I was once a Navy photographer, so I have a box full of camera filters that are rarely being used theses days. I have two new Weaver scopes, a Classic V 4-16x42 AO and a 40/44 3.8-12x44 AO Aspherical. The first scope appears to have 49mm threads and the second scope seems to have 55mm threads. FWIW, those are the respective standard filter thread sizes for most Olympus OM Zuiko and Canon FD series lenses of the 70's, when I was buying 35mm SLR gear. Nikon was mostly 52mm, and maybe Pentax as well. Minolta may have been Canon size, I forget. I used Canon, Nikon, Pentax SLR and Leica rangefinder in 35mm photography. Any camera store should have 49mm, 52mm and 55mm filters in a wide selection as long as they cater to the older 35mm SLR cameras. Smaller sizes like 42mm will fit the Leica rangefinder cameras if I remember correctly, and might be useful on scopes with smaller objectives. You can buy adapter rings that convert 55mm filters to 49mm or 52mm threaded lenses. For instance, Canon55 to Nikon52 and Olympus49. From what I know about light, the most useful filters for rifle scopes will come from filters used in black and white photography, but a few color filters as well (Polarizer is used in both casses). The human eye does not see like color film and needs strong correction of light to make a useful difference when using filters for visual enhancements. The filters I have that will be the most useful are: 85B, which is orange. It gives stronger blue reduction than yellow, cutting off some green as well. Number 8 (K2), which is yellow and very similar to yellow shooting glasses. For overcast days and distant shooting, where blue light predominates and needs reduction. G(X1), which is green, and eliminates red and blue. It's main purpose is to lighten foliage and darken things that are not green, such as wild game in this case. Polarizing, which gets rid of reflected light that has been polarized from nonmetallic surfaces, such as water, foliage, snow. Known as a glare reduction filter, used often by drivers and boaters in sunglasses, but can kill you in winter as it gets rid of ice glare. Number 61 dark green is darker than the G(X1) and will give more lightening of foliage by making everything else darker still. R(25A) will lighten red and darken everything else. A red furrred animal in green foliage would be lightened and stand out against a dark background. Number 47 Dark Blue has no value I can see, unless you want to increase UV Haze or run up against blue skinned aliens someday. Rifle scopes are not apochromatic as far as I know. By getting rid of either blue or red, you have a narrower spectrum of light to bring into focus. Orthochromatic lenses can focus blue through green into a sharp image, but red makes the image blur. Your scope probably has trouble bringing a red dot into focus when blue and green are focused, this is the nature of all complex lenses that are not apochromatic and are not made from slecial glasses such as ED types. BUT MORE IMPORTANTLY, when blue and green are parallax corrected, red will not be as it focuses in a different place. You can parallax correct abig black dot, then put a smaller red dot in the center of it, move your eye around, and see the red dot move on the black dot. Just sandbag your rifle and check it out at a long range target. Something you might want to remember when shooting at furry animals like a red fox at great distance. FWIW, I have found camera filters to often be better than cheap astronomy eyepiece filters that were supposed to suitable for high power telescopes. Tiffen filters were the most common brand among pros, but filters put out by the top camera manufacturers were always good, but usually higher priced with no gain in quality over Tiffen or Hoya that I could tell. I hate to break the news to you, but your rifle scope is probably a piece of crap compared to a good telescope (cheap department store telescopes are NOT good, and you probably NEVER looked through a really good telescope, as they typically cost thousands, and not hundreds of dollars. Mine is a professional quality Russian LOMO; they made at least 90% of the high end optics used by Russian military forces. Your spotting scope is also a piece of crap up against a good telescope, and the cheap image correcting prism is a major culprit there.), as they have to run at 200X power (normal "seeing" limit on a good night, imposed by earth's atmosphere) and even much higher on occasion, while hunting scopes rarely exceed 24X power. Rifle scopes do not impose the extreme optical standards that professional quality telescopes do. While Celestron and Meade talk about 1/4 wave as being good in their amateur telescopes, nothing in my LOMO scope chain is less than 1/10 wave, a huge difference in optical quality.