Technical Discussions

Bad Times with Bad Filters

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Published October 16, 2013

A long time ago I wrote a blog post called Good Times with Bad Filters about how cheap UV protective filters are different from good ones. It was mostly in fun.

Today I’ve got a post about how cheap UV filters may hurt your lens. It’s not in fun.

Here at Lensrentals we see lenses come back with scratched front elements every so often. Not a big deal, it happens. But since the Canon 24-70 f/2.8 Mk II lens came out we’ve seen a whole lot of them come back with scratched front elements. The weird thing was it was always in the center of the lens and often circular in pattern like the one below. (Ignore the dust, this front element had been taken out for replacement and sat on my desk for an hour before we took the picture.)

 At first I thought maybe there was a problem with the new coating Canon was using, but it seemed a coating issue wouldn’t occur just in the center.

It turns out that the combination of the slightly bulging front element of this lens and a ‘less than best quality’ thin or ultra-thin filter is the culprit. Let me make this point first, though: The vast majority of filters do NOT touch the front element of this lens. I went through a number of filters before I found one that did. But it can happen and that’s worth knowing.

This 24-70 had a front element that was about to be replaced because of some scratches near one edge (which is why I didn’t mind putting filter after filter on it to see if any caused a problem), but the center was absolutely clear.

 I went through 8 filters with absolutely no issues. The 9th filter, though, seemed to come in contact with the front element. It’s hard to be certain about that by just looking and feeling. So I dusted on the back side of the filter with a little carbon black. Notice I covered a fairly large area of the filter with it.

 

Then I put the filter on the lens, took it back off the lens, and took a picture of the front of the lens. Notice the circular pattern of the carbon,which is fairly clingy. Other than a few specs, it doesn’t come off the filter except where there was glass-to-glass contact. This is a much smaller area than the large smear of carbon I put on the filter.

And when we blew the carbon off the lens, there were a couple of scratches that hadn’t been there before.

READ THIS PART

This is a good demonstration about what MIGHT happen. I will add that I’ve put another dozen brand name filters (Heliopan, B&W, etc.) on this lens with absolutely no problem and no sign of glass-to-glass contact. It seems that you need the proper combination of a thin-line filter with glass close to the back of the filter, and a lens with a slightly bulging front element (this lens has one, but so do lots of others) to even worry about it. I would also think that wider front elements (this is 82mm) might allow more play or sag in the center making this more likely.

For those of you who can’t wait to go post something about how the Canon 24-70 f/2.8 II has a problem, let me assure you that’s not the case. I had to try a number of filters and the one that I used in the demonstration is a ‘discount’ filter that someone sent back to us in place of the name-brand filter we sent them. The other name-brand filters I tried were all fine.

I’ve also seen this ‘center circular’ scratch pattern on a few other lenses and we’ll start watching for it now that we know what it is. But I don’t have enough records to go back and figure out which of the numerous front element scratches we’ve seen were of this type.

My suggestion, though, is that you stay away from ultra-thin filters on these lenses, especially discount ultra-thin filters. If you look across the front of your lens from the side, you can get an idea how far up the center of the lens bulges. Then look at the back side of your filter and see how far the glass is from the bottom of the threads. If those two distances seem similar – well, be careful!

Roger Cicala

Lensrentals.com

October, 2013

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

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  • I haven’t really had great experiences with the Uvs either. Seems like I lose some sharpness. I do like them for the element protection factor but nowadays I am only using NDs and polarizers.

  • Roger Cicala

    Zoltan, I’m afraid I have no experience with that filter.
    Roger

  • Zoltan Ajtay

    Thank you Roger for the very important article!
    In my opinion no matter the filter is cheap or expensive, the problem coming from the geometry, of course.I plan to buy the Kenko ZETA EX CPL filter, which is really not cheap and maybe the one of the best about the coatings and image quality, and the thickness only 3 mm without the back thread. However because of higher chance of this problem. Have you tried it, and recommend or not?
    Thank you in advance,
    Zoltan

  • Jason Bleibtreu

    Same thing happened to be. A cheap UV filter put tiny little circular scratches on my 24-70 f2.8 lens. I thought a client may of done this. However I was wrong.

  • TTMartin

    Did you test the Hoya Pro1 filter?

    Actually, while I understand not wanting to list the filter name that did make the scratch. Why not list all the filters you tried that did NOT make contact with the lens?

  • Roger Cicala

    Bob, we found B+W, Singh-Ray, and Heliopan all seemed fine. YMMV though, since I suspect there’s some copy-to-copy variation in both lens filter rings and filter filter rings.

  • Dragos Nculescu

    This effect can also came from cheap lens cap. I have bought an replacement cap for my Sigma 70-200 2.8 EX DG, because I am a little slipshod with those caps, generally. And I saw that this cap is touching the lens. The original cap did’nt.

  • So, what was the brand name of the “discount” filter? Am I safe with continuing to buy B + W, which certainly isn’t a discount brand?

  • Crudman

    Regarding the ‘switcharoo’ comments:
    Lensrental could easily deal with what is essentially theft by charging a deposit that would cover the stolen stuff (not wasting time and money on recovery); or have a system in place to reduce deposits for return honest customers; there’s not even any prominent notice. While such things may not remove all cases outright, it will reduce it, it’s essentially risk management. This may reduce first time renters which they probably already know, as such all the swapped and missing stuff is really their problem if they keep choosing to ignore better preventative and curative measures.

  • Roger Cicala

    P, I can’t imagine it would affect photos significantly. But it would affect resale value, and of course our customers expect flawless glass 🙂

    Roger

  • P

    Would a tiny scratch like that have much effect on the photos taken? I’m sure it would cause complaints from rental customers, but if it happened to my lens I’d be really annoyed but wouldn’t fix it unless it was really causing image problems.

  • Vlad Razvan

    This does not seem to completely a filter issue. It seem that the filter thread is too near the lens front element; just 2-3 mm further, probably the problem would not appear with any filter…

  • I am a fan of not using a filter unless it’s needed. Frequently I find myself next to the splashing ocean, a misting waterfall or maybe just a sprinkle. In these circumstances I will use a clear filter every time, even Canon says their L lens won’t be weather sealed without a protective filter.

    I’m happy to report that my slim Marumi protective filter doesn’t contact the front lens element on my 24-70L f2.8 II lens.

  • Stephen

    David, so you are saying the problem only occurs with bent/bulging filters (ie very cheap and dodgy filters with poor manufacturing tolerances) rather than filters that have the glass element very close to the filter threads (ie good slim filters). Most non-slim filters have a gap between the filter surround and the rear of the filter glass – but slim filters are deliberately made to reduce or eliminate such wasted space. It is normally good practice to buy a good brand slim filter for use on an extreme wide angle lens, as there is less chance of vignetting. So I ask what is the gap between the front element of this lens and the start of the threaded filter mount.

  • Roger

    Sometimes when using a CPL filter I use one, one size bigger with a stepping ring on the lens. It makes rotating the filter easier and certainly leaves plenty of space between the two so you could use this technique for any filter. It may also provide further protection from knocks and dings. Problem is it’s a bit more expensive and you can’t use a lens hood.

  • Paul McCauley

    For those of you that don’t use a protective filter, while photographing a helicopter taking off, I had a stone thrown stone thrown into my lens. It chipped and cracked the UV filter, but the front element of the lens remained pristine.

  • David

    Stephen I think you may have missed part of the article….

    “For those of you who can’t wait to go post something about how the Canon 24-70 f/2.8 II has a problem, let me assure you that’s not the case. I had to try a number of filters and the one that I used in the demonstration is a ‘discount’ filter that someone sent back to us in place of the name-brand filter we sent them. The other name-brand filters I tried were all fine.”

  • Roger Cicala

    Dan, most do on their supertelephotos. For most of those the front is simply a protective flat element.

  • Dan

    Why don’t the lens companies have a built in lens protection as part of the lens? It wouldn’t add that much to the cost, since it is only flat glass and a bit of coating.

    I myself do not use filters and have found the image quality is far better without the filter when shooting BIF PLus the big lenses do not have a way to mount a front filter, only in the back end is where you can put one.

  • Stephen

    So what is the clearance between the front element and the filter? It sounds to me like this particular lens has insufficient (or possibly negative) clearance between the centre of the lens and the bottom of the filter thread. Filters with a large gap between the base of the threads and the filter glass are OK. Slim filters with the filter element almost flush with the filter threads are colliding with the lens. In that case it is a serious design fault. An 82mm filter should be large enough that there can both be sufficient clearance and avoidance of hard vignetting. The previous generation of the same lens managed OK with only a 77mm filter.

  • DrGerm

    Hood and cap to protect the lens…

    Yeah, great idea… Except that it’s usually the cap that scratches my Nikkors, since it’s so darn hard to put on exactly centered. Especially when you have to reach deep into the hood in the process…

  • mckenzy

    Why don’t we just stick to hoods instead? Albeit a little more expensive but at least it does the job of protecting the lens from falls as well as the occasional flare. also there’s the old saying, “why put cheap glass in front of expensive?”

  • Roger Cicala

    Gerald, I see your point, but this isn’t the only lens that’s designed this way. And the majority of filters cause no problems.

  • Although the emphasis here is on ‘cheap filters’ surely it’s the design of the lens which is to blame? Ultra Thin filters are hardly new, so why is the front element protruding past the lens barrel? Thinner filters cost considerably more, due to brass mount, stronger glass and in the case of polarizing, rear mounted glass. I think you can only blame the filter if it has been designed with glass that enters into the lens cavity? Canon are aware the front element bulges out, so why are the ‘tolerances’ so close. I call that bad lens design:)

  • The easiest way to check if there is any contact between the lens and the filter is to throw inside a small piece of lens tissue paper… then screw the filter and check if it is easily going around as you play with the lens.

  • Lexxie

    I never understood why someone would put a protection filter before a really good lens, I really don’t understand to put a cheap protection filter before a good lens. So if you really need protection only the best filter. Still, for the little chance that a scratch happens (you will normally not see tis on a picture) the replacement of the front lens (scratch chance times cost) it is much cheaper to not use a filter at all..

  • Randy

    When I worked for Leitz we used to tell people to not use a filter unless absolutely necessary–and we were the B+W distributor at the time. The story was why put an $83 filter on a $2500 lens. Fortunately for all concerned I don’t think too many people took that advice. BTW, we did have defective B+W; mostly de-cemented polarizers but occasionally UV/skylight that had a slight curve and caused AF problems(obviously not on Leicas) So bad filters are not necessarily a “cheap” filter issue.

  • Dave Gaines

    A thin filter on a normal to wide zoom seems unecessary. A normal filter will not vignette or show in the corners of a 24 mm lens. I’ve used normal filters on my Olympus 11-22 mm f/2.8-3.5 WA lens (22-44 mm EFL) with no problems. I’ve even stacked multiple ND filters on this lens, albeit mostly at narrow apertures. Maybe at wide apertures with some WA lenses on some systems a thicker filter might produce corner shadows, but I haven’t experienced it. A thin filter often often does not have front threads, which keeps you from placing the typical lens cap. I always have a good clear or UV filter on all of my lenses.

  • Mark

    Thank you for this Roger. I have never used cheap filters but also never thought that the two could come into contact. A useful piece of knowledge that I can keep for future reference — before I make a mistake.

  • Doug

    I found this problem in 1974 with my new Zuiko 21mm for my new Om1. As I put on a HOYA UV and noticed it looked different in the middle. I didn’t tighten it down and saved the front element, however all the other UV filter brands I had also touched the lens, so, I never had anything other than an occasional polarizer on it. The polarizing filter was a bit too fat to stay out of frame.

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