No one can say for certain what the effect of the scratch will be. As mentioned, it appears that scratches on the front element have less effect on overall photo quality than scratches on the rear element. However, the scratch cannot be said to have 'no effect'. Of course it has an effect. The question is how detectable it is, and how much it bothers you.
I suspect that you won't see any difference, even if you had two otherwise identical lenses to test, in a print. If you scan your negatives with a reasonably high definition scanner, such as the SD IV on up, you MAY be able to detect the effects at 1:1 viewing after a high-resolution scan. However, I agree with the others that I tend to doubt you'll see it.
In my experience, problems of this sort tend to manifest themselves as an overall slight lowering of contrast, or very minor flare problems - of the sort you'd otherwise expect if you didn't use a lens hood, or had a dirty / scratched filter on the lens, etc. And again, I doubt you'll be able to actually detect it - but that is not the same as saying it is not there.
There is a fellow on PNET who posted some scans of photos he took with a lens that looked like it had been dug out of a cesspool and then kicked about by footballers for a year or so. The photos were fine, quality more than acceptable for web-posting. How much difference there would be between that lens and the same lens in undamaged condition, I could not say. It is possible that he'd never be able to do a huge enlargement with the 'bad' lens, whereas he could with an undamaged one - but as someone else here said, are you likely to be doing that?
And of course, one must consider the effect of owning such a lens on the owner. I'm not joking. I know some folks who are inveterate glass-peepers. They have their bright lights and their high-powered loupes, and they peer intently into their Nikon or Leica (just picking, sorry folks, but it does seem to be mostly you guys) glass and if they see ONE SPECK of dust, it's off to DAG or the equivalent. If they bought it off eBoy, God help the seller.
I'm not putting those people down. If that's you, then well and good - but you might want to think about how owning a lens flawed in this way is going to bother you sleeping at night, even presuming that you're satisfied that you can't see the difference in your prints or scans. Everyone has their lower limit of acceptability, and that's fine - one should just know where that is and purchase accordingly.
Every lens has flaws. Some flaws are so common to some lenses, that they even become celebrated as signatures of the lens' character. And this scratch will affect the image in some way, whether you can see it or not. I think the advice you've received is excellent - take some photos in the way you normally would, and have them developed and printed or scanned as you normally would. Look them over carefully and see how you feel. If you see no flaws, then be happy, and if it bothers you, then sell it!
I do wish we'd stop saying things like 'lens has swirl marks' or 'minor cleaning marks' and then assure people that 'won't affect the photos.' Of course it will affect the photos. The only questions are how much and how bothersome will the effects be?
Best Regards,
Bill Mattocks
