UV Filter is for what purpose?
 

I understand UV filters are meant to absorb less UV light. I'm trying to film some cloth that is illuminated with UV light. Would this filter help me get better footage?

A UV filter is meant to prohibit the passage of UV light rays in outdoor bright sun conditions. The presence of excessive UV leads to a slight hazing of the image, effectively reducing contrast and color saturation in the process. The UV filter on the front of the lens is for combating those issues.

Indoors, it is a 'clear lens cap' as some have called it. Filters never add anything. They always take away something which is why they are called filters.

-gb-

Good pt about filters not adding anything. I forgot about the basic principles! Thks.

I'd advise you to avoid doing this. UV radiation is a life killer. It can change the DNA (cause cancer), can affect the immune system and can seriously damage the eyes (up to blind) etc. Even a plastic exposed to UV for some time will be damaged!

BTW, a UV filter is completely useless in places other than the highest mountains. It has no effect in low heights -except as a lens protection that is worth having only if it is a multilayer type.

AFAIK, the output of typical UV lamps (such as flourescent "black lights") should really be way below the threshold of creating problems like this. You are probably getting much more UV exposure when you step outside on a sunny day...

He didn't say what lamps he will use... I hope I'm wrong guessing the worst case (you can buy harmfull lamps for medical, industrial etc use as well). I've seen posts about geting such lamps and even installing(!) for not creative purposes...

don't use tanning bed lights... :)

There are UV filters intended to be used with UV illumination, vs. those intended to cut UV in a landscape. UV used in illumination is usually used to induce fluorescence in the illuminated scene. The light coming from the fluorescence is in the visual range.

Really Jim =) ?? Actually I'm trying to film chinese Dragon dance. The dragon is painted in UV absording paint and the dancers prance in the dark with some UV lights only. FX1 doesnt work really well in low Lux and we have toi shoot from quite a distance. I heard from someone that there are UV filters that will help but I have no clue if it really works with my usage! For those who may be interested to see what a dragon looks like, please click the link below:

http://www.thephotostation.net/photo...97dragonhd.jpg

From your description Sean a UV filter isn't going to help at all. Glass itself absorbs UV, so the 13 elements in your lens's lineup don't need you to add another one. The filter you add will not be absolutely spotless (impossible in the real world) so you're losing some light, adding some flare and spending money better put into a tripod or mic.

tom.

Oops, I think I didnt explain myself clearly. What I film is a moving dragon, held by 9 men in a dark area illuminated by UV Florescent lamps. The dragon itself is painted with UV absording paint. Hope this is clearer now. Anyway to get better footage in this case?

I'd be more afraid of that dragon eating your camera from your arms. Have you seen one of those things drink a glass of water? Its mind boggling

Ha ha :p I spoke to DSE in person today. He feels adding a wide len may help get an additional F stop. Some gain would definately help too! Will try it out and let u guys know..

I'd cross DSE off your photographic buddy list then. Adding a wide-angle converter always loses you light, never the other way around.

Any converter costs you two f-stops the way I understand the rule. If anyone has any other theories I'd love to hear them. I've always wondered if all lens converters were equal when it came to acting like .06 ND filters.

No no Jeff. I'm sure Sean is talking about an A-lens converter, one that attaches to the front of a zoom lens and either increases the focal length (a telephoto converter) or decreases it (a wide-angle converter).

In such a case these zoom-through converters are generally constructed of two to four elements, and as a little bit of light is reflected back from each surface of the individual elements, a losing a small part of a stop will be missing at the chip.

Most times it's so little that the camera's aperture readout won't notice it, but the camera itself will be making the small adjustment necessary.

Your '2 f-stop' thinking is about tele-converter lenses that go between zoom and body. These do indeed (depending on their power) lose a great deal of light.

tom.

Doesn't a wide-angle light put more light onto the sensor?

For example, lets say a wide-angle captures an image

ABC
DEF
GHI

Without, the camera will just see

E

which would be a smaller area... and less light.

Look at it another way Glenn. You set up your camera perpendicular to an evenly lit grey wall. You can choose whatever focal length you like, whatever lens converter you like, the results will always be the same because the wall continues to reflect the same amount of light back into the room.

So a camcorder with a wide-angle converter in place will want f/4 (say) to correctly expose the wall, and the same camera without a converter will also want f/4 - at whatever focal length you've set it at.

Let's say you have a simple, single element prime lens on your camera that's f/2. You swap it for a 14 element zoom lens that's also engraved f/2, but its T stop (true transmission rather than a mathematical f number) is f/2.4. Now you'll see that adding glass might not alter the f stop, but it sure does alter the T stop, and exposure allowances will have to be made.

Each piece of glass you add in front of your camera absorbs light. It's the law of the land.

tom.

I guess I wasn't thinking on this. I had heard this theory on another board recently and didn't think about how much 2 f-stops really were different. I remembered just what you said above from my photography class long ago - that any glass in front of your senson (or film) means less light getting through. But not 2 f-stops.

I checked the theory on my still camera. I took a photo with no lens adapter in place and checked how the auto function of the camera set the f-stop and shutter speed. I tried the same photo (framed exactly the same but with the adapter on) and lo and behold the camera set the exposure to the same level exactly. There was a barely perceptible difference in how the photos looked (the one with the adapter looked a little darker) but there certainly wasn't even one f-stop of difference much less two.

I guess I'll go back to the other board and correct some things now. Sorry for the mistake.

Yes, please do that Jeff. The 'wide-angle converter will let you work in dimmer surroundings' is a common fallacy that I'm often having to correct.

tom/

That wasn't the problem on the other board. It was the suggestion that a lens converter reduced the light by a substantial margin (2 f-stops). It was a pretty reputable person that said this which is why I accepted it without really thinking it through.

It's pretty obvious that adding extra glass will darken the image but I realize some people think a bigger lens means more light is getting in. I see that said pretty often too.

Anyway thanks for the help.

My WA adapter does not lose any significant light. Tele adapters, particularly those put behind 35mm SLR lenses to double the focal length, may be what you are referring.

To get back to the original question, the light reflecting off the dragon will be visible-spectrum light that the paint converted from UV. I can prove this by referring to the picture. See the orange and green in the dragon? Orange and green, by definition, can not be in the UV spectrum. I would say that you should put no filter in front of the lens since you want to see the fluorescent reflected colors and maybe even into the violet spectrum a bit anyway. I seem to recall that CCDs can't even see UV very well anyway, so a filter for it is probably almost never needed. CCD cameras have built-in infra-red filters to block out interference from those frequencies, but that is not the issue in this scenario anyway. I tested my VX2000 in front of a metal forge, and it did not have IR interference.

I think the real issue one may have shooting "blacklight" subjects would be the tremendous contrast range in the scene that would confuse the auto-exposure. Try using manual exposure or maybe the "spotlight" mode on your camera's automatic exposure. I would also fix the white balance to the practical lights in the scene that are not the UV lights. Assuming it is an indoor/night performance, you might try just sticking the camera at 3200K/incandescent.