*Updated Dec. 9, 2009.*
I had some lenses from various sources sitting around and wanted to devise an easy way to use them with my camera phone to take magnified pictures and videos. At first, I just had small metal bezels that I used to hold the lenses up to the camera lens. This was clumsy and upon getting the camera close enough to focus, all of the light was crowded out. These are the problems I intended to solve with this device. This is what I came up with.
Mine is built specifically for the iPhone. However, you can adjust the design to fit the phone you have. I am aware of the magnification apps for the iPhone that use digital magnification. They do not achieve the same magnigication strength and clarity as optical magnification.
*Step 7 is the update.

Video of an ant war captured with this device:
http://www.youtube.com/watch?v=Em9TzHY8lmc

Step 1: You Will Need:

Tools:

Ruler
Compass
Center Punch
Drill (drill press is best)
Soldering iron

Materials:

Jar lid (I used a lid from a pickle jar)
Lenses (from scrapped cameras)
3/4" Suction cup
Rubber O ring that fits around the "mushroom" part of suction cup
White LED
JST Connector (scrapped computer)
Small piece of extra wire
Switch (AM/FM switch from old walkman)
3v battery holder and battery (scrapped computer)
Super glue
Hot glue or silicone

All of these materials (except the glue) were easily scavenged. I have a case on my iphone. Many cases leave a hole to reveal the Apple symbol on the back. This makes a perfect place to consistently attatch the suction cup in the same place, but I had a hard time finding the perfect sized suction cup to fit that hole so I ended up buying a pack.

Step 2: Making Measurements and Marking

First, you need to find the center point on the jar lid and mark it. Next, choose where you want to attach the suction cup to your phone. Measure from that point to the centerpoint of your camera lens. You want to be fairly precise. This will be the radius (on the iphone 3gs it is 1 1/8" from the center of the apple) for the circle that you will scribe on the lid with the compass. Mark your lens positions centered on the arc of the circle. Remember to make a mark for an empty hole for taking regular pictures and place your lenses in ascending order of magnification. Find the diameters of the lenses and the "mushroom" part of the suction cup.

Step 3: Drilling

Punch all marks with center punch before drilling. Make all holes for the lenses slightly smaller than their diameter and arrange them in order of magnification. The center hole for the suction cup should be small enough that you have to work at forcing the "mushroom" through the hole. Leave ample space for your light circuit.

Step 4: Assembling the Light Circuit

Leave the wires on the JST connector long enough to allow the lid to swivel once it is all assembled. Strip the ends of all the wires. Solder one lead of the JST to the negative post of the battery holder. Attatch the other to the negative post of the switch. The small piece of wire gets soldered to the positive posts of the battery holder and the switch. Insert battery and test the circuit.

Step 5: Put It Together

     Before assembly, I took a wire wheel to the top of the lid to remove the paint.

To Assemble:
     First push the "mushroom" part of the suction cup through the center hole. Stretch the O ring around the cap of the mushroom and onto the stem. This should be tight enough to provide resistance when swiveling the lid.
     Glue the lenses in their respective holes with the more curved side facing up out of the bottom of the lid (away from the phone). I tried a couple different glues and epoxy worked best for me. Be careful not to get glue on the viewable part of the lens!!!
     Super glue the JST connector to the top of the suction cup (this way the light stays stationary as the lid swivels), the battery holder and the switch to the lid. Use hot glue or silicone to cover any open circuitry to prevent shorts.

Step 6: Try It Out

To attatch the device to my phone, I just grab the JST connector and with the light pointed toward the camera lens, push it directly onto the apple on my iphone. I don't know the actual magnifications of the lenses so I just labled the samples as first lens, second lens . . .

Step 7: Update

I did a little more work on my magnifier. I installed a spring with a slider to automatically keep the lenses flush with the camera lens, added a cover for the battery, cut away most of the rim for the jar lid, built up around the lenses with a two part epoxy putty, lined it with reflective aluminum tape, glued brass rings around the lenses, and finished the edges with tool handle dip.