HD Pentax-DA 55-300mm F4.5-6.3 PLM WR RE

Aberrations

No lens is totally free of optical flaws, but some get closer than others. Below are listed the main defects that a lens can suffer from. You've probably seen all of these from time to time.

Name
Description
Chromatic aberration Different colors don't 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 in sharpness. This occurs mostly at wider apertures. It's easy to correct via software, and can be mitigated with 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 This is sometimes caused by chromatic aberration effects, but can also occur because the RGB color filter in front of a sensor's pixels create differences in sensitivity. The result is a purple band along high-contrast edges. This occurs mostly at wider apertures, and is easy to correct via software.
Flare Reflections on various internal lens elements can cause a decrease of contrast, the appearance of a bright veil, or ghosting. This occurs if an image includes bright light sources, and especially if the light source is near the edge of the frame. Using a lens hood helps to control flare, and better lens coatings greatly reduce the effect. 
Ghosting This is a type of flare that causes artifacts (orbs) to appear in an image that includes bright light sources, especially if they're near the edge of the frame. These can be used artistically.
Coma Flaws in the optical design cause highlights (such as stars or other distant lights) on the edges of the frame to appear elongated and ovoid, rather than round. This is dependent on lens design.
Distortion Geometric distortion causes straight lines to appear curved, and is dependent on the lens design. 
Spherical aberration Light rays hitting the sides of the lens do not have the same focus point as those passing through the center. The effects can be 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, so we tested them together. Flare and ghosting will also be measured as a pair. Distortion has already been covered. We didn't test the other aberrations, since their effects are harder to isolate, and less prevalent in modern lenses like the 55-300mm PLM.

Flare

To test flare resistance, we aimed the lens at the sun and fired away, placing it at various points in and just outside of the frame. Below, we've excerpted shots with the sun in the top right corner of the frame and just outside the frame in the same corner.

To a certain extent, the 55-300mm PLM is naturally resistant to flare thanks to its range of focal lengths. With such a narrow field of view, the chances of getting the sun in the frame are very low unless you're intentionally shooting a sunset or sunrise. Its flare resistance is also bolstered by the use of Pentax's latest HD and AeroBright coatings, which do an excellent job of warding off unwanted rays.

Hood On, Sun in Corner of Frame, f/8 Hood Off, Sun in Corner of Frame, f/8 Hood Off, Sun Outside Frame, f/8
55mm
70mm
100mm
200mm
300mm

In our testing, we found that the 55-300mm PLM will indeed flare at 55mm and especially 70mm., though only a very small amount—pretty much a non-issue. The amount of flare isn't affected by the aperture setting. When the sun is in the frame, you won't get sunstars when the lens is stopped down. This is due to the rounded, 9-bladed aperture.

Fringing and Chromatic Aberrations

For this test, we turned again to our sharpness test images (RAW), checking high-contrast edges for any sign of cyan, magenta, or purple fringing. We found some, but not a lot. Have a look for yourself:

55mm

At 55mm and wide-open aperture, you can see some purple fringing around the street light and evergreens toward the edge of the frame. This sort of fringing is common against blue skies. It also appears around the edge of the white tarp covering the boat.

120mm

No fringing or CA is evident at 120mm—not even on the edge of the white tarp covering the boat. This is very impressive performance.

200mm

At 200mm, cyan CA begins to appear along the edge of the boat tarp, but it's exceedingly minor and easy to remove in-camera or in post.

300mm

At 300mm, the cyan CA on the tarp intensifies but remains minor.

Verdict

Though you can provoke the 55-300mm PLM to show chromatic aberration and fringing, it does an admirable job of suppressing it, even under the most difficult circumstances. If you need more proof, here are a few other challenging scenes it handled with aplomb.

Click on each image for a cropped area highlighting aberrations or lack thereof, or click the link below each image for the original RAW to JPEG conversion.


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