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. The fastest camera lenses designed for DSLRs and widely available are between f/1.4 and f/1.2, but lenses as fast as f/0.75 have been made in quantity for special applications, and some of those are available quite cheaply via scrap yards, surplus stores, or eBay.
These ultra-fast lenses usually are branded either Kowa or Rodenstock and were designed for use in medical or semiconductor industry equipment, etc. They are not well-suited for use on DSLR cameras, and 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. Here's how to use one on a DSLR....
Step 1: Get a Lens
Start by getting one of these ultra-fast lenses.
The example here is a Kowa 1:1 55mm, an f/1.0. There are lots of others to choose from, mostly branded Kowa or Rodenstock. The Kowa shown here cost $25, including shipping, via eBay. Personally, I've purchased ultra-fast lenses from two eBay suppliers: ctr_surplus and svcompucycle.
Fundamentally, these are commonly junked lenses and should not be too expensive. However, these lenses were used in very expensive equipment and probably cost a lot when new, and people often go nuts when they see fast apertures, so pricing varies wildly. The Kowa 1:1 55mm is among the cheapest and most commonly available, but there are many alternatives. I've heard that one can find ultra-fast lenses for $1 in junk yards if you're diligent and dress like a buck is all you can afford. However, surplus stores and eBay sellers are commonly listing ultra-fast lenses anywhere from $10 to $400. Some of the more exotic ones, or ones with DSLR-compatible mounts, are between $600 and $2,000.
In any case, you want a lens with a rear element that fits inside the diameter of your DSLR's mount. A lens with a larger rear element generally might imply vignetting, as well as being harder to mount on your DSLR. Independent of the rear element diameter, large diameter lens barrels can complicate mounting by colliding with an overhanging finder or a front hand grip on your camera body.
Step 2: Clean It
Unfortunately, these lenses were usually stripped out of equipment with the minimum possible level of care, so the odds are you'll need to do a bit of clean-up.
Because the lens was mounted inside expensive equipment, it probably lived a sheltered life before being ripped from its home. The lack of a focus mount also means the lens block didn't have air sucked through it during focus. The happy result is that despite a filthy exterior, there probably isn't any nasty stuff between the lens elements. There are probably fingerprints on the outer elements, lubricant (graphite dust) on the mounting threads, etc.
My lens had a cardboard strip glued around the front, which I first removed by scraping underneath it using a box cutter. I used artist's oil paint thinner with a non-scratching kitchen cleaning pad to soften and remove the remaining glue and other nasty stuff on the lens body. Then I used paper towels to wipe-off the thinner and a paper towel moistened with soapy water to remove any traces. Finally, I used cotton-tip swabs moistened with alcohol to clean the lettering and threads.
None of that should touch the glass. Clean the glass by first giving a few puffs from a blower to remove dust (which might otherwise scratch the elements). If there are greasy fingerprints, use lens cleaning wipes or a swab moistened with alcohol to remove them. I like to do final touch-up using a lenspen.
Through all that, you should never have the lens elements or body wet -- using too much of any liquid can allow that liquid to seep inside the lens. Considering the kind of equipment these lenses often were used in, I recommend treating the crud as potentially toxic "medical waste" and taking appropriate care in dealing with it.
It has been rumored that some of these lenses might have become radioactive due to exposure to X-rays. More likely, they deliberately might have used slightly radioactive materials in their glass, like the famous Takumar 50mm f/1.4. If they are radioactive at all, they should not be particularly dangerous in normal use... but I wouldn't keep one in my pocket 24/7 (not that it would fit).
Step 3: The Bad News
Ok, so you've got this impressive hunk of metal and glass... the bad news is that it isn't just a pain to mount on your DSLR. Odds are that the lens cannot be fully functional on your DSLR no matter what you do.
Take your lens and a sheet of paper where you can see the sun. Use the lens to focus the image of the sun on the paper (look at the back of the paper). How much space is there between the rear of the lens and the paper? For my Kowa 1:1 55mm, the answer is only about 1/4 inch!
My Sony A-mount DSLRs have a lens flange mount to sensor distance of 44.5mm, which is about 1 3/4 inches. The shortest APS-C mount to sensor distance in commodity use is the EVIL Sony E-mount, and that's 18mm, or a little less than 3/4 inch. Thus, you can pretty much forget about infinity focus. Even if you did get the lens pushed back far enough, for example by modifying the lens body to fit deep inside a Sony E-mount, the image circle at infinity is nowhere near big enough to cover an APS-C sensor.
Having accepted that this lens is for macro only, it is time to talk about the lack of a focus helical. Your lens doesn't have one, does it? It probably does have a screw thread, so you could use partially unscrewing it as your focus mechanism. However, in such a macro domain, that's not likely to be worthwhile. You could always mount it on extension tubes or a bellows for really extreme macro use....
More bad news: your ultra-fast lens isn't actually as fast as it is marked. Why? At extreme close focus, the extra distance behind the lens typically increases the effective focal length. This happens for most lenses, including ones designed as macros. My f/1.0 thus behaves more like f/1.7, thankfully also increasing the image circle to just about cover APS-C.
The final bad news: at least at this level of macro, image quality is technically horrifically BAD! In fact, IQ is probably significantly worse than any lens you've ever tried. Contrast is very poor, flare is highly visible and bizarrely shaped, and sharpness is mediocre in the center and non-existent at the edges.
So, why bother? Because it effortlessly makes abstract images like the one shown here, it may be worth it. You simply can't make images like this any other way. With this type of lens, enough light gets in to do this hand held, and the ultra-thin depth-of-focus paired with undercorrected aberrations makes an amazingly smooth blur of the background. The bokeh is uniquely appealing so long as flare does not mess it up.
Step 4: Mount It
Ok, how do you stick this on your camera? Well, I'm not proud of this, but sticking it is definitely the right idea. We'll literally stick it on an adapter.
An adapter? They make adapters for these? No. We'll simply glue it onto an adapter made for something else. If you have machining skills and equipment, feel free to do something a little less homemade, but metal-to-metal bonding with a lot of surface area works quite well with cyanoacrylate glues (e.g., Krazy Glue).
In general, what you want is an adapter that is thin but still has a flange covering the camera mount. Glassless M42 adapters are reasonable choices for DSLRs using Sony/Minolta, Canon, Nikon, or 4/3 mounts, and usually cost as little as $5. The M42 thread is 42mm across with no obstructions, so rear lenses with diameters less than that will fit without modification.
Before attaching the lens to the adapter, try holding it in place to see how far back the rear element protrudes. If it sticks out in the back more than any of the lenses designed for your camera, there is a serious chance of a collision with your DSLR's mirror. A wide lens barrel can also hit the finder or body grip. These problems can be remedied by simply using a thicker adapter (e.g., a T adapter). To push the lens out even more, you can use an extension tube.
Once you have confirmed the fit, spread a thin layer of cyanoacrylate glue on the rear edge of the lens barrel, and attach it to the adapter. Be quick centering it; this type of glue sets within 30 seconds or so. Let it dry overnight for best strength.
Step 5: Make It Pretty
Although the mount is usable as it is, this still leaves a lot of raw screw threads exposed. Despite my best efforts to clean them, there is still some black powder (graphite) deep in the threads, just waiting to leave traces on something when it is least convenient to do so. It's worth covering up the threads.
For my cover, I used black craft foam. It costs no more than $1 for a large sheet and can be cut to size with scissors. Sheets are available with a self-stick backing and even a piece of tape under the foam will work, but I used ordinary white glue to attach this one. Simply cut a sheet of the material to fit the exposed threads and fix it into place. Do not use the cyanoacrylate glue to attach this.
Step 6: Take Photos!
Put the camera on your lens -- yes, it is a lot heavier than most DSLRs -- and find something to photograph. There's probably something small nearby.
Depending on your camera and the adapter you used, you might need to tell your camera something about your homebrew lens. For example, some Sony cameras need you to tell them it's ok to take pictures when the body doesn't know a lens is attached.
Without focus, always shooting wide open, there's not much to do. You can vary exposure by changing the shutter speed and ISO "film speed." The really bright finder image makes it easy to see what you're doing and enough light is admitted to allow fairly fast shutter speeds and low ISOs. You'll be able to take extreme macros handheld in most lighting conditions.
The catch is the images will look like the one shown here -- kind of gray. Yuck!
Step 7: Post Processing
The images look awful as captured with default settings, but that's ok. You'd be stuck if you were shooting on film, but digital processing can fix a multitude of sins. After much playing, I determined that the images are not really as bad as they look. In fact, there are really only two things wrong with most photos captured this way.
The first is that the lens is not well corrected outside a small central area of the image circle. There is not much we can do about that. Call it part of the "look" of the lens and be careful where you put stuff that you want to be sharp in the image.
The second is severe lack of contrast. Look at the histogram in the previous step's image. It doesn't use much of the sensor dynamic range, does it? Auto levels sometimes can do a good job fixing this, but has a tendency to shift colors, not just luminance levels. Instead, use the curves tool to change the luminance values. Move the black point to the left edge of the histogram and the white point to the right edge. The colors were there all along! The only problem is that the image often looks grainy after stretching contrast this way, so noise reduction may be appropriate.
You might be able to approximate this PP in the camera. Try turning-up contrast and saturation. You can turn off any shadow enhancement options; the lack of contrast is doing that for you.
Step 8: Conclusion
Is this worth doing? Well, maybe. This is not going to be a lens you use every day. Even for macros in general, other lenses, especially a real macro lens, will create images that are technically superior. Then again, these images have a very special look....
Some people are bothered by the lack of focus adjustment, but it really only limits the range of applications of the lens. For this type of macro, moving the camera back and forth is a pretty good focus method.
The same goes for the lack of an aperture iris control. You could easily make a Waterhouse Stop -- a piece of opaque material with a hole in it to reduce the lens aperture. For most of these lenses, you can probably just put the Waterhouse Stop in front of the lens, making it very easy to change between perfectly-circular apertures. That might be useful, but stopping down isn't going to make this lens compete with a lens designed for your camera.
Step 9: A 3D-printed Focusing Sony E-mount
As of January 2016, I've actually taken-apart the mounts I made here and switched to 3D-printed ones described in 3D-Printed Focusing E-Mount Adapter for Ultra-Fast Lenses. If you have a Sony E-mount body and access to a 3D printer, that's definitely a better way to adapt these lenses....