HD Pentax-D FA 15-30mm F2.8 ED SDM WR

Aberrations

No lens can be totally free of optical flaws. The following list describes the main defects that a lens can suffer from.

Name
Description
Chromatic aberration Different colors do not have the same focus point. The result is colored lines (usually red or green) on edges showing a sharp transition from clear to dark tones, and a general decrease of the sharpness. Occurs mostly at wider apertures. Easy to correct via software. Mitigated by the use of achromatic lens elements. In simple terms, lateral CA occurs in the in-focus zones, while longitudinal CA occurs in out-of-focus zones.
Purple fringing Sometimes caused by chromatic aberration effects. Can also occur because the RGB color filters in front of pixels create differences in pixel sensitivities. Creates a purple band on edges showing a sharp transition from clear to dark tones. Occurs mostly at wider apertures. Easy to correct via software.
Flare Internal reflections on the various lens elements cause a decrease of contrast, the apparition of a bright veil, or ghosting. Occurs if an image includes bright light sources, especially if the light source is near the edge. Using a lens hood helps to control flare. Better lens coatings greatly reduce the effect. 
Ghosting A type of flare causing artifacts (orbs) to appear on an image including bright light sources, especially if the light source is near the edge. Can be used artistically.
Coma Flaws in the optical design cause point sources (such as stars) located on the sides of the frame to appear elongated. Dependent on the lens design.
Distortion Straight lines appear curved. Dependent on the lens design. Tested in another page.
Spherical aberration Light rays hitting the sides of the lens do not have the same focus point as those passing through the center. Mitigated by the use of aspherical elements.

Not all of those optical effects are easy to test independently. Purple fringing and chromatic aberrations are almost always coupled, and will be tested together. Flare and ghosting will also be measured as a pair. Distortion will get its own page later in this review. The other aberrations will not be formally tested as their effects are both harder to isolate and generally better controlled by design.

Flare

Flare will affect images in which a bright light source, such as the Sun, is present in the frame or near its border. The use of a lens hood helps reduce the effect for side lighting, as does a recessed front element. High-quality lens coatings play a very important role in minimizing flare, by improving light transmission and minimizing internal reflections.

The Pentax 15-30mm is a modern lens designed by Tamron, but using some of Pentax's newest and most renowned coatings. In particular, it uses the HD generation of coatings which superseded the venerable SMC. On the other hand, there is no indication that it benefits from Aerobright or other Pentax technologies. Its extreme wide angle will likely make it more sensitive to flare than most lenses.

Flare Test One - Center-of-Frame Sunlit Flare

For this test, we placed the sun near the center of the frame, directly illuminating the sensor.  We offset the sun slightly in order to see eventual ghosting which could be hidden if there was a straight line between the light source and the sensor. Pictures were captured at various apertures, at 15 and 30mm to evaluate the effect of the ultra wide angle. You can click on the thumbnails for larger views.

15mm 30mm
F2.8 15mm Center F2.8 30mm Center F2.8
F4 15mm Center F4 30mm Center F4
F5.6 15mm Center F5.6 30mm Center F5.6
F8 15mm Center F88 30mm Center F8
F11 15mm Center F11 30mm Center F11
F16 15mm Center F16 30mm Center F16
F22 15mm Center F22 30mm Center F22

Images appear almost devoid of flare when the sun is in the center. These results are impressive and commendable. There is the hint of a halo around the center, which is more visible at 30mm, but nothing overly noticeable.

Flare Test Two - Edge of Frame

For this test, we placed the Sun in the bottom left corner of the frame, directly illuminating the sensor. You can click on the thumbnails for larger views.

15mm 30mm
F2.8 15mm Corner F2.8 30mm Corner F2.8
F4 15mm Corner F4 30mm Corner F4
F5.6 15mm Corner F5.6 30mm Corner F5.6
F8 15mm Corner F88 30mm Corner F8
F11 15mm Corner F11 30mm Corner F11
F16 15mm Corner F16 30mm Corner F16
F22 15mm Corner F22 30mm Corner F22

The images at 30mm do not show any hint of ghosting or flare. At 15mm, some small, well-defined spots appear at F8 and smaller apertures.

These results could suggest a lens performing far beyond expectations. And in the right circumstances, it can be the case. However, it is still possible to create situations where flare will occur.

The image below is a scene comparable to our test images, but with the Sun lower on the horizon (later in the day), and located at the top. The lens is set at 30mm and F22. As can be seen, in this case ghosting is clearly present.

Ghosting

Another image shows a smaller light source, in this case a street lamp, just outside the field of view. This image was captured at 15mm and F5.6. Again, in this case ghosting is obvious.

Ghosting

These results show that, when used in the right conditions, the 15-30mm can shine regarding its control of ghosting. Push it too hard and it becomes obvious that like every ultra wide angle lens, it is not without flaws. In all cases, flare is kept tightly under control.

Chromatic Aberration Test

For this test we used a well-lit, sharp transition from dark to bright, in order to make manifest any chromatic aberration present in the image.

We then looked at three parts of the image: the focus point, the top and the bottom (beyond and before the focus point). You can click on the images to see 100% crops, and navigate by using the left-right arrows.

Center
Top
Bottom
F2.8 F2.8 Center F2.8 Top F2.8 Bottom
F4 F4 Center F4 Top F4 Bottom
F5.6 F5.6 Center F5.6 Top F5.6 Bottom
F8 F8 Center F8 Top F8 Bottom
F11 F11 Center F11 Top F11 Bottom
F16 F2.8 Center F16 Top F16 Bottom
F22 F22 Center F22 Top F22 Bottom

There is no hint of any color shift on our test images. Just like the 24-70mm, the 15-30mm shows stellar control over chromatic aberrations, both lateral and longitudinal.

Purple Fringing

There is no evidence of purple fringing in our test images. 

Verdict

The D FA 15-30mm shows excellent performance regarding chromatic aberration and purple fringing, with no hint of either across our test images. Superb!

Flare is globally absent from our test images. This is commendable. Ghosting can occur in some cases, but is globally kept under better control than many other modern lenses. These results are better than our expectations, especially for an ultra wide lens.


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