As would be expected from any super zoom lens, the Pentax 18-270mm has quite a bit of geometric distortion, meaning that straight lines may appear curved even when perspective distortion doesn't play a role.
At the wide end, the zoom lens shows significant barrel distortion, as is evidenced by this test chart photo (note that the uneven lighting was not caused by the lens):
Enabling the in-camera geometric distortion correction (available in all current Pentax cameras) will correct the distortion at the expense of slightly reduced resolution and field of view:
This photo of a door may help you better understand how barrel distortion would affect a real photo. Use the slider in the middle of the photo to switch between the corrected and uncorrected versions of the photo.
The distortion is the least severe in the 90-150mm range. This is a photo shot at approximately 135mm:
At full zoom, the distortion worsens again, but still isn't as pronounced as at the wider end of the spectrum.
The 18-270mm produces significant distortion, but despite the frightening test chart results, in practice, we didn't find the distortion to be a major issue. In our landscape sample photos we rarely needed to apply lens corrections, and when we did, they were quite effective. Thus, this lens's distortion shouldn't turn you away if you're just looking for a single lens to carry around.
The distortion also says something above the lens in general: it is much sharper in the center than near the edges or in the corners. You can read more about this on the next page.
Please note that in-camera lens correction requires additional processing and will force you to wait a few seconds before seeing the photo, so we recommend that you apply lens corrections during post-processing instead (they will be just as effective if you have the latest version of the Pentax software, or a third-party application with a profile for the 18-270mm lens).
Do be advised that whenever you apply lens corrections, you lose a small part of the frame (along the edges) as well as some sharpness, especially in the corners of the image. Removing distortion (partially or entirely) will not always produce a more pleasing image, so be careful! Below are some examples.
Removing Distortion Improves Image
Removing Distortion Worsens Image