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I am using a Hoya UV Filter which the coating is inside the filter, not outside. I have today noticed that there is a small failure in the coating which is very small but annoying.

My question is, do I have to change the filter or may I continue using it without any bad effect on the images?

Thanks in Advance

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Multicoating on the outside may not be necessary. The outermost element, if it reflects, will simply reflect light out and away from the camera. That won't actually cause any problems for your photos, since the light never enters the lens and therefor never reaches the sensor.

A coating on the inside of the filter is more important, as it is the internal reflection that bounce around off of the various lens emenents and even the back of a filter that can create ghosting. Ghosting is that telescopic set of usually aperture-shaped reflections that occur when a bright nearly-off-axis light source (i.e. the sun in the corner of your image) causes those reflections to occur.

By multicoating the inside of the filter, Hoya is able to cancel out a lot of any potential ghosts that reach it. The inner coating is much more important than the outer coating. Having an outer coating as well may help reduce flare and maintain higher contrast, as it could potentially allow light to pass through the filter and into the lens without the same kind of losses as without a coating (an uncoated glass element is going to reflect more than a coated glass element), but I think those problems would be more subtle than ghosting, which is very likely the "failure" you are seeing.

If you are indeed seeing ghosting, it don't think switching to a filter that is coated on both sides is going to really change anything. With a powerful enough light source, multicoated lenses are still going to flare and ghost. The sun can pretty much cause both in pretty much any lens. The only way to significantly reduce ghosting and flare from such powerful light sources is to use lenses that nanocoat (rather than multicoat) the most critical internal elements that usually cause the most egregious problems.

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  • Dear jrista, thanks for the fast reply. The point is, I have the failure in the filter not in the lens. Because of that I wanted to ask whether to change the "expensive" filter or not. The "failure" is like a transparent dust particle which can only be seen when I put the filter between me and the light source horizontally and let the light reflect. I haven't notice a ghosting in the images till now . But I didn't check them for a ghosting problem also. On the other hand I have heard that using the camera without filter especially under the bright sun brings better images. But I have no idea
    – Rutilicus
    May 25, 2014 at 16:38
  • If it's dust on the filter, I'd just clean the filter. Pick up a pack of kimwipes and a soft brush (either camel hair or something synthetic like a lens pen). Brush off the filter, clean it with the kimwipe (dry, they work like magic, and don't worry about the rough feel, it's by design and WHY they work so well.) If that does not fix the problem, then it's probably a nick in the glass or coating, and you'll probably need to replace the filter to get rid of the problem.
    – jrista
    May 25, 2014 at 16:41
  • Yes I have tried to clean it too many times, also with methyl alcohol. No chance. That means, I have to buy me a new filter unfortunately. Many many thanks for answering me by the way
    – Rutilicus
    May 25, 2014 at 16:50
  • Forget the filter and use a lens hood. A hood provides better protection from both off axis light and bumps to the front of the lens. A properly sized and installed lens hood will NEVER degrade image quality. EVERY filter, even the expensive ones, degrade the performance of your lens to one degree or another.
    – Michael C
    May 25, 2014 at 19:47
  • This what I am really thinking now.
    – Rutilicus
    May 25, 2014 at 20:42

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