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03-21-2008, 06:49 AM   #1
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pentax kit 18-55

so I got the camera with the kit lens. but I am perplexed by a removable cover on one side of the lens hood that reads PH-RBA 52 mm what is it for ?

03-21-2008, 06:52 AM   #2
JMS
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It comes out to allow you to use a circular polarizer while the hood is on the lens.
03-21-2008, 06:54 AM   #3
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QuoteOriginally posted by JMS Quote
It comes out to allow you to use a circular polarizer while the hood is on the lens.
yep. it allows you to spin filters that can can do so, without removing the shade, or having to try to fit your fingers in next to it
03-21-2008, 07:01 AM   #4
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oh right thanks but I have a filter holder that ertainly won't fit into the hood so are you refering to a filter that screws stright on ? I was always told to use a circular because some lenses turn when focused but of the three lenses I own (one being the fixed lens on my Fuji S6500fd) only the tamaron 70-300 turns when focusing. are normal polarizes better than circular ?

03-21-2008, 07:32 AM   #5
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linear polarizers can play havoc with the phase detection AF system. it's always best to use circular polarizers with an AF camera. and yes, it's for using standard round threaded filters. if you have a system like the Cokin P system, they offer hoods that attach to the end of that to reduce flare
03-21-2008, 07:34 AM   #6
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so if a circular polarizer must be used whats the poind of the hole in the hood ?
yea I want to get or make some hoods to go over my filter holder
03-21-2008, 07:40 AM   #7
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QuoteOriginally posted by simons-photography Quote
so if a circular polarizer must be used whats the poind of the hole in the hood ?
yea I want to get or make some hoods to go over my filter holder
polarizers, once screwed onto the lens, have a portion which rotates, so you can align the polarizing foil to the best angle to get the most out of it. if you were to try to rotate this with the hood on, without the hole, you'd have to fit your fingers in down next to it, to grip it. unfortunately, i don't have a cirpol for my lenses, but.. i think you can understand what i mean
03-21-2008, 07:57 AM   #8
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QuoteOriginally posted by simons-photography Quote
I was always told to use a circular because some lenses turn when focused but of the three lenses I own (one being the fixed lens on my Fuji S6500fd) only the tamaron 70-300 turns when focusing. are normal polarizes better than circular ?
The difference between circular and linear polarizing filters has nothing to do with turning or non turning front element. Circular polarizing filters "depolarize" light that exits filter and enters camera, while linear polarizing filters leave light polarized. That's all. Circular polarizers are solving the problem of auto-focus systems being confused with polarized light and failing to work properly. If you use manual focusing then it does not really matter which filter type you use.

Both types or filters work the same way: by turning the filter you filter out polarized light or let it pass (depending on the polarization angle). In general, all light reflected from shiny surfaces at sharp angle is polarized. The sky is also polarized early in the morning or late in the evening, with peak 90 degrees to the sun (e.g. facing straight north or south). LCD displays and monitors work based on polarized light: put filter in front of any LCD monitor and turn it. At one point you will get completely black circle -- this is when polarized light from LCD monitor is at 90 degrees to polarizing plane of your filter. (There should be a dot on the filter to indicate plane of polarization.)

Bottom line: to adjust the filter to produce the effect you want it is necessary to turn the filter. However, if you put lens hood on you can not really do that. So to allow you to use polarizing filters with lens hoods AND turn filters Pentax added that little opening. Simple.

03-21-2008, 08:04 AM   #9
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ok I understand now so will rotating my circular polarizer alter its effect on the image ?
so a linear polarizer is really specific for taking pictires of stuff behind glass and through shop windows etc.
03-21-2008, 08:10 AM   #10
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QuoteOriginally posted by simons-photography Quote
ok I understand now so will rotating my circular polarizer alter its effect on the image ?
so a linear polarizer is really specific for taking pictires of stuff behind glass and through shop windows etc.
yup. if you rotate a polarizer of any sort, it will change the intensity of the polarization. you can watch the sky go from blown out white-blue, to deep dark blues, and back, you can also see glare from non-metallic objects fade away. linear and circular should both have the ability to turn, it's just that the way the circular polarizers are produced, the foil used to polarize the light doesn't effect the AF system the way it would from a linear.
03-21-2008, 08:16 AM   #11
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QuoteOriginally posted by OniFactor Quote
yup. if you rotate a polarizer of any sort, it will change the intensity of the polarization. you can watch the sky go from blown out white-blue, to deep dark blues, and back, you can also see glare from non-metallic objects fade away. linear and circular should both have the ability to turn, it's just that the way the circular polarizers are produced, the foil used to polarize the light doesn't effect the AF system the way it would from a linear.
ok thanks thats vital info as like as ass I went round with my fuji S6500fd for the last 6 months with the polarizr on it at all times thinking it didn't need rotating hm
03-21-2008, 10:31 AM   #12
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QuoteOriginally posted by simons-photography Quote
ok thanks thats vital info as like as ass I went round with my fuji S6500fd for the last 6 months with the polarizr on it at all times thinking it didn't need rotating hm
No, it does and that is the key feature. Take polarizer off your camera and just look through it, rotate and see how things change. Especially glossy and shiny surfaces, water, windows, car windshields and so on and at a sharpangle (looking straight at it does not change anything). Go and look through it at parked cars around you and you will see what happens.

Also, leaving the polarizer permanently mounted is NOT good: it darkens the image and you usually loose about 3 stops of light (which means that if a scene requires 1/125s shutter speed with polarizer mounted that will drop to 1/15s!!!). Use polarizing filter ONLY if you need to remove reflections, increase contrast in landscape, darken morning/evening sky and such. Do NOT use it all the time, it is not the same as UV or Skylight 1B filters that can be used all the time.

And one more time: remember that there is NO DIFFERENCE between circular and linear polarizing filers in therms of how to use them and what they produce as effect. The only difference is that circular polarizing filters can be used with auto focus systems and linear can not be used with auto focus. If you focus manually there is no difference at all and both would do precisely the same job.
03-21-2008, 10:36 AM   #13
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yep thanks for that I'll have to play with my polarizer some only thing is that does mean that I will have to hold the filter wilst my tamaron 70-300 turns as it is focused
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