Saturday, May 19, 2012


Camera lens filters and camera lenses explained. In this basic video guide from Adorama TV, Mark Wallace demonstrates exactly how to establish which size filter you require for your lenses, and the best type of camera filters to meet your needs. UV filters are the best for stopping UV rays, as well as safeguarding your precious lens. Polarizing filters assist in reducing reflections and improve saturation in blue skies.
UV filters are supposed to block UV light. So, for the newcomers to photography let's first look at what UV light is and why you would want to block it. The "traditional" visible spectrum runs from red to violet. Red light has the longest wavelength and violet the shortest. Light which has a longer wavelength than red is called infrared, and light which has a shorter wavelength than violet is called ultra violet or UV. The wavelength of light is measure in units of nanometers (abbreviated as nm), and 1nm is a billionth of a meter (that's a US billion or 1000 million, not a UK billion which is a million million!). Light shorter in wavelength than about 400nm is called ultra violet, light longer in wavelength than 700nm is called infrared.
So, now we know what UV light is, why would be want to block it? Well the answer lies in the way that color film works. There are basically three color sensitive layers, one sensitive to red light, one to green light and one to blue light. The blue layer not only responds to blue light, but also to UV light, so if there is a lot of UV around the blue sensitive layer gets extra exposure and the final image takes on a blue color. Since film isn't normally sensitive to infrared, you don't need an infrared blocking filter. Interestingly though, digital sensors are infrared sensitive and most digital cameras have an infrared blocking filter built in. Now there isn't usually a huge amount of UV around at sea level. There is some (that's what gives you a suntan or a sunburn) but most of it is scattered by the atmosphere. However as you gain altitude, for example by going up a mountain, the amount of UV increases. Under these conditions a UV filter can prevent a blue cast in photographs. Since UV filters look clear and neutral to the naked eye, some people also use them as a protective filter which they leave on their lens at all times. Some people think this is a good idea, other question the wisdom placing a $20 filter in front of a $1000 lens and potentially affecting image quality. Both schools of thought have some valid points. It's your choice. So if you buy a UV filter, you'd expect it to block UV right? Well, sometimes you'd be wrong as the results of this test show. I've looked at the range between 350nm and 400nm for UV blocking since the glass used in almost all lenses will itself block any light with a wavelength shorter than 350nm, so you don't need help from a filter there. The Tests The filters were measured using a calibrated UV/visible spectrophotometer which I had access to at the time of the tests. The plot below shows the transmission characteristics of a number of "UV filters". There are 3 "generic" type filters, a Millennium (marked "made in Japan"), a second Millennium (this one marked "made in China") and a Promaster, plus 3 "name brand" filters, a Tiffen UV protector, A Hoya UV filter and a B+W UV filter. As you can see from the plot, the 3 "generics" along with the Tiffen UV protector really did not cut any appreciable UV down to 350nm. The Hoya and B+W filters showed definite UV absorption, the Hoya being more effective at UV blocking.
Looking more closely at the plots for the four filters which did not show much UV absorption you can see that they are all quite similar. In fact the Millennium UV (Japan) and Promaster UV filters appear to be identical. They may well have been made by the same factory and branded with two different names. The Tiffen looks close enough that it too might even come from the same factory, or at least use the same type of glass.
In addition to the UV filters I also looked at a three filters often used in place of a UV filter (i.e. filters which some photographers keep on the lens at all times as protection). These are the Hoya 81B, the Tiffen 812 and the B+W KR1.5. All three are warming filters in that they shift the color balance towards the red (warm) end of the tonal range. I also included a Hoya circular polarizer, just because I had one around. As you can see all three of the warming filters were effective UV absorbers, as well as slight absorbers in the blue and green regions of the spectrum (which is what makes them warming filters). The polarizer absorbed slightly more in the UV than the visible, though I wouldn't call it an effective UV absorber.
Perhaps a more informative way of plotting the data is as a bar graph comparing transmission in the visible range (400nm to 650nm) to transmission in the UV range (350nm-400nm). This is shown below. Added to the group is data for a Tiffen Haze-1 filter, which you can see is VERY effective at blocking UV and a Hoya 1B, a slight warming filter. The four filters on the left clearly don't really absorb UV any more than they absorb visible light. They may be fine as lens protectors but don't make good UV blockers.
Another interesting way to look at UV blocking is to calculate the effective number of stops that the filter attenuates for the wavelengths between 350nm and 400nm compared to transmission in the visible. The data are plotted below on that basis. For example the Tiffen Haze-1 filter looks like a 5 stop filter to UV wavelengths in that range, while the low cost generics and the Tiffen UV protector show less than 0.1 stop attenuation of UV.
This last plot makes the order of UV absorbing effectiveness quite clear The Tiffen Haze-1 is best. It's a neutral filter so color balance is unaffected. Next is the Tiffen 812. Good UV blocking if you also want a warming filter The Hoya 81B is very similar to the Tiffen 812. The Hoya UV filter comes next, neutral, but with 2 stops of UV blocking. The B+W KR1.5 gives about 1.5 stops of UV blocking with slight warming. The Tiffen polarizer gives less than a stop of UV blocking, but that's not why you use a polarizer! The B+W and Hoya 1B aren't very good UV blockers. The 1B is slightly warming The three "generics" and the Tiffen UV protector are pretty useless for blocking UV, though they may make fine, neutral, lens protectors.

  My Pick What I actually use when I need a UV filter or a protective filter is a Tiffen 812. Usually, for the type of work I do, a warmer image isn't a problem, indeed it's often desirable. I also like to minimize the number of filters I carry so my 812 serves three functions. It blocks UV, it protects the lens and it's a warming filter. Some people use an 81B for this, but I slightly prefer the color shift of the 812. Not everyone wants a warming filter, so the clear winner for a neutral filter that really bocks UV is the Tiffen Haze-1, though the Hoya UV should also be pretty effective.

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