Lenses are an important part of the PENTAX system. There are cameras with different sensor sizes for which there are different lens series.
Cameras of the K-1 series are equipped with a so-called 35 mm full format sensor. Due to the sensor size, corresponding lenses are required here. In the current Pentax system, these are identified by the designation "DFA". Since the sensor of these cameras is exactly the same size as the 24x36 mm 35 mm film, the lenses that were used in analogue photography can also be used. These are lenses of the FA series, for example. Other series can also be used, but with restrictions.
In all digital cameras other than the PENTAX K-1 series, such as the K-3 series or K-70, KF and many other models, there is a smaller sensor in APS-C format (23.6 x 15.8 mm). Because of the smaller sensor size, lenses are used that illuminate a smaller area. This is done with the lenses of the DA series.
The focal length specification of all lenses always refers to a focal length specification with reference to the 35 mm full-frame sensor. On a camera with an APS-C sensor, the image section of the identical focal length is 1.5x larger. To find out the correct focal length value for cameras with an APS-C sensor, this must be multiplied by the cropfactor 1.5x.
A lens with a focal length value of e.g. 16-50 mm, corresponds to a focal length effect (image detail) of 24 - 75 mm on the APS-C sensor, or a lens with a focal length value of 70-200 mm works like a lens with a focal length value of 105 - 300 mm on a camera with an APS-C sensor.
The lenses of the D-FA series can be used without restrictions on cameras with the APS-C sensor. Lenses of the DA series, can only be used with restrictions on the larger 35 mm full-frame sensor. Due to the illumination of a smaller image field, vignetting (shadowing at the edges) may occur.
Lenses have a variety of features that influence usability and quality. You can find out more about lens characteristics and the abbreviations used in lens designations and what they stand for in addition to focal length and maximum aperture in our Lens Glossary ... (Link)