Every now and then there are threads on whether or not filters degrade image quality. Quite often you see remarks like "don't put cheap glass in front of your expensive L lens" etc.

While I personally don't use UV filters unless I really have to, stories of filters degrading image quality (other than risk of flare and "ghosting" in some circumstances), these remarks are exaggerations at best (IMO, of course).

A year or so ago I did a simple test and just stumbled upon these pictures, so I though I'd post them here as well.

To prove how little significance the quality of the glass directly in front of a lens has, I used the cheapest glass possible - sliding glass from a bookshelf.

The first shot is the lens alone.

The second is the bookshelf glass about an inch in front of the lens.

The third is the bookshelf glass ANGLED about 20 degrees, just for the heck of it.

All shots are 100% crops. The subject was about 10 feet away.

Lens alone
<img src="http://tarkhan-mouravi.com/test/lensonly.jpg">

Lens with glass in front
<img src="http://tarkhan-mouravi.com/test/lenswithglass.jpg">

Lens with glass ANGLED
<img src="http://tarkhan-mouravi.com/test/lensglassangled.jpg">

keep in mind, this is a 100% crop (pixel level)- you are looking at about 1/8th of a frame.

In any event, I doubt you'd find a UV filter of quality similar to a bookshelf glass (rolled glass vs cut glass), unless you really go looking for one :)

post the 3 images, and ask everyone to identify which was taken under which condition...

An uncoated or poorly coated filter will increase the likelihood of flare, especially in an ultra-high-contrast scene with light and dark elements on the image plane on opposite sides of the optical axis.

A scratch or other localized imperfection on filter will decrease image contrast, not sharpness. If the entire surface is slightly textured, a soft-focus effect occurs.

Sharpness is a result of the design and manufacturing tolerances of the internal components of a lens, and won't be affected except by a SEVERELY abberated front element.


I *do* keep a clear filter on the front of my lens, and I use an expensive one (B+W brand) to reduce the possibility of flare. It can be easily removed for ultra-high-contrast scenes if necesary.

-Alex

If I got results like that from a L-series lens I'd be on the phone bitching at Canon ASAP

let's start marketing Tilted Crappy Glass Holders for major lens brands.

Crappy Glass itself not included, but available at extra cost

what is definitely there is lack of contrast, which also contributes to images appearing softer.

And again, you will never (hopefully) find a filter equal in optical quality to bookshelf glass :)

Alex (TS4F) is correct in his assertion that even poor quality filters will likely have no effect on sharpness (lens resolution and "micro-contrast). It's too far away from focal point.

What really poor quality glass (or surface imperfections, or bad coating and lack thereof) can do, is reduce contrast due to the way it disperses (if that's the proper term) light