September 12 2007
The move to bayonet mount lens in the 1970s meant photographers needed adaptors to use their existing screw mount lenses on new cameras.
Why bother with old equipment that does not have full functionality.
The optical performance of older professional quality lenses is still very good, and this is not always reflected in their price on the second hand market. Maybe you have favorite lens that has that special quality and you want to keep using it.
The sensors of most DSLRs are smaller than the size of 35mm film so digital cameras use only part of the view through the lens. This can help older lenses in their performance as lens faults are more prevalent at the edge of their view area. Using more of the center section ignores these problem areas, so the old lens may perform better on the digital than they ever did before.
Recognizing the value of the screw mount lenses, even in the digital age, there are still new and second hand adaptors available to mount these lenses on a variety of digital SLR cameras. While the lens may mount on the camera, the adaptors generally do not provide any mechanical or electrical connection between the lens and the digital camera body.
Manufactured before autofocus became the norm screw mount lenses need manually focusing.
This is less convenient as it is done manually often at maximum lens aperture and then the lens is stopped down (more on this later) for metering and the shot. The autofocus system may help as the sensors still work and giving an indication in the viewfinder, at least on Pentax cameras where a custom menu option enables the function. This gives a confirmation of correct focus while the manually focusing the lens, even though the camera is set to manual focus.
Focus trap technique.
Set the camera to autofocus, not the sports focus mode, so the digital camera will not release the shutter until the autofocus system has confirms the lens is focused.
Then press down on the shutter release while adjusting the focusing ring on the lens, when the autofocus system detects the scene is in focus it will release the shutter.
Stop down metering.
In most lenses, the aperture remains fully open and automatically changes to the aperture setting for the shot when the shutter button is pressed. However, when using an adaptor for screw mount lenses the connection between the lens and the camera is lost.
In “stop down metering”, the aperture is adjusted to the desired size before pressing the shutter button. The inbuilt light meters in Pentax DSLRs will operate in manual or aperture priority mode giving some degree of automatic metering for using screw mount lenses.
There may be menu options to enable use of the lens aperture ring when it is not set to the A position, or in the case of the screw mount lens there is no connection between the aperture controls and the camera.
When using small aperture settings focusing may difficult so set the lens to its maximum aperture for focusing and composition and then do the metering last.
If the in-camera metering presents problems, as long as the lens mounts properly on the camera, use an external light meter. Another alternative is by trial and error using the LCD screen and the camera histogram.
Install the adaptor
Screw the adaptor onto the lens then mount the lens on the digital camera the same way as a normal bayonet mount lens.
Once in the lens can be unscrewed to change to another screw mount lens, the adaptor should stay locked in the camera body. To remove the adaptor on the Pentax system there is a small springy lever in a cut out on the adaptor near the red reference mark, pull this towards the center of the hole in the body. Use a fingernail or a small flat tool, releasing the adaptor allowing it to rotate for removal.