Pentax-FA 43mm F1.9 Limited (APS-C)

Sharpness

Now, let's move on to the FA 43mm's image quality, starting with its shaprness.  We tested the lens's sharpness in our studio using our typical star charts.

The test charts were shot with a Pentax K-50 as JPGs at ISO 100.  The later the lines converse, the sharper the image.  Click on any thumbnail for the full-size image.

	Center	Corner
F1.9
F2
F2.4
F2.8
F3.5
F4
F4.5
F5.6
F6.7
F8
F9.5
F11
F13
F16
F19
F22

Center sharpness peaks at F5.6 and F6.7 with F4, F4.5, F8 and F9.5 close behind. Due to diffraction, F11 is slightly worse than even F2.8. The MTF program of the K-3 keeps the aperture in the range F2.8 to F8, with a preference for F8. We'd say that F9.5 is perfectly acceptable as well, so if center resultion is the main concern, use the interval from F2.8 through F9.5 in your everyday shooting. F1.9 and F2 are visibly softer and have less contrast than F2.8, but not to the point where these apertures should be avoided altogether.

Corner sharpness is a different story. Not until F4 do we get sharp and contrasty results, so using the lens wide open is only recommended if the subject being photographed doesn't have any fine details in the corners. On the other hand, this makes the lens suitable for portrait photography at the wider aperture settings, since corner softness and narrow depth of field are often desired for portraits. Resolution and contrast peaks at F8 and F9.5. Corner sharpness at F9.5 is comparable to center sharpness at F3.5, but corner contrast is visibly lower.

As indicated by the test charts, this lens is able to produce excellent resolution and detail across the frame when stopped down some.  We illustrate this in the photograph below, which was taken at F9.5 with a Pentax K-3. We focused on the building in the background. 

Click the image to enlarge or click the link to the full size JPG image developed from DNG RAW.

Click to enlarge | Link to full size JPG image

The next image was also shot at F9.5 with a Pentax K-3, and then heavily cropped (click to enlarge).

Click for 100% crop

Verdict

The smc Pentax-FA 43mm F1.9 Limited is able to produce excellent sharpness and contrast when stopped down by about 2 stops. We recommend the range F4 to F9.5 for general photography. The corners are disappointingly-soft when the lens is used wide open and even at F2.8 and F3.5. This can benefit portrait photography of course, but using the larger apertures for scenes where corner detail is important is not recommended.

Although discernible at F11, diffraction has no significant impact until F16 and smaller.

Note that we verified our findings with respect to shaprness with two separate copies of the FA 43mm.