Re: Newbie

[Clive Warren <Clive.Warren@xxxxxxxxxxxxxx>]

At 2:30 pm +0200 19/4/01, Andreas wrote:
>
>Thank you all for your comments.
>
>To Helmut:
>
>It is a EOS. Are you saying that my "modern" 50mm lens system is useless
>for UV? If so, then I'd go through with trying to remove the coatings
>before buying a new or rather an old lens system.
>
>
>bhphoto.com have an assortment of B&W Black Glass Lenses, some really
>cheap (~35$ for a 52mm). It says that
>
>"Although this dark violet filter transmits a small amount of visible
>light, it completely blocks wavelengths longer than 360nm."
>
>which is conflicting information, as I see it, since the upper limit for
>our vision is usually given as 400nm.
>
>I'd like to know if anyone has experience with the B&W filter.
>
>Thank you again
>Regards
>Andreas

Andreas,

Generally you will obtain better results using a non-coated lens with 
only a few elements as previously suggested.  If you will be 
photographing flowers etc as an insects' eye view of the world then 
using a set of bellows with a camera would be an option for the 
close-ups. This then also allows you to use almost any lens using a 
manual stop-down - high quality older non-coated lenses can be found 
for next to nothing on old junker cameras - some may even have 
shutters that work :-)

The ideal lens is made from quartz rather than glass.  The crystal 
allows most UV light to pass - these lenses are expensive and rare.

Vision - that's a can of worms you've opened up.....  Our peripheral 
vision is mediated mainly by rods (receptors) in the eye which are 
most sensitive to UV light - this is called scotopic vision (B&W). 
There are not so many rods in the area of the eye used for 
central/focal vision - the receptors there are called cones and this 
is known as photopic vision (colour).  When enough light hits a rod 
or cone then the threshold is exceeded and it fires off a signal to 
say that there is light. However, there is a difference in 
sensitivity under dim and high illuminations known as the Purkinje 
shift. Then there's this huge lump of gray matter, the visual cortex, 
which processes the information from the receptors.

Sensitivity to light is not an invariant quality of rods and cones - 
it also depends on the previous activity of the visual system. With 
suitable adaptation time, the absolute sensitivity of the eye to 
light may increase as much as 100,000 times!

So it all gets horribly complicated and then there is individual 
differences - different people have different absolute ranges of 
sensitivity to light wavelength....

However, The human lens contains a filter that blocks U-V light from 
reaching the retina. The U-V filter in the human lens has a yellow 
appearance and also absorbs heavily in the violet and blue. This 
filter is not present in the lens of cats, dogs and predators. They 
receive much of the U-V light that we filter out.  Basically we live 
longer and the cumulative UV damage matters more for us than shorter 
lived mammals.

Generally you can state that 400 nm is the wavelength break point 
between visible and ultraviolet light. This is because the average 
human lens absorbs 94% of the light at 400 nm and its absorption 
increases dramatically for wavelengths shorter than that. We do not 
see U-V because it never
reaches the retina.

The bottom-line - some people will see more UV than others. I bet the 
B&H filters do not completely block visible light and that maybe 80% 
or so is blocked at 400nm.....

Let us know how you get on.





-- 
All the best,

              Clive   http://www.cocam.co.uk
                      Photographic Services, Filters and Equipment, Infrared FAQ
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