Sharpness Comparisons for several digital cameras

Sharpness results derived from Websites

Several websites publish images of ISO 12233 test charts that can be entered into Imatest SFR to obtain detailed sharpness measurements. This page contains sharpness results derived from charts downloaded from two sites:

• Dpreview.com (DPR)  
• Imaging-resource.com (IR)  

The measurements in the tables below come from two areas of the ISO 12233 image, illustrated by red rectangles in the image below.

• First row: near the center. The rectangle just above the center of the chart (about 15% of the distance to the corners).
• Second row: near the corner. The rectangle in the lower right of the chart (about 78% of the distance to the corner). Results include Chromatic aberration, which cannot be measured near the center.

These measurements are limited to

• One lens. For DSLRs this is usually one of the sharpest available, such as the 50mm f/1.4 prime (non-zoom).
• One focal length. Sharpness is a strong function of focal length for zoom lenses. The sites aren’t very consistent in their choice of focal length for zooms in compact digital cameras; long, short or intermediate focal lengths may be chosen.
• One aperture, typically around the optimum: f/8-f/11 for DSLRs; f/4-f/5.6 for compact digital cameras.
• One ISO speed, which strongly affects noise and dynamic range, but has little effect on sharpness.
• One sharpening radius, typically 2, used in standardized sharpening calculations, indicated by (corr.) and red columns.
• One or two vertical edges (in most cases), resulting in horizontal MTF measurements. Sometimes vertical MTF (from horizontal edges) is different.
• One RAW converter (usually the JPEG converter built into the camera) with one setting, usually the default. The choce of converter and sharpness setting has a strong effect on sharpness measurements.
• Gamma assumed to be 0.5, typical for digital cameras. A 10% gamma error results in about a 2.5% MTF50 error. Though gamma error is rarely severe, gamma should be measured with Stepchart or Colorcheck for greatest accuracy.
• Exposure and lighting are not always optimum. If the image is too dim or bright there may be some clipping on data (where it goes pure black or white), which reduces the accuracy of the sharpness measurement. Lighting may not be as even as it should be.

If you want to compare your own camera or lenses with the published charts, test lenses at various focal lengths and apertures, or observe the effects of ISO speed and signal processing, you’ll need to download Imatest. The trial version allows up to twenty runs.

Explanation of results

Tables of results

DSLRs: Digital SLRs, which take interchangeable lenses. Used by professionals and serious amateurs. Sensor size is at least 22 mm diagonal. Larger and heavier than the compact digital cameras, below. This table is more complete than the table for compact digitals because it’s where my interest lies (and there are far too many compacts to keep up with). For each camera, the results in the upper and lower rows are for the image center and edge, respectively, as illustrated here.

Reminder: Remember the limitations listed above: one lens, one focal length, one aperture, one ISO speed, one RAW converter & settings (usually) built-in, one sharpening radius (2 unless otherwise indicated), etc., when interpreting these results. Also remember that several important image quality factors such as noise and dynamic range have been omitted.

Compact digital cameras: These are primarily consumer cameras, though many produce excellent image quality, especially for letter-size or A4 prints. Most have non-interchangeable zoom lenses. Maximum sensor size is 11 mm diagonal.

Interpretation of MTF50

What MTF50 do you need?  It depends on print size. If you plan to print gigantic posters (20x30 inches or over), the more the merrier. All the cameras in the above tables can make excellent 8.5x11 inch (letter-size; A4) prints. At that size the best of them wouldn’t look much better than the 4 megapixel Canon G3 (or any other high quality 4 megapixel camera).

With fine lenses, careful technique, and a little extra sharpening, my 6.3 megapixel Canon EOS-10D (corrected MTF50 = 1335 LW/PH) makes very good 12x18 inch prints. Prints are quite sharp from normal viewing distances, but pixels are visible under a magnifier or loupe; the prints are not as sharp as the Epson 2200 printer is capable of producing. Prints made with the EOS-20D (corrected MTF50 = 1581 LW/PH) are have a slight edge: I’d call them excellent.

Softness or pixellation would be visible on 16x24 inch prints made from the EOS-10D. The EOS-20D would perform slightly better, and several of the cameras listed in the table— especially the full frame (24x36mm sensor) DSLRs— perform even better.

The table below is an approximate guide to quality requirements. The equation for the left column is

Example of using the table: My Canon EOS-10D has MTF50 = 1335 LW/PH (corrected; with standardized sharpening). When I make a 12.3 inch high print on 13x19 inch paper, MTF50 is 1335/12.3 = 108 LW/in: “very good” quality; fine for a print that size. Prints look excellent at normal viewing distances for a print this size.

This approach is more accurate than tables based on pixel count (PPI) alone. Pixel count is scaled differently; the numbers are around double the MTF50 numbers. The EOS-10D has 2048/12.3 = 167 pixels per inch (PPI) at this magnification. This table should not be taken as gospel: itwas first published in October 2004, andit may be adjusted in the future.

Some observations

There has been a good deal of skepticism about 7+ megapixel compact digital cameras. They live up to expectations for sharpness, but their noise may be objectionable at high ISO speeds. Dpreview.com has found the Sony DSC-828’s noise at ISO 64 to be as high as the Canon EOS-300D (similar to the EOS-10D) at ISO 800.

The 6 megapixel raw/12 megapixel interpolated Fuji S7000, which uses Fuji’s SuperCCD, has sharpness comparable to the Sony F828. It also has some problems with noise. The SuperCCD is based on an interesting concept. Thanks to gravity, most detail in nature is vertical or horizontal, and evolution has made our eyes are more sensitive to those directions. SuperCCD pixels are located on a diagonal grid where the spacing between x and y-pixel spacings is 1/sqrt(2) = 0.707 that of a normal grid the same number of pixels. The diagonal grid is interpolated to a horizontal grid that has twice the pixels, keeping the reduced pixel spacing. This results in an impressive gain in horizontal and vertical resolution, though sharpness is slightly below that of a sensor with the same number of uninterpolated pixels.

I analyzed the sides of Fuji F7000 because dpreview.com noted a problem with softness on the left side. Uncorrected MTF50 is the appropriate metric for comparing portions of a single image. The problem is indeed very evident. It seems to be slightly worse on the left, where MTF50 (uncorrected) is 62% of the center value. The lens alsohas a problem with chromatic aberration. The Sony DSC-F828 does much better: edge MTF is 93% of the center value. It apears to have a finer lens, at least at the focal length and aperture chosen for the test.

The Sigma SD9 has high resolution for its pixel count because it doesn’t have, and probably doesn’t need, an anti-aliasing filter. If only Foveon could break the 3.4 megapixel barrier…

Links

Scanner Bake-off 2004  Jim Hutchinson used Imatest results in his comparison of scanners.