The ratio between the focal length and the aperture (diameter) of a lens is called the f/number. The smaller the f/number, the more light is let in. Fast lenses start around f/2.0, and the light let in goes as the inverse square. Compared to f/2.0, f/1.4 lets in twice as much light, f/1.0 four times, and f/0.71 eight times. As I observed in Using Ultra-Fast Lenses on DSLR Cameras, there are a variety of ultra-fast lenses ranging from about f/1.6 to f/0.75 and cheaply available via scrap yards, surplus stores, or eBay.
These ultra-fast lenses usually are branded Kowa, Rodenstock, or Delft and were designed for use in medical or semiconductor industry equipment, etc. They are not well-suited for use on DSLR or mirrrorless cameras, most cannot focus to infinity (and, like all lenses, have somewhat higher effective f/numbers when used for close-ups), have no aperture iris nor focus mechanism, and generally are no substitute for an f/1.4 or f/1.2 lens that was designed for your camera. However, they easily can produce very distinctive images.
My old Instructable described a really simple way to use them involving little more than glue and and a sacrificial M42 adapter -- but that was years ago. Here's how I more recently made a 3D-printed Sony E-mount adapter that not only looks better and holds the lens better, but also allows a little focus adjustment.
Step 1: Preparing the Lens
It should go without saying that you need to have one of these lenses before you can mount it. They should cost less than $100 -- a lot less. The most common seems to be a Kowa marked 1:1 55mm, which is a 55mm f/1.0. My ultra-fast lenses all have a huge external mounting thread -- as seen in the photo. I don't know how many different sizes these lenses were made in, but mine have either 60mm or 80mm diameter, and I suspect there weren't too many different sizes despite a variety of focal lengths and apertures.
When you get one of these lenses, they're usually pretty dirty. Clean them as described in my old Instructable. However, in my old Instructable, all we did with those huge threads was to cover them up. Here, we're using the thread to attach the adapter and as a focus adjustment -- so we need the thread clean. For me, that meant a lot of passes over the threads using Q-tips moistened with alcohol.
If, like me, you already have the lenses but with M42 adapters crazy-glued on them thanks to the old Instructable, it's time to undo that. Take the craft foam off the thread and clean the thread as described above. Then, how do you remove the M42 adapter? Well, got a hair dryer? Basically, the lens body is metal and so is the adapter, but they're probably not the same metal and definitely have wildly different masses... which means that if you blow hot air on the M42 adapter, it will heat and expand more than the lens body it is glued to, quickly breaking the glue bond. With the adapter removed, scrape-off the remaining glue residue.
Step 2: The 3D-printed Sony E-mount Adapter
We're ready for some 3D printing. I've posted STL designs for the two different sizes of external threads my lenses have at Thingiverse as thing 1289753, and you'll also find printing instructions there. They are easy prints that I made in purple PLA using my MakerGear M2 printer, with a little black paint light sealing the inside of the mount and a white paint spot on the outside for the mounting alignment dot. Why purple PLA? Why not? These are fun lenses, so they might as well look like it.
The designs are a bit complex, actually. Here's why:
- Sony has essentially made two somewhat different versions of the E-mount body flange. Although Sony-made stuff is compatible with both, the E-mount flange design most commonly being used for 3D printing doesn't fit with the newer all-metal E-mounts on the A7II series bodies. The E-mount I designed for these has been confirmed to work on both NEX-5 and A7II, although it can be slightly loose on the older bodies.
- Some of the lenses have really tiny rear elements that stick out the back, but the longer focal length ones don't. In fact, my Rodenstock XR-Heligon 95mm f/1.3 has a rear element that is recessed, but has a larger diameter than the inside of my standard E-mount flange. Thus, you'll see that the 80mm threaded adapter actually has a different E-mount flange from the 60mm one -- it is carved-out in the center to minimize vignetting. It still seems strong enough.
- To enable the lens to focus to the greatest possible distance, it needs to be close to the sensor. Mirrorless Sony E-mount helps with that, but the truth is this needs to be very close... so, the 80mm adapter just barely clears the grip of my NEX-5 and both adapters cover the body lens-release button enough to make pressing it awkward (but still possible).
- The design needs some support structure as it prints with E-mount facing up, and neither cura nor slic3r is able to automatically build correct support. No problem; I simply designed-in the support needed.
Anyway, you don't have to worry about that as long as your lens takes one of the two adapter sizes I have posted.
Step 3: It Works!
Here are a couple of photos shot with these lenses using the 3D-printed mounts. The first was with the 42mm f/0.75 on a NEX-5; the second was with the 95mm f/1.3 on an A7II.
As you can see, the images are very soft and you cannot focus very far away. The Rodenstock XR-Heligon does better than the Kowas in this, and can focus far away enough for a tight head shot, while the Kowas don't make it very far out of the macro range. You can focus by turning the lens within the adapter, but it's best to think of that as adjusting the magnification a little -- focus is usually best done by setting the magnification and then moving the camera to the appropriate distance for the live view to be in focus. Incidentally, don't expect focus peaking to show much because most of these lenses have too low contrast; magnified live view tends to work better.
Then again, if you want critically sharp focus, you probably don't one of these lenses. They're about the dreamy-looking bokeh, etc.