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Bad Times with Bad Filters

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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

63 Responses to “Bad Times with Bad Filters”

LensRentals Employee

Roger Cicala said:

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

Crudman said:

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.

Bob Phillips said:

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?

Dragos Nculescu said:

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.

LensRentals Employee

Roger Cicala said:

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.

TTMartin said:

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?

Jason Bleibtreu said:

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.

Zoltan Ajtay said:

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

LensRentals Employee

Roger Cicala said:

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

Ron Miller said:

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.

Simon Lupton said:

Hi Roger,

Have you seen anything like this on the Canon 24mm TS-E Mk II?

I just bought myself the lens and the bulging front element looks like it would get mighty close to the rear face of a filter

LensRentals Employee

Roger Cicala said:

Simon, I haven’t seen it on 24s but we don’t have nearly as many of them. It certainly seems like it would be a good candidate.

Roger

Simon Lupton said:

I’m going to put a slim Hoya on it. No news here will be good news!

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