A Viewfinder Darkly

Photography tips and tutorials

Pentax K1 and Old Lenses

March 21 2017

by Philip Northeast

The Pentax K1 is the first full frame digital SLR from the Japanese camera manufacturer. Pentax is late to this field after years of concentrating on the smaller APS-C sensor for their dSLR. The latest owners of the Pentax brand have succumbed to market pressure and produced a top of the line dSLR with a sensor the same size as 35mm film, or full frame digital.

There is a range of new full frame lenses for the Pentax K1, covering focal lengths from wide angle through to super telephoto. However, many potential Pentax K1 owners will have a collection of existing lenses. As a long time Pentax photographer, I have a large collection of film era lenses and newer APS-C size lenses for K mount cameras.

landscape photo

Landscape photo made with the Pentax 55mm f1.8  lens on the Pentax K1 body.


Image size

The attraction of the older film era lenses is they produce an image that is the full size of the digital sensor. Conversely, the modern Pentax DA lenses have limitations on the Pentax K1. Lenses designed for the smaller APS-C digital sensor give the same image area as an APS-C camera when used on larger full frame cameras. Because Pentax dSLRs only had APS-C sensors, they designed their DA lenses to suit the smaller image area. This kept the size and cost of the lenses down.

Pentax designed the K1 with this in mind and photographers are able to use only the APS-C area of its sensor. Zoom lenses I have tried have a different image area depending on the focal length. At their shortest focal length the image are is smaller than the image area at the maximum focal length of the lens. Even though it may be an APS-C lens the useable area can be significantly larger and take advantage of the larger sensor.  This amount will vary between lenses.  However, you can use the full sensor and deal with the areas outside of the normal APS-C image area by cropping in post processing. It is often possible to use more than nominal APS-C image size.


This is a 50-135mm APS-C zoom at 90mm focal length. There is some darkening in the corners, but cropping the image  produces a larger image than APS-C size.

Metering compatibility

One feature Pentax retained with the K1 is the ability to use any K mount lens ever made. The modern DA lenses have full metering functionality. Some film era lenses lack some functionality, such as autofocus and most automatic exposure controls, they still work as manual lenses.

Manual exposure methods  

Setting the exposure mode to M for manual opens up alternative methods for setting the exposure.  An important factor is adjusting the lens aperture and there are three configurations found in K mount lenses. The original series K mount lenses do not have any connection between the camera body and the aperture control on the lens. Then Pentax added an A position on the aperture ring that enabled control by the camera body or the aperture could be set on the lens. Finally, on the newer lenses, the aperture ring disappeared and aperture control is only on the camera body.

camera lens control ring

A Pentax film era lens with the A position on the aperture control ring.

The early K mount  lenses are limited in their metering capabilities on dSLRs. They lack the detailed real-time exposure level display in the viewfinder. The only way to measure the light is using the Green button. The other method is taking a test shot and using the camera’s histogram display on the rear screen to adjust the exposure. Photographers compose the photograph and press the green button on the rear of the camera. Then the camera sets the shutter speed for a satisfactory exposure based on the light coming through the lens with the lens aperture set by the photographer. In effect, it is an aperture priority system, except it only adjusts the exposure when the green button is pressed.

For newer lenses, where the camera controls the lens aperture, photographers can use the full array of automatic exposure control modes. The full manual control option is also available where photographers adjust the shutter speed and aperture using the camera dials while monitoring the exposure in the display viewfinder and the rear screen.

Autofocus sensors  work

The Autofocus system on the Pentax K1 is active and confirms when focusing the lens on an object. Although photographers still have to manually adjust the lens focus.

Image Quality Of Old Lenses

Of course, this varies between manufacturers and models of lenses, but I compared my first Pentax lens from the mid-1970’s a 55mm f.8 prime lens. This was the kit lens on the Pentax KX SLR. Comparison shots with the K1 between the Pentax lens and a modern Zeiss 50mm f1.4 prime lens suggested the old Pentax had lost nothing to its more modern equivalent.

camera lens comparison images

Two 1/5 cropped sections of photos comparing the Zeiss 50mm and Pentax 55mm prime lenses. Larger version is available at Flkr


This confirms that a good film lens is still a good lens on a digital camera. Some of my older “classic” lenses did not live up to their promise and while competent lacked crispness. The image quality differences are difficult to appreciate in photographs shared on the web, where even phone camera pictures look good. The improving quality of computer monitors makes these differences more apparent.

The point of buying high-resolution full frame digital cameras is the pursuit of image excellence. This allows you to use the photographs for high-quality printing or large size prints and retain crispness of object edges.


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