Photographic Lens Filter

What is photographic lens filter?  Its a thin piece of glass that is mounted in front of lens.

What is their function?

• To protect the precious lens.  Lens generally costs few hundreds to thousands of dollars, therefore people sacrifice filter - that costs a lot cheaper - to protect the lens from airborne dust that will scratch the lens or rain drops.
• To change colour and white balance (usually not needed for digital camera)
• To cut stray light from entering the lens, therefore ensuring clear photograph.
• ND Grad filter is used to make a sky bluer and darker, therefore balancing the exposure between the sky and subject, then finally improving the dynamic range of a camera.
• Polarised filter will allow only direction of light in the camera, virtually reducing the light who enter the lens into half and removing all available stray light therefore removing flares.
• Some filter even claims that their filter can improve sun stars, do macro close-ups and more.

Now continue to my comparison of EF-S 18-55mm lens, equipped with Kokaii UV Coated Filter, with Hoya Pro1 Digital DMC Filter and naked.

Testing method is very simple, just by creating an angle of light source and the filter.  A bad filter will induce more flare and ghosts, while good filter will help eliminate them.

My testing technique is totally safe and can be tried at home.  It is not blindness inducing, since I use a normal 18w SL tube!  If that does not satisfy you, a tungsten street light can be considered as the light source.  It will create a much bigger flare than an 18w SL tube.  The point is the angle that creates ghost, and the light source doesn't need to be a very strong light source. 

Ghosts

Ghost is the reflection of light source on the lens, therefore their shape usually resemble the shape of the light source.  This is how ghost occurs:

Kokaii UV Coated Filter

Kokaii Filter is the worst, it creates 2 ghosts, both are clearly visible near the light source and the other is just above the copyright mark on the image!  Well, Kokaii is the cheapest filter anyway, it costs Rp 38.000 (USD 3).  It will only offer physical protection to the lens anyway.  Please see the image below as the reference! 

Hoya DMC Pro1 Digital

Hoya DMC filter is much better, the ghost is much less visible than Kokaii.  I've had to circle the ghost in red to show where it is.  Considering that Hoya costs 6 times as much (Rp 225.000, or USD 20) as Kokaii, all I could say, it worth OK.  My filter is made in Japan by Tokina.

Naked

On my case Naked lens is the best!  It has no ghost visible, moreover this is the cheapest option.

Flare

Flare is caused by stray light that colour the lens, it usually come from a sharp angle and can be fixed by covering he source of stray light using one hand or lens hood and changing angle.  If that does not help, try to clean the lens since lens dirt also induce flare.  FYI, flare might occur even when the light source is not on the frame!

Kokaii UV Coated Filter

On Kokaii, the flare is visible from the view finder.  Its circled in red near the light source on the bottom left and right of the image.

Hoya DMC Pro1 Digital

Hoya is excellent in this case.  It has no visible flare on the image.  In fact its very hard to get a flare there

Naked

Naked lens also did not produce any flare. 

Conclusion

Some lens is more ghost and flare prone than other lens, and a filter helps to remove those ghost and flares.  However since my lens is good - it won't show ghosts on the photograph - the filter is the one who actually bring the ghosts. 

Kokaii filter can't defeat ghost and flare, it only give physical protection to your lens.  Try to remove the filter if you find ghost and flare in your image.  For photographing action with bright light shinning to the lens, forget Kokaii, since you might have only 1 picture taking chance.

Hoya DMC Filter can't totally defeat ghost, however it might defeat flares.  I'll have to check it with more flare inducing lightning to conform that.  Moreover, if the standard lens is severely ghost inducing, the Hoya will of course help reduce the ghosts.

Final word, if your image is ghostly or flarely, please try to remove the filter.  If it still ghostly, then change your position relative to the light source, or recompose the image.

FAQ on photographic lens filter

This is the most common question and answer asked by beginner:

1. Do photographic lens filters robs sharpness?  The answer is yes - as long as the filter is not the very bad one - with current camera and lens, filter will rob the sharpness by an invisible degree.  Taking picture behind a dirty screen will help you to understand this!
2. Do lens filter reduce the amount of light transmitted to the camera?  The answer is yes.  Normal filter usually reduce the amount of light transmitted less than 1%, however polarised filter could reduces the light by 1 exposure stop or more depending on their specification!
3. Will the filter alter the natural colour of the subject?  Unless the filter is a colour altering filter, they only shift the colour hue by the slightest amount.  Therefore having almost no effect on the photograph.
4. If a filter rob sharpness and alter the natural colour of the subject, can a filter improve the image quality?  It entirely depends on you!  Colour altering filter (warming or cooling), ND Grad filter and will enable you to explore your creativity, therefore improving the overall art in your image.
5. Will I use my lens filter after running those ghost and flare tests?  Yes, I will use the HOYA DMC filter I've got to give a physical protection to my lens!  Moreover, even with the Kokaii, flare or ghosts only occurs in less than 1% of my photograph, therefore having a Hoya will reduce that even further!