Lenses of the Pentax Auto 110

The Pentax Auto 110 is a really tiny Single Lens Reflex camera from the late 1970s. It is a real camera, with quality interchangeable lenses, using 110 film - which is 16mm film in a drop-in cartridge. The image size is 17x13mm. I have the camera and four lenses in my cupboard, resting as it has been these many years, so I thought it would be worth getting it out and trying the lenses on my Fuji X-H1, which is APS-C sized. Actually Micro 4/3 would be a better match to the original Pentax 110 frame size, but never mind, we can crop down the resulting pictures from the larger 23.6x15.7mm APS-C frame. As we shall see, we need to!

The Pentax Auto 110 with standard lens, motor drive and dedicated flash

Pentax 110 outfit - camera, dedicated flash, and four lenses; 18mm, 24mm, 50mm, and 70mm, with a British pound coin for scale.

Some preparation is needed of course. The first step was to order a mount adaptor from China, which came in a couple of weeks for £11. Next, to deal with an issue arising from the design of the P110 (as I shall call it) - the lenses don't have a diaphragm. Aperture control is automatic using the camera's built-in meter, achieved by opening the shutter partially, to an extent required by the amount of light. The behind-lens shutter and aperture adjustment are combined.
The rear of the lens is 3mm forward of the shutter/diaphragm in the P110, so to stop down the lenses for a modern mirrorless digital body we wish ideally to put an adjustable aperture about 3mm back from the lens flange. Inspecting the Chinese adaptor when it came, I concluded that the easiest way is to make a set of aperture "washers" of a few sizes which can drop in the back of the adaptor, retained by a slightly oversized O-ring pushed into the hole. The aperture turns out to be about 4mm behind the lens, which should be good enough.

Rear view of the adaptor, with the f/7 "washer" retained by an O-ring

The P110 standard lens on my Fuji X-H1. It looks comically small!

So now on to trying out the P110 lenses on the Fuji camera. All the test pictures were taken of the same scene, on a tripod in the same position. We start with a picture of the scene taken with the Fuji's own lens - the 16-50mm zoom set at 24mm and f/7.1 - to give an idea of what a modern system makes of it:

And now with the P110 24mm standard lens, using the f/7 washer. The white box shows the crop corresponding to the Pentax's actual image size:

There is serious vignetting and loss of sharpness at the edges, but it barely extends into the area that the lens is intended to cover. We see the same thing with all the lenses, not surprisingly at its worst with the wide-angle 18mm.

Expanding to 100% we can see that the modern lens (16-50 zoom at 18mm) and the Pentax 18mm perform very similarly within the intended coverage of the Pentax lens (both at f/7):

Fuji

Pentax

The same can't be said of the 70mm Pentax lens, which seriously underperforms the Fuji 55-200mm zoom set at 70mm:

Fuji

Pentax

At 50mm this problem goes away, with similar results from the Pentax and the Fuji zoom at its 50mm setting:

Fuji

Pentax

The final comparison is with the standard focal length of 24mm (Pentax) and the 16-50 Fuji zoom set to about 24mm, which gives us:

Fuji

Pentax

... this time the Pentax is not quite up to modern standards, a little softer and less contrasty - but a respectable performance even so for a lens of its period.

I conclude that the Pentax lenses, with the exception of the 70mm, are remarkably good for 40-year-old designs, and can stand up well to a comparison with modern expectations. Although not ideal for APS-C because of their limited coverage and the need therefore always to crop, they should do well on a Micro-4/3 digital body. I would avoid the (rare and expensive!) 70mm, and was particularly impressed with the tiny 18mm lens. Remarkable!