When does poor filter homogeneity actually affect image quality?
Asked 8/9/2013
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2 answers
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I saw a circular polarizer review that tested filter “homogeneity,” meaning bubbles, streaks, uneven thickness, or uneven polarizing properties in the filter. I understand those defects in theory, but I’m not sure when they become visible in real photos.
If I can’t see any problem in my images—even when zooming in on a computer screen—does homogeneity really matter? Are these defects usually too subtle to notice without a direct comparison, or can lens quality hide them? What image problems should I look for if a filter has poor homogeneity?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
3
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.
Originally by user3422. Source · Licensed CC BY-SA 4.0
user3422
13y ago
0
Generated from our catalog & community — verify before relying on it.
Poor homogeneity usually won’t show up as obvious lines or bad pixels in a photo. Because the filter is far out of focus, defects tend to affect larger areas of the image more subtly.
What you’re more likely to see is:
- slightly reduced sharpness
- lower local contrast
- a faint unevenness across the frame
- in some situations, uneven polarization effects
These problems are often hard to notice unless you compare two otherwise identical shots: one with the filter and one without it, using the same aperture and adjusting exposure for the filter’s light loss.
A weaker lens can make the filter’s effect harder to isolate, since the lens may already limit sharpness/contrast. On the other hand, long telephoto lenses can make filter defects easier to notice because they magnify the effect.
So yes: it matters, but usually only a little unless the filter is poor or the shooting conditions make it obvious, such as strongly polarized light. If you’re not seeing a problem in your real images, it’s probably not a practical issue for your setup.
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UniqueBot
AI13y ago
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