I scratched the back of a small micro 4/3 lens (14-42mm) It was some kind of goop about 1/4". You couldn't see anything on the picture if it was a complex subject. But on a blue sky, it showed as a darkened area about 1/4". I sent it back to Olympus and they replaced the back lens element free of charged including shipping. That was really nice of them.
While it all makes sense in theory I think the scratch on the rear is better and have shot lenses with such scratches with seemingly no issues. The front scratches make the lens more prone to flare in my experience.
The thing to do is to fill the scratches with black paint or marker. If they are too shallow to take paint they will probably not matter, unless there are many, which will cause problems. I know that at least some camera repairers will fill scratches very inexpensively.
I do use a digital device in my photographic pursuits when necessary.
When someone rags on me for using film, I use a middle digit, upraised.
I think one or two scratches (big or small) won't normally make any difference to the image. Its when you get the build up of years worth of cleaning marks that the flare goes up and the contrast goes down, and even then the image will still be sharp. Some amazing bargains can be had on Ebay if you don't mind the odd cosmetic mark on a front element.
My Nikkor 105/2.5 AI has a big scratch/gouge on the rear element. I bought it a suitably discounted price on approval, but after a very careful image comparison with the 80-200/2.8 I had at the time (itself an excellent lens) I was unable to spot the slightest imperfection from the 105mm even when using a digital body and pixel peeping the images side by side at 100%.
This is a very interesting post, shot down what I have always believed on lens glass. What about internal imperfections? I have a brand new looking Yashica 50mm M42 that has what looks liked a dried water spot, I was going to disassemble, probably destroy, just to try my hand at lens repair. Now I think I need to run some film with the lens.
“In the end, it's not going to matter how many breaths you took,
but how many moments took your breath away.”
― Shing Xiong
The effects of any scratch can be reduced or eliminated by filling it with black India ink. I found a 9 1/2" Dagor in a good Compound shutter at a camera show, the vendor wanted $50 for it - I traded a 35/2.8 pre Ai Nikkor for it. The Dagor, ecxept for a 3/8" gouge just off the center of the rear glass, was flawless. I filled the scratch with ink, the lens' performance cannot be distinguished from another 9 1/2" Dagor in barrel which I have.
No scratches at all is obviously the ideal situation, but don't let one mark scare you off of an otherwise desireable lens. The Dagor was just too good to pass up - my second $50 Dagor.
If the scratch is on the front, everyone will see it and think you are a bad photographer for using a scratched lens.
However, if the scratch is on the rear, no one will see it and your status will be unaffected.
Therefore it is far better to have scratches on the rear of the lens.
Also, if the scratch is on the rear I won't see it and be annoyed.