Do Filters Affect the Resolution of Lenses?

Just like the old “film vs digital” or the “Nikon vs Canon” debates, lens filters often create endless discussions on the Internet. Some people argue that one should never use protective filters, since it is another piece of glass in front of the lens that reduces resolution and emphasizes other optical problems such as ghosting / flare, while others argue that filters make it easier to protect the front element of the lens and make it easier to clean that element. I personally have been recommending use of protective filters for years, as long as they are of high quality. The filters that I have been using do not seem to affect the resolving power of lenses they are mounted on and mostly do not seem to heavily affect ghosting / flare either. Having spent the last couple of weeks in a lab testing many lenses, I wondered if I could actually measure the resolution of a lens with and without a filter. I recently purchased a used lens that came with a crappy plastic filter, so I decided to run two separate scenarios – one without a filter, one with a high quality B+W filter (more on B+W products below) and one with a cheap plastic filter. The results of the study came out very interesting!

Armed with the Imatest software, I first measured the sharpness of the Nikon 70-200mm f/2.8G VR II lens, which is usually my reference lens for telephoto sharpness. The focal length was roughly 150mm and the aperture was set to f/5.6, which is the sweet spot of the lens. I then mounted a high-quality 77mm filter by B+W and did it again. Here is the result of Imatest measurements:

If you look at the above charts, they look pretty much identical. There is a slight variance in numbers, but that’s expected when going from one image to another – Imatest scores can be slightly different even when you do not change anything. Judging from the above case, using a high quality filter does not affect the resolving power of the lens at all. This is a good scientific proof for those that claim that all protective filters decrease resolution. Myth debunked!

Let’s now move to a second case scenario. What if you were to use a cheap, low-quality filter? As I have mentioned above, I recently purchased a used lens – the Nikon 55mm f/2.8 AIS that came with a cheap plastic filter (more on this project on a separate post). So it was a perfect case scenario to see what the resolution would look like with and without the cheap filter. Let’s take a look at what Imatest measured (shot wide open at f/2.8):

Now this is an interesting result. Looks like using a cheap filter does indeed affect lens resolution. There is about a 10% drop in resolving power across the frame! And I am sure, if I were to take this lens and shoot it against the sun, this particular filter would surely make ghosts and flares appear worse.

Summary

This was an interesting study, because it proved that high quality filters have no affect at all on lens resolution. If you keep them clean and in good shape, they also do not affect the ghosting and flare properties of the lens. You might see a little more ghosting/flare on some filters, but if it annoys you, simply dismount the filter and put it back on once you are done shooting against the sun. It is not like you would shoot against bright light every day right? From my experience, the multi-coated B+W filters have no affect on ghosting/flare, so I never bother to dismount filters when shooting against the sun.

At the same time, make sure that you are not using cheap, low-quality filters. As you can see from this article, cheap filters do indeed affect the resolution of the lens and they can seriously affect the ghosting and flare properties of the lens. I have seen reduced contrast and really nasty ghosting / flare with cheap filters before, so I have learned my lesson on not buying those.

Filter Recommendations

If you are looking for recommendations on what filters to use, I have been a huge fan of B+W filters. All filters that I buy (neutral density, polarizing, clear) are B+W brand. They are made of high quality glass and they are manufactured in Germany – so you can expect the best performance from them. A key feature you should always look for in high quality filters is MRC, which stands for “Multi-resistant coating”. No matter what brand you go with, always make sure that the filter comes with this particular coating. This anti-reflection coating is the key to getting little or no additional ghosting/flare in your photos.

The one I usually get for pro-grade lenses is the B+W 77mm UV Haze MRC 010M Filter. At $72, it is not a cheap filter, but it is worth it as you can see from the above test results. I have been using this particular filter for several years now. The last thing you want to do is mount a $10 filter on a lens worth hundreds, if not thousands of dollars! There is a new XS-Pro version of the above filter, which is slightly more expensive at $87 (adds nano coating), but I doubt that it will make a huge difference. I have purchased a couple of those recently and I cannot say that they are better in any way. Note that UV protection is not needed on digital cameras, because sensors have a built-in UV filter. So when you mount a UV filter on a modern DSLR, it simply acts as a clear filter. So either buy a UV or a clear filter – both should be as good.

Obviously, you have to choose the right filter size for each lens you own. The above links are for lenses with 77mm threads, so if you need another size, simply type something like “B+W 67mm MRC” in the search box at B&H and you will find a clear filter quickly. You will see that as the size of the filter get smaller, the price of the filter is reduced as well. For example, a 67mm B+W MRC filter costs around $43, which is significantly cheaper than the 77mm filter.

If you do not know much about filters, I wrote a detailed article on using lens filters. Give it a read – it is pretty extensive and covers all of the above-mentioned filters.