Lens Filter Quality: Testing UV and Skylight Filters
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
This post is about lens filter quality. There seems to always be a debate about whether UV or Skylight filters should be used on a lens. I’m not going to enter the debate but will present some information so that you can make a choice.
Filter quality is important because a poor filter will degrade the image from a quality lens. The perfect filter will have two plane parallel sides. If the sides are not plane parallel, the filter will act as a lens. And not a very good one. The two filters I’ll be showing here are a Hoya 52mm skylight (1B) and a Niko 55mm Skylight. I am using these two for illustration because they demonstrate excellent quality and not so excellent quality.
I have been testing filters for many years now and use a simple technique with an autocollimator. An optically flat mirror is placed on the collimator base and the filter under test is placed on the mirror and rotated while observing the image of the Siemens Star Target in the eyepiece (photo two). The reflected image will actually be two images, one from the front of the filter and another from the rear of the filter. Ideally, these images will be superimposed in a perfectly plane parallel filter. The greater the error in superimposition, the greater the error in the surfaces not being parallel.
The first video is taken through the collimator as I rotate the Hoya filter. As you can see, there is quite a bit of angle in the differences of the target image. In fact, reading from the reticle, it is about six degrees of angle. The second video shows the Nikon filter being rotated and the images are perfectly superimposed. This is about as good as it gets.
Now, this information may not be helpful as some Hoya filters are better than others. I see a lot of variation with samples in the various brands. I will say that the current B+W filters that I have tested all look very good.
