I found this on another forum:
Dirty Lens Article
Very interesting indeed.
Front of- yeah!
Try specs of dust on the rear element
Hmmmmmmmm.......might just go out the back and hit the front element of my 300 f2.8 and see if it's true ....back in a mo..........................
Nah seriously, it does take a lot of muck on the glass to have any effect, I even have a large scratch on the back element of my Tammy 28-75 with no adverse effects which did surprise me.
IN GENERAL you really only start to notice the affects of scratches and dust onelements if two conditions exist.... shooing into a source(haze or loss of contrast) and stopping down to apertures like f/11 in some cases and smaller.. I've generally found f/16-f/22 to be apertures to avoid if your has dust on it.
usually, bad technique, misfocusing, bad , lack of mirror lockup(if the bad is still being used), placing one's fingers in front of the element (such as when handholding Cokin GND's ) .. all contribute to more loss of IQ, than a few scratches and/or some dust and haze will.
This is all well and good, but if ahas been treated carelessly and allowed to get scratched and marked, then there may be inside damage invible to the eye. So on the buying front, I'd go for the well cared for every time.
I've often noticed an incredible amoount of muck on mine while out shotting in windy /dusty conditions, but as you say, it does not seem to effect the photos.
“Can't keep my eyes from the circling sky”
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The truely is interesting. I know I'm not a clean nazi when I'm out and about. I don't set out to get my gear dirty, but if I get a collection of dust or rain mist on the front element I typically leave it until I can get home and clean it methodically and properly.
On a side note that has terrible vignetting.
Using Nikon & PS CS5.
I found this quite interesting: http://www.lensrentals.com/news/2008...ment-scratches
scratches and dust won't directly manifest itself on the photograph, but they will indirectly through increased flare and reduced contrast
This also hold true for filters btw.
The fine scratches they take don't show up in any photo - I know, I've pixel peeped to try and see them. For the record, my come out of the box and get a hoya UV on them before I even put them on the camera, so if they were going to be scratches anywhere they'd be on the filter.
Also consider that a reflex has a huge centre area that it can't see through and yet there's no black circle in the middle.
Only cause I am proud of it, my gear;
Sony A700 | Sony DT 18-70 f/3.5-5.6 | Sigma 24-70 f/2.8 EX DG HSM | Sigma 70-200 f/2.8 II EX DG HSM | Lensbaby Composer | Lensbaby Optic Kit | HVL-F56AM | VG-C70AM | Slik Pro 713CF Tripod | Slik Pro 381CF Monopod
Please feel free to rework my photos, but give me step-by-step instructions on what you did. Thanks