Testing the HD Pentax-DA 1.4x Teleconverter with the D FA 150-450mm on Full Frame
No lens can be totally free of optical flaws. The following list describes the main defects that a lens can suffer from.
|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. The other aberrations will not be formally tested as their effects are both harder to isolate and generally better controlled by design.
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.
A teleconverter adds more glass elements in the light path. Because of this, it is likely to offer a decreased control over aberrations. The Pentax 150-450mm showed some flare when the sun was positioned in the center of the frame, and ghosting when it was placed in the corner. Our tests will show if the teleconverter made things visibly worse.
Flare Test One - Center-of-Frame Sunlit Flare
As usual, we used the sun as our light source for flare testing. It is bright and covers a wider range of wavelengths than most artificial light sources. 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 210mm. You can click on the thumbnails for larger views.
Flare is obvious at all apertures, and becomes more prominent as the aperture closes. It is advisable to avoid this kind of scene when using the teleconverter. Luckily it does not represent a likely use case.
Flare Test Two - Edge of Frame
For this test, we placed the source in the top right corner of the frame, directly illuminating the sensor. You can click on the thumbnails for larger views.
Ghosting is present even wide open, in the form of a green dot. At F8, a red patch also shows, joined by a third spot at F16. There is also some flare occurring at all apertures. The results are not alarming but the presence of the teleconverter clearly influences the results.
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. The lens was set at 270mm.
The control of the teleconverter over chromatic aberration is impressive. There is a hint of green on the "Top" image wide open, and a similar, red shadow for the "Bottom" image. They can still be seen at F11 and are gone by F16. There is no indication of purple fringing. This is almost as good as when the lens was used alone, which means that the teleconverter manages to preserve the lens' quality.
The combination of the D FA 150-450mm and teleconverter almost no chromatic aberration effects, and no purple fringing. This is impressive.
Flare is always present with a strong source in the frame. Ghosting also occurs when the light source is near the corner. This is to be expected, but users should be aware of it when shooting.