Note whether there are any symptoms that indicate the elements may be out of order, flipped, or misaligned. There are other reasons lenses may not focus as expected that aren't related to elements being misplaced.

• Focus may need to be calibrated.
• The lens may be soft wide open.

Draw the lens diagram, as you have it. This can serve as a reference if you want to put the lens back to how it was. You may also be able to spot elements in positions that just "look wrong".

Try to find an official lens diagram. Diagrams of similar lenses may be helpful. Or you may find another copy of the lens to inspect.

Trial and error.

• Lens groups tend to follow patterns. Some sequences are highly unlikely. The following pages should be helpful:

  • Lens Rentals: Lens Genealogy Part 1

  • Lens Rentals: Lens Genealogy Part 2

• The first and last element in a group can often be determined by size. The largest element is usually in the front of the group. The smallest element is may be in the middle or rear of the group.

• The spacers that are adjacent to elements can also be determined by size.

• Elements are usually convex away from the center of the lens or group. However, there are notable cases where elements are flat or concave away from the center.

• Glued elements are usually toward the front or center of the group.

• Whether elements will hit each other can help determine their order and which way they face.

• The above constraints should limit the number of sequences to try.