3.9 Cleaning Lenses

While cleaning the imaging sensor of a DSLR is a tricky business, cleaning lenses is a snap.  The tools are cheap, and can be easily carried into the field.  In fact, I typically only clean my lenses in the field, during a lull in the activity.  I clean my lenses using a simple four-step procedure.  Any of the later steps of the procedure can be omitted if the earlier steps were successful at completely cleaning the lens.
    The steps are:

1. Blow off loose dirt with a rubber blower
2. Brush off remaining dirt with a brush
3. Wet-clean using a safe cleaning fluid
4. Remove remaining smudges with a microfiber cloth

We’ve already encountered rubber air blowers in section 2.8.  The figure below shows the Hurricane Blower in action.  Note that while cheap rubber blowers are not recommended for cleaning imaging sensors (because they usually don’t have a filter to keep dust out of the air stream), they should be safe for most optics.

Fig. 3.9.1: Blowing loose particles from a lens using
a rubber air blower.  Rubber blowers are small,
cheap, and easy to keep with you in the field.

Some people just blow on their optics using their mouth.  The problem with that is that you can get saliva on the glass.  Rubber air blowers are small and lightweight, and therefore easy to carry into the field.  But they only blow off dust that’s loosely adhering to the glass.
    The next line of defense is the lens brush.  The (very dusty) brush shown below is actually one end of a Lens Pen; we’ll discuss Lens Pens in a moment.  The brush obviously allows you to loosen up dirt particles that won’t come off with just the air blower.  After applying the brush, another round of blowing with the rubber blower helps.

Fig. 3.9.2: Loosening particles from a lens
surface using a lens brush.  After particles
have been loosened, another pass with the
rubber air blower can be useful.

For dirt that won’t come loose during brushing, a lens fluid is useful.  I use Zeiss Lens Cleaner, which is cheap and effective.  Whatever you use, just make sure it’s made by a reputable manufacturer, and that it’s guaranteed safe for coated optics.  Recall from section 3.3 that many lenses are coated with a clear, antireflective coating, which can significantly improve light transmission and contrast.  I’ve personally damaged optics in the past by using a lens cleaner not specifically designed for coated optics.  Since Zeiss makes world-class binoculars and camera lenses, I trust them to make safe lens cleaners.

Fig. 3.9.3: My favorite lens-cleaning fluid:
Zeiss Lens Cleaner.  It’s both affordable and
effective.  Use only lens fluids made by
reputable companies and guaranteed not
to damage lenses with optical coatings.

    It’s very important that you do not spray the lens cleaner directly onto the lens glass!  I did this once on my Sigma 800mm lens, and found that the fluid that had run to the edge of the glass and had seeped through to the inner space behind the objective lens element, causing the inner surface of the len to completely fog up.  Unfortunately, a beautiful Common Yellowthroat (Geothlypis trichas) showed up just as I noticed the fogging of the lens.  Here’s a photograph of the bird taken through the fogged lens:

Fig. 3.9.4: Common Yellowthroat photographed through fogged lens.
Spraying lens fluid directly onto a lens can result in internal fogging,
which can be very difficult to remove.  Instead, spray onto a cloth
and then apply the wet cloth to the lens

It’s remarkable that the image came out as well as it did, considering that the objective lens was totally fogged up.  I’ve spoken to other photographers who say that a lens can accumulate quite a bit of filth without noticeably affecting image quality; apparently, dust or water droplets on the objective lens (on either side) are generally so out-of-focus that they are virtually never visible in images.  Their effect is probably to scatter light rays so as to reduce overall contrast.  For this reason, I don’t bother wet-cleaning my lenses when they get a little dirt on them; I wait until it builds up enough that I think it might begin to affect image quality.
    Instead of spraying the cleaning fluid directly onto the glass, I now spray it onto a microfiber cloth, and then use the wet cloth to clean the lens.  The cloth doesn’t need to be microfiber for wet-cleaning, but that’s what I keep with me in my photo vest, so that’s what I use for wet-cleaning too.  I wipe from the center of the lens outward toward the edge.  This pushes all the filth out toward the margin of the lens, where it’s less likely to affect optical quality.  Then I wipe around the edge to remove as much of this accumulated filth away as possible.

Fig. 3.9.5: Using a microfiber cloth for wet cleaning.
Microfiber cloths are now extremely cheap, and highly
effective for both wet and dry cleaning of lenses

The important thing is to use very little pressure when doing the wet cleaning.  I bunch up a fold of wet cloth and drag that flimsy fold across the surface of the lens.  My fingers don’t push against the back of the cloth during this process.  Keep in mind that any abrasive particles remaining on the lens can scratch the lens or the antireflective coating.  You just want to gently waft those particles way, without applying any pressure that might cause scratching of the lens.
    The microfiber cloths that I use (such as the one shown above) are the ultra-cheap kind that you can get at Wal-Mart in the automotive
.  I did at one point buy an expensive Kodak microfiber lens cloth, but I never use it anymore.  The Wal-Mart ones are better, larger, and cheaper.  They’re the size of a wash-cloth and come in packs of five.  They’re truly amazing.  If I accidentally touch a lens and get a fingerprint on it or notice some other smudge on the lens, I just rub it off with the microfiber cloth (if I’m sure there are no abrasive particles on the lens or cloth that might scratch the lens). 
    Before switching to microfiber cloths, I used to use Kodak lens paper, which comes in packets of 50 or so, and can be expensive unless purchased in bulk.  The figure below shows a single sheet of this paper.  The lens paper is intended for wet cleaning, but I’ve always found it to leave streaks on my lens.  The moistened microfiber cloth leaves some streaks also, but generally fewer, in my experience.

Fig. 3.9.6: Kodak lens paper: the old standard
for wet-cleaning of lenses, now largely supplanted
by cheaper and more effective microfiber cloths

After wet-cleaning, any remaining streaks left on the lens can be easily remove by a gentle wiping with a dry part of the microfiber cloth.  This isn’t a completely risk-free operation.  If the wet-cleaning somehow missed any abrasive particles, this dry removal of smudges can scratch the optics.  I do it, but it’s probably a bad idea, and I don’t necessarily recommend it.
    Finally, a popular method for dry-cleaning of lenses is the Lens Pen, which is shown below.  One end of the pen is a brush, as already shown above.  At the other end is a special surface that makes use of a dry, carbon-based cleaning agent that gets re-applied to the applicator every time you replace the cap (not shown in the figure).

Fig. 3.9.7: Dry-cleaning a lens with a Lens Pen.

I recommend using a Lens Pen only after wet cleaning, when you’re certain that all abrasive particles have been completely removed.  Otherwise, the pressure which is typically applied during use of the pen may grind those abrasive particles into the lens and cause damage.  The pen is useful for removing any subtle streaks that remain after a wet cleaning, though a microfiber cloth will do the same, and typically more cheaply.