Testing the HD Pentax-DA 1.4x Teleconverter with the D FA 150-450mm on Full Frame
In simple terms, vignetting is the darkening of the corners of an image that occurs at wider apertures. Every lens, wide open, will exhibit some. Most of the time, it need not be a problem for modern photographers. First, it is always possible to shoot at smaller apertures, mitigating the effect. Second, cameras released in the last few years often have built-in tools to remove vignetting in a way that's invisible to the user. Advanced software also often incorporates lens profiles which can seamlessly correct vignetting. Last, it is always possible to purchase higher-quality lenses that will show milder vignetting.
The 1.4X teleconverter is a DA product, meaning that in theory, it should not work properly when used in full frame mode on the K-1. Our test will verify this using the 150-450mm.
The test was performed by pointing the lens at a blank wall. In order to avoid the bias of the in-camera corrections, it was deactivated prior to the test. Resulting files were scaled down, converted to grayscale for improved visibility, then exported. The following settings were used:
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The following images show the vignetting at various focal lengths and apertures. Click on any thumbnail to compare the vignetting at the given focal length.
It is our habit to plot the amount of vignetting on a chart. In this case, this is not possible. At 210mm and F6.7, vignetting is quite strong, at 2.28 stops. All other settings return completely black corners. This type of vignetting cannot be corrected in post-processing, as there is no information to salvage.
Despite many users' wishes, the Pentax teleconverter cannot be used in full frame mode with the 150-450mm. In almost all conditions, corners are completely black and unusable. Cropping to reject the black areas would yield a field of view slightly wider than APS-C but less than full frame.