Front/back focus; lens v.s. camera

I didn't want to hijack other similar threads yet hesitant to start another, but I'm curious as to how the lens can cause front or back focusing?

I have adjusted my camera in debug mode and it seems to have solved most of my issues. Now I'm looking to get the 55-300 lens but read about some of them possibly having focusing issues.

My confusion comes in how the lens might play a role. As I understand things, the camera is looking for edge contrast to determine where to focus. It makes sense that if the camera is off a bit that the area you're focusing at will not necessarily match precisely with the camera.

But if the lens is off, won't the camera still try its best to attain sharp focus on the specified area? The camera may have to go further forward or backward than with another otherwise identical lens, but still the same spot has only one point at which the edge contrast will be "correct".

Does my question make sense?
What am I missing?

Since the K20D has several auto focus adjustment slots, I would assume there is actually a need but I cannot think of how the lens would make much difference.

I can assure you the lens does make a difference. When I tested my lenses on my K10D, I had some front focussing, some back focussing, and only one needed no adjustment.

Actually, that one lens does need adjustment, but it needs two. My DA* 50-135mm lens BF's at 50mm, and FF's at 135mm. Go figure . . . 'cause I can't.

FWIW: I've had to make a +40um adjustment to get acceptable results from all of my lenses.

I wish I could do the same for my *istD but it's debug mode lacks these adjustments.

Oh, I trust ya. There has been enough talk about lens being an issue.
But how?
I just see the camera adjusting the lens to focus until a single point becomes sharp regardless of how long it takes for the lens to shift and settle into a good spot where the edge contrast is acceptable.

There is obviously something I'm missing?
Probably more hypothetical than anything else, but I just have trouble sleeping at night not knowing these important details.

Me too.

Given that even the virtually omniscient Matt doesn't know for sure, it's a worry.

Same thing puzzles me Amoringello. And your reasoning is the same as mine. So, if anyone can elucidate, it would be most appreciated.

Dan

I've come across this phenomenon before with a Canon 28-105mm lens - it backfocused very slightly at the wide end, but front focused considerably at 105mm.

Jonathan

EDIT: Typo

Hey lads, lots of big words being used on this thread topic, got to go now and find the dictionary

And George hasn't even posted yet But i'm sure you won't find many of his words an any dictionary

Amoringello: What sort of price are you looking at for the 55-300 over there.:

Im looking at getting one but they are quite scarce over here and expensive AU$600.
I was looking to the US for one.

The 55-300 is running about $400 here. Not a bad price as the lens seems to be getting decent reviews for the most part.

Just had a look at B&H and they have it for US$359 + $41 postage. But they don't accept pay-pal from Aussies : but they will accept mastercard if i send them a pic of my card front and back Not sure i would want to do that though.

You have to give them all the same details though, don't you?

Or is it one of these customisable cards with a particularly unattractive picture of you on it?

I wasn't the winner of the Tom Cruise look-a-like competition for nothing you know

I guess your right Chris but it still concerns me that they will have a pic of my signature on my mastercard in there possession for as long as they want to keep it. A little disconcerting thats all.

That's a good point.

I imagine they want proof that you physically have the card - see if they'd accept pictures where the signature is partially obscured.

I expect they'll look at the quality of the photo and decide if you deserve to own this sought after high quality glass.

Good idea i might just obscure it a little anyway. Whats the worst that can happen, they refuse it.

Mikew. If thats the case i should get it for nothing