Instructables
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....
 
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Step 1: Get a Lens

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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.
goldenshuttle6 months ago

impressive..so many lenses out there on surplus; with idea s like yours, they would be useful again,

Pyk2 years ago
Thanks for this great tip, I ordered a Kowa 1:1/55 right after reading and haven't regretted it. I noticed (as your images show) that it has a sort of light-bleed towards the edges. Light seems to get smeared away from the center of the image.

Here is how I was able to fix this: On my lens, there were three screws on the back (the part that touches the camera) so I unscrewed them and found that the back piece of the lens could be completely removed (removing a couple of the lens elements along with it).

When I tried the Kowa with the real piece removed directly against the camera body, the focal plane was a good 10 feet from the camera. At this point you can move the lens further away from the camera body to bring the focal plane closer, but you will also be losing quite a bit of light.

So I further modified the lens by using a 10x opteca macro lens attachment between the lens and the camera body. This brought my focal plane right back up close to the camera allowing me to take very similar macros to the original Kowa lens, but without the light bleeding (and, of course, a ridiculously thin depth of field).

I have been amazed at the low light performance of this setup (which I believe is better than the Kowa on it's own). I was able to shoot 1/100 on ISO 100 with a single 60W incandescent room light using a Canon 7D.

I've really enjoyed this attachment ever since buying it 5 years ago: http://www.bhphotovideo.com/c/product/825023-REG/Opteka_OPT10X72_10x_High_Definition_II.html

This was always a great lens attachment (as far as lens attachments go), especially before I owned an actual macro lens. With four elements it works much better than any single element macro attachment. And though I haven't used it much since I bought an actual macro lens, it has proved to be one of the most useful elements in my exploration of DIY lenses as it seems to have very different focus properties than single element macro attachments.

In fact, you can even used this attachment as a "attachment" as it's own lens: I happened to have a Canon attachment for a spotting scope which included a 2" extension tube so I simply stuck the opteka onto the end with some sticky tack. This works very well as a super fast macro lens. And because of how thin the attachment is, you can use a shorter extension tube and actually achieve infinity focus (which would only be possible on the Kowa if you had the skills / equipment to cut off some of it's length). This could also be easily turned into a tilt-shift!

Anyway this comment has gone on too long, I hope someone finds it helful!
avatar_i3 years ago
Oh- that is so VERY COOL!

I have several OLD barrel lenses [large format, 360~500mm] I have acquired over the years, and one was mounted on a long aluminum pipe with a camera mount on it, probably made it about */- 800mm focal langth [?].
Not too stable, though.

Always wanted to put it on my Nikon D700, but couldn't figure out...
Thanks!
ProfHankD (author)  avatar_i3 years ago
I have an Apo Goerz lens from a 30"x40" vacuum-back copy camera that also has a very long focal length (and slow aperture, unlike the lenses in this Instructable). However, the long focal length means it can focus to infinity on a DSLR.

Thus, my mount for it consisted of a custom back for a 4"x5" camera to mount my 35mm SLR (I did this long before digitals) and a custom lensboard to extend the lens way past the normal maximum extension of the 4"x5" bellows. Very scary to use and not really worth the trouble for the IQ, but impressively huge and strange to look at. :)
electronbee4 years ago
Whoops, sorry about that ProfHankD! I just read the intro and went straight to looking at your pictures!
electronbee4 years ago
Actually, the stated f/ will only be at the designed specifications of the lens itself. A lot of these lenses have an extremely short back focal length. So, if your camera has a lens-mount to sensor distance of say 43mm and your lens has a back focal length of 15mm then the projected image will be larger than designed and the amount of light hitting anyone area will be reduced. Similar to a macro lens will have you have to increase exposure the higher your magnification.

The lenses will still be fast, especially for the money, but not as fast as one thinks.
ProfHankD (author)  electronbee4 years ago
This already was explained in step #3.

Here's the math. The effective focal length of a lens typically is increased by the extra distance that the lens is moved from the film/sensor plane when focus is set closer than infinity. An Xmm lens that is f/Y and moved Zmm more distant from the sensor still has an aperture of (X/Y)mm, but now has effective focal length of (X+Z). Thus, the effective f/number would be (X+Z)/(X/Y).

Close focus using a lens designed for your camera typically will have the same impact on it's effective f/number. The most common exceptions are lenses that have internal focus mechanisms that cause them to change focal length independent of the distance to the sensor, and even for them there is usually a similar effective change in f/number at close focus.
nukemall4 years ago
Good work!