Perhaps the ideal addition to the camera bag for those who like to push square pegs through round holes, the iPhone 4 SLR Mount does pretty much what it says on the box. Available in either Canon or Nikon-mount forms, it allows iPhone 4 owners to attach lenses for either mount type to their camera phone. Advantages noted by Photojojo include the ability to focus manually, and to achieve depth-of-field effects that aren't possible with the stock iPhone 4 camera.

Since there's no way to directly attach a mount to the iPhone 4, Photojojo's SLR Mount includes a chunky aluminum sleeve into which the phone must first be inserted. This provides both a threaded mount onto which either the Canon or Nikon mount adapter can be fixed, and a protrusion that serves double-duty as a finger grip and tripod mount. The mount adapter, meanwhile, includes a focusing screen onto which the image from the attached lens is projected, and it's this image which the iPhone's camera is actually taking a photo of. The combination is said by Photojojo to lose one or two stops of light. Canon shooters should note that EF-S and FD lenses are incompatible with the device.

There are a few drawbacks to the design, beyond the obvious ergonomic issues of attaching a bulky SLR lens to a cellphone, and the fact that camera phone image quality isn't going to rival that of a large-sensor DSLR. Firstly, the focusing screen gives images shot with the iPhone 4 SLR Mount a grainy texture, and can also easily pick up dust during lens changes, although these could perhaps be seen as a benefit, for fans of lomo photography. Because there's no way to drive a lens iris or autofocus motor, both aperture and focus must be controlled manually, which of course relies on the lens providing a manual aperture ring, something that's fairly rare on modern lenses. Since the lens projects an inverted image onto the focusing screen, images must also be framed upside down using the stock iPhone camera app, and then rotated 180 degrees in post-processing, although we'd imagine a dedicated camera app could cure this framing issue relatively easily.

Bearing these points in mind, Photojojo's iPhone 4 SLR Mount is clearly not intended for the casual shooter, something that's backed up by a ~$250 price tag in either Canon or Nikon-mount form. For the target audience, all this is unlikely to matter. There's no question that there's significant geek value in the ability to attach SLR lenses to your camera phone, and the combination is likely to prove something of an attention-getter, especially with larger telephoto lenses.

More details on the iPhone 4 SLR Mount, which ships from mid-August, can be found on Photojojo's website.