The View Up Here

Random scribblings about kites, photography, machining, and anything else

Cleaning Camera Optics

Posted by Tom Benedict on 28/06/2012

I’m still prepping for my trip to Amsterdam for SPIE. Kindle batteries charged? Check! AA and AAA batteries for my KAP rig charged? Check! Camera batteries charged? Check! Camera optics? HOLY COW! What’s that on my lens?!

For a lot of reasons, it’s better to keep optics clean than to let them get dirty. First and foremost, clean optics perform better. There’s no sense dropping $2.5k on a lens if you’re going to let dirt stick around and degrade its performance. By the same token, there’s no reason to let a cheapo kit lens perform any worse if that’s all you’ve got. The cleaner your optics, the higher quality your photos. It’s that simple.

Another reason to keep your optics clean is that once dirt gets on a lens, it’s free to move around and do further damage. Modern optical coatings are actually quite hard. But sand, cinder dust, and other particulate contaminants can and will scratch them. Better to get them off before an inadvertent wipe with a cloth makes a permanent mark on your glass.

I’ve seen all kinds of stuff on the market for cleaning camera lenses and filters. My favorite is the one we use at work to clean our optics. These optics:

CFHT with Megacam

We have two cleaning methods we use:

Method #1 is to blow the optics off. We have cylinders of high purity nitrogen at our summit facility, so that’s what we use. But a good bulb blower would work just as well on camera optics. These are less than $20 at any camera supply shop. I got mine off of Amazon for about that much. And before you ask why we don’t use bulb blowers at work, keep in mind that some of our optics are large enough to fill a room. Using a bulb blower, it would take weeks to clean some of these things:

Tedium

Method #2 is what we refer to as a “wet wash” – good ol’ lens tissue and methanol. Depending on the optic and the direction it faces (up or down) we may do wet washes once a month, once a year, or only when transmission tests indicate they’re dirty. On my personal camera, I like to do them any time I can see anything on my lens that isn’t glass.

But there’s a trick to it… As with most things, the success of a wet wash depends a lot on technique. Here’s how I do mine:

First, wear gloves. If you’re constantly re-depositing finger oils on your optics, all the cleaning in the world won’t mean a thing. Latex gloves degrade rapidly at 14,000′ of altitude because of the higher percentage of ozone in the air, so we use powderless nitrile gloves. At sea level, latex gloves should work just as well. Members of my family have latex allergies, so I use nitrile gloves at home, too. Any time you plan to touch optics, wear gloves.

Second, don’t touch the optics. Not with your gloved fingers, anyway. The only thing that should touch the glass is your lens tissue. And even then it should only touch glass when it’s got methanol on it. The gloves are there to keep your finger oils off of the lens tissue. Don’t let your fingers touch the glass.

I mentioned lens tissues. We use Berkshire Lensx 90 wipes. They’re 4″x7.75″ in size, and come in packs of 500 sheets. We use these universally, regardless of whether we’re cleaning an optic half an inch across, or one that’s three feet across. These are the lens tissues we use for everything. To use one, we pull a single wipe from the bag and fold it in half, rotating 90 degrees each fold, until we have a single folded edge about 1″ across. (We’ll fold one additional time if the optic is small.) At that point, the wipe is ready to use.

I also mentioned methanol. We use high purity stuff from a chemical supply house, but you can get it elsewhere. Some of the lens cleaners you can get at the camera store are actually straight methanol. You can also check with your local camera store to see if they have contact information for a supplier in town. Chances are they use methanol or ethanol in their repair shop. Whatever you do, don’t use rubbing alcohol from the pharmacy. Most of these are 70% alcohol, with the other 30% water. In the next paragraph I’ll describe why this is not what you want.

One important point with any alcohol: they are hydroscopic. That means they’ll suck water straight out of the air. Once an alcohol becomes hydrated, it is useless for cleaning optics. As you wipe across the glass, you’ll see microdroplets left behind the wipe. These are water drops that stick around after the alcohol has evaporated away. They can and will leave water spots on your glass. Once you get water spots on a coated optic, it’s practically impossible to remove them. Don’t let it happen! If you see droplets forming while you clean your glass, wipe them away and stop what you’re doing until you can replace your alcohol. I keep my methanol in a squeeze bottle. At the end of a cleaning session, I squeeze the excess air out of the bottle and flip the lid closed. This slows down the hydration process, but it can’t stop it. Check your alcohol before use and discard it once it becomes “wet”.

Back to the lens tissue!

To load a lens tissue with alcohol, you only want to wet the very edge of the folded tissue. Don’t saturate the whole thing. It’ll leave a wet trail of alcohol across your optic that will suck water out of the air and still leave microdroplets, even though your alcohol is “dry”. You only want enough methanol on the edge of the tissue to wet the edge. No more.

Once loaded, a tissue is good for one pass across the optic. One pass, then toss it. No matter how well you blow your optics off first, there will be microscopic dust particles on the glass that your lens tissue will remove. Once you lift the lens tissue off the glass, don’t let it touch the glass again or the dust it picked up will act as tiny abrasive particles that can scratch your glass. One pass, and toss it. Learn the rule and live by it.

When applying a lens tissue to an optic, use only light pressure on the tissue and move at a slow enough speed that you can barely see a band of alcohol film on the glass as it evaporates away. I try to keep the evaporating “tail” behind my wipe at only a millimeter or two of width. If it’s wider than that, I’m moving too fast. Narrower, and I’m probably moving too slow.

For an optic the size of a camera lens or filter, I’ll fold up a tissue, drop methanol onto it, and take a single swipe across the center of the optic. >toss< Then I’ll load another one and wipe a circle around the outer edge of the optic.>toss< Then I’ll take a good hard look at the optic before possibly applying a third on any remaining trouble areas. If the optic is clean, I’ll flip it over and repeat on the other side. Using this technique, you should be able to clean an entire camera bag of lenses in thirty minutes or less.

What about bigger optics? Take it in stripes. Here’s an example:

Big Focal Plane

This is one of our detectors at work. It’s 40 2048×4608 pixel CCDs arranged in a mosaic that’s roughly 300x350mm in size. I fold my lens tissue so it’s about 1.5x as wide as the narrow edge of one of these chips. My first “stripe” is from top to bottom along the leftmost edge of the frame, covering just those two chips. My second “stripe” is from top to bottom along the next line of chips. I get about a 50% overlap this way so I know I don’t have gaps. The stripes proceed this way from left to right, wiping from top to bottom. After each stripe the lens tissue goes on the floor and I fold a new one. By dropping the tissues on the floor I run no risk of accidentally touching one and contaminating my gloves. Once the entire surface has been cleaned in stripes, I’ll prep a tissue and wipe along each edge. (Yes, that’s 12 more tissues.) This catches any goop I might have bulldozed across the optic with one of my stripes. Once that’s done I’ll take a good close look. If it’s still dirty, I’ll repeat the entire process until it’s not. And before you ask, the 40 CCDs in this camera live inside a vacuum cryovessel. I don’t touch the chips themselves with this technique. What I’m cleaning is the optical window that lets light into the cryostat.

So I’m one step closer to being ready for my trip to Amsterdam. My camera, the one lens I’m taking, and all three of my filters are clean. Now where did I put those battery chargers?

– Tom

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One Response to “Cleaning Camera Optics”

  1. […] in 2012 I wrote a post about how to clean optics like a pro. I mentioned stuff like wearing gloves, making sure your methanol doesn’t get hydrated, not […]

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