Mystery Lens Spot
Friday November 9, 2007
Sometimes our lenses get dirty no matter how hard we try to protect them. As pointed out by a recent reader email these spots can sometimes be difficult to remove."Hello, I have a little bit of a problem regarding one of my lenses. I was doing a routine cleaning of my 18-135mm Nikkor, and I noticed that there was a stain on the lens. It wasn't a very big stain. As a matter of fact it's just a little dot.
I tried to wipe it out with my lens cleaner but it didn't work. Also, this 'dot', is only visible in certain angles when light hits it a specific way. I've kept my lens behind a UV filter and fail to guess how the stain could've gotten on there. As I've said previously, it isn't a very big stain. However, it causes a red dot to intermittently appear in my photos at certain focal lengths."
In cases like this there are a couple of likely suspects for the mystery spot.
- If you clean the lens itself as well as the filter you may have rubbed off a bit of the oil coating on the lens
- It may be a spot of dust or grime worked its way into the lens and the spot is actually under the glass (this does happen occasionally as lenses are not completely sealed)
- It may be a lens defect, such as a minor flaw in the lens coating, that is worsening with time
Using a filter is the number one line of defense for the front of your lens. These small circles of glass are generally much less expensive than the lens itself and can actually improve images. By using a UV filter you can help cut down haze and a polarizing filter can eliminate reflections or make some colors more intense. Whatever filter you use, remember that its purpose is to catch dirt and damage (like the photo shows). Clean the filter, not the lens, the lens is being protected by the filter. Lenses generally have a micro-coating of oil or other chemical on the lens, if you rub this coating off the lens is damaged.
Spots of dust (even inside the lens) do not usually become visible on photographs due to their small size. However, if you use your lens in harsh conditions frequently then enough debris can accumulate over time to cause a problem. One way to avoid this is to use a protective cover over your camera and lens. These range in complexity from a gallon zip bag with a hole cut in it for the end of the lens to waterproof nylon cases designed for specific camera bodies and lenses.
Lens defects are uncommon but they do occur. Unfortunately, some of these defects are not readily apparent when a lens is first purchased and may worsen with time. If your lens suddenly develops image problems with no sign of cause, there may be an inherent defect that the factory must address.
In order to minimize problems such as these, it is essential to take good care of your lenses. There are more tips for lens care in the article "Proper Care of Your Photographic Lens".
Photo © Liz Masoner licensed to About.com, Inc.
Comments
I disagree on all counts about keeping a filter on your lens at all times. One reason the lens is so expensive is that a lot of work goes into getting the optics just right. Why would you then cover it up with a cheaper piece of glass? And no filter is completely flat, which means that you’ll be distorting the optics slightly (not much, but when fighting chromatic aberrations at the edges of your images, every little bit helps). Secondly, modern lens coatings are much more durable than their 1950s and 60s counterparts — if you use a soft, lint-free cloth and make sure you blow dust off the lens before wiping it, you’ll be fine.
I’ve had images absolutely ruined by a filter, either because the wide angle of the lens picked up the filter edges or because a long exposure in low light levels in nearly monochromatic light picked up “Newton rings.”
That said, there are a few situations where a high-quality UV filter may make a difference:
1. When shooting at high altitudes (where more UV light is prevalent)
2. When shooting glacial ice in low light conditions (glacial ice has a naturally blue tint, and UV light can create a bluish fog)
3. When you know there is a greatly increased risk of lens damage, like covering a game of paintball or the Hindu festival Holi
This is a risk-reward scenario, and you have to balance the risk to the lens against the risk to the image. Any advice that starts with “always” is almost always wrong.
Lastly, this column didn’t touch on the effects of lens damage. A spot on a front lens element does not show up as a spot on the image. It shows up as a slightly dimmer image (in proportion to the area of the aperture) with more chromatic aberration (since the path of light passing through a stain or around its edges is not the same as the path for which the lens was designed). That’s why a dusty lens takes a picture without the dust showing. A splotch of paint will ruin a lens, but small stains do not.
Thank you for your comments Scott. This is a perfect example of how much a “hot button” topic filter use is. There are widely varying opinions on filter usage.
As far as the effects of lens damage goes. As I said, a spot does not usually show up in an image. However, there are certain instances when they can show up. As the letter indicated, the visibility of the spot is intermittently and at certain focal lengths. That suggests that it is only a problem when the light strikes the spot in a specific way.
Yes, lens coatings are more durable than they were 40 years ago. However, they do still degrade occasionally.
When a photographer has a problem such as the one described in the email it is not a case of everything working as it was designed to do. Something went wrong and the point is to find the solution, not discount possibilities because they are “supposed” to happen.
Again, thank you for sharing your opinion and if you have an alternate idea of what the problem may be please share it with us.
One alternate theory: if the spot is only visible at certain angles and when the light hits it a certain way, it could be that the spot is not actually located on the front element, but appears that way due to reflections. I’d suggest unmounting the lens and performing a thorough visual inspection of the rear element, followed by a careful cleaning. If that doesn’t resolve the problem, and particularly if the problem is reproducible at specific focal lengths, then I would take it to a pro shop and see if they have someone there who can take it apart and repair it.
Depending on the lens and camera system, it may be less expensive to replace the lens than to repair it.