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I just purchased a used Tamron 45mm lens, and its rear element is a bit dirty. Am I allowed to clean the rear element using microfiber and lens cleaning solution + lens cleaning tissue?

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You can absolutely clean the rear element as you've discussed - it's done exactly the same as the front element is cleaned.

Caution must be used because if, for example, a piece of grit were on the rear element when you used your cloth, and you damaged the element's coating, the damage is far more likely to affect your image quality than it would be if you did the same damage on the front element. The rear element is very close to the film/sensor plane and the image is nearly in focus at this point, whereas the front element is far from the capture plane and far from the point of image focus, so a surprising amount of dirt or damage can be on a front element with minimal effects on image quality (except if you are shooting into strong light, like the sun, when the issues will tend to cause flare).

Blow off any particles or gently brush them off, using a blower brush, then use a microfibre cloth, or a piece of lens cleaning tissue moistened with lens cleaning solution, to clean the element. The standard advice is to clean from the centre out to the periphery/edge.

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    "a surprising amount of dirt or damage can be on a front element with minimal effects on image quality"; the classic link for that is this one. – inkista May 23 '18 at 18:31
  • Wow, that's really impressive. Good read @inkista – Kjeld Schmidt May 23 '18 at 21:02
  • @inkista: That's impressive. Do you know which lens it is? I think it would be much more of a problem with wide angle lenses than with telephoto. I can put my whole hand in front of a 200mm at f/2.8. There's a clear drop in contrast but that's about it. – Eric Duminil May 24 '18 at 7:50
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    @EricDuminil, found it! (because I remember when he posted it that it was on fredmiranda.com), Cicala said that it was an old Sigma 80-400 that had been dropped. So your theory holds. – inkista May 25 '18 at 6:20
  • @inkista: Excellent, thanks. I remember reading somewhere that scratches are worse on rear element for telephoto and front element for wide angle. I've never scratched a telephoto's rear element, but any dirt on front element is clearly visible on my ultra wide angle lenses. – Eric Duminil May 25 '18 at 8:30
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You can absolutely clean a rear element on a lens, much as you'd clean the front element. Exercise the same caution and you'll be fine.

The only danger with a microfiber cloth is that it should be grit-free (you may inadvertently scratch an element because whatever you got off a lens the last time might still be in the cloth), which is why some folks use cleaning solution and cleaning tissues or Zeiss single-use wetwipes (I get mine at Walmart for both my camera lenses and my glasses).

Another common tool you can use is the Lenspen, which uses a dry carbon compound to clean the lens (but this may leave small carbon particles behind, so be sure to use a blower or brush afterwards). Lenspens can also be used for sensor cleaning.

If you really want to go to town on cleaning your lens, you could read about the methods used by lensrentals.com for their high-volume turnover of lens cleaning. But it's serious overkill for most photographers.

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Use a rocket blower on it first to remove debris, make sure the rear element isn't recessed into the lens body (it definitely shouldn't be with a prime), and polish gently with a lens cloth or lens pen. If you use cleaner, spray it on the cloth first and don't use much.

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    What do you do differently when the rear element is recessed into the lens body? (tele-zooms and long tele-primes, for example) – Hueco May 23 '18 at 15:18
  • You don't use lens cleaner, and if you can't get the rear element to come flush with the body you don't try polishing either. – BLHolliday May 23 '18 at 15:53
  • Why shouldn't the rear element of a prime lens be recessed? Pretty much all of the Canon and Nikon primes with focal lengths over 100mm have recessed rear elements? Are you saying all of those lenses are defective? – Michael C May 23 '18 at 17:29
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    I mean like in a zoom lens with a trombone mechanism, where the rear element can move. When the rear element is in a recessed position, certain parts of the lens mechanism are accessible and shouldn't be exposed to fluids like lens cleaners. – BLHolliday May 23 '18 at 17:36
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    Lens elements should not be exposed to drops of fluids like lens cleaners, either. They should be cleaned with cloths or swabs that have been moistened with lens cleaning fluids. It really makes no difference whether the element is recessed or not if there are no free drops of fluid involved. – Michael C May 23 '18 at 19:57

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