How can I know if two lenses have interchangeable elements?

Question

This simple question might have a not so simple answer, and I want to ask it about a single lense, but assume that would be off topic, because I'd basically be asking 'should I buy this to repair my other lens?'

Here's an example: are the lens elements from the nikon 18-55mm vr ii compatible with the elements from the 18-55mm vr? They look cosmetically similar. Is there a spec sheet?

Same with other lenses, like the 50mm 1.2 ai-s and 50mm 1.2 ai. Compatible elements? Spec sheets?

My example comes with real world motivation: I got a free lens from a friend, wanted to try my hand at repair for cheap. I saw a lens with the same specs, same generation, but one model newer or older and it's af motor is damaged, or something similar.

Feel free to edit my comment if you feel it can be better phrased.

Answer 1

Sadly, you can't, without taking them apart to find out.

Unfortunately, it's not reliable to assume anything about the construction of photographic equipment.

Even if two instances of what is marketed as the same lens contain optically identical elements, which isn't always guaranteed, you may find that a bevel on the edge of a particular element (or somesuch) is slightly different, because the manufacturer adopted a slightly different way of mounting it in later versions.

You can't rely on repair manuals or other documentation either; you'll often find that the published description is different from the equipment in front of you.

Most typically, you will discover that the manufacturer found a way to do the same things slightly more cheaply.

Having said that, often you will find that the same elements and other parts are re-used across different models, because that too helps reduce costs, so though you get no guarantees, the chances can be good.

Answer 2

Almost every piece of every modern lens from the major manufacturers has a part number. If you can access the service manuals for the two lenses in question you can compare the part numbers. If the numbers are not identical, they're not the same part. Perhaps one lens uses part # 123xyz and the other lens uses part # 123xyz-a. This would indicate that a minor change was made to the later version which may, or may not, prevent them from being interchangeable. Each lens element may have a part number, or, in the case of lens elements that are only available as replacement parts together as a group or as part of a larger subassembly, they may not.

That's not all there is to it, though. Many lenses use shims or spacers to align the optical elements as closely as possible to the the intended design. This allows adjustment for the manufacturing tolerances of parts that are theoretically identical. As a lens is being assembled in the factory, optical alignments may be checked in several stages and shims/spacers of varying thicknesses (or even varying differences in the thickness of one side of the ring compared to the other) used to improve the optical alignment of the lens. Such shims/spacers may or may not be marked to indicate their exact thickness/geometry. That's why it is important to carefully mark the position of such parts when a lens is disassembled. But when one swaps a lens element from one lens for another from a different lens, the lens will need to be carefully measured optically as it is being assembled, and a different shim/spacer may be needed to account for the slight difference between the two theoretically identical pieces of glass.

Answer 3

It would basically be impossible for anyone other than the manufacturer to answer that question about any given pair of lenses. Two very similar lenses might have some of the same outer elements (particularly the front lens, which is the most commonly damaged part), but they could also be subtly different.

For example, the Canon 16-35 L, L II, and L III look almost identical, and both have 16 elements, but

• L: 10 groups, 14 elements, 77mm front thread
• L II: 12 groups, 16 elements, 82mm front thread
• L III: 11 groups, 16 elements, 82mm front thread

You probably should assume that the lens elements themselves are not interchangeable between lenses. That said, you might be able to take the working autofocus motor from the free lens and use it to repair the other one. Parts like that are often the same across a large number of lenses.

If you can find a parts diagram for the two lenses, that will tell you somewhat definitively whether they have any parts in common.