I've had the Vivitar Series 1 600mm f8 Solid Catadioptric lens for some time now and have been somewhat disappointed in the quality of the images that it produced, they always appeared very soft and unappealing. Mirror lenses haven't been renowned for producing the same image quality as normal type lenses, but the Series 1 was supposed to be very good, being specifically made for the US military special ops or some such. And being a solid piece of glass, there are no elements to go out of alignment. This is what it looks like: I was looking at the lens the other day and it occurred to me that I've never tried the lens without the rear filter installed. The only way to attach filters is to the rear and I was always under the impression that the filter formed part of the optical system and therefore should always be installed. My Nikon 300mm f4 has a drop in filter and the same apparently applies, though it's location isn't near the end of the lens. So I thought why not and, after removing the filter and taking a few test shots of various subjects, I had another look at the results and was surprised that the images were much sharper and cleaner than what I'd achieved with the filter installed. The filter is the original Vivitar Series 1 filter, but I wonder if it wasn't playing nicely with the digital sensor, as it would with film for which it was designed. Whether that was the cause or not, removing the filter has improved the image quality significantly, something to at least consider if using older lenses that take rear mounted filters. I look forward to finding better subjects than tree bark and leaves, something a little more worthwhile showing.