Tischer's Review of the Sigma 28mm F1.8

The reason I bought this lens was because of the large aperature. This lens lets in about 10 times the light as my Sigma 12-24mm f4.5-5.6 at 24mm.

Unfortunatly, I noticed focus problems with this lens straight out of the box. The following experiments verify my lens is front focusing. The lens was returned to Sigma to be rechipped and recalibrated. The lens was returned to me from Sigma with no improvement in focusing accuracy. I subsequently bought a second copy, and it was identical to the first. If this lens could focus correctly, it would be impressively sharp wide open. I've decided not to keep this lens, and based on reports of others recieving four badly focusing lenses in a row, I decided not to try other samples.
AF MF - 2mm MF - 4mm MF - 6mm


The left most picture in the series above was taken with auto focus on. The subsequent pictures were taken with the lens in manual focus. For each subsequent picture, the target was moved aproximatly 2mm closer to the lens.

All pictures were taken with the camera mounted to a tripod. The focus ring was always disengaged. Shutter speed was 1/60" and a flash was used. Only the center AF sensor was enabled, and it was centered on the bar code. The test pattern was aproximatly 6 inches from the lens (11 inches from film plane) well within the range of focus.

The results from this test estimate the lens is focusing aproximatly 6 mm in front of the target.



In this test, the lens front focused about 14CM at a distance of about 130CM. The subject is clearly out of focus.


Sigma auto focused at 36" away 200% center crop f/4 Sigma set to man focus, then moved to 33" away f/4








My test pattern was downloaded from http://www.photo.net/

In this test, the pattern is photographed at a 45 degree angle. Only the center autofocus point is selected, and aimed at the line located at 0 cm. The numbered ticks are scaled at 1.414 centemeters, this coresponds to a distance of 1 cm distance from the lens at 45 degrees.

The results from my test show the lens is clearly front focusing. I placed a red dot where I considered the center of focus to be. The lines before and after the red dot appear to be similarly focused, the next set of lines beyond those appear similarly out of focus. The first "1" is much more infocus than the second "1".

The results from this test show the lens is focusing approximately 5mm in front of the target. The result from this test verifies the first test's results.



this is what the test patern looks like uncropped.

Despite the close focusing issues I tested this lens against my other lenses in the 28mm range. The following pictures are 100% crops about 300 pixels from the corner of the frame. The Sigma 28mm f1.8 had a polarizer on because it was too bright to shoot at 1.8. I took several shots in manual focus and auto focus at f/1.8. The autofocus seemes to be ok focused at infinity.
Sigma 28mm f/1.8 VS Canon 18-55mm EFS VS Sigma 12-24mm f4.5-5.6

Sigma 28 @ f1.8
Sharpened in Photoshop



Sigma 28 @ f1.8 Sharpness = 77



Sigma 28 @ f2.8 Sharpness = 87



Sigma 28 @ f4 Sharpness = 91

Canon 18-55mm @ 28mm f4.0 Sharpness = 67

Sigma 12-24mm @ 24mm f5.6 Sharpness = 79

Sigma 28 @ f8 Sharpness = 93

Canon 18-55mm @28mm f8.0 Sharpness = 80

Sigma 12-24mm @ 24mm f8 Sharpness = 85

Sigma 28 @ f10 Sharpness = 91

Canon 18-55mm @28mm f11 Sharpness = 84

Sigma 12-24mm @ 24mm f11 Sharpness = 84

Sigma 28 @ f16 Sharpness = 88


My sharpness rating is based soley on file size. A rating of 91 means the file size is 91Kb. Don't laugh, it seems to work!




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Macro

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