Using protective filters on lenses

A topic of much debate is whether or not to use clear glass filters on the front of lenses, for protection. Some would argue that putting on a clear filter protects the lens against objects touching it, and against water, dust, and so on. If something hits the front of the lens, they would say, it breaks the filter and not the lens itself.

Others argue that the front lens element typically is strong, and does not need extra protection. Some would even say that adding a filter may be negative, that it breaks more easily, and that the glass shards could add more to the damage of the front lens element.

Another argument is that an extra glass filter deteriorates the optical properties of the lens.

Now, I wouldn't want to drop my lens on the pavement to examine if using a filter helps protect it or not. Besides, as all lens accidents are different, one drop would not prove much anyway.

But to look at the resulting images when using a filter, as opposed to not using a filter, is possible. To examine the effect of using a clear glass protective filter, I took the same picture with and without a filter.

Example @ 200mm

Here is an example picture taken using the Panasonic GH2 with the Lumix G 45-200mm f/4-5.6 lens at 200mm f/5.6.

without filter	with filter

This is an image with fairly low contrast, and on first glance, the two images are pretty similar. Let's look at an enlargement:



So is there any difference? Not really, I would say. Perhaps one could say that there is a tad bit more contrast in the left image, without the filter.

Example @ 14mm

Another example taken using the Panasonic GH2 with the Lumix G 14mm f/2.5 lens at f/5.6.

without filter	with filter

In this case, we barely need to look at an enlargement to see the differences. Clearly, there is more flare when using a filter. The right image also has some odd "phantom lights" in the frame, from light reflections between the filter and the lens. Here is one example in the centre of the frame:



Beyond the flare, it's hard to argue that the right image is worse in terms of sharpness, for example.

Example @ 45mm

Finally an example picture taken using the Panasonic GH2 with the Lumix G 45-200mm f/4-5.6 lens at 45mm f/4.

without filter	with filter

In this case, it is very clear that the presence of the filter deteriorates the image. Not only is there much more flare, there is also a severe reduction in the contrast and sharpness across the whole frame.

Here is an enlargement which demonstrates this:



Conclusion

In low contrast situations, it is hard to find much evidence of negative impact from using a filter. However, when there is more contrast, e.g., a strong light source in front of the camera, the presence of a filter generates significantly more flare, and potentially also less contrast.

People who are proponents of using clear protective filters may still not agree with me. They could argue that if I had used higher quality multi coated filters, I would not have gotten these negative impacts. And perhaps they are right.

Since I don't believe in using filters for protection myself, I haven't invested in the most expensive filters. So it could be that higher quality filters would have reduced the deterioration.

Still, I believe it is fair to say that adding a filter will generate some deterioration of the image quality. But if using a protective filter gives you more peace of mind when using your expensive lens, perhaps that is an acceptable cost to you.

Personally, I prefer using hoods for basic protection of the front lens element.

Appendix

The filters used were: "Kenko Digital Filter UV" (52mm) for the Lumix G 45-200mm f/4-5.6 lens, and "Green Digital Filter UV" (46mm) for the Lumix G 14mm f/2.5 lens. Both filters are littered with terms like "High Quality" and "Premium Quality", but I would take that with a grain of salt.

Home made stand off ring

A stand off extension ring is a ring for attachment to the filter threads of a lens, and allows for mounting something further away from the lens. It has the same diameter threads on the outside as on the inside.

This kind of ring has many uses. One can be to offset an attachment to the Panasonic Lumix 20mm f/1.7 pancake lens. Mounting a step up ring to the Lumix 20mm is not advisable, since it will cause the lens to jam when powering down. The front lens assembly is withdrawn into the chassis when powering down, which will cause anything wider than a normal filter to jam. But using a stand off ring between the lens and step up ring solves this problem.

Stand off rings are uncommon, and can even be pretty expensive. What's far more common and inexpensive, though, are UV protection filters. These can be bought from various auction sites at a small cost. You can remove the glass from these, and what remains is a stand off ring.

Here is an example UV protection filter:

Let's break the glass off it:

Be sure to use eye protection, and gloves. I used gloves then breaking the glass, but not in this illustration photo. Note that I put the ring into a plastic cup, to avoid having the shattered glass flying all over.

Breaking the glass takes a surprising amount of force. The round shape of the glass makes it extra strong. This is the reason why airplane windows are rounded: A rectangular shape is much weaker, especially in the corners. Hitting the glass towards one side may be an easier way to break it.

Once broken, remove the shattered remains:


It turns out that the glass was held in place with a threaded locking ring. Unscrewing this ring would have released the glass, without having to break it! Anyway, this was a cheap filter, so no harm done.

What I have now is a stand off ring, used to mount front lens accessories at an offset from the lens.

Here is an application of a stand off extension ring. A 52mm screw in hood is attached to the Lumix 20mm f/1.7 Pancake lens. As the lens has 46mm front filter threads, a step up ring is needed. However, connecting a step up ring directly to the lens is a bad idea due to the jamming risk discussed above. Using a stand off ring between them solves this problem.

Of course, attaching all this stuff to the moving front assembly of the lens is not a good idea. If you want a hood, it is better to use a light, low profile one.