Jump to content

kevinteo

Members
 View Profile  See their activity

  • Posts

    289

  • Joined

  • Last visited

 Content Type 

Posts posted by kevinteo

    Polarizer and Nd filters

    in Accessories

    Posted

    <p>First off, a little background on polarizers to help you understand the difference... A polarizing filter is a natural-crystal or manmade substance that linearly polarizes light transmission

    as it enters the lens. In the case of manmade filters, a diachroic substance (such as quinine iodosulphate) is sealed between two pieces of optical glass. As the term diachroic implies, the

    quinine iodosulphate absorbs light in all but one plane of polarization. This is possible since quinine iodosulphate can be manufactured with a parallel molecular chain structure form

    throughout the filter.</p>

     

    <p>Polarizing filters generally come in two types, linear and circular. You should choose a circular polarizer if your camera uses TTL metering (this is explained below). Linear polarizing

    filters do not confuse your camera's auto focus sensors; but they can confuse it's light meter. (Better if you used a handheld light meter.) A circular polarizer is just a linear polarizer followed

    by a ?quarter-wave retardation? plate set at 45° to the axis of polarization. A quarter-wave plate is made of a material in which light polarized in one particular direction travels more

    slowly than light polarized in the perpendicular direction. This plate is just thick enough that after passing through it, light polarized in one direction is delayed 90° (or one-quarter

    wavelength) relative to light polarized in the other direction. Since the quarter-wave plate is set at 45° to the polarization, you can think of the incoming light as having two equal

    components in the principal directions of the quarter-wave plate. After passing through the plate, one component is delayed 90°, and the resulting light is circularly polarized.</p>

     

    <p>A problem with linearly polarized light in your camera, is that when you bounce it off a mirror near or at Brewster's angle (Brewster?s Law states that the greatest amount of polarization

    occurs with light that a reflecting surface at an angle of incidence whose tangent equals the refractive index of the reflecting material), it may be nearly or completely eliminated. (The

    application of Brewster?s law works out to 53° for water and 57° for glass.) If the light meter measures the light after it bounces off a mirror (as in the case of some modern SLRs

    with TTL metering), the amount of light arriving at the meter may be drastically different from the amount of light that will arrive at the film with no bounce, since the mirror has to be

    flipped out of the way.</p>

     

    <p>But note that a quarter-wave plate is only exactly a quarter wave for one frequency of light. That frequency is usually chosen to be a yellow in about the middle of the visible spectrum

    so that on the average, the light will be circularly polarized with various degrees of elliptical polarization mixed in. If you were photographing something that was primarily red, or primarily

    violet, your metering might be slightly off, even using a circular polarizer. And of course, since there's another chunk of material (the quarter-wave plate) in the way, there will technically be

    slightly more degradation of the image with a circular than with a linear polarizer.</p>

     

    <p>Many polarizers on the market are uncoated or double coated. This is partly because until the 1990s, when B+W and Heliopan figured it out, there was no known way of multicoating

    polarizers due to the polarizer foils melting during the multicoating process. The new technique involved applying the coating at a lower temperature that significantly lowered the hardness

    of the coating. As such, these multicoatings, though effective, are not as tough as multicoatings on regular filters. Lastly, polarizers (like all special effects filters that require a rotating

    mount) are going to be thicker than other filters; a slim version should be bought to avoid vignetting on wide-angle lenses. Slim filters do not have a front thread.</p>

     

    <p>Käsemann polarizers are manufactured with a process introduced by Erwin Käsemann in 1955. The polarizers foils for these filters are produced with colloidal film made with

    polyvinyl alcohol. The result is a uniaxial crystalline structure. These polarizer foils are claimed to yield consistent neutrality of color and the highest extinction ratio of any type of polarizer.

    (The extinction coefficient or extinction ratio is the measure of the effectiveness of a polarizer. Heliopan have also stated that linear polarizers have a higher extinction ratio than circular

    polarizers, but circulars give higher color saturation; this could be attributed to the absence of the quarter-wave plate. B+W however claim that there are circular polarizer foils available with

    the same extinction ratios as linear polarizer foils.) Käsemann polarizers are completely edge-sealed for better durability. The edge sealing is also supposed to make them optically

    superior, as the polarizer foil is flatter, being "under equal and constant tension in all directions". The optical glass used for K䳥mann polarizers is also specially prepared to suit the type of

    photography and you can select the type of wave plate (quarter or full) which will be tuned to the appropriate wavelength- 135 nm (quarter) or 540 nm (full). And yes, you can also order

    them multicoated. With most photographers now using TTL-metering SLR cameras, K䳥mann polarizers you buy from your camera store are likely to be the circular variety with a quarter-

    wave plate tuned to 540 nm. There are many more options available through special order such as polarizer foils optimized to specific wavelengths.</p>

     

    <p>Warming or enchancing polarizers are simply polarizers, as described above, combined with a specific optical glass (equivalent to a Wratten 81 series) or didymium glass.</p>

    Which filter? B+W or Heliopan

    in Accessories

    Posted

    <p><i>I do not have problems with my filters getting scratched from the cap - I think

    that's blown out of proportion.</i></p>

    <p>I agree. The rubber cap is more flexible than a plastic snap-in cap, but I store the

    polarizer in its case when not it use. It is unlikely to get damaged there.</p>

    <p>I like the construction quality of the filter more in the sense of it being

    psychologically-reassuring rather than confidence-inspiring. What makes a difference for

    me a little notch along where the front thread usually is. I'm not sure if this feature was

    deliberate/coincidental, but that little notch indicates the orientation of the the polarizer

    foil, with the 6/12 o'clock position giving maximum polarization. This is an added

    convenience if you ever use a rangefinder camera. :)</p><div>00ND8Z-39584984.jpg.0f6ad15f6c97720769f9d1edbb1e90b3.jpg</div>

    would you buy an imperfect lens?

    in Mirrorless Digital Cameras

    Posted

    <p>Off-topic:</p>

    <p><i>Instability in modern fighter aircraft is not so that they can produce a 'slippery', low

    detection profile.</i></p>

    <p>It depends... Planes like the B-2, F-117 or X-45 were designed with aerodynamics as a

    secondary consideration to minimization of their radar cross section. They thus have inherent

    instability in all three axes, hence the need for computer correction.</p>

    would you buy an imperfect lens?

    in Mirrorless Digital Cameras

    Posted

    Modern planes are aerodynamically unstable due to constraints imposed by the need to

    maintain a stealthy profile, or have certain limitations imposed by wing loading, etc. In

    warfare, lowering the risk of detection may take precedence over actual aerodynamic

    performance. I'm pretty sure a lens designer would worry more about optical performance

    than the physical shape of a lens barrel, or the weight of the glass elements.

    back-lit cibachrome

    in The Wet Darkroom: Film, Paper & Chemistry

    Posted

    <p>There's another Australian studio called <a

    href="http://www.createdforlife.com/">Created for Life</a> located in Gosford, about an

    hour north of Sydney. They also do Cibachrome/Ilfochrome. I've never been used them

    though...</p>

     

    <p>But I have had stuff printed by Chroma Colour on a number of occasions and can attest

    to the quality of their work. They even include a pair of white gloves when they ship the

    package back to you.</p>

    Aperture of shorter length lenses

    in Nikon

    Posted

    <p>Vignetting becomes a bigger problem as aperture size increases; and wide-angle lenses

    also have the additional problem of illumination fall off (the Cos<sup>4</sup>law). Trying to

    design a lens around those limitations is no easy task.</p>

    Missing Lens

    in Medium Format

    Posted

    <p>I take it this is about the 645 in this thread... <a href="http://www.photo.net/bboard/q-

    and-a-fetch-msg?msg_id=00N79p">http://www.photo.net/bboard/q-and-a-fetch-msg?

    msg_id=00N79p</a></p>

    <p>Stuff from Kyoto Ceramic is handled by Tasco; I'm not sure if they do Contax though, I

    could only find Yashica stuff on their site...</p>

     

    <p>Tasco Sales Australia Pty Ltd<br>

    19 Roger Street, Brookvale NSW 2100<br>

    Phone: (02) 9938 3244 / (02) 9939 2972<br>

    <a href="http://www.tasco.com.au">http://www.tasco.com.au</a></p>

     

    <p>In Melbourne, try the usual secondhand places like Camera Lane and Camera

    Exchange.</p>

     

    <p>Camera Lane<br>

    391 Little Bourke Street<br>

    Phone: 9670 0560<br>

    <a href="https://cameralane.store.net.au/">https://cameralane.store.net.au/</a></p>

     

    <p>Camera Exchange<br>

    377 Lonsdale Street<br>

    Phone: 9670 1929<br>

    <a

    href="http://www.cameraexchange.com.au/">http://www.cameraexchange.com.au/</a>

    </p>

     

    <p>If not, try Camera Electronic in Perth, or Foto Risel in Sydney.</p>

     

    <p>Camera Electronic<br>

    <a href="http://www.cameraelectronic.com.au/online-shop/online-

    location.html">http://www.cameraelectronic.com.au/online-shop/online-

    location.html</a></p>

     

    <p>Foto Riesel<br>

    <a href="http://www.fotoriesel.com">http://www.fotoriesel.com</a></p>

    Film for street MF

    in Black & White Practice

    Posted

    I'm assuming you've got a pretty old folder with no rangefinder if you're resorting to scale

    focus. A lot of those old folders like the early model Voigtlander Bessa or Folding

    Kodak/Brownie cameras do not have shutter speeds fast enough to use with high speed

    film in daylight.

     

    Also, many of those old shutter speeds are not the standard speeds you might be used to

    on your digital gear. If you have a 1/100 or 1/125 sec shutter speed, Kodak Plus-X would

    be a good place to start for shooting sunny 16; depending on the enlargement factor, you

    may choose to ignore the reciprocity-of-your-focal-length rule about handholding.

×
×
  • Create New...