Whither Filters?
The debate rages onwards regarding the use of clear filters for protecting a lens. Whether the filter in question is a 1A Skylight or UV filter, or a Nikon NC (no colour) or whatever it is, some folks say that you should keep such a filter on every lens to protect it from abuse. I am one of those folks. My big Tamron lens was saved from a nasty beating through the self-less martyrdom of the Tamron Skylight filter that was on it that day. When the edge of the filter hit the dirt, literally, it kicked up particles that scratched the filter in a spray... half a dozen scratches from near the edge to about half way across the filter. The front optic of this lens is only a few millimeters behind the filter. It would have been *doomed*. Also, the filter threads on the filter (the threads used to attach more filters to the skylight filter) were bent, and I have no doubt that the filter threads on the lens would have been bent, and that costs money to get fixed.
A benefit of clear filters is that if you clean the lens and the inside of the filter carefully and then screw the filter on, then you will have no dust in there, and the only cleaning you have to do from then on (providing you don't remove the filter) is on the easily accessible front of the filter.
The main complaint people have about using protection filters is that you lose image quality. They all say that. "Why would you put a $10 piece of glass in front of a $300 lens" they all say. Well, the Tamron Skylight filters that I use cost $22.99 each for a 62mm thread at my local camera retailer. The lens that I'm protecting costs $675 at the same retailer. So, should I be worried about the quality? No, I think not. Why? This lens has 15 elements. That means there are 15 individual pieces of ground glass in there. Even if the body of the lens, aperture mechanism, microchip, focusing mechanism and all of that stuff was *free* to manufacture, that would still be an average of $45 per lens. That's about twice what the filter costs. Now, consider that these are ground lenses. Ground to a particular, exacting profile to bend light rays just the right amount. Consider that the filter just needs to be flat so that it *won't* bend light rays. Why shouldn't it be cheaper to make flat glass than ground lenses? Even throwing in a few bucks for coatings leaves me comfortable with the idea that I'm not losing quality there. But comfortable isn't good enough. No, it's time for testing!
I set up a Canadian 20 dollar bill on the still table, and took a shot with and without the filter. Nothing else was changed. Same exposure, lighting, zoom, everything. Here are details from those shots:
Here we have the Queen's right eye. Twice. Same scale, same everything. These are the pixels as-captured with no funny business in post-processing except to crop out the same bit from each shot and put them together into one image. No level adjustment, no scaling up or down, or anything like that. One shot was taken without the filter, and one with. Can you see a difference? I don't see much of a difference. Maybe the bottom one has a bit more detail due to contrast. Maybe. The bottom one is the shot taken *with* the filter.
So should I be worried? I don't think so. The filter has saved my lens more than once, and it's staying on there.
So it's strictly win/win? Not quite. There is one serious drawback to protection filters and that's caused by shooting into the sun. You can get wicked lens flare from filters because they're flat. So if you're shooting into the sun, and you don't have a dust-storm going on, then take the filter off. Just remember to put it back on again afterwards...