If you have a standard Yukon Photon 5x42 you will have found it is extremely focus fussy and the best you can get is almost focused, particularly at short ranges.
The Photon is too responsive for our own good. The result of being able to gather so much light has a side effect that narrows the depth (or range) of focus.
It's basically a very efficient low light camera fitted in front of a scope with a tiny monitor in the eyepiece. Most efficient cameras have an iris in the lens to reduce light to a manageable level when there is plenty of it about so they can achieve a greater range of focus.
Yukon supply a very small fixed iris with a filter in it for daytime purposes, but daylight is maybe 1000 times more intense than night time.
The Photon has not been given the means to moderate the amount of light going to the sensor at night with an illuminator - so it is very difficult to get a good focus from it.
These lens caps place a fixed iris in front of the objective lens so you can get a longer field of focus - or if you like, a better focus in general with much less eye bloom from eyeshine.
The lens cap is a flexible plastic that will friction fit to the front of your Photon objective bell. The one shown fitted to the Photon has a recess for an experimental Ir pass filter, the item being sold here does not have that recess. There is sufficient space between the inside of the cap and the objective lens to place a 52mm filter behind it it if you wish to use one.
Anyone with a Photon in standard 5x42 form will know how difficult is to focus clearly. I have machined some plastic lens caps with a fixed iris in them which helps the very low light camera cope with more light than it needs.
Make sure you watch the video:
Forum members can order it direct @ £20 each posted, or £10 each if ordered at the same time as any other 501B item I sell if it's going to be in the same £6 RMSD package.