Incense

by Bart Arondson

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An article appeared on Apartment Therapy that claimed that a scratched camera lens can be repaired using peanut butter and/or toothpaste. This strikes me as a horrible idea that could make a bad situation worse due to surface coatings or just adding new scratches.

Is this a correct assessment, or can you actually remove scratches from a lens with light abrasives in such a way that the optics are not impacted?

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As far as I know this technique only works on plastic surfaces –  Vian Esterhuizen Jul 12 '13 at 16:03
    
It also works on brass and silver musical instruments, at least the toothpaste. Never tried peanut butter on them. –  Michael Clark Jul 12 '13 at 16:06

4 Answers 4

up vote 6 down vote accepted

Most minor scratches on lens' have an infinitesimal impact on image quality. They look a lot worse than they are, especially in terms of the front element of a lens. This is because the light from a point source in the scene being photographed is spread over the entire area of the lens as it enters the front element before being focused (hopefully) to a point on the image plane behind the lens.

As they say, a picture is worth a thousand words. Look at the results, and then the lens they were shot with at this blog post from lensrentals.com.

enter image description here

By introducing an oily substance such as peanut butter to the surface of the lens you are just going to increase the amount of the lens' surface that is affected by less than optimum performance, and will probably cause more dirt and dust to "stick" to the lens. The toothpaste, especially if it is clear, would be less problematic than the peanut butter but would only serve to possibly make a scratch on the lens less noticeable when inspecting the lens at the cost of optical performance when actually taking pictures with the lens.

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True, but on the same token, what's the impact when you stop things down like they did in the article? If you only shoot wide open you might not notice an issue with the IQ, but what about the coatings as well? –  rob Jul 12 '13 at 16:28
    
Even stopped down, dust and scratches on the front element have very little impact on image sharpness. They only affect such a tiny percentage of the total surface of the lens. Smearing something all over the surface of the lens, on the other hand... –  Michael Clark Jul 12 '13 at 16:35
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@rob The most important coatings for a lens are the ones inside the lens that keep light from bouncing around and causing ghosting and flare. The coating on the front element is mainly to prevent usable light from bouncing off the from of the lens instead of entering the lens. Now if there is a filter in front of the lens, the front element of the lens is now internal because the filter has become the front element. –  Michael Clark Jul 12 '13 at 16:40

Toothpaste (and more professional polishers, like Brasso) is able to remove scratches from things like CDs and lenses because it has very small micro-abrasives that (when combined with force) smooth-out the area around the scratch.

This works well for CDs, but because lenses have such strict tolerences, making the lens even a little bit thinner by polishing it will cause the light that enters the camera through that area to hit the sensor slightly off-center. This will result in your images being slightly more blurry. A scratch, on the other hand, will scatter the light even more, but because it's scattered so much, it will probably actually be less visible in the final image because the intensity of the scattered light will be so much lower.

Thus, unless your scratch is extremely shallow, you are probably better off leaving the scratch in the lens than trying to polish it away, using any method.

(Though, note that the placement of the scratch is important - a scratch in the center of the lens may have a large effect when using small apertures, where only a small portion of the lens is actually used)

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Toothpaste and PB act more like a caulk or filler that puts a material of roughly equal density in the void left by the scratch than an abrasive that buffs the rest of the surface down to the bottom of the scratch. –  Michael Clark Jul 12 '13 at 18:46
    
@MichaelClark: Do you have a source for that? I'm fairly certain that's not true. See for example this HowStuffWorks article on resurfacing CDs with toothpaste. –  BlueRaja - Danny Pflughoeft Jul 12 '13 at 18:49
    
If toothpaste is abrasive, it would remove the thin layer of enamel from your teeth. Ask ANY dentist. The last thing you want in your mouth is an abrasive. Peanut butter has no abrasives in it either, at least not creamy PB. And I don't think anyone has ever suggested using crunchy PB to repair a CD or polish anything. –  Michael Clark Jul 12 '13 at 18:52
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@MichaelClark: en.wikipedia.org/wiki/Toothpaste#Abrasives "Abrasives constitute at least 50% of a typical toothpaste." You might be right about the peanut butter though, I'll edit that out. –  BlueRaja - Danny Pflughoeft Jul 12 '13 at 18:54
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@MichaelClark - it will however remove some coatings from glass. Toothpaste is used for both removing factory residue from scuba masks that will contribute to fogging. This may also lead to damage to optical coatings, even if they are less critical than internal coatings. –  AJ Henderson Jul 12 '13 at 19:24

I would expect it would cause more harm than good. Minor scratches are insignificant, but it's going to be just about impossible to get the oils from the peanut butter off the lens without damaging the coating which does matter, particularly when using filters.

I am pretty sure that the article is wrong about why it supposedly works. I believe the oil in peanut butter or tooth paste tends to fill in the crack and refract light such that it is less of a distortion than the crack would normally cause, however, toothpaste particularly has slightly abrasive properties that may have a bigger impact on the optical properties than the scratch it "heals" by removing image coatings from the lens. Additionally, both peanut butter and toothpaste would leave an oily residue that is hard to remove and would cause a negative impact on performance.

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Toothpaste and PB work more as fillers or caulk than buffing agents. –  Michael Clark Jul 12 '13 at 19:03
    
@MichaelClark - that was what I thought as well and my initial answer indicated that, but the article linked to said it was for the abrasive properties. Either way, the conclusion doesn't change, the approach is BS and is going to cause more problems than it solves. –  AJ Henderson Jul 12 '13 at 19:22
    
I think the article is wrong about how the toothpast or PB is working. Back when audio CDs first came out we would repair scratches with toothpaste by filling the scratch. Eventually the toothpaste would come off the disc and we would have to do it again. The scratches were never buffed out, we were caulking them with toothpaste. I've also repaired CDs/DVDs with a SkipDr that does use an abrasive wheel to remove plastic material until the scratch s buffed out. That process produces a LOT of white plastic dust. But the "buffing fluid" is distilled water for cooling purposes. –  Michael Clark Jul 12 '13 at 19:31
    
@MichaelClark - I agree with you. I think I'll change my answer back since we both think it is that. Oil filling in and smoothing the scratch makes much more sense. –  AJ Henderson Jul 12 '13 at 20:29

I know that toothpaste (not tooth gels) are used on goggles to keep them from fogging. I have been using the same with an underwater camera that fogs and so far it has done pretty good.

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How does this relate to scratches? –  mattdm Jul 25 at 16:18

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