Putting it all together
So far, we have talked about all the parts required to mount longer lenses. In this last installment, we are going to mount the Ross London 5 inch f4 Wide Angle Xpress lens, as an example how everything fits together. The 5 inch lens (equivalent to about 127mm) is not really that long, but fits in kind of a sweet spot; a Goldilocks focal length if you will, and I really like this lens (and the focal length). Besides, I have been shooting with this lens in the last few days and it's already setup, therefore less work for me :)
The flange to sensor distance of this lens is roughly equivalent to its focal length. This provides plenty of space to mount this lens on pretty much any current lens interchangeable digital camera, this includes all Sony E-Mount (or NEX-Mount).
Preparing the lens for the helicoid
The first thing to do, is to assess if the lens can be mounted on the helicoid. Some are easy and some are very difficult to do. Any lens that allows a standard filter size, like 49mm, 52mm, 55mm, 58mm, etc,. to be attached to its rear, would be the easy ones. The Ross London 5 inch f4 lens is one of those. It allows a 55mm filter ring to be attached with just a little bit of work. What we want to do is to epoxy a 55mm filter ring to the rear, and then screw the lens to the tube/helicoid for focusing.
The rear of the lens is just slightly too large to allow a 55mm filter ring to pass through. We need to file the filter rim slightly with a Dremel grinding tool until the filter ring can go through. Once it does, we can apply epoxy to the filter ring to attach itself to the lens. Be sure to check that the filter is perfectly aligned, or else we will get pictures with one side sharp and one side soft. Once the epoxy is set, it's ready to be used.
Mounting the lens on the helicoid
Now that the lens has a 55mm thread and ready to be mounted. It's time to decide what combination of helicoid/tube/filter rings to use to get the lens to focus as closely as possible, but also able to focus to infinity. The distance between the lens and the sensor is not long enough to stack two Vivitar 2X macro teleconverter helicoids; it's just slightly shy of infinity focus. This means I will be using just one helicoid, and extension tube/filter rings. As it turns out, all I needed was on extension tube and I didn't need any filter rings. With the Vivitar 2X Macro Focusing helicoid and the filter rings, the minimum focus distance is about 5 feet, which isn't too bad, considering most old lenses have very long minimum focus distance. If closer focus is needed, you can add two or three filter rings to extended the space between the lens and the helicoid.
Well, there you have it. Mounting a different lens just means adding or removing extension tubes/helicoid. Hope you find this mini-series useful and helpful. It has been a fun exercise for me. Good luck if you are trying it out.
All the parts together from left to right: 1 mm thick M42 to E-Mount adapter, 42mm-62mm step-up ring, Vivitar 2X macro focusing helicoid, extension tube, lens with 55mm mount.
All screwed together
Sample picture: Sony A7 & Ross London Wide Angle Xpress 5 inch f4