Homogeneity of a filter, when does it matter?

Question

I read a review of circular polarizing filters in www.lenstip.com and one of the tests is to judge the homogeneity of the filters.

Quote from the test explanation page:

Homogeneity is a qualitative parameter consisting of the bubble content in the glass and streakiness of the glass. While you can always reclaim an element with air bubbles in it, it’s harder to show streakiness to the sales clerk. This aberration occurs when the surface is uneven or the components of glass (foils) are badly mixed. Light refracts and, as a result , the resolution of images decreases. Precise measurements require so-called shading method, but in the case of filters there’s no need to for this much precision (using shading method you can detect convection of warm air over a hand!)

While I understand what streaks and bubbles would do, I fail to see what does it matter if I can't see it myself. When/where does it matter?

• Am I simply not seeing it with my unexperienced eyes, or is it that the effect of streaks in a filter are not visible at computer screen resolution? I don't see it even when pixel peeping.

• What part is lens quality playing here? My low-quality kit-lens might just do such a poor job that streaks in filter foils disappear in the overall mush?

• In case I did, after all, not really understand the effect: What faults in image quality exactly should I be looking for to actually see the effect of poor filter homogeneity?

My polarizing filter is a Hoya Pro1 Digital MC PL-C which performed very well in other parts of the test but vignetting (which I don't see either) and homogeneity part.

Answer 1

"Am I simply not seeing it with my unexperienced eyes, or is it that the effect of streaks in a filter are not visible at computer screen resolution? I don't see it even when pixel peeping."

It's not visible like that in the image. As the filter is completely out of focus, any flaws will affect most of the image, not single pixels.

It would mostly be seen as a slight loss of contrast or detail, and that is very hard to see without having a reference image to compare it to. You would have to take identical images with and without the filter, with the same aperture (only changing the time to compensate for the light that the filter blocks), to be able to make out the difference.

"What part is lens quality playing here? My low-quality kit-lens might just do such a poor job that streaks in filter foils disappear in the overall mush?"

Yes, if the lens for example is bad at reproducing detail, it would be hard to see what effect the lens would have. However, even bad lenses usually have a sweet spot where the quality is pretty good. Somewhere in the lower to middle of the zoom range, and with an aperture a few stops from wide open is a good place to look for it.

Theoretically it would of course be best to have the best possible equipment in every place in the path of the light, but in practice it's just not cost effective to put a very expensive filter on a cheap lens. It would of course make a difference (as everything in the path of the light affects the quality), but the difference would be so small that you wouldn't want to pay for it.

Answer 2

The word homogeneity itself refers to the uniformity of a substance. A substance that is of uneven thickness, uneven colour or uneven polarising properties will cause a slight measurable difference in the uniformity of the resulting image in different parts of the frame.

In particular, the homogeneity test you linked will be sensitive to differences in thickness across different parts of the filter which may manifest as streaks in the projected image in the test.

It can be hard to see small differences in thickness with your eyes, thus the reason they test the way they do.

If a filter has poor uniformity of thickness it may manifest in streaks or a general unevenness under some lighting conditions, such as pointing it at strongly polarised light.

It is unlikely you'll ever notice an issue in real world circumstances unless there is something seriously wrong with the filter.

It's likely that flare will still be the most noticeable result of using the filter, even if the flare is rated as low.

Answer 3

You will notice it most when using it on a long telephoto. This will magnify the effect. The image will appear lower in local contrast, and slightly soft.

This is true of all filters, not just CPL's. But CPL's are a glass sandwich so they have to get two layers of glass, two layers of glue, and a sheet of polarizing plastic right. It's a tough gig.