I’ve been trying a few vintage 50mm lenses on my Sony a7r3 recently. I use a cheap adapter I found on amazon. There are no contacts, the adapter is there purely so that the focal plane of the lens is exactly on the sensor. The mount of a mirrorless camera is much closer to the sensor than the film was on the reflex cameras the lens was made for so the adaptor allows to change any mount (here a m42 screw) to the sony FE mount while placing the lens at the correct distance from the body.

The barrel of the Pentax lens is about 4cm long whereas the sony 50mm 1.4 in FE mount is nearly 11cm long. If you count the adapter, that adds 3cm, the total length is about 7cm. That is 7cm vs 11cm : a real gain in size. It actually places this combo at around the length of the sony FE 55mm 1.8 lens. The Pentax Asahi 1.4 is manual focus whereas the sony FE 50mm 1.4 is an autofocus lens and logically bulkier too.

Let’s talk about image quality a little. For me, the 50mm focal length is a great all round lens. I use it a lot for portraits but also for street and travel photography.

I like the colours and the pop I get out of this lens. The photo looks nice and sharp until you zoom in 100% (but who does that ?)

You can see that the eye is in focus and pretty sharp but there is a dreamy quality to the picture. As I understand, these are spherical aberrations because the light passing through the outer portion of the lens wide open refracts differently to the light that passes through the centre of the lens. So when you close the diaphragm, the lens seems sharper.

To be honest though, I don’t mind this dreaminess and you wouldn’t notice it on a print.  Another portrait :

On both these portraits, I like the bokeh on the background. On this second photo, taken at a photography exhibition, you can see sturdy metal grids with photos hanging off them. A busy background to blur out and I like the result.

So how it is for outdoor, more candid photography? Well, the only thing to master is the focusing. All the photos in this article were taken at f/1.4. Why get a 1.4 lens if you can’t use it at this aperture? That doesn’t mean to say you should always use a lens wide open, quite the contrary in fact, but if it works at f/1.4 then I can use it at any aperture.

It is a difficult task to photograph someone moving, even in a predictable way with a manual focus lens. On a small screen this photo looks roughly ok but a 100% crop shows a severely blurred image.

If we look at another one :

This one looks much better, the man was shuffling forward quite slowly. The 100% crop shows a lack of sharpness on the sharpest part of the photo :

I could get away with this one I think. The last photo I have taken during my test is nice and sharp, the subject however wasn’t moving. I still had to take the photo as quickly as possible before she moved away.

I really like the background blur, I find that the lens has a real 3d pop to it.

Some conclusions: 

• It is a great lens, I love the rendering, especially for portraits.
• Bokeh is very nice, even on difficult backgrounds
• The transition from sharp to out-of-focus is smooth. This gives the lens a nice three dimensional look.
• Focusing is difficult on moving subjects when the lens is wide open. I had more failed attempts than successful ones. I know that sharpness isn’t everything, but standards have changed a lot since the 1960’s and a reasonably in focus subject is mandatory in my mind.

So if you find one second hand in a flea market that looks in good condition, get one! You won’t regret having such a tiny lens that has so much character!