I mentioned digitally optimized lenses before, but just what does that mean? While these lenses have several features that make them especially well suited to digital photography one of the main benefits lies in the coatings applied to the glass of the lens. Standard lenses have coating applied to almost every internal and external surface yet they tend to only have an anti-reflective coating on the exterior surface of the front element only. Digital lenses have the anti-reflective coating applied to all surfaces of the glass and this is a key component.
The image sensors used inside digital cameras is highly polished and reflective. This isn't a design flaw, it's the only way they can be made in order to achieve sharp and focused images. Unfortunately it's the very reflectivity of the sensor that causes imaging problems.
As light from our subject passes through the camera lens it focuses on the image sensor. But because the sensor is so reflective some of that light bounces back up through the lens. Older lenses or those not digitally optimized lack the anti-reflective coatings on the back sides of the glass elements. The light that has bounced off of the image sensor strikes these uncoated elements and reflects once again back down to the image sensor. This twice reflected light is now out of focus and causes ghosting and loss of contrast in our image.
We can see that if the lens is made with anti-reflective coatings on the backsides of the elements that we can eliminate a significant source of image ruining stray light. The same principle applies to lens filters. It's difficult to find a filter without coatings; all of them in fact are coated in one way or another. However like the lenses just discussed standard filters lack the coatings on the backside of their glass.
Filters optimized for digital cameras have anti-reflective coatings on the back side of the glass element. Our digital lenses do a great job of not bouncing back reflected light to our camera's sensor, filters made for digital complete the job and allow the camera to capture the very best image.