Lens Scratches

[frederick_muller]

I have a 35mm f2 Canon Black lens with a noticable scratch on the front element

that extends very slightly from the outside towards the center. It's length is

maybe a third of the radius of the front element's circle when looking straight

in. The lens is otherwise in great shape and is extremely sharp. In fact, it's

every bit as sharp as another sample I have of this lens that is mint minus.

 

Why is it that scratches on the front element are less of a problem than

scratches on the rear element?

[jim nichols]

My understanding is that the effects of scratches on the front element are reduced as the other elements bend the elements of light, whereas the scratches on a rear element have effects that fall directly on the film or sensor. Just my two cents.

[ronald_moravec1]

Defects to the rear are worse. Scratches on the front catch light and cause flare. One small scratch, ignore it or fill with india ink.

[terry_rory]

India ink is a good idea. It reduces/eliminates flare from the scratch. This has been done for hundreds of years with telescopes and microscopes that had scratched lenses.

 

A 'Lenspen' contains microscopic particles of carbon (to absorb grease) and may have a similar flare reduction effect if the tiny carbon particles lodge in the scratch. I cannot be sure of this though. (Never had a scratched lens yet.) The India ink trick is well documented though.

[frederick_muller]

I'd like to try the India ink idea. Do you just "write" into the scratch with a fine pen dipped in India Ink? Or use the tip of a needle?

[mohir_ali]

Try a fine tipped black Sharpie.

[winfried_buechsenschuetz1]

IMHO it makes little difference whether a scratch is on the front or rear lens surface. If light is scattered by a scratch it will never be "collected" properly, i.e. follow the correct light path, and will cause some flare. Light which is irregularly scattered on the front lens element surface will not be "collected" or "bundled" correctly - there is only one correct way for the light rays.

 

However, scratches usually have more influence on the (financial) value of a lens than on image quality. Scratched lenses usually end up in the grab box of dealers and can be picked up very cheaply - and in most cases will perform close to an immaculate lens.

[kelly_flanigan1]

Scratches tend to drop the contast a tad. Thus using a Koh-in-noor engineering/drafting technical pen was a typical WW2 fix for an aerial lens with some micro flak; pock marks or scratch. The Indian ink is used to mask off ie hide the bad area; like it didnt exist. The drop in surface area is nil; the contast is kept decent. From an optical standpoint a scratch, dig, or ground thru air bubble/hole on outer glass can gather light almost 90 degrees to its surface since its rough and of varied shape. Thus this stray light can sometimes find its way into the lens via reflections and drop the contast of the image. With a rear scratch the effect can be less for stay light; BUT with a non symetrical design a rear scratch can be worse optically for ruining an image. <BR><BR>From a value standpoint a lens stratch often radically drops the worth of a lens for a Lieca; since owners often stress looks over function. Its sometimes to the oddball extreme; like discounting a dumptruck or shovel 50 percent due to some scratches that cannot be measured for a performance drop. In many folks minds there is this multiplier that gravely exagerates the performance drop.

[christer_almqvist2]

re. India Ink:

 

what is the generic name, or is there a brand name that is well known in Europe, and in

what sort of shops does one find this (artists' supplies)?

[terry_rory]

Any good art supplies shop or stationers will sell you Indian (or India) ink. For instance Rotring drawing ink...

 

http://www.officepad.co.uk/stationery/rr59117.shtml

[terry_rory]

Or Windsor & Newton black Indian Ink....

 

http://www.artsupplies.co.uk/item.php?item_id=4422

 

http://www.winsornewton.com/products.aspx?PageID=171&ProductID=736