Given the purchase of a Filter is always a little confusing decision to make, we thought we will share some clarity on the use / purpose of these various types of filters (especially concerning the Clear / Protection vs UV vs UV+IR). We will cover topics on ND & CPL filters in a separate blog.
1. Clear / Protection filters - as the name suggest provides outright protection of the lens from dust, water, smudges etc. These are clear glasses so they have negligible impact on the image quality of the lens, regardless of them being used either on a Digital sensor or a Film based camera. The multi-coating, which is basically done on both the sides of the filter ensures that internal reflections do not cause ghost images & flair in strong lighting conditions. Shooting with these filters on the lens is as good as shooting without them. Which means they are meant to be left on the lens all the time only for protecting the front element of the lens.
2. UV-Haze filers (speciality) - these filters were made particularly for Film cameras as Films are more prone to UV radiations, adding haze to the images. With Digital sensor cameras, the anti-aliasing filter element on the sensor does provide quite a bit of UV protection (every sensor has its own UV cut-off point), however, they are still prone to haze, especially if you are shooting landscapes and distant subjects. The camera will not be able to capture clear view of the distant landscape subjects such as mountains, rivers, etc. A UV - Haze filter for that scenario is a great boon. Remember that it is only useful for distant shots where the humidity / dust in the atmosphere can compromise the quality of the image by causing Haze. They are not useful otherwise and should not be left on the cameras as a protective filter. They are advised to be used only when you are shooting landscape and distant subjects.
3. UV + IR Filters - also known as Digital or HD or Pro filters by different brands (not to be confused with multi-coated filters). These filters have been treated with Dichoric coatings that are specially made for Digital sensor cameras because both CMOS & CCD sensors are still prone to UV & IR - either under natural or artificial lighting conditions. Whether the sensor has an anti-aliasing filter or not, they are still pretty much prone to IR. These filters although are expensive but provide the most optimal image quality your lens & camera is capable of capturing. Usually, people buy only one of them in the biggest size of the lens they own and then use step up/down rings to use with other lenses. These filters must always be used with lenses ranging 40mm or above and to be avoided with wide angle lenses due to their characteristics to reject light entering the lens at an elevated angle.
How to use filters in combination then?
1. You can always directly screw a speciality filter (like a ND, CPL etc.) together on top of your Clear / Protection filter. However, in this scenario make sure the Clear / Protection filter is always a 16 layer MRC coated to minimize additional scope of reflections between these additional glass elements. If its not a MRC, then best thing to do is to remove them and mount the speciality filter directly to the lens when going to shoot. So for e.g. your camera system will look like body > lens > clear filter > nd filter. Notice here that the clear filter is sitting in between the lens and the ND Filter hence MRC is an important aspect.
2. If you decide to invest in a HD Filter (UV + IR) and intend to use it all the time, then a Clear / Protection filter is not required. Further, if you have to use a UV-haze for landscapes, you will have to unmount the Digital Filter because its not supposed to be used with a wide angle lens. For a ND filter, CPL or any speciality filter, they can be mounted in conjunction with your HD filter so your system will look like body > lens > HD filter > ND filter or CPL etc.), You may have to try these combinations to see which combo works the best eventually because every camera sensor has a different UV & IR cut-off thresholds and may render differently.
Please note that apart from a single or multi-coated Clear / Protection and HD Filters, all the other varieties (ND, GND, Skylight, UV-Haze, CPL etc.) are basically speciality filters - i.e. they are meant to be used only occasionally for their respective use cases and not supposed to be left on the lens all the time to just protect the lens. In some scenarios if used for scenes not suited, they may do more harm than good.
Tip - the AWB may exhibit a bluish / yellowish cast in some cameras especially when used with an adapted lens, hence setting to daylight / night / incandescent may be required to keep the cast as neutral as possible. This problem should not be attributed to the filters though.
Purple Fringing - some people say or notice that a UV Haze or a UV+IR filter reduce any visible purple fringing. Is it true? Although most of them except the severe ones can be corrected in Lightroom, but the answer to this is "yes", "somewhat" or "no". This is a little complex issue but the simplified version is that if a camera's in-built UV cut-off is far lower than the lens's native UV cut-off, then the blue pixel of the sensor will exhibit a lateral chromatic aberration which cannot be completely rectified in Lightroom. This type of chromatic aberration is what is known as purple fringing. A UV filter in that situation, if can cut-off closer to the lens's native UV cut-off, then purple fringing may either disappear completely or reduce to a point where it can be further corrected in Lightroom. If not, then it may simply have zero impact. Purple fringing is usually common when adapting lenses to a non-native body, but it is mostly more of a teething issue with legacy lenses only, not with any of the modern day lenses. Most of the lenses depending on their type & age of coating also absorb a significant amount of UV on their own.
At Mystic Focus, we recommend using at least a minimum of a Clear Protective Filter on your precious expensive glass. If you go cheap with a 500 rupees filter then you dont really need a good optics because that cheap filter will ruin the image for you anyway! Our collection of highly acclaimed and hand crafted filters from Heliopan will ensure that your lens gets the best lighting conditions!