Pulizia Sensori CCD

robbynaish · Inviato: Gio 24 Giu, 2004 2:58 pm Oggetto: Pulizia Sensori CCD

Ciao,

come suggerito da Izutsu posto questo 3d oltre che su Nikon anche su questo perche effettivamente puo essere usato su tutte le reflex digitali

Cleaning your CCD
or why you might want to eat at Wendys or shop at Michael's

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Are those spots you're seeing, or have the dreaded dust bunnies inhabited your camera?

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updated: 2/8/04
Nikon cleaning info: 5/8/04
This article applies to all Nikon-based digital SLR bodies (D1, D1h, D1x, D2h, D70, D100, S1, S2, 14n, 14nx, and SLR/n).

It happens to all of us: eventually you'll find an image that, on close inspection, seems to have gotten the freckles (see example, below). No, you don't have bad photosites on your CCD, you have dust on the sensor filter.

Don't go trying to eyeball the dust on your CCD, though. Most small spots that show up on your lens are not visible to the naked eye. Yes, it doesn't take a giant glob of dirt to make your camera exhibit the dreaded spots--itsy bitsy dust particles can be just as annoying.

A simple enough scene in Southern Utah. I was testing out a lens and thought the delicate clouds and fall foliage would give me plenty of detail to look at (they did). I've brightened and enlarged the very middle of the shot. Now you can see the dreaded dust bunnies that had managed to sneak into my camera during a two-week Utah sojourn. I counted 63 in the sky area alone, so I probably had 100 or so little imperfections waiting to mess up my shots. Of course, the healing brush in Photoshop 7.0 makes fast work of these annoyances, but still, who wants to spend all their time post-processing photos?

You can’t send the camera back to Nikon, Fujifilm, or Kodak every time you get a dust speck on your CCD. If you did, you’d pile up quite a freight bill and be without your camera for significant periods of time. And while Capture now has the ability to use a "dust reference" shot to remove dust from NEFs, this doesn't help you much with JPEG images. Nor does the dust reference photo idea work well in the kinds of environments I shoot in, where the dust accumulation on my CCD changes daily (I'd have to take several reference photos a day and then carefully track them, adding yet more complexity to my already complex workflow). So, eventually, you’ll come around to the decision that you’ve got to learn how to clean the CCD yourself.

First, don’t be put off by Nikon’s disclaimers (both Fujifilm and Kodak endorse user cleaning if done properly). The Lithium Niobate filter over the D1 and D100 CCDs is somewhat difficult to scratch if you use the right tools (on the MHOS Scale of Hardness table that ranges from talc at 0 to diamond at 10, Lithium Niobate is a 5, the same as Apatite, and a bit lower than Orthoclase and Quartz; Fujifilm and Kodak don't identify the material they use [nor does Nikon on the later cameras], but it seems just as durable). And if you manage to leave a bit of streaking behind, as most first-timers do, it just means you’ll get to practice your cleaning technique again.

A lot has been written about different cleaning techniques, such as using an air compressor or a small turkey baster type of hand blower as suggested by Nikon, but eventually you'll find some dust that so stubbornly sticks to the CCD that it forces you to use the "wet method": a light swipe of a lint-free cloth that’s been wet with a small bit of methanol. The dampness grabs the dust and makes it adhere to the cloth rather than the filter. You use lint-free material because you don’t want to generate more specks to clean off (trust me on that one--I decided to see how bad a regular cloth would be, and ending up spending most of an afternoon trying to get rid of all the residual threads and spots left behind). Methanol is the liquid of choice as it is less prone to leaving water streaks behind (contrary to popular opinion, methanol can pick up water vapor, though it does so less readily than isopropyl alcohol).

In Japan, Nikon actually sells a cleaning kit (for camera body, lenses, and CCD). The kit contains training videos as well as cleaning material. This US$80+ kit (!) apparently is only sold in Japan at present, but it does provide guidance in cleaning a CCD using Nikon-approved methods (more on that in a bit).

So, let’s get down to brass tacks: where do the rest of us get the lint-free cloth and methanol needed for a wet cleaning? Eclipse is probably the most convenient form of the liquid, and one bottle will last you a long time. You can find Eclipse at most good photo stores these days. You’ll also find something called Sensor Swabs at those same locations. A Sensor Swab is basically a precut bit of lint-free cloth attached to a Q-tip like stick. It looks like a funny white, thin lollipop. For more on Sensor Swabs, see the Photosol Web site.

The original Sensor Swab was designed for cameras other than the D1 (Kodak bodies, actually; Kodak specified the process and materials used to create them). Unfortunately, those Kodak cameras had larger openings around the CCD. If you try to use the original Sensor Swab on a D1, D1h, D1x, or D100 body, you’ll find that you have to angle it in the opening to reach the CCD. To use an original swab you do one sweep with the swab angled one direction, another sweep angled the opposite direction, change to another swab and then do the same thing in the vertical direction. Unfortunately, that doesn’t work very well. If you follow those instructions, you’ll generally move the dust into one or two corners, but not fully off the CCD. With practice, you can do a reasonable job with the original Sensor Swab, but I’ll bet you that you’ll use more than two to get it done, and at US$4 a swab, that starts to add up quickly.

A D1-specific Sensor Swab was introduced which solves the size problem. This smaller swab allows you to get the swab squared in the sensor area, and thus gives you a decent shot at getting the dust completely off the CCD. But the Sensor Swab design has an Achilles heel: it doesn’t provide support to the cloth outside the central area. Thus, you have to make sure that you keep the stick portion exactly perpendicular to the filter lest you collapse one edge and lift the other edge up (missing the dust, obviously). You’ll also find that truly stubborn dust (for some reason there’s always a couple of dust spots that aren’t easily dislodged) just doesn’t get removed by a Sensor Swab unless you have absolutely perfect technique, and even then you may miss one or two dust bunnies.

[The Sensor Swab design and the clean-room assembly was originally dictated by Kodak. Making changes to the design at this point involves getting buy-in from Kodak, which won't continue to approve use of the swabs unless Kodak gets a chance to review each and every change. Thus, the current folded-material-on-stick design is likely to remain for the time being, as is the need to hand assemble them in clean rooms, which keeps the cost of producing them high.]

What we need is a slightly wider, sturdier stick to hold a lint-free pad. I contemplated that one afternoon and went on a scavenger hunt around the house. The chisels in my toolbox looked a bit inviting, but I’m not that dumb—one slip and metal against filter is going to result in one bad (and expensive) experience. What I needed was something that was stiff but way softer than metal. I tried some cardboard I had lying around, but the stuff that I found that was stiff enough was too thick (if the “edge” you wrap your lint-free cloth around is too thick, you can’t get the edge off the CCD at the side, as there’s very little room). The thick laminating material I had sitting around was almost stiff enough, but as Maxwell Smart used to say: “missed it by that much.” A flexible plastic ruler I chopped up almost made it into the running, but it too wasn’t quite stiff enough.

For some reason I looked in the kitchen, and in my utensils drawer there it was: a plastic knife from Wendys. When it had gotten there, I had no idea; I don’t remember the last time I ate at that fast food chain, and couldn’t imagine bringing home a wrapped plastic knife. But the butt end of the knife was almost as wide as the CCD, was made of a soft enough material that I wasn’t worried about scratching, and was very stiff. By wrapping a bit of lint-free PecPad around it, I was able to get a good, secure swipe with plenty of leverage across the CCD. The knife’s plastic is a little too thick, making it difficult to get the dust all the way off the CCD, though. I eventually filed the knife down a bit, and got close to my “perfect” support for my cleaning cloth. (Note: if you file the plastic, you'll end up with plastic dust adhering to the knife. Put the knife through a couple of dishwasher cycles before trying to use it in the camera!) (Further note: Wendys changed the design of their knife so that on some the butt end now has a bit of a dip in the middle. I now cut the knife at the widest portion of the knife end, file the cut so that it is perfectly flat and thin, and use that.)

[Others have taken my original idea and looked for other materials that fit the bill. Rubbermaid makes a narrow rubber spatula that can be cut down to fit perfectly. Apparently, the thought of the Wendys knife busting through the PecPad (has never happened in hundreds of cleanings) scared folk into looking for softer materials. While the Rubbermaid utensil works okay, personally I find it to have two problems: it isn't as rigid as my modified Wendys knife, so requires firmer pressure and good technique in holding it perpendicular (though it's nowhere near as bad as the Sensor Swab in losing it's support at the cleaning edge); and you end up with a long handle sticking out of the camera. But I can see how it works just fine for others, and it certainly isn't as intimidating sticking rubber into your camera as it is to use stiff plastic. I was slightly facetious when I wrote my original article, trying to poke fun at Nikon's knee-jerk liability avoidance at the same time as providing a useful technique. The point is this: there are plenty of things you can use that provide a stiff edge for the cleaning material. I use a filed-down Wendys knife (really!). Nikon uses wood sticks. Others use rubber spatulas. Find something that works for you and use it! A quick visit to the local Michael's art supply store found three different possibilities that'll work (wooden and plastic clay tools, plastic ink palette "knives"). All were under US$3, and they gave me three different "softnesses" to play with; I ended up liking one of the plastic clay tools best.]

So, when people ask me how I clean my CCD, I say: “First, I eat at Wendys…” The moral of the story is simple: if you look around, you can find something that may work better for you than the expensive Sensor Swabs. The ingredients are simple:

Lint-free cloth (PecPads work okay; you wrap them around the support stick you choose, making sure not to touch the area that will be used in cleaning the CCD, as you don't want to introduce skin oils to the process--Despite the Rube Goldberg aspects of this cleaning process, you must maintain high levels of cleanliness and discipline).

Methanol (to wet the PecPad).

A stiff, thin support just a bit narrower than the sensor opening, around which you’ll wrap the lint-free cloth; the support needs be of a non-hard material just in case you wet the pad too much and break through (practice on another surface before trying it on your CCD!).
And so is the technique:

Wet the edge of your “cleaner” with two or three drops of methanol.

Quickly, before the methanol "dries," do a gentle swipe from one edge of the CCD to the other, making sure to get the edge of the cleaning pad past the edge of the sensor.

Flip the cleaning tool (so that you use the other side of the edge) and repeat to cover any area you didn’t get on the first pass; again, make sure to get the edge of the cleaning pad past the edge of the sensor.
It’ll probably take you a few tries to get it right. The usual mistakes I see from first-timers are:

Too much liquid is used, and a streak is left behind.

You don’t start at one edge of the sensor and swipe past the other edge, leaving dust at both edges.

Swiping too gingerly, which tends to leave behind a few stubborn spots (and may even make them more stubborn).

Not holding the support nearly perpendicular to the filter (I use a very slight tilt towards the side I'm moving towards), lifting one side and leaving dust on the filter on that side.

Trying to “rub” the filter clean (you tend to just move the dust around--once it is on the lint-free cloth, it can migrate back to the CCD if you use improper technique, such as not enough fluid, or rubbing motions).

Doing the cleaning in an environment where there is lots of dust in the air!
Note that I still use the D1-sized Sensor Swabs some of the time. In particular, I'll use them when I'm trying to travel light and need to do a quick field cleaning. With proper technique, you can get the CCD cleaned well with Sensor Swabs, though even tiny mistakes in angle or wetness may force you to use more than one. The material used on the Sensor Swabs is not the same as the PecPad, by the way--in some ways it's better (not as coarse).

Now let's talk about Nikon's cleaning instructions on the Japan-only cleaning kit video. I was surprised by several aspects of their cleaning method.

First, Nikon's supplied "stick" is wood and very narrow. This means that they can't do a "swipe" across the CCD to clean. Second, Nikon appears to use a lens cleaning tissue instead of lint-free cloth. Lens cleaning tissue is thinner than PecPads, though this advantage is negated by the way Nikon uses them.

Above is a frame from the video supplied with the Nikon cleaning kit. Note a few things: first, the stick is much narrower than the CCD. Second, the cleaning tissue has been wrapped around the stick in a way that essentially forms a "brush." Nikon wets the tip of this "brush" and then uses the same technique on the CCD that they use cleaning a lens: use a circular brush motion outward from the center of the item being cleaned. Personally, I have multiple issues with this technique, especially if used outside a clean lab environ: (1) it takes more time (I usually get my CCD cleaned in two quick swipes); (2) it involves more rubbing using the same portion of the cleaning material (which can drag the dust across the sensor and end up reapplying it); (3) the wide wrap at the brush end makes it more likely that you'll touch the sides of the frame holding the CCD, which dislodges more dust; and (4) Nikon wets their brush far more than I would (which means that you must keep your Methanol from picking up water vapor). Since I often have to clean while traveling, these things give me a bit of hesitation about Nikon's technique. Of these, #3 is the most interesting: I've noticed a clear tendency for the frames on my cameras to get "dirty," and any touch against them tends to mean I have to do another pass on the sensor itself. Because of the narrow stick and wide tissue bundle on it, Nikon's technique seems to make touching the frame sides much more likely.

In many ways, though, this is all a moot point. First, the cleaning kit is overpriced and only available in Japan, so not many of you are going to be going that route to start with. Second, you'll note a commonality amongst the three cleaning methods described (Sensor Swabs, Nikon Cleaning Kit, Thom's Wendys method): lint-free material supported by a "stick" of some sort, with the end wet with methanol.

Whichever cleaning technique you decide to use, work slowly, deliberately, and think about what you’re about to do before you attempt it. If you’re unsure of your ability to work gently in tight quarters, watch an experienced D1 user perform a cleaning a few times before tackling it yourself. Yes, if you do it wrong with the incorrect tools you can damage the filter, which, unfortunately means that Nikon has to replace the whole CCD unit. But it’s not that difficult to get right. If you're afraid to experiment or can't maintain very clean conditions while wrapping the PecPad on the Wendys knife, then use Sensor Swabs. If you have extra money to spend, travel to Japan and pick up Nikon's cleaning kit, watch the videos, then make up your mind as to how to proceed.

Here's a couple of added tips for cleaning:

Blow it. When traveling, I use a Giotto blower bulb (Atomic Rocket or some other silly name; the thing looks like a rubber rocket that sits on its tail). Most of the time, a hand blower like this can get rid of most of the "loose hanging" chad, uh, I mean dust (perhaps Florida should have used Giottos in voting booths in 2000). A couple of quick puffs usually do the trick, getting me at least 80% of the way to clean, sometimes more.
Get a headlamp. The Petzl LED headlamps work great, though you look like a geek using one to clean your camera. Using a headlamp lets you put light right where you need it, and even makes it easier to see the largest dust bunnies, if any.
Watch the humidity. Dry climates get dust into your camera, but it's easy to get out. Indeed, in Southern Utah I find that often just a quick blast from the Giotto cleans my CCD just fine. But when you encounter humidity changes, dust seems to "weld" to the filter, and you'll need to use a wet, contact cleaning technique to remove it.
Minimize the time. Because the Bayer filters in DSLRs use dye inks, I try to keep the amount of time they're exposed to light, especially bright light, to a minimum. That's not to scare you into thinking that if you leave your CCD exposed for five minutes that it'll produce poor color next time you use it; just that light accumulation to dye inks really should be minimized. If you clean in a darkish enviroment using an LED headlamp and don't take more than a few minutes, don't worry about it. But don't leave the camera's shutter open while you walk around the house or office looking for your cleaning supplies, decide to have lunch, watch a football game, and then come back and clean your camera! (Besides, the longer you leave the shutter mechanism open, the more dust gets into the box that you'll eventually have to clean out.)
Don't try to see it. A photosite on most DSLR bodies is about 64 square microns. You could fit several hundred photosites onto this hyphen: -. Thus, a dirt particle that covers a photosite or two can be very small--essentially invisible to the naked eye. That's one reason why I discourage use of the Speckgrabber to clean CCDs--you can only see the really big stuff.
My legal counsel wants me to run even longer legal disclaimers than Nikon, but I’ll keep it simple: when you work on your own camera, you do so at your own risk. I try to provide accurate, useful information that reflects the way I work, but I can’t be held liable for what you do with that information. Use the procedures listed here at your own risk.

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