From the Filter FAQ Thread;
Q1: Should I use a UV, Skylight or Protective filter on my lens?
A1: Photographers are divided on protective filter, some do not feel this is necessary and never or rarely fit a protective filter. Other photographers routinely fit a filter perhaps taking the filter off for some types of photography.
For digital photography a clear protective filter is all that is needed as the sensor does not have the sensitivity of film to UV and blue light that UV and Skylight filters are used for with film. If you use your lenses with film or a mixture of film and digital you may prefer to fit a UV or Skylight filter. The effectiveness of the UV rejection depends on the make and type, see http://www.bobatkins.com ...hy/technical/filters.html
Canon DSLRs do not seem to be too sensitive to UV although this seems to be more of an issue with Nikon SLRS and some digicams. Information is scarce however, it seems to be reasonable to assume UV is not a major issue (see below link). Note that if looking for effective UV blocking Bob Atkins tests above showed Hoya to provide more UV rejection than B+W for example, so makes are not equivalent. The best UV block was a Tiffin but see the link in A8.d
Q2: Will a filter damage my image quality?
A2: Again photographers are divided, some feel that any extra glass in from of the lens may or will introduce a reduction of image quality, other maintain that if good quality filters have a minimal effect under normal conditions and even harsh conditions.
Q3: What makes a filter good quality?
A3: There is more consensus in this area although individual photographers will have their favourite brands they are used to, like and so recommend. In general a good filter will be made of optical glass that is well annealed and has been accurately ground flat with parallel sides.
Another factor is the quality of the filter coatings, these reduce the amount light reflection, the less light reflected the less reduction in image contrast and the less issues with ghosting (very bright areas of the image being seen in other parts of the image).
Filter coatings come in a number of types, each with differing amounts of reflection. From lowest to highest quality they are:
Coating Type ---- Typical Reflectance --------------- Comment
Uncoated ------------ 9% --------------- Very Rarely seen except with square filters
Single Coat ---------- 3-5% --------------- Many Cheaper end and lower cost filters
Multi Coat ------------ 1-2% --------------- Many mid-range filter [3 layer coating]
High End ------------- 0.3-0.5% --------------- Best filters [5-8 coatings]
Because high-end filters have such low reflectance values many photographers feel that adding a protective element in front of the lens will have minimal impact. However other photographers feel that any loss of quality however small is unacceptable. See Q8 for what issues to watch out for with filters and test examples of good and bad filters.
Another point to check is that the coatings are applied to both sides of the glass and if the manufactures quoted reflectance is for one or both surfaces.
Other points that some find significant are:
a) the mount base material, some are brass whilst others are aluminium,
b) the mount finish, these are normally some form of electro plate finish which might be Black chrome or anodised depending on make,
c) easy clean top coatings
d) black painted glass edges
e) low profile mounts
Here are some tests showing the effect of different quality filters;
Q4: So what brand of filter should I use?
A4: Three brands are most recommended by photographers: Hoya, B+W and Heliopan. Within these brands there are different grades. These tests compare Canon UV, Hoya Pro 1 Digital and Hoya SHMC Pro 1.
a) Hoya, B+W and Heliopan standard grade filters are all single coat designs.
b) Hoya HMC are three layer multicoat, Kenko MC Pro 1 Digital and Hoya MC Pro 1 Digital are 3 layer multicoat
c) Hoya SHMC, Hoya SMC Pro 1 (discontinued), B+W MRC and Heliopan SH-PMC are all High End multicoated filters.
A simple table is here http://www.zen20934.zen.co.uk ...ests/Flare/FilterData.htm
Please refer to the manufactures data:
a) Hoya: http://www.hoya-online.co.uk/Hoya.pdf
b) B+W: http://www.schneideroptics.com ...goryDisplay.aspx?CID=1161
c) Heliopan: http://www.heliopan.de/Heliopan-Filters.pdf
Q5: So what protection does a filter offer?
A5: A filter will give protection to the front element in from the following:
a) Accidental touching
b) Touching by subjects like children and pets (with stick fingers and noses)
c) Wind blown debris, sand and low velocity flying material like stones
d) Accidentally bumping into twigs when doing macro photography
e) Corrosive liquids like salt spray.
f) Accidental damage due to the lens cap coming off in the bag
g) Some L series lenses are weather resistant and require a filter to be fitted to complete the weather sealing
However, a filter made from optical glass is not toughed, armoured or laminated (these would cause optical problems) so if hit sufficiently hard it may crack and even break. Brakeage has occurred by filters being hit by flying stones near racetracks, paintballs and by dropping the lens front element down onto a stone.
Q6: So how easy is it to damage a front element?
A6: The glass of the element is quite strong, anything that hits it hard enough to brake, crack or chip the glass is likely to cause internal damage or misalignment to the other lens components.
However, the front element coatings are very thin vacuum evaporated or spluttered materials, often metal oxides. These can be relatively soft, even a gentle touch with something hard such as metal or stone will damage it.
Although the coating will stand up to careful cleaning, many fear that repetitive cleaning will cause micro-scoring damaging the optically perfect finish and so prefer to fit a protective filter and clean that, being able to replace it at need.
Q7: So what happens if I damage the front element?
A7: A scratch or micro-scoring will not generally be visible in the image, however it will lead to light scattering, if the accumulated damage is bad it will lower the lens contrast and flare resistance. This test result shows two extreme cases.
Q8: So what problems can I get with a filter?
A8: This is discussed below. The problems tend to only manifest in extreme situations, so many of the test cases used to investigate them tend to involve very harsh lighting conditions or extremely accurate resolution testing. These are a long way from every day photography conditions and requirements.
Problems fall into several categories:
a) Loss of sharpness: This is very difficult to test for; the tester needs to photograph the same view with and without the filter. Each shot will require refocus and due to variations of manual or autofocus each shot will not always be ideally focussed. So it is difficult to just take two shots and compare them. Statistical methods have been used and show no discernable effect on sharpness and AF accuracy with a filter made of good optical glass (see http://www.zen20934.zen.co.uk ...Tests/Filter_AF/index.htm ), however some photographers will always have reservations in this area.
b) Problems with strange out of focus double images. Some photographers have reported this problem when using a cheap filter. A similar effect can be seen if shooting through domestic glass windows. This effect seems to be due to very cheap filters that do not use well finished optical glass. Good quality filter have not been shown to cause this problem.
d) Reduction of contrast in heavy backlighting. This is a very significant with cheaper single coat filters although is much less of a problem with high end filters. (see http://www.kenandchristine.com/gallery/1054387 ). For everyday use it seems fair to say a high end filter is unlikely to have a discernable impact.
e) Ghosting and flare; This occurs when a very bright object or light source is against a dark background and looks rather like a double image or an area of blue or green haze. This can have various causes and occur in a lens naturally but can be exacerbated by fitting a filter, particularly a cheap filter. Typical situations can be night photography with bright light sources or day photography with the sun or a specula reflection of the sun in frame.
Lens performance with and without filters varies from lens design to lens design and is also dependent on aperture. This is relatively easy to test for using a domestic light in a darkened room (see http://www.zen20934.zen.co.uk ...LensTests/Flare/index.htm ). In general in everyday photography this will not be an issue, but for sunsets and night photography it may be worth not using a filter.
Note that lenses with well recessed front elements seem to be more susceptible to ghosting possibly because of the greater distance between the filter and front element. The EF 50/1.4 falls into this category for example.
Q9: So what sort of photography should I be wary of using a filter?
A9: Situations with bright light sources in the frame:
a) night photography,
b) sun rises and sunsets although the strength of the sun is often not so strong in these conditions,
c) heavily backlight or high key situations
If high end filters are used you may well not have a problem but it is advisable to do some tests. Note that even the best lens is quite capable of producing the problems discussed in Q8, so you need to take identical shots with and without filter before concluding the filter is the issue.
Q10: Hay I can just claim off my insurance if I damage my front element!
A10: Yes you can. However, each time you claim your premium will go up as will the excess for those sorts of claims; eventually you will not be able to be insured for that risk. Insurance companies exist to make a profit and prefer clients that pay up each year and never claim.
This does not mean you should not insure your camera equipment, quite the reverse. However, you should consider insurance as some cover against major loss rather than cover repair fees for routine risks.
In the end you need to access the likelihood of damage to your front element and decide if the risk is worth it.
Many photographers point out they have never used protective filters and have had no damage in an extended period such 20 years or more. Other photographers have reported the lens cap coming off in the bag with a brand new L-series lens and the front coating being damaged.
Some photographers see even relatively expensive high-end protective filters as a form of insurance, one that can’t be withdrawn if it is used too much.
Q11: Can’t you just give me straight answer, should I use a filter or not?
A11: No this is something the individuals need to make their own mind up on. Recent polls of the POTN community showed a 50/50 split between semi-permanent use of protective filters and no use of protective filters unless in a hazardous environment.
What can be concluded is if you do want to use a filter for protection either occasionally or more or less permanently you should use a high end filter to minimise any adverse effects, however even reasonable single or multicoat filter will probably be OK under benign conditions although single coat filters are really not recommended.
It is probably best if you consider the sort of photography you do and the environment you are in.
If you rarely do the sorts of photography mentioned in Q9 or are regularly in an area or situation with hazards mentioned in Q5, you may prefer to by default fit a protective filter and only remove it for potential problem situations.
If you regularly shoot at night or in extreme against the light situations in safe environments, you may just want to have a protective filter handy in you bag for those occasions when conditions are less safe for your equipment.
Q12: Can’t I just use my lens hood for protection?
A12: You should always use a lens hood with a few exceptions such as flash photography where the hood may cast a shadow or stop you using a macro flash.
The main function of the hood is to stop stray light hitting the surface of the lens front element, which can cause a reduction in contrast and even cause flare.
Whilst a hood for a telephoto lens does provide quite good protection of the front element from accidental handling of the front element and from off axis blown debris, grit, sand and salt spray, anything arriving on axis will still be a threat.
For wide and ultra wide lenses the hood is really no protection, even standard lenses have only minimal protection.
So to conclude, yes a hood can provide some protection but it’s main function is to shade the lens.
Q13: What about other filters like CP and NG and GNG?
A13: A lot of the comments about optical glass and coating quality applies in many cases, however these filters are being used for a specific purpose rather than being fitted on a semi-permanent basis. Please see the General and Creative filter FAQ coming soon.
Q14: I want to use a protective filter on my Canon macro lens but can’t fit my Canon macro flash on if I do?
A14: Most of the Canon macro lenses have a little ridge that the two macro flash guns (MR-14EX and MT-24EX) clip on to. Adding the filter stops you mounting the flash gun as the mounting ridge is now too far back.
The work around is to add a Canon Macrolite Adaptor ring to the filter (obviously the filter must have front threads for this). The Macrolite Adaptors are little rings with a filter thread one side and the mounting ridge the other and are intended to allow non-macro Canon lenses and non-Canon lenses to be used with the macro flashes. They come in 3 filter thread sizes, 52C, 58C and 72C (example http://www.jr.com ...2053273#productTabDetails).
The MT-24EX does have a filter thread in its mounting bracket so a protective filter could be fitted there. However this is actually intended to fit the special lens hood for the MP-E 65mm when used with the twin flash (http://www.bhphotovideo.com ...oughType=accessory_detail ).
Q15 If Canon thought a UV filter was need they would put one there, so why should I?
A15 As already noted some lenses are weather sealed and Canon state that a filter needs to be fitted to complete the seal. Also Canon actually recommends using protective filters in its article on lens care http://www.usa.canon.com ...FLenses101/lens_care.html but you the user must weigh up the cost, risks and benefits.
Q16 Will a protective filter may introduce a colour bias?
A16 The answer is in general yes, but very small bias, and in this test undetectable. There are measurements at http://www.kenandchristine.com/gallery/1054387/3 and http://www.zen20934.zen.co.uk ...F/Colour%20Bias/index.htm.
These are very small and will tend to be removed by AWB or click balance of digital systems and may be lower with a Clear Protective filter rather than the UV filter tested. Differing bands and types may vary, but high end filters should be of a similar order.
Examples are shown below.
----- No Filter ----- Hoya SHMC Pro 1 UV(O) ----- B+W UV-Haze 101 MRC
---------------Q17 Will a Filter Cause Vignetting?
A17 Vignetting is a reduction in image brightness in the image periphery compared to the image centre. http://en.wikipedia.org/wiki/Vignetting
It is possible with ultra wide angle lenses such as 10mm or below on APS-C or 16mm or below on Full-Frame or 135 Film. However in general this is not found to be a problem although some people use special thin filters on such lenses to minimise any issue. Some of these thin filters lack front threads and so do not work well with a lens cap, some come with a special lens cap for this reason.
This is one of many reasons to avoid multiple stacking of filters.
Ultra wide lenses tend to vignett somewhat anyway simply due to the physics of their design, this reduces as the lenses stops down as will any vignetting due to physical obstruction like a filter.
A test of EF-S 10-22mm at 10mm and f3.5 shows no significant vignetting with a Hoya SHMC Pro 1 UV(O) http://www.zen20934.zen.co.uk ...r_AF/Vignetting/index.htm
---------------Q18 Will UV filter get stuck if left on lens all the time?
This is probably a myth possibly exacerbated by people over-tightening the filters. I can say I have had aluminum filters (mostly Hoya HMC) on my Canon FD lenses since the early 1980s; these filters can still be removed with no difficulty.
This is in the days when the filter threads were aluminium, these days they are mostly plastic, including the L lenses, filters are anodized or black chrome depending on make so this should not be an issue even with an aluminium lens thread.
B+W and Heliopan promote the use of brass on the basis it reduces filter sticking. There does not seem to be any materials science quoted to justify this on B+W's or and Heliopan's web sites or generally within the community although many photographers seem to feel the effect is real. However is is clear that two brass filters can get stuck as is seen in this forum thread
If you do get a filter stuck through over-tightening put a rubber band around the outside to improve the grip and don't squeeze to hard.
Separating polarizing filters from a lens or indeed another filter can be tricky but this seems to mostly be down to the mechanics due to the thin area to grip. Hoya Pro 1 filters have a knurled region to aid removal.