Photography and UV-filters - sense or nonsense?

After the great response to my German article I decide, to translate it into English.

Introduction

Have you ever bought a new lens and the seller was asking you for an UV-filter as well? Unfortunately the known brands (within the shop) are quiet expensive and the only economical solution seems to be the house brand for 39 Euros. So the question is: Do you really need an UV-filter and what is the aim of such a filter? Should you take it or leave it?

_{UV-filter on an ultra wide angle lens [own image]}

I know, the usage of UV-filters is polarising the photo community. My aim is to deal with the usage from a neutral point of view. First I am discussing the causes of front lens scratches, while secondly I am focusing on the six most named pro’s and con’s. As a conclusion I am telling you what I do.

The arguments are the followings:
(+) The protection against ultra violet rays
(+) The mechanical protection against "foreign matter"
(+) The protection against front lens scratches and streaks
(-) The flare susceptibility
(-) The deterioration of the optical system in general
(-) The lens hood makes everything better and is more useful at all

Lens scratches and the causes

Good news first. You will never see a front lens scratch as a sharp line, dot or anything else in your picture. Because of the front lens surface is not within any sharpness region of your lens, this can not happen. The "only" physical cause is the light scattering effect. You will see it as blurs or hazes. As so often: One scratch does not much, two a bit more, one deep scratch is worse then two light scratches and so on … . I often read, that professionals are painting here scratches black to avoid light scattering effects. For some of them no difference is visible after, for others it is a real disaster. So lets have a look on the arguments.

Argument Nr. 1 - The protection against ultra violet rays (UV)

The UV spectra of the light continuous the visible spectra to higher frequency's. That means, it has a high energy. In older times UV light exposed the film and caused a visible haze. That is why the filters were often called “0-Haze filters”. It was a good idea to carry one with you, especially during the midday sun. Since the sensor cover glass is blocking the UV spectra completely - and all of them do that - UV-rays are not a topic any more. So forget about the protection function against UV-rays.

0:1 against the UV-filter

Argument Nr. 2 - The protection against "foreign matter"

It sounds like a space story but “foreign matter” only means things you run into - with your lens of cause. But be serious! What will happen? If your filter glass breaks, it does not matter if your front lens breaks as well. You will have enough work for your edding elsewhere. But if you are a bit lazy and you are putting your cam without a lens cap - inadvertently of cause - in your bag, a UV-filter might help to protect your lens from scratches. I’m honestly, this does happen some times but it’s not a real issue for me. All in all this argument is a draw.

1:2 against the UV-filter

Argument Nr. 3 - The protection against front lens scratches and streaks

If you was ever cleaning your lens, than you know about the difficulty to do this without streaks. Quiet apart from that, the smallest sand corn causes fine scratches within the coating. Dust is faster on your lens than you think. In a few hours I will go to the coast for some long exposures. But it does not really need the salty spray to pollute your front lens. I see this also by waterfalls in the Alps, steam engine close ups and so on. In this situation you will not improve the condition of your lens, when you carry out the microfiber cloth, at least in the long run. To say it clearly, the situation is the same for the filter glass, but you can change it easily.

2:2 - in sum a draw

Argument Nr. 4 - The flare susceptibility

Flares are mostly pictures of the aperture, caused by reflections within the optical system. A usual glass surface has a reflectance of approximately 4 %. That means, 4 % of the light will be reflected the way back. If the “following” glass surface has a reflectance of 4 % as well, the light will go the way to your sensor with an intensity of 0.16 % of the incoming light. By this we have two problems: Because of its optical way, back reflected light is hesitating the optical quality of your lens. Further more we do have this problem on every lens. With 14 lenses within your optical system, you have this effect 13 times! So you might have approximately 2 % hesitating light on your sensor. This would be a photographic disaster. The optical industry is coating the glass surfaces to avoid this. By manipulating the scattering index you can reduce the reflected light from approximately 2 % to 0.001%. Sounds good right? The problem with a filter is, you bring an additional surface within the optical system. If its surface is badly coated (single ore double coated for example) you are rising up the reflected light. The problems will occur, when you bring light sources in the frame. Unlikely pictures of the aperture will be the result. That is why a coating of the inner filter glass surface is that important! If the outer side is not coated, you will only loose 4 % of the total light. If the inner glass surface is not coated, you will get flares! Unfortunately I saw filters, where only the outer side was coated. This is will be a “marketing coating” only.
In summery an UV-filter can never improve the optical quality of your camera. At least it should hold it. This is not a point for the UV-filter.

2:3 against the UV-filter

Argument Nr. 5 - The deterioration of the optical system in general

This is a very curious point. Theoretically it is possible to worsen your optical system - beside the flare topic - by a plan parallel UV-filter glass. This deals with the fact, that not every light ray has the same incoming angle. Without calculating the influence I can say, that I never noticed effects practically. Corner unsharpness effects are more a reason of a worse lens build quality. Never the less, we can expect significant effects using poor UV-filters, where the glass is not even. Here we can prepare for inhomogeneous incoming and outgoing angels. High quality filters will not have this problem. An other point is a spectral shift due to cheep filter glasses, but you will not see this by quality filters as well. All in all both arguments are without a practical orientation. A draw!

3:4 against the UV-filter

Argument Nr. 6 - The lens hood makes everything better and is more useful at all

I will not discuss the use of a lens hood. It is definitely worth to use it in a lot of situations. For the use as a front lens protector you should consider some points. If you have a lens with a moving tube, this could be a serious option. If you have one with a moving tube, you should better avoid this. The problem is the higher force on the zoom shafts, caused by the leverage of the lens hood. Any movement within your baggage leads to an unusual burden. After a period of time this could lead to a mechanical slackness of your tube system. If you see unusual unsharpness effects, mostly at two corners of the image, it is too late. Changing the zoom shafts often exceeds the value of a used mid range lens. This way I ruined a lens some years ago and I will never follow the lens-hood-advice again! While argument Nr. 6 is not applicable for any lens, this is a draw for me.

4:5 against the UV-filter

Short summary

The discussed arguments are showing, it is not easy to find a clear “Yes” or “No”. It depends a lot on your personal preferences. Are you going better with a lost capture due to a flare or by keeping up the optical quality of your front lens and your images? Do you really hold a lens for the long run or not? And do you change the UV-filter if it is scratched? So the decision is not a football game and 4:5 is not a clear result anyway!

What I am doing?

Generally I use UV-filters. I can clean them on the road, what I would never do with my front lens. There is no worry about sea spray or anything like this. On the end of the day a wet cleaning is possible as well.

_{Surface of an UV-filter after a shooting at the coast [own image]}

For a shooting against the direct sun I dismount the UV-filter. This is the case for 3 % of my images.

What kind of UV-filters I am using and what are the important characteristics?

To say it clearly, I get nothing for any advice and there are many good brands on the market. I prefer the UV(c) filters from Hoya (HMC version). A 77 mm filter charges me ~ 30 Euros via the net and I know about the excellent quality. Exactly this is the charm! Unscrupulous replacements are not an issue. Generally I prefer the normal slim version (4 mm thickness) over ultra slim versions. The lens cap fits much better in the longer front thread. Problems with vignetting never occurred on wide angle lenses (tested with a focal length of 12 mm on an APS-C sensor).
Pay attention on a multi layer coating, especially for the inner filter glass surface. Do not choose the cheep uncoated, “coated” or “quality coated” versions. Good coatings are made of 7 or more layers. Every brand will use the wording “Multi Coating” or similar to show you a good optical build quality. Be aware of “house brands”, you will never know what you get!

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Thanks a lot for coming and have always an unscratched front lens!

Cheers,
David