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Photo Forum / Digital Photography / Digital Photo / May 2007Playing with polarisers
I just bought a couple of circular polarising filters. Both are Hoya. I wanted one to improve my landscape photos. The reason for two is that I was dithering as to which lens to buy it for when the guy in the shop offered me a good discount for the two. My camera is a Canon 300D. The lenses being considered for polarisers were the: 18-55mm kit lens and the 28-135mmm IS lens. The 28-135 should be the better lens but I am quite happy with the kit lens and I like the extra width for landscapes. Anyway, problem solved, I now have both sizes. One will also fit my 75-300mm but it would rather dark at the 300 end so that may not be very useful. Question 1, the instructions on the box are minimal. There is an arrow on the rotating ring. When this is at the top does it mean that vertical polarisation is selected? Since I had two filters, I took them out of their boxes and looked through them. As I rotate them, they do change (so it seems that they really are polarisers) but not quite as I expected. The change seems to be more of colour than density. When lined up the filters appear slightly orangey and when 90 degrees apart they appear grey but not as dark as I hoped. If they were perfect, I would expect black. I guess that is hoping too much but I did expect a darker grey. Has this experiment produced unexpected results because these are circular polarisers? Maybe, since the first will output circularly polarised light, I should not expect rotating the second to have any effect at all. This has just occurred to me, if I turn one around so that they are face to face, should I get the effect that I expect? Should I find a linear polariser or a pair of Polaroid sunglasses and put it in front of one of my filters to test it? Of course, another test is to put the filters on the camera and go and take some pictures. The problem with that is that the weather is lousy here and it is raining as you may expect for November rather than late May. -- Seán O'Leathlóbhair I just bought a couple of circular polarising filters. Both are Hoya. I wanted one to improve my landscape photos. The reason for two is that I was dithering as to which lens to buy it for when the guy in the shop offered me a good discount for the two. My camera is a Canon 300D. The lenses being considered for polarisers were the: 18-55mm kit lens and the 28-135mmm IS lens. The 28-135 should be the better lens but I am quite happy with the kit lens and I like the extra width for landscapes. Anyway, problem solved, I now have both sizes. One will also fit my 75-300mm but it would rather dark at the 300 end so that may not be very useful. Question 1, the instructions on the box are minimal. There is an arrow on the rotating ring. When this is at the top does it mean that vertical polarisation is selected? Since I had two filters, I took them out of their boxes and looked through them. As I rotate them, they do change (so it seems that they really are polarisers) but not quite as I expected. The change seems to be more of colour than density. When lined up the filters appear slightly orangey and when 90 degrees apart they appear grey but not as dark as I hoped. If they were perfect, I would expect black. I guess that is hoping too much but I did expect a darker grey. Has this experiment produced unexpected results because these are circular polarisers? Maybe, since the first will output circularly polarised light, I should not expect rotating the second to have any effect at all. This has just occurred to me, if I turn one around so that they are face to face, should I get the effect that I expect? Should I find a linear polariser or a pair of Polaroid sunglasses and put it in front of one of my filters to test it? Of course, another test is to put the filters on the camera and go and take some pictures. The problem with that is that the weather is lousy here and it is raining as you may expect for November rather than late May. -- Seán O'Leathlóbhair Hi, A circular polarizer has a quarter wave retarder behind the main polarizer to circularly polarize the light coming off the regular polarizer. It's purpose is to allow proper exposure on cameras that send the light through a partially silvered mirror as many AF cameras do. The quarter wave retarder causes the polarizer not to seem to look right when peering through a couple of them. Place the FRONTs of the to polarizers facing each other and turn the. You should see the polarizers work as they should when you turn one. For anyone in the US who is near a Meijers store. For $13 each they sell "Targus" branded CPL filters in 52 and 58mm sizes. My tests shows they work just fine, but they don't include a case. Local camera store wants nearly $40 for a "Promaster" brand (rebranded Hoya). John > I just bought a couple of circular polarising filters. Both are > Hoya. I wanted one to improve my landscape photos. The reason for [quoted text clipped - 43 lines] > of them. Place the FRONTs of the to polarizers facing each other and turn > the. You should see the polarizers work as they should when you turn one. <snip> Yes, if I turn one polariser around, they behave as expected: when lined up, they look pale grey, the same as one by itself; when one is rotated 90 degrees, they go very dark, not completely black but a very deep purple. For a laugh, I tried them back to back, there is no noticeable difference at all as you turn one. Considering the circular polarising layer this makes sense. The effect in the original orientation is still a puzzle: a change in colour not density. Anyway, I am happy now that I have two functioning polarisers. It has stopped raining so I will pop out for a few photos before dinner. -- Seán O'Leathlóbhair >> It has stopped raining so I will pop out for a few photos before >> dinner. Polarizers are often very effective imediatly after the rain. Often you get patches of blue sky. And everything is wet, great to try a polarizer in those circumstances. ben >>-- >>Seán O'Leathlóbhair > >> It has stopped raining so I will pop out for a few photos before > >> dinner. > > Polarizers are often very effective imediatly after the rain. > Often you get patches of blue sky. And everything is wet, > great to try a polarizer in those circumstances. So I have read but I didn't get anything good tonight. Oh well, keep trying - practice makes perfect. -- Seán O'Leathlóbhair >Yes, if I turn one polariser around, they behave as expected: when >lined up, they look pale grey, the same as one by itself; when one is [quoted text clipped - 7 lines] >The effect in the original orientation is still a puzzle: a change in >colour not density. You have stumbled on what filter makers stumbled on years ago. Now add in a layer of clear plastic behind the circular polarizer and add a linear polarizer behind that. You can then dial-in whatever color changes you want this way. Try various cellophanes (cigarette-pack wrapper plastics give you an intense blue to vivid orange option) or other perfectly clear packaging plastics that twist the wavelength. You'll have the same as those over-priced variable-color filters. Depending on the chosen clear plastic If you want to use 2 polarizers as a variable ND filter, then the front one will have to be a linear polarizer and the back one (camera side) the circular one. Or just use 2 linear polarizers. BTW: Some of the most expensive polarizers are actually the worst in being able to filter out polarized light sources (reflections from flat surfaces). I tested some brands a few years ago. Tiffen's being the absolute worst. I have a $15 generic that beats out a $60 Tiffen. To test them have a polarizer of good quality, meaning if 2 of them are placed at 90 degrees to one another nearly all bright light is extinguished completely and evenly across the faces of the filters. Then test other polarizers against one of them. You'll find that very few of the high-priced polarizers are able to extinguish the light adequately. Even worse is that most of them have the polarizing layer laid down unevenly, revealing strong banding and blotches in them when turned 90-degrees to a polarizer of known good quality. I suppose they get away with it because most people don't think to test them this simple way. They just assume if it costs more it must be higher quality. When in reality nothing is further from the truth these days. On 30 May, 03:04, nob...@noplace.org wrote: > >Yes, if I turn onepolariseraround, they behave as expected: when > >lined up, they look pale grey, the same as one by itself; when one is [quoted text clipped - 15 lines] > wavelength. You'll have the same as those over-priced variable-color filters. > Depending on the chosen clear plastic Sounds fun, I will try it at the weekend. > If you want to use 2 polarizers as a variable ND filter, then the front one will > have to be a linear polarizer and the back one (camera side) the circular one. > Or just use 2 linear polarizers. Not a plan. I was just playing and testing the polarisers. > BTW: Some of the most expensive polarizers are actually the worst in being able > to filter out polarized light sources (reflections from flat surfaces). I tested [quoted text clipped - 7 lines] > revealing strong banding and blotches in them when turned 90-degrees to a > polarizer of known good quality. These ones managed a very deep purple when crossed. It is a bit harder to check how even it is since only bright objects (e.g. bulbs) are visible. > I suppose they get away with it because most people don't think to test them > this simple way. They just assume if it costs more it must be higher quality. > When in reality nothing is further from the truth these days. I got these from a reasonably good price (by UK standards). -- Seán O'Leathlóbhair >>I just bought a couple of circular polarising filters. Both are >>Hoya. I wanted one to improve my landscape photos. The reason for [quoted text clipped - 65 lines] > -- > Seán O'Leathlóbhair Circular polarizers are not perfectly achromatic. That is, the quarter-wave plate is only a quarter-wave plate at one wavelength, unless they use a very expensive achromatic quarter-wave plate, which I doubt. (I've had them made for my research and paid $1000s.) So the linear-to-circular conversion is only "perfect" at one wavelength, and as you depart from that wavelength, some residual linear polarization will remain. Even the purely linear polarizers are not completely achromatic (unless they are very expensive) and will not give complete cancelation when crossed for all wavelegths; often blue light leaks through, though it can be red. The result is that, if you cross polarizers of various kinds in various ways, you can get some funny color effects. Two successive circular polarizers can give these color effects for these reasons. The first one will "leak" some wavlength-dependent linear polarized light that will interact with the second polarizer to various degrees, depending on its orientation. I hope this helps. Joe > >>I just bought a couple of circular polarising filters. Both are > >>Hoya. I wanted one to improve my landscape photos. The reason for [quoted text clipped - 83 lines] > > I hope this helps. Yes, it does thanks though I think that I will read a bit more to revise my own old A level (*) physics. Partly for fun but also because a niece that lives with us is studying A level physics so learning this will be useful for support. My son will probably follow in a couple of years. (*) A levels are the exams that we study for between 16 and 18 and hence immediately before university. A typical good student does 4 or maybe 5 subjects at this level. I think that polarisation of light comes up, it did in my day. She was studying lenses recently: 1/u + 1/f = 1/ v etc. Once, I turned one polariser around, I was quite impressed by them. One by itself is as close to neutral grey as I can tell. Two in a row when aligned appear the same as far as I can tell. When crossed, they are not totally black but only very bright objects are visible such as light bulbs. These appear a deep purple so I guess that the highest wave lengths are leaking though. Talking of the high wave lengths leaking through, is there any point in stacking the polariser on top of the UV filter? I read that digital sensors are not sensitive to UV so I guess not and the UV would lose its protective value in this case. How about with film: polariser alone or with UV filter? (The film question is just a curiosity and not important) -- Seán O'Leathlóbhair And lo, Seán O'Leathlóbhair <jwlawler@yahoo.com> emerged from the ether and spake thus: >> >>I just bought a couple of circular polarising filters. Both are >> >>Hoya. I wanted one to improve my landscape photos. The reason for [quoted text clipped - 107 lines] > polariser alone or with UV filter? (The film question is just a > curiosity and not important) Somewhat OT, but I heard a story about a professional stock shooter who, when asked his opinion about using UV filters as a protective measure, he said, "UV filters? I don't use LENS CAPS. It takes too much TIME to take them OFF." He shoves everything into his bag without UV filters OR lens caps, just bare elements into the bag. Considering he was carrying around a Canon 500mm L, I think he is somewhat successful. To each their own! At least he put the 500mm into the individual case it came with...  SignatureAaron http://www.fisheyegallery.com http://www.singleservingphoto.com >>Circular polarizers are not perfectly achromatic. That is, the >>quarter-wave plate is only a quarter-wave plate at one wavelength, [quoted text clipped - 13 lines] >> >>I hope this helps. <snip> > Once, I turned one polariser around, I was quite impressed by them. > One by itself is as close to neutral grey as I can tell. Two in a row > when aligned appear the same as far as I can tell. When crossed, they > are not totally black but only very bright objects are visible such as > light bulbs. These appear a deep purple so I guess that the highest > wave lengths are leaking though. Actually it is the other way around. In the visual wavelengths (blue-green through near-red), the polarizer is extremely good, effectively 100%, so crossing them gets rid of all that light. But at the shortest wavelengths the filter is less than 100% effective, so that crossing two of them will still transmit a bit of unpolarized purple light. Probably there is a bit of red light leaking through, too, as these filters often lose their effectiveness somewhere in the red. > Talking of the high wave lengths leaking through, is there any point > in stacking the polariser on top of the UV filter? I read that > digital sensors are not sensitive to UV so I guess not and the UV > would lose its protective value in this case. How about with film: > polariser alone or with UV filter? (The film question is just a > curiosity and not important) There is no need for the UV filter, both because the sensor won't see it and quite often the polaroid filter is a poor transmitter of UV light itself. If I'm lazy, I don't remove my UV filter when I use a polaroid filter, but I should. There's no point in going through an extra piece of glass with two air-glass boundaries unless you have to. The extra glass can reduce contrast unless it and the polaroid filter are very well coated. As for film, it depends on the polarizer and the camera. Many film cameras don't benefit significantly from a UV filter (the glass used for camera lenses really doesn't transmit well in the UV), though some do, and as I said, the polarizer itself can often be a good UV filter. Joe > >>Circular polarizers are not perfectly achromatic. That is, the > >>quarter-wave plate is only a quarter-wave plate at one wavelength, [quoted text clipped - 24 lines] > > Actually it is the other way around. Yes, I spotted the mistake myself just after posting. I asked Google to delete it and posted a corrected version but somehow you saw the original incorrect version. In the visual wavelengths > (blue-green through near-red), the polarizer is extremely good, > effectively 100%, so crossing them gets rid of all that light. But at > the shortest wavelengths the filter is less than 100% effective, so that > crossing two of them will still transmit a bit of unpolarized purple > light. Probably there is a bit of red light leaking through, too, as > these filters often lose their effectiveness somewhere in the red. A bit hard to tell that by visual inspection. > > Talking of the high wave lengths leaking through, is there any point > > in stacking the polariser on top of the UV filter? I read that [quoted text clipped - 16 lines] > > Joe Thanks. -- Seán O'Leathlóbhair > >>I just bought a couple of circular polarising filters. Both are > >>Hoya. I wanted one to improve my landscape photos. The reason for [quoted text clipped - 83 lines] > > I hope this helps. Yes, it does thanks though I think that I will read a bit more to revise my own old A level (*) physics. Partly for fun but also because a niece that lives with us is studying A level physics so learning this will be useful for support. My son will probably follow in a couple of years. (*) A levels are the exams that we study for between 16 and 18 and hence immediately before university. A typical good student does 4 or maybe 5 subjects at this level. I think that polarisation of light comes up, it did in my day. She was studying lenses recently: 1/u + 1/f = 1/ v etc. Once, I turned one polariser around, I was quite impressed by them. One by itself is as close to neutral grey as I can tell. Two in a row when aligned appear the same as far as I can tell. When crossed, they are not totally black but only very bright objects are visible such as light bulbs. These appear a deep purple so I guess that the highest frequencies are leaking though. Talking of the high frequencies leaking through, is there any point in stacking the polariser on top of the UV filter? I read that digital sensors are not sensitive to UV so I guess not and the UV would lose its protective value in this case. How about with film: polariser alone or with UV filter? (The film question is just a curiosity and not important) -- Seán O'Leathlóbhair The practical use of polarizing filters is empirical, depending on what you see through your SLR rather than where little marks on the ring are. The big initial surprise will be that the degree of polarization varies depending on the angle of the camera vs the position of the sun. This will be a big problem for really wide angle shots, where polariaing filters don't work so well, because of an ugly variance of darkness across the sky. You can estimate the area of the sky most prone to dramatic polariaing effects. It is 90 deg off the position of the sun. To find that spot, point your index finger at the sun, and your thumb will be pointing to the area of the sky that will be most affected by polariaing filters. Of course it is a circular line across the sky, partially obscured by the earth that gets in the way. Dave > The practical use of polarizing filters is empirical, depending on what you > see through your SLR rather than where little marks on the ring are. Yes, of course, but I like to understand the theory behind what I do. So, I expect to adjust the filter as you describe but I would like to check the orientation afterwards. Most of the shots in which I expect to use the polariser will not be hurried so I plan to bracket the angle. How sensitive is the angle? Would you recommend this bracketing and, if so, how many shots and what (approximate) angle step? > The big > initial surprise will be that the degree of polarization varies depending on [quoted text clipped - 6 lines] > that will be most affected by polariaing filters. Of course it is a circular > line across the sky, partially obscured by the earth that gets in the way. That will be a problem for me. One type of photo that I have some success with is extreme wide angle (up to 360) created by stitching multiple shots together. I have already learnt things such as: use manual exposure, focus, and raw conversion to get seamless joins. I guess that I will have to omit the polariser for these shots. Maybe I need a neutral density filter to darken the sky. That will have to wait, I need to get some good shots with the polariser to convince the wife that they were worth buying. -- Seán O'Leathlóbhair >That will be a problem for me. One type of photo that I have some >success with is extreme wide angle (up to 360) created by stitching [quoted text clipped - 4 lines] >wait, I need to get some good shots with the polariser to convince the >wife that they were worth buying. Correct, don't use a polarizer for panoramas. The band of sky (90-degrees from the sun) that becomes deep blue from a polarizing filter will be difficult to stitch together later and just look wrong. |
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