The subject of using or not using protective lenses can invoke heated debates among photographers, with both sides often fiercely defending their choices. I am not going to debate whether it is right or wrong to use protective filters – that’s certainly a personal choice. I have been using them for a number of years now to protect my higher-end lenses and make it easier to clean lenses with recessed front elements (such as on Nikon 50mm f/1.4G / f/1.8G). Having had bad experience with purchasing a low-quality no-name brand filter when I just started photography (it was sold to me as a “must-have” at a local photo store), I learned what such a filter can do to my photos the hard way. Since then, I have only been purchasing multi-coated B+W filters that use high-quality Schott glass. I have been very happy with these filters and have been telling our readers to either use the best they can find, or not use filters at all.
A couple of weeks ago, I received a bunch of filters from a new filter manufacturer in Europe. The company wanted me to test their filters and see what I think of them. I asked them if they would be comfortable with me comparing their filters to B+W and they told me that they did not mind. As I was testing a lens in my Imatest lab a couple of days ago, I first shot a test chart without filters at f/5.6, then stacked 4 of my 77mm B+W XS-Pro MRC filters and took another shot, then finally mounted 4 filters from the new manufacturer and took the last shot. Here are the numbers that were produced by Imatest:
As you can see, the B+W filters had no impact on image quality. The differences in numbers between “No Filter” and “B+W” you see above are within the margin of error. However, if you look at the third graph, it is pretty clear that the filters that I was testing were of low quality. There is a rather significant drop of approximately 5% in the center frame, a drop of 7% in the mid-frame and a pretty significant drop of close to 15% in the corners. To make sure that I was not making any errors, I re-ran the test several times and used completely different sets of filters from the same manufacturer (I had a total of about 20 77mm filters that manufacturer regarded as “high quality”). Other batches yielded similar reduced numbers, some worse than others.
I wanted to see what a 15% difference in score represents visually, so I took extreme corner image crops from an image that was shot with 4x B+W filters and the same with 4x lower quality filters. Here is what the comparison looks like:
As you can see, the difference in sharpness is pretty clear – lower quality filters certainly result in visible image degradation. Details are blurred and the straight lines are no longer sharp. And this is just sharpness alone. If I shot with the two in a high contrast situation (say shooting against the sun), I bet the low-quality filters would introduce all kinds of artifacts to my photos.
Yes, this is a rather extreme example and I know that nobody would stack 4 filters like I did above. If one were to look at images with a single filter, the differences would be very minimal. However, that’s not what I am trying to show here. The point of the article is to show that high quality filters have no effect on sharpness and do not necessarily reduce image quality, as some photographers claim. It is certainly the case with lower quality filters though. The only thing that filters might introduce to images, is more flare and ghosting when shooting against a bright light source. In those situations, it might be best to remove filters altogether.
Lastly, keep in mind that the filters I tested were regarded as “pro” filters. If you are curious to know what a single cheap / bad quality filter can do to an image, see this article that I posted last year.
Moral of the above research: not all lens filters are made the same, even from the same “high-quality / pro” category…