Where is the lowest point of your golf swing ?

I recently loose up the consistency in iron play, tend to be duffing them a lot on course. Longer clubs, not so.

Which leads me to think about where the lowest point of the full swing is. I don't get much visual feedback on the range mats looking for "whitened" patches.

Mine generally tend to be left armpits for irons and right armpits for driver.

Just after the ball for irons and before the ball for driver
For hybrids and FW, tend to depend on situation.If AOA is steep then after the ball, if AOA is shallow then just before the ball.

For range practice, you can draw a line with chalk before the ball or use masking/ electrician tape
It gives you visual feedback if you are hitting before the ball

Duffing may not have to be something wrong with impact position. Sometimes it could be the clubhead is too upright at impact cause you to catch the turf. If you address the club and it is upright, without altering the clubpath in your downswing you would most likely duff with the heel of your club. 
I have been working on ballstriking and I am glad to say i have managed to make my MB sounds like a GI at impact, the loud and crisp sound.
Without seeing your swing, I would suggest working with a kind of shot that get you the best sound you get at impact. For me, i start working on 3/4 swing which i dont have to torque up my body due to some flexibility problems but i have to achieve a certain criteria like a straight, "soft" left arm and etc.

For me, the low point is at the center of my left chest for all clubs. Note that the low point does shift if one changes the tilt in his/her spine, i.e. tilting towards the right to get higher launch. I try to maintain the tilt for all clubs and just play with ball position for different clubs.

If you are duffing your short irons, I think you have too much tilt towards the right which brings your low point behind the ball. Best is to consult a coach or go to a grass range to find out the characteristics of your divots.

Looking back, i think it should be when i started to golf again.
Every time when i range or game, I try to improve on my swing and aim to be more constant.

Many a time duffing is not due to ball position but maybe due to the fact that one changes his/her spine angle at impact ... Just look at the legendary Jack Nicklaus's ball position ... He plays his ball positions at the left side for all clubs ...

Iowowa, as some have rightly suggested, there are many reasons for duffing. Having the head positioned too far behind the ball, or hanging back on the downswing and not transfering your weight to the left are just a couple of other reasons for duffing. Flipping the hands through impact is another.

With regards to the low point of the swing, there is only one, approximately where the left breast pocket is. If the low point for driver is off the right armpit, it would be physically impossible to launch the ball without some kind of massive compensatory move in the golf swing.

Do see your coach or seek out one to diagnose the actual reason why you are duffing, and you'll soon be on your way to more enjoyable golf.

 Beside what Birdman has mentioned above, the correct swing plane is vital too to minimize fat or thin short ... Here is a video on Sean Foley's tips may be helpful to prevent fat or thin short ... Enjoy!

its hard to contribute something if there's no video showing how you swing.
But one tip you can do is this...

On the fairway, try to "practice" hit the ground, but do not Dig too much.
Look where the irons or woods hit the floor.
Those "Ground hits" is your lowest point for that shot.
So adjust your stance and smoothly swing the ball with a nice follow through.

Try reading this article by Paul Wilson
http://thesandtrap.com/t/42960/swing-machine-golf-by-paul-wilson

when the weight did not shift to towards the target may cause a duff as well

weesern wrote:

when the weight did not shift to towards the target may cause a duff as well

Absolutely ... If one fails to do a proper weight shift, not only one will duff a shot but a host of problems would occur too ... One big problem is lack of distance ...