Full frame. See 100% detail crop just below...
After looking over a fair number of images from the 28-105mm Pentax lens, when used in tandem with the K-1 body, I've come to the conclusion that it's really good lens and if you decide to embrace Pentax's vision of a full frame camera system this might be the lens to buy. It's not the fastest of the Pentax full frame zooms being offered but it does have the most usable focal length range and it doesn't mind being used wide open, at its maximum apertures.
Let's start with a physical description of the lens: It's small for a lens that cover the 24-36mm film gate but it feels dense, and heavier than expected, when you pick it up. It does not have an external aperture ring so you'll always be using one of the camera controls to set your f-stop. The filter ring at the front of the lens is 62mm and the supplied cap is the pinch type and it's made of thick plastic; not thin or chintzy stuff. The lens is supplied with a cloth pouch and a petal-type lens hood that can be reversed onto the front of the lens for transportation. Please don't be a dweeb and shoot with the lens hood reversed. It just doesn't make any sense at all. If you are too lazy to use the lens hood you should just throw it away. Anything else looks lame.
There is a small, almost vestigial focusing ring positioned closest to the the back end of the lens (the camera side) and I can't imagine wanting to use it instead of the AF in one of the K series cameras. The front ring is a very long (from front to back of the lens) zoom ring which has a nice rubber grip. Focal lengths are marked on the grip at 28,35, 50, 70, 90 and 105mm.
In deciphering the lens description from the lens barrel markings the "HD" stands for high definition and means that the lens was constructed and coated to work with high resolution digital sensors. The D-FA means it's a digital lens that covers the full 24x36mm frame. ED connotes the use of extra low dispersion glass in the optical construction while "WR" means that the lens has been made weather resistant with the application of gasketing. Finally, the lens is "DC" which means it has built in motors and doesn't depend on noisy "screw-driver" mechanical connections to the camera in order to focus.
The lens features a nine bladed, rounded aperture which generally means better bokeh, and I have found this to be the case at nearly every focal length and focusing distance. The lens weighs in at 1.33 pounds and has an optical construction of 15 elements in 11 groups. Included are two precision aspherical elements, an ED element and an anomalous dispersion element. A bit more sophisticated than a typical "kit" lens, for sure.
I've been shooting with it in low light situations and also in bright sunlight (trying to achieve maximum performance with the lowest ISOs) and have been able to rely on the maximum apertures to delivery good to great performance (best at the wider angles but still good at the long end). I've had very little flare even in flare prone shooting situations and when I stop down one stop the performance of the lens equals that of my 24-105mm f4.0 Panasonic Lumix lens at the same f-stops; and that lens is nearly three times the price of the Lumix. The only real advantage of the Lumix lens over the Pentax is the constant aperture and a closer minimum focusing distance.
The Pentax 28-105mm trombones (extends) as you zoom but the focusing is internal. Even fully extended, at 105mm, the barrel and extensions are tight and not at all anxiety provoking. The lens inspires confidence because it feels "right" mechanically.
I selected this lens over the available Pentax 24-70mm f2.8 because I value the focal length range of the 28-105 more than I do the extra stop at the middle of the range or the extra two stops at the longest end. If I need shallower depth of field at the 100-105mm range I have the Pentax 100mm macro lens which is an f f2.8 and which is nicely sharp and contrasty even when used wide open. The 28-105 is just a great walking around lens for 90% of the imaging most people would do. By adding the 50mm f1.4 FA and the longer 100mm macro along with the zoom I feel like there's not a lot of general photography I can't cover well. Additionally, the K-1, at 36 megapixels, gives me room to crop and fine tune.
The camera and lens work in concert with the camera applying lens corrections to Jpeg files and writing the corrections into the raw files. The camera is making some obvious corrects to lens geometry but once made they are largely unnoticeable and I'd rather have the camera correcting them than spend time doing it myself. Of course, I would prefer a lens that didn't require computational (my new buzz phrase) corrections but I'll gladly trade that compromise for a lens that is nicely sharp, contrasty, has great color and is fun and easy to carry around. And that's the 28-105. I'd buy it again as long as my intention was to use it with a Pentax K-1 or K-1 mk2 body. More samples below:
100% crop from the image above the written review.