I'm sort of curios. I had physics, so I get the basics, but what I do NOT get is the following:
hold up your UV filter to a TL tube. it looks like a TL tube. now look at the -reflection- of the TL tube in your filter. it (should be) very green =)
So my thought would be "the UV filter doesn't reflect UV and visible shades of violet" but why does it do that, and why does it seemingly allow the UV to pass through the glass anyway?
I'm stumped. (or possibly missing something here)
#1. "RE: UV filters.. how do they work?" | In response to Reply # 0f5fstop Basic MemberThu 13-Jul-00 08:37 AM
LAST EDITED ON Jul-13-00 AT 12:38 PM (GMT)
The filters I know of do not reflect UV lightwaves; they absorb the UV lightwaves according the Beer's Law. Beers' law is defined as the linear relationship between absorbance and concentration of an absorbing species. (There is also another well-known Beer's law that is found in the southern US. It relates to the amount of beer a "good old boy" can consume before passing out. }> )
A = a() * b * c
(A is the measured absorbance, a() is a wavelength-dependent absorptivity coefficient, b is the path length, and c is the analyte concentration.)
Most filters are multi-coated to reflect stray light. The filter will allow the light that is perpendicular to the its plane to pass through to the lens; however, light at a certain angle to the filter's plane will be reflected. (This angle may vary depending on the filter? Not sure at what angle stray light is reflected; however consideration must be taken into account for the possible lens's angle of view.) This helps to prevent flaring caused by stray light entering the lens on an angle.
I believe the statements by filter manufacturers regarding the layers of multi-coating and the reflection of light does make one believe that this includes the reflection of UV light. However, if you read the company's literature regarding UV light you will find out most, if not all, of the filter manufacturers state the filter absorbs UV light, similar to sunglasses. (Not sure of what UV spectrum filers absorb; A or B.)
I know Tiffen and Hoya advertise that their UV and Skylight filters absorb UV light.
Hope this helped….
"Take only photographs, leave only footprints"
#2. "RE: UV filters.. how do they work?" | In response to Reply # 1Mon 17-Jul-00 04:49 AM
LAST EDITED ON Jul-17-00 AT 08:50 AM (GMT)
I can't add to Doug's technical comments, but on a practical side, I'm wary of UV filters except as a means of preventing damage to my lenses. Usually I don't bother with them.
I'm pretty sure that modern, multicoated lenses, especially from the big manufacturers, have plenty of UV filtration to start with. Please don't accept this as Gospel Truth - only my private opinion, but screwing a filter on the lens is just as likely to cause the image quality to deteriorate.
You're adding two extra air-to-glass surfaces to a light path that already has, on some zooms, around 18 bits of glass in it. Nikon (and C*n*n, M*n*lt* etc.) lens designers and optical engineers go to an awful lot of trouble to design their lenses, select the right glass, coat and multicoat every single lens surface. Does it sound like they went to all this effort, made a perfect lens, but just forgot to add UV filtration? I don't think so! Maybe in the sixties, before multicoating was popular, but not today.
Also, the filter is now even further forward and less protected by the lens hood (a far more useful accessory!).
I've never done any kind of scientific testing, but I've found that if you have a hazy day, you'll get hazy photos even if you screw a whole bag full of UV filters on...
A UV or skylight filter will, however, be cheaper to replace than the lens if it gets scratched up.
Hope this helps!
#3. "RE: UV filters.. how do they work?" | In response to Reply # 2Mon 17-Jul-00 07:32 AM
LAST EDITED ON Jul-17-00 AT 11:33 AM (GMT)
I don't think they coat the lenses with built in UV filters... Lens manufacturers realise the importance of being able to start with a clean image for the photographer. Therefor, all the coatings will only be to make sure the light reaches the film in a proper way, while also reinforcing the glass (just a little). It probably won't filter anything out significant (like UV) out, because that might cause people to *not* buy the lens because it's already got something built in that they might not want.
That's why Nikon, and so many others, still makes their own filters. Of course, if you're going for protection, you'd take a UV filter with 0 haze, but then why are there UV filters with higher haze values? it's probably got to do something with "painting with light", but I don't know how =)
I'm not even going to concidere the absoluteness of using a UV filter for only protection, or only as UV filter, but I do wonder why it's such a popular choice as protecter. a 0 ND (or just circular polariser) is just as good. Sure, sometimes it's inconvenient, but so's a UV filter at times
Perhaps Lens manufaturers should, aside from the regular hood, supply a padded lock-hood with some type of quick-release mechanism. like a filter screw with a click on for the hood... hmm.. invention-time =)
I'm assuming that lenses are "dust-tight" but not airtight... I wonder what would happen if you pumped it's surrounding vacuum, and THEN take a picture. would the abscence of airi in the lens affect the image much, or not.. hmm.. questions questions. I think I'll post this one in another forum, perhaps someone has even tried this <g>
if not, perhaps *I* can try this =)
But the other one. (how many Mike's do we have?)
#4. "RE: UV filters.. how do they work?" | In response to Reply # 3Mon 17-Jul-00 08:24 AM
Mike (the other Mike),
Hey, there must be dozens of Mikes out there... I'll just sign with my surname to protect the innocent, OK?
Why don't we do some testing to see who's right? I'm not thinking like those people who spend more time studying lens resolution charts than using their cameras, but how about shooting some slide film up on a hazy day, with and without UV filter? Just a simple experiment? I only own 2, a Nikon and a Hoya, both claim to be high-end.
Assuming we can find a day where the sun comes out for more than an hour - are you having better weather in Holland than we are in Germany? Right now you can't see the horizon without radar... I've almost given up on summer!
#5. "RE: UV filters.. how do they work?" | In response to Reply # 4Mon 17-Jul-00 01:07 PM
Sounds good. I only own one UV filter, a Hama which is "midrange" as far as that goes for filters. (it cost about 60 DM for 72mm).
What lens(es) do you use?
The weather here is autumnish. If it weren't for my great love of cooking, coffee and jazz, I'd be depressed.
come on, sun!
#6. "RE: UV filters.. how do they work?" | In response to Reply # 5Mon 17-Jul-00 03:42 PM
OK, How about we use a humble 50mm f1.8 Nikkor for our tests? Have you got one of those? Got an idea about a haze test... and I really mean HAZE, Mike! As soon as the weather changes, I'll take a Piper Cherokee up to about 5000 feet and shoot off some slide film out of the clear side panel. You can't do serious photography with one of these, because the wing gets in the way, but if I have a brave volunteer take the filter on and off we should see some results.
By the way, you Netherlands folks must have some incredible pilots! We were in Holland last year, and they were flying little Cessnas back and forth along the coast in 30 - 40 knot gusting winds, no problem! I guess all you'd have to do to get airborne off the runway would be to just take your feet off the brakes...
#7. "RE: UV filters.. how do they work?" | In response to Reply # 6Tue 18-Jul-00 04:41 AM
I don't own one, but I'm sure I can rent one for a week or so.
I'm not sure how much effect my filter will have (Hama M72 UV390 0-haze) but it's worth the try =)
I only have a drivers liscence, no controlled airxperience. (yet..
If someone can pilot a light prop through 30-40 knots, I have a deep respect for that person. Either they know their plane in and out, or they're insane. either way, they rate high in my book =)
#8. "RE: UV filters.. how do they work?" | In response to Reply # 7Tue 18-Jul-00 04:51 AM
Aviation and insanity go hand in hand, believe me!
PLEASE don't spend any money! Anyway, your 28-300 Tamron will do fine - we're not testing lenses, we just want to see if UV filters actually cut down haze, right? This is about as non-scientific as it gets...