Diffraction Effects (Example Photos)

Started Nov 27, 2011 | Discussions thread
Mike Davis
Contributing MemberPosts: 686
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Re: Excellent point Lance!
In reply to Mike Davis, Dec 17, 2011

(...Continued from above)

For calculating either the CoC diameter that you can specify in your choice of DoF calculator (of those that permit user-specification of max. CoC) or for calculating the f-Number at which diffraction will begin to inhibit a desired final image resolution at an anticipated enlargement factor, here are some definitions of the two variables used in each equation:

Enlargement factor is simply the ratio of the final image dimensions to your sensor or film dimensions. (Divide print diagonal by sensor diagonal, or print width by sensor width, for example.)

For your desired final image resolution, I recommend a value of 4- to 8-lp/mm when you want the print to survive scrutiny at a viewing distance of 10 inches. 4 lp/mm will satisfy most people at that viewing distance and anything greater than 8 lp/mm would be overkill (despite Ctein saying that some people can appreciate resolutions as high as 25 lp/mm in a print viewed at 25cm). If you only want to satisfy a viewing distance of 20 inches, you can cut my recommendation for desired resolution (at 10 inches) in half.

A desired final image resolution of 2 lp/mm requires an unresampled image file resolution of 100 dpi (for a sensor that lacks an AA filter) or 144 dpi for a typical CMOS sensor, due to a 30% loss of resolution typically imposed by the RGBG Bayer algorithm and AA filter.

A desired final image resolution of 3 lp/mm requires an unresampled image file resolution of 150 dpi (no AA filter) or 216 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 4 lp/mm requires an unresampled image file resolution of 200 dpi (no AA filter) or 288 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 5 lp/mm requires an unresampled image file resolution of 250 dpi (no AA filter) or 360 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 6 lp/mm requires an unresampled image file resolution of 300 dpi (no AA filter) or 432 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 7 lp/mm requires an unresampled image file resolution of 350 dpi (no AA filter) or 504 dpi (with 30% Bayer and AA losses).

A desired final image resolution of 8 lp/mm requires an unresampled image file resolution of 400 dpi (no AA filter) or 576 dpi (with 30% Bayer and AA losses).

Before you can select a "desired" final image resolution, you have to be realistic when selecting an enlargement factor.

For example, the Nikon D300's 15.7 x 23.7mm sensor captures 4288 x 2848 pixels (12.21 MP) at a moderate pixel density of 181 pixels/mm.

Taking into account the loss of resolution caused by the RGBG Bayer algorithm and AA filter, typically a 30% loss relative to actual pixel count, if you "desire" to record subject detail in the final image at a resolution of 5 lp/mm, you'll be limited by pixel count to the following print dimensions:

Max. 5 lp/mm Width: 4288 pixels / 360 dpi = 11.91 inches
Max. 5 lp/mm Height: 2848 pixels / 360 dpi = 7.91 inches

Assuming you plan to make 7.91 x 11.91-inch prints (where the Pixel Count will support a desired print resolution of 5 lp/mm), your enlargement factor (without cropping) would be 11.91 inches / 23.7mm = 302.54 mm / 23.7mm = 12.8x

Now let's run the two equations for an anticipated viewing distance of 25cm (9.84-inches):

FOR CONTROLLING DEFOCUS:

CoC (mm) = 1 / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement factor

CoC (mm) = 1 / 5 / 12.8 = 0.0156 mm

Your largest CoCs, which will occur at the near and far limits of DoF, must not exceed 0.0156 mm at the sensor, before enlargement. Plug this CoC diameter into your choice of DoF calculators (www.dofmaster.com, for example), then follow the calculator's recommendations for setting both your working aperture and your focus distance.

FOR CONTROLLING DIFFRACTION:

N = 1 / desired print resolution (lp/mm) / anticipated enlargement factor / 0.00135383

N = 1 / 5 / 12.8 / 0.00135383 = 11.54 (or f/11.54)

Thus, even when your DoF requirements indicate that you should use an f-Number larger than f/11.54, doing so would cause diffraction to inhibit your desired goal of 5 lp/mm in prints that require an enlargement factor of 12.8x.

(Continued below...)

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