Joined: Sat May 19, 2007 1:14 pm Posts: 6183 Location: Kelso, Scotland
All of this is meaningless without reference to lens design and focal length. With many zooms and with all retrofocus wide angles, dust or damage on the front element is serious because the lens is in effect a rear prime group focusing through a negative front group and at small apertures the front element will be 'sharp' enough for marks to show in-picture. With telephotos, the front element is generally the prime lens in the design (it does the imaging) and the rear groups serve to reduce back focus distance by refocusing the image to a closer focal plane. Marks on a telephoto front element rarely show and you can take a chip out of the side of a 300mm f2.8 without noticing. Unless it happens to catch sunlight or something and throw up a problem. You can shoot with a cracked fluorite element in that kind of lens, wide open, and get a normal looking pic. You can even stick carpet tape over the middle and get a normal shot.
To confuse matters, some lenses have a sacrificial front element, especially fast apo teles with internal fluorite elements. This element either has minimal effect, or none. It's there to seal away an expensive soft element which can discolour if exposed to UV light and air (old apo fluorite lens elements go yellow). So chipping away at such an element or cleaning it with wire wool will just turn it into a kind of softener.
Fine scratches, cigarette smoke film, grease film or dust all degrade images badly. A really perfect, factory fresh, new lens will often outperform the same lens as found in a studio after a decade of use and cleaning - and checking under a magnifier, or hold the lenses up to light and looking through them, quickly shows why.
As for filters, the reverse situation applies with wide and tele lenses. A badly made UV filter will not lose sharpness on a wide-angle shot, for the same reasons you can safely shoot through glass windows with a wide-angle - especially when stopped down to f/11 or so. The distortions created by the filter are local, and are seen as local loss of sharpness. But try shooting through a non-plane-parallel UV on a 300mm f2.8 and you may not even be able to get focus. The entire ray bundle is affected by the poor filter. Same with a window. Try this - find a window with some old, wavy glass (maybe a greenhouse, I live in a house with 200-year-old glass and most of mine is very wavy but modern houses often have almost perfect window glass). Take a pic using a 28/18mm lens about a foot from a window pane at full aperture looking out. Now try the same with a 300/200mm lens. Then see the effect of moving the lens right next to the window in both cases.
Bottom line - it pays to keep all lenses as clean as possible, and never to clean them by rubbing with a cloth. Good practice means fitting a UV or protector. For short focal lengths and smaller apertures (i.e. 18-70mm kit lens) this does not need to be super-quality, but plastic/resin ones and very cheap glass in plastic rims are worth avoiding. Marumi, Izumar, Kenko, Hoya, Sigma*, Soligor, Vivitar and the German makes are all OK. I'd avoid Jessop, Tiffen, Cokin, Singh-Ray, Photax, own-brand chain store, Raynox etc. For fast lenses (85mm f1.4) or long lenses (70-300mm SSM G) the best quality you can get is essential - even more so with 300mm f2.8, 500mm f4.5 etc designs. Sony, Zeiss, B+W, Rodenstock, Heliopan, Minolta, Nikon, Hoya top line, Sigma*, Olympus, Canon, Contax, Rollei.
*Sigma - not sure - they may be good or very good. They are certainly well made and priced.
Joined: Sun May 27, 2007 3:25 pm Posts: 6136 Location: Townsville, Qld. Australia
Worthwhile info there David, which reminds me I must get a good filter for my 17-35, I only have a 77mm polarizer that fits at the moment, ( when I ordered the lens from B&H way back when I ordered the wrong size 1-B filter 72mm instead of 77mm, trusting too memory...duh), straight up 77 (and not stepped) polar might be ok for APS-C but probably will vignette on FF at the widest angle, so an even bigger one might be needed...either that or get one of those super dooper thin pro ones. BTW I have often seen for the longer high grade lenses, Telephoto and Super Telephoto's, that they mention rear filter design, this may be all well and good for getting a high grade filter for such a lens that doesn't cost the earth but I for one would still feel naked without a protective filter over the front element of some sort, if nothing else it keeps the dust off so one does not have to risk trying to clean and possibly damage that front element, and the less cleaning the better I reckon, also stops the gradual wearing away of the coatings as well....but where can one find good filters of such large size...if one wanted/needed one that is. Greg
I usually use linear polarizers but didn't have one to fit my 28-135 which is whats on my camera most and probably a better one to fit with a polarizer of the lenses I have. the only CPL I have is a cheap tiffen my other 2 are linears so I picked up a Marumi CPL instead (they had no linears) and this one seems to have a more radical effect on the pictures than even the linear ones I have. if I look through it it actually can almost completely obliterate the picture on my LCD tv across the room. the others don't. I've never sen this effect before. the other polarizers just darken the picture somewhat. if there is sky blue in the scene on the LCD it comes through as a faint purple.
Joined: Tue Nov 06, 2007 7:55 am Posts: 4870 Location: Japan
David, thanks for that excellent info about filters. I certainly learned something. I have always kept protective filters on my lenses. Kenko and Marumi bought in Tokyo. For my Sony 18-250mm and Sony 11-18mm I have Hoya Pro1 Digital filters though. I do have a 62mm Cokin CPL filter for the 18-250mm, but it is rarely used. Not the best, I guess. I also have Kenko 72mm and 55mm CPL filters, but they also rarely get used.
Useful tips there, David, thanks. One thing though. While I almost always use protective UV filters I have on occasion cleaned the front element of a lens with Zeiss cleaning cloth (the blue silky stuff). Is this not OK to do? It seems to me a lot less drastic than using one of those lens pens but maybe that is not the case.
Joined: Sat May 19, 2007 1:14 pm Posts: 6183 Location: Kelso, Scotland
Sometimes you do have to clean a lens. The safest way is to use an air blower, and then a brand new lens cloth - microfibre. They cost about £2 here in a sealed pack. Use it carefully on the pre-cleaned lens, if necessary breathing on the lens to help clear any smears with a bit of moisture. Don't use liquids unless it really essential, in which case top grade optical cleaning fluid followed by a brand new cloth is the best method. After the cloth has been used a few times, it should be kept for cleaning the viewfinder eyepiece, rear screen etc. I normally clean my lenses in a batch. If one lens needs cleaning, or I have bought one with a smear mark or filming, I will check all my other lenses to see if any could do with a freshen up. Then I start with the cleanest (not the worst!) and clean three or four, ending with anything which looks as if it might leave residue on the cloth.
I have occasionally cleaned all the gear - camera bodies etc, and filters - this way. At the end of it the cloth would not be safe to use on a new lens but it's done a lot of work. It then gets put aside for cleaning my specs, laptop keyboards and similar mucky surfaces. Finally it ends up in my workroom cleaning piano keys or guitar strings. I always try to have both a used microfibre cloth and a brand new one in my bag when travelling. Used one for obvious stuff like wiping beer off my gear
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