Lens Spotlight: M50f1.7 - Jason Doss

Lens Spotlight

smc Pentax-M 50mm f/1.7

The Pentax SMC-M 50mm f/1.7 is another standard normal focal length lens that was sold as part of a kit with many of the M-mount film cameras. Like all lenses in my collection, this one is fully manual with an aperture ring and manual focus ring.  I obtained this one as part of a camera/lens kit from either eBay or KEH, and often you can get a setup like this for about the same price as the lens alone.

Normal focal length are officially described as anything between 43mm thru 58mm, or any focal length that produces a field of view that would be "natural" for the average observer. I guess it could have been called a "natural" focal length, but I guess they are all natural, aren't they? 55mm is at the upper end of what's considered "normal," but in use feels about as normal as a 50mm. There is, however, a distinct difference in feel between 40mm and 50mm, but that's for another day.

The lens is one of four pancake lenses I have, as defined arbitrarily as any lens bearing a length of one half or less than the width.  Pancakes were very popular in the film days, as 35mm was considered as the small format (even though there were 110s and APS-C units in those days), whereas medium format was preferred for high quality photos.  As interested as I am in medium format, I won't go there until Pentax whips out a mirrorless 645 which can accommodate K-mount lenses... but that has very little to do with today's topic, so on we go.

An extensive number of user reviews can be found here. The lens is 185g (0.4#) and, focused at infinity, 31mm long (1.2 inches) and has 6 aperture blades and 49mm filter threads. The fast aperture of f/1.8 is about average for normal lenses of the day, although some were considerably faster. The lens bears the usual aperture ring and has a rubberized grip for easy manual focusing. Minimum focus distance is 45cm (18in) producing 1:6.7 reproduction ratio (0.15x), which is pretty good unassisted. Of course, extension tubes and diopters can greatly increase the magnification.  My lens is in good condition, with the singular exception of the rubberized focal ring grip that seems to have loosened from the barrel.

So here we are.  Above, you'll see a photo I took at our friend's place.  They bought a new property, a hotel in Jasper called the Arkansas House.  We stayed there once a few years ago, and were shocked when we discovered it was sold to people we knew.  There is a multi-sleeper cabin in the back (background here) with a little pond and statue fountain water feature here.  I opted to use this statue to show the super-sharp detail possible with this lens when used wide open.  Again, this is f/1.7 and ambient light metering.

Below left is a scenic view taken on Highway 39 in southwest Missouri between Springfield and my place.  I did some work on the background of this shot focused at infinity and taken at f/11.

On the bottom right is a photo taken while driving (not recommended) of a roadsign that I found very very odd.  You see, I don't talk to my parents very much for a number of reasons.  However, just yesterday, I exchanged a few words with my mom, probably the first back/forth discussion in 2019.  It wasn't long, but it was longer than typical, and as usual, it ended badly.  So, as I drove unfamiliar roads hoping for photo ops after dropping my wife off at the Springfield Airport, I am surprised to see this sign bearing my parents' names, and nothing else.  Not Jerry & Cindy Smith, or Jerry & Cindy's Stop-n-Rob, but just Jerry & Cindy.  This is what is known in psychological and mystical realms as a synchronicity, coined by Carl Jung, and is at this time of unclear significance.  The mystical types I know say that when this kind of thing happens, it is the universe talking to you... so I share this here with you hoping something will come of it.

This particular 50mm lens is a popular favorite among Pentaxians. Reasons are abound, and include low price, high availability, small size, sharp photos, decent bokeh, and, well, it's probably one of the best 50mm lenses of the era available for Pentax. Because of its diminutive size, I have paired it with the 1.4x-S teleconverter which almost doubles the weight in order to get to a short telephoto focal length of 70mm. Why do this? Because it can be done. Also, I kind of like the short-tele field of view that 50mm can bring, and initially I was interested in what effects a TC can have on such a lens.

Anytime a teleconverter is attached to the back of a lens, it should be considered a *different* lens because the optical formula is now different. In fact, many things are different including physical dimensions, field of view, fastest aperture, and potentially the application of the lens. So, sticking with that strategy, the story will continue (until this series is finally complete) which treats this combo as a separate optic.

As stated before, the lens is 185g (0.4#) and, focused at infinity, 31mm long (1.2 inches) and with the teleconverter attached, the combination weighs 305g (0.67#) and 65mm (2.5 inches). The fast aperture of f/1.7 is about average for normal lenses of the day, and the addition of the TC adds another stop, so the actual fastest aperture is f/2.5. The lens bears the usual aperture ring and a rubberized grip on the focus ring, and has 6 aperture blades and 49mm filter threads. Minimum focus distance is 45cm (18in) producing 1:5 reproduction ratio (0.2x). The TC increases magnification without reducing minimum focus distance.

We have photos from our Thanksgiving bash to share this time, and an attempt was made to show more capabilities and limitations of this lens combination. First we show a macro photo of Pandora as she toots her horn. She has taken up the trumpet because she's into music, as all intelligent and creative types are. The lens was very close to the trumpet bell, as you might discern from the upside-down reflection lower left. There is some sharp detail, despite the position of the detail in the corner of the frame. The lens was set to a wide aperture (either full wide or 1 click down) to show performance here. Bokeh is fairly smooth except for the hexagonal specular highlights on the trumpet itself.

Second here is her father's guitar, again up close, but this time stopped down. The mother of pearl patterns were striking here, and the hope was that flash lighting would exaggerate the colors and patterns, which did happen. The aperture was mid-range, probably f/8 or so evidenced by the gradation into bokeh in the foreground and background. He was playing at the time, so timing to keep his hand from blocking the pearl was important.

The last musical photo we have on display is the trumpet itself. Imagine this photo at 50mm by expanding the crop to include 1.4x more white background, and unintended details like stitching of the couch (that's the background), the nearly complete but still incomplete front and back end of the horn, and possible uneven flash coverage due to the close focus. 85mm is the next focal length up, in my collection at least, yet this combination can focus closer with greater magnification than the 85mm. Why not use an unencumbered macro lens? Surely, this would have been the better tool, but the goal was to show that this one is also capable.

The detail evident in the trumpet pipes look good to this observer. A macro lens could certainly have done better, as there is sharpness drop-off toward the edges of the frame.

Finally, you shall be assaulted by a larger-than-life photo that ought to be in a horror film. This is Itchy the Pug, and he can't help that or anything that you're seeing here. The pug posed just briefly enough during the musical and photographic exploration for me to nail this shot. There was no time to focus, but as he approached, the lens was set to a close-focus distance, the aperture was stopped down (probably f/11), and I just took a chance. Success in the eye of the beholder.

Again, click any photo to see larger versions (except the pug) comments can be left on this page or under each photo if you are so inclined.

That's all I have for now... See you next time.

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