print resolution in a given print size.

I find the popular Sony Cyber-shot RX100 II to be remarkable in that it does not offer any maximum apertures (at any focal length) that would inhibit a desired print resolution of 5 lp/mm in a non-resampled 360 ppi print (which would yield dimensions of 10.1 x 15.2 inches). Diffraction would not begin to inhibit a desired print resolution of 5 lp/mm at the 29.25x enlargement factor until one attempts to use f/5.1, but the RX100 II's maximum apertures across its zoom range vary from a relatively fast f/1.8 (at 10.4mm) to f/4.9 (at 37.1mm). That lens speed allows a couple of extra diffraction-free stops from which to choose (vs. a slower lens on a like-sized sensor), when attempting to actually exploit the resolution offered by its 20 megapixels.

But the RX100 II does allow the use of f-Numbers as large as f/11 (actually f/11.3), which thanks to diffraction, would effectively reduce the print resolution by a factor of 2.2 (vs. shooting at f/5.1), yielding a 10.1 x 15.2-inch print with true subject detail rendered at a resolution of 2.3 lp/mm (equivalent to 163 ppi) vs. the 5 lp/mm (equivalent to 360 ppi) that can be had at f/5.1 or smaller f-Numbers.

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