Posted on Tue 01 December 2009
I've been trying to learn a bit more about the optics involved in mid- to pro-level cameras, and I've been finding that getting out the tools and doing some hacking serves as a great mechanism for instruction. Just recently the idea of optical couplers - that is, a system to transmit light from one optical device to another to make an image - has gotten my attention.
Mainly just to see if it was possible (and to teach myself about optical couplers), I thought I'd try my hand at building an adapter so I could attach SLR lenses to my iPhone. The result is pretty cheesy, but nevertheless - I present to you, the Phone-O-Scope:
Just to get the inevitable question of 'why' out of the way - well, why not? As far as I can tell, I think this is the first - I couldn't find any similar SLR lens to camera phone attaching attempts anywhere else online (I'm not including the mini-lenses you can get as they're not 35mm lenses). The Phone-O-Scope doesn't take especially superb images, and it's a bit clumsy to handle. On the other hand, it's fun to shoot with and produces very analog (almost Holga-like) results. You also get the advantages of SLR lenses - that is, DOF effects, and the wide range of available focal lengths (i.e. macro to telephoto).
There are plenty of interesting designs for cameras floating about, and plenty of nice phones with reasonable optics attached. I'm hoping that this experiment takes the concept of a camera phone to it's illogical conclusion.
In keeping with my other optics experiments, I wanted to try something that required the minimum amount of dis assembly of expensive components (i.e. no warranty voiding!), as well as being cheap and relatively easy to assemble. The parts list (roughly) is:
- Camera phone (in this case, an iPhone)
- SLR lens (in this case, two Canon EF lenses)
- Old surplus canon lens end cap
- Hard phone case (cheapie iPhone case, ~US$10)
- Old optical mechanism from a CD or DVD player (scrap - cost me $0)
- Bit of old sticky putty (again, $0)
- Cardboard tube for testing
- 2-inch PVC pipe couplers (about US$3 each)
- Duct tape (of course)
I already had the lenses and phone, so the project cost me less than US$10.
Get the case off the phone first. This particular design doesn't do anything destructive to the phone or the case, but it's a good idea not to tempt fate :)
You'll need to take laser pickup (see left inset, above) out of the CD player and start to pull it to bits. I was able to get three lenses out; you'll need to verify that each one is a magnifier. These can be stacked to achieve more magnification of the lens aperture (when it is attached); the more you can magnify, the less vignetted the image will be.
I wrapped a thin layer of putty around the edges of the stacked lenses to stick thin strips of paper around the edges of the lenses. This is so that they can be assembled together and handled as a single unit.
Take the lens stack and insert it into the cameraphone case; I again used a bit of putty to hold it into place. Just out of interest - this turns the cameraphone into a pretty decent microscope. The depth of field is rubbish (in my case, about 1-2mm) but the magnification is huge.
Now that you've got the magnifier lenses in place, I'd recommend building a simple test rig to check roughly the distance you will need between the camera and the back of the SLR lens. The proper thing to do is do some proper calculations involving the measured actual magnification of the lens stack and the flange focal distance of the SLR lens. However, if you lack the equipment for proper measurement, you can estimate by building a set of sliding tubes with an SLR lens attached.
Start off by taking two sections of tubing, one slightly narrower than the other so it can slide inside the other. Cut strips along the tube with wider diameter so you can hold the lens on it using a rubber band. Attach this whole mechanism over the camera phone and magnifier.
The tricky part now is to get this working mechanism to make a picture. You'll need to slide the outer tube back and forth as well as adjusting the focusing ring on the lens, and there'll be a lot of experimentation until you get a sharp picture. If you find that all your images are heavily vignetted, you'll need to add another lens to the magnifier stack.
Components for the actual adapter. I used an old end lens cap, and two pieces of two-inch PVC pipe connectors. It was important that the connectors could be connected via a screw thread - this way we can move the whole SLR lens back and forth for secondary focusing.
Cut a hole in the lens end cap (inset top left). Use the measurements for the top and bottom cardboard tubes (step 4) to figure out how much you'll need to trim the PVC connectors. The distance that you get from the cardboard tubes should be the same as when the PVC connectors are roughly screwed together halfway, so you get lots of room to move them back and forth. To the top connector, attach the end cap. If you need, cut a notch out of the bottom connector (inset, bottom right) so it fits nicely on the case. Attach the SLR lens just and verify that your measurements are right.
Duct tape madness. Attach the bottom connector to the camera phone case (inset, middle bottom) using thin strips of duct tape. Make sure you get a nice tight fit, but don't go overboard with the tape. Use a bit more tape to also firmly attach the lens cap to the top PVC connector (inset, middle top). Don't forget to attach a bit of cardboard at the back to prevent too much light leakage into the mechanism. Screw the top connector to the bottom, and the Phone-O-Scope is complete (right).
Attach the lenses!
50mm lens attached to the Phone-O-Scope:
18-55mm lens attached to the Phone-O-Scope:
As I'd mentioned earlier, the point of this mod is to learn what is needed to successfully build an optical coupler. One important thing to keep in mind is that even if it doesn't work perfectly, it's still a win - I've observed what doesn't work and I'll have a much better idea about how to do it better next time.
The image quality issues come down to two main factors:
1) Optical axis alignment. The phone, lens stack and SLR lens alignment isn't good enough yet. I've found that small alignment errors in the axis - even as little as one or two millimeters - is enough to cause significant portions of the image to become out of focus
2) Image stack lenses used. The lenses are out of a CD player; they're not perfect magnifiers, and, they're designed to work with a specific wavelength of light. This is a significant contributor to the chromatic aberration observed.
Technical issues aside, I'd also say that there is something to be said for having an unpredictable mechanism that prevents you from getting the same photo twice. I've taken a lot of photos with the plungercam, and I've found that although I do take a lot of images that are no good, there are a few that are pretty awesome. And the best bit about that is that I can say with reasonable certainty that it'll be impossible to ever get those same shots ever again. If that doesn't appeal to you, then this type of modification is not for you.
The Phone-O-Scope produces fuzzy, Holga-like images. I think a lot of the image artifacts (strong chromatic aberration, bizarro lens distortion) are down to the extreme magnifier stack. However, I've tested it with a few Canon EF lenses now and it does seem to work reasonably well with every one. At the very least, it seems to work like a telescope for the iPhone, and it is fun to shoot with (not to mention the odd looks I get when I'm using it :)
I've got a series of comparison images below; an iPhone picture is on the left, and a Phone-O-Scope image is on the right. In the interest of transparency: I've made some small corrections to the Phone-O-Scope images (exposure, white & black points):
Epilogue: will this type of design work in practice?
The summary: I don't think that cellphone SLR lens adapters are ever going to work particularly well, based on experience I have building one myself and looking at the design of a commercial one - http://photojojo.com/store/awesomeness/iphone-slr-mount/.
The adapters have to get around two main problems - 1) fact that the iPhone sensor is tiny and the image that the SLR lens projects at the registration distance is large and 2) light loss.
For my own design, I got around 1) by using a magnifier out of a CD player in a similar fashion to the rear element on a set of binoculars to reduce the image size so that a real, but shrunken image is projected more-or-less straight on to the iPhone sensor. I got around 2) by only using one optical element.
It didn't really work. The problem mainly boiled down to having used a poor quality intermediate lens, which created a grainy image and chromatic aberration artifacts.
The commercial adapter (manufactured by Turtleback ) approaches the problem differently by projecting the image from the attached SLR lens on to a frosted screen, and having the iPhone focus on that instead.
This does work - but the light loss is massive (two stops, at least) and, apparently, the focusing screen gets dirty very easily (see the set of cleaning instructions from the manufacturer ). Also, the frosted element adds a large amount of grain to the image. It is a better approach - but not by much (or so I think - at this point reiterate that I haven't tried it for myself, so please that that into account as you read this).
The adapter-based-approach is a cute idea, but it's essentially doomed by the fact that SLR lenses are complicated optical apparatus that are designed to work with very specific sensors - getting a good quality image out of any iPhone SLR adapter is nearly impossible.
I should emphasise that these adapters do work, but only in a limited way - there has to be enough light, and there is an upper limit on the quality of the image that is achievable. If all you want to achieve is shallow depth-of-field, then there are other approaches that produce better results (see below).
Without adapters, the fact that the iPhone sensor is super tiny, and it's physically impossible for it to have shallow depth-of-field for anything other than objects that are less than a couple of feet away. If you want to get shallow DOF out of the iPhone, there are a couple of synthetic aperture approaches that might get you most of the way there. For example, see synthcam.
TODO PDF: Russian - http://www.iphones.ru/iNotes/43166
TODO PDF: Chinese - http://apple4.us/2009/12/phone-o-scope-attaching-slr-lenses-to-iphone.html