The Limit of Infinity

Before Nikon Z mount, we had the Nikon F mount. Before the Canon RF mount, there was the Canon EF mount. I could list brand after brand, but the fact is that most modern camera companies on the market today have had a multitude of mounts in their history.

Pentax is no exception. Before the bayonet Pentax K mount, Pentax adopted the M42 thread mount (not to be confused with the T-mount, which is also technically has M42 threads, but at a different thread pitch). Despite not being the creators of the mount, it's become synonymous with the Pentax brand. Most people that inquire about M42 adapters bring up their favorite Takumar lenses.

For the creators devoted to the Pentax brand, an M42 to Pentax K adapter is the perfect adapter to bridge the gap between the two historical mounts. This month, we rented a Pentax K III, marking the first time we've had a Pentax camera in-house since the Pentax Q! I figured it would be the perfect camera to test out our M42 to PK adapter. When I picked the item from stock, I was surprised to find that our Pro adapter design had been changed. It was now reminiscent of our M42 to EF adapters.

M42 to PK (Previous Version)

M42 to PK (Current Version)

At Fotodiox, most of our adapters are engineered with infinity focus in mind. As long as infinity focus is maintained, a lens can basically be used as it was on its native system. There are certainly exceptions, such as our C-mount to DSLR adapters or select M42 adapters.

Both the M42 and Pentax K systems have the same flange focal distance, so I knew this design would not allow for infinity focus. I measured how much space is added with the adapter, and it looked like the lens was 0.84mm too far from the sensor for infinity focus. Let's find out just how much that would affect viability.

After mounting a lens on the camera, a quick walk outside definitely shows I can't focus as far as I could with a native lens or the lens was adapted onto a mirrorless camera. To show the difference, here are two setups:

Using M42 to PK on Pentax K camera

Using M42 directly to mirrorless camera

This doesn't render the lens completely unusable, however. You can still utilize it for macro for sure, and portraiture is also workable.

The lack of far focus is even more apparent on the M42 to Nikon F adapters without glass. With the M42 V2 adapter mounted on my Nikon F camera, the lens is effectively 1.9mm too far from the sensor to focus to infinity. That being said, even when the lens is set to infinity, the closest I can focus is somewhere between three to four feet.

Some adapters require glass for infinity focus. The glass adds a 1.4x magnification (and about 1 stop of light loss). Mirrorless cameras generally do not need the internal glass. Note the difference in field of view in the images below:

Using glassless adapter on MILC

Using Glass adapter on DSLR

While some customers are concerned with those factors, as well as the possibility of image quality, it's worth noting these glass elements can be removed. With the adapter removed, infinity focus will no longer be possible, unfortunately, but the benefit is that you aren't dealing with any glass between the lens and camera. For example, the OM to PK adapter requires a glass element. After removing the internal glass, the adapter acts as a 14.22mm extension tube. This much extension past the standard flange focal distance is similar to one of our DLX Stretch adapters, so infinity focus ends up looking more macro than anything:

We've touched on this many times before, but flange focal distance is a necessary factor when it comes to adapting lenses. It's a finnicky thing; even a fraction of a millimeter too much is enough to remove infinity focus. The majority of our adapters allow for infinity focus, thankfully, but for the few that don't, the amount of far focus you lose truly depends on the lens and how much space is added.