[Snap-On] Great protection and easy to use. Should be a must lens cap for every 600mm nikon lens. Easier to use than the cloth covered lens cover that comes with my lens.

[Plastic] I have one for all of my Canon Great Whites. Easy to put on and remove and really protects lens. Only drawback is does not collapse but certainly not a dealbreaker.

[Proprietary] I got frustrated with not having protection over my 500mm, without having the lens hood in place. This was a perfect solution. I changed out a few of the smaller rings for the larger supplied rings and it's a very snug fit!! Thank you for a great product.

[Plastic] I have these for my 300 F/2.8, 500F/4 and 600F/4. I would never travel without them on the lens. They are far more protective then the silly sock that Nikon includes. They come with two sets of different sized O-rings, so it is fully adjustable for a perfectly snug fit. If you can't get it to fit properly, you probably only shoot jpeg.

[Proprietary] I bought this for traveling so I didn't need to take the large lens cap and lens hood along. It seems to work perfectly fitting on the front of the lens snugly but is easy to put on and take off.

[Plastic] This lens cap is far and away much more convenient to use than the stock sock cap that comes with the Canon 600mm lens. It is also not impeded by the LensCoat wrap I have on my lens. The cap goes on quickly and provides a very positive,secure seal over that large front element. If you invest in expensive glass, this cap is also worth the investment.

[Plastic] Just to clarify this is on my Mark 1 not the newer Mark II.

The O ring system easily twists on the front of the lens (NOT THE LIGHT SHIELD). The light shield slips on and off with the cap in place. The cap is easily put on and off with the light shield in place by just reaching in. Comes with a few extra O rings.

[0.425 lb] This cap works with friction.
Use the appropriate o-rings to create the right amount of friction when you twist the cap into the end of the lens. This makes for an easy-on/easy-off solution (especially when you have a lens coat on).
The handle adds some extra protection when face-down in a camera bag, but if you want something that is just flat so you can stand your lens on the floor, you can contact Don and get a custom made version without the handle.

[Snap-On] It is unbelievable that Canon gives us horrible caps for their very expensive great white telephotos when Don Zeck makes these caps.
Don Zeck caps are better than any other after market caps, I can not recommend them enough.

[Front] This is a very clever idea to provide a really effective lens cover for large lenses. The 'sock' style cover provide by Canon for the 600mm f4 lens is heavy and very impractical. This is a far better solution - quick and easy to put on and remove. The addition of a set of oversized O rings enables a perfect fit by replacing some or all of the standard sized rings. I really can't fault this product and would highly recommend it.