This was interesting, and it seemed to be true in my experience with the F70.
The T2 and the F75, when used in discrimination mode, usually provide better electrical interference rejection at low discrimination settings, than at high discrimination settings. This contrary to the pattern of most discriminators.
Also Treasurebone stated that for him 2F on the F70 is a little less prone to EMI and I also believe this is true.
As far as pop tops...they are my arch enemy.
I hate them, so many of my sites are awash in a huge number of these things at some of my older parks and many of those sites have a huge amount of good high tone targets I want to dig so a blanket of these things at different depths really slows me down.
Using the standard elliptical coil that came with my F70 they are fairly easy to spot, the numbers aren't usually coin numbers at all.
When I switch to the F75 DD coil that changes and the numbers come way closer if not right on dime and quarter numbers a lot...even into some pretty solid 90's numbers from time to time.
I have divided these up into a few categories because this is the different kinds of behavior I have seen and this is how I deal with each of them.
Always looking for a fast and efficient way to recognize them and avoid digging them with no "what if" feelings in the back of my mind and these techniques seems to be working for me pretty well so far.
These things are made of different materials depending on age and manufacturer, and exactly how they are laying in the ground seems to affect the way our detectors see them and behave.
1. The jumpy ones...These might come in at all numbers from the 60's to the 90's and even mimic coin number areas but no matter how I move the coil over them they jump in the numbers more than 2 or 3 and even sections a lot of the time.
Raising the coil over them will usually show a dramatic number drop as mentioned in the post above, and "rimming" the target running the edge of the coil over them, any edge but I use the front, you will see a big drop in the numbers and sometimes even down to iron just about every time.
Using 2F or any other tone setting and low disc where you can hear that iron grunt helps to speed things up in this situation, but even in 3-4H tones and higher disc you can notice a big number drop on the screen and a tone change.
2. Stable ones...Sometimes the numbers are pretty right on where coins fall, the numbers don't hardly jump at all which is the case with most coins.
I don't do that coil raising thing much in any of these situations because that rimming thing usually is faster for me.
On these types a few quick swipes with the rim of the coil and a number drop and I just move on confident I have not left a good target in the ground.
3...The ones that fool you...Some of the older steel ones that are rusty and flattened and especially laying flat in the ground will just come in like a coin at good numbers with no change in the numbers, sections or tones by raising the coil or rimming.
Quick side to side swipes they might also stay very solid and stable and in this case you just have to dig them and see what they are hoping for a good target.
Luckily, these are the rarest type I come across...they are out there but not in the huge volume of the other types.
A few of the modern crimped aluminum ones could act this way especially if they are flattened, but most times they fall into one of those first two categories.
There might be other clues to differentiate the more difficult ones and tell me they are trash and not good targets, but if there is a technique for doing that I don't know it yet...so I dig them.