Darrell did a great job sharing some of the view points. By the way, the link he gives showing the affects of the one UV filter on that persons camera is an excellent one. It walks through the exact process I was trying to explain in words. Great link and process. IF you look closely, in that link, the affects of the filter also seem to be less noticeable with subjects close to the camera in comparison to those that are far away. THAT is another affect. The other area that can be an issue with filters is your auto focus system. That works by measuring the contrast of the target. So, shooting pictures in the fog etc can often cause issues with auto focus. Same as with the dark, your camera will struggle to find enough light to focus. The more you darken the image coming in and the more you reduce the contrast, the harder it will be for the auto focus to find it's mark. Especially on birds that are tiny in the view finder with bright skys. It can't help but see the sky and the tiny bird is such a small contrast point that a filter may goof up the precision of autofocus. That is why I suggest a black and white sign. That is the highest contrast available for your lens and will tell you if the glass is CLEAR and does not affect the true image quality. BUT, always be aware that any filter can have an affect on your camera's ability to focus. So, using the link that Darrell provided, I would say that filter itself only affects the IQ of the picture a very slight bit. I do notice in the various shots that taken of close subjects a definite drop in contrast of the colors which will have a serious affect on perceived sharpness. So let's say this filter is $20 would I buy that filter? Depends on my personal circumstance. If my subjects will always be close and I am willing to edit every picture or adjust my camera settings to shoot a higher contrast setting, and I had little or no money and still wanted a filter on my lens, yes I would buy. BUT, I would also be keenly aware that as soon as I get into situations where birds are far away, where colors are tending to blend, etc, I HAVE TO TAKE IT OFF. My auto focus can not handle it. I have to know the limits of my equipment. Now, If I want to leave it on no matter what, better cough up a 50 and buy a much better filter. Even then, I would go through and see how much IT affects the picture. Remember, as Darrell stated, many serious photographers of birds (and wildlife for that matter) do not use ANY filters, including the expensive ones. Why? Darrell hit the nail right on the head with the affects they have on light. AND, since we are often cropping our pictures quite heavily, ANY slight affect either on the IQ of the picture or the focus of the camera lead to unacceptable results.
I will give some personal set up observations. Remember, these are settings and techniques I have figured out for me and my equipment.
In birds and wildlife shots, rarely use any filter at all. I shoot a 400 5.6 and the 5.6 is often a challenge to get decent shutter speeds to handle birds and moving subjects. One exception, if I am shooting birds on the water and they are basically static or just swimming and I am working from a tripod, I will often use a polarizer to kill glare and reflection on the water help the subject pop a bit more in the picture. I have used it with eagles in flight in very controlled conditions. When on the Mississippi, I sat in one place, the sky was perfectly clear, and I used it since it was bright sun and it helped me darken the blue sky and gave great control over the reflection of light off the white heads and tails. It did cut my shutter speed from about 1000 to 640. I was comfortable at that speed so things worked but that is getting close to minimum shutter speed for that lens and my personal body. Any other time, it is no filters in bird and wildlife photos. Now, with landscape and portrait work, I have filters on quite regularly.
Many here are shooting lenses that are often 70-300 range with a 5.6 on the top end. Throw a polarizer on the end of it (just like my 400 5.6) and the light is cut heavily and it becomes the same as shooting with an F11 fstop. A better technique to consider is to shoot with no polarizer and see if you have enough light to shoot at f8. MOST lenses do not give high stellar IQ when they are wide open. SO, many of the different 300 zooms are known to be some what soft at 5.6. By bumping to f8, focus appears sharper. You do loose more of your depth of field. More will be in focus due to the fstop change but if that is what you can afford, blurring the back ground in the computer might be the option compared to spending considerable money on a new lens. A new lens is always best though!!!
I use a tripod or some sort of rest as much as possible. Filters or not, if you are moving, it does not help. In the days of 35mm, there was a rule that stated you need a shutter speed equal to the reciprocal of the lens length.
What does that mean? Take your 300mm, you would need to shoot a shutter speed of 1/300 or faster. IF you are shooting a full frame camera, this still apples in a general sense. MOST of us from what I have seen, are shooting what is called a 1.6 crop camera. That means, it only records about 40% from the center of the view that can be captured by that lens. In layman's terms, it crops it by 60% thus giving the affect that the lens is longer than it is. For us with crop bodies, the above rule works a bit different. Let's start with your 300mm. We first multiply that by 1.6 and we get 480. In other words, your lens acts like a 480mm on your camera and it also records your vibrations at that increase as well so we need to have a shutter speed of 1/480 (1/500) as a minimum shutter speed. Couple that with a 5.6, and as you have experienced, that takes a bit of light. What about vibration reduction lenses and programs on our cameras? Remember, they can help basic camera shake but the slower you go, with moving targets the shutter is still slower so blurring of the image is still an issue if your subject is moving.
Sorry for the length and hope it helps. Darrell, you have come a long way in a short time. It was not that long ago you were posting your first shots. I am impressed. Keep it up. You have a good handle on the filter issue for sure!!
Share more of what you have learned. It not only helps others, but it helps solidify it in your mind as well. I am certain I learn as much teaching as I do "learning". Thanks for the great input!!!