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Old August 17th, 2006, 01:32 AM   #16
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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.
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Old August 17th, 2006, 12:00 PM   #17
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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.
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Old August 17th, 2006, 12:32 PM   #18
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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.
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Old August 17th, 2006, 11:30 PM   #19
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Quote:
Originally Posted by Tom Hardwick
Each piece of glass you add in front of your camera absorbs light. It's the law of the land.
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.
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Old August 18th, 2006, 01:00 AM   #20
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Originally Posted by Jeff Phelps

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/
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Old August 18th, 2006, 01:24 AM   #21
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Originally Posted by Tom Hardwick
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.
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Old August 18th, 2006, 01:25 AM   #22
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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.
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