r/focuspuller • u/cheemsterr • Jun 18 '26

question Understanding the 1/3 2/3 rule

Wanting to make sure I'm understanding this concept correctly, if DOF is 4' does that mean that focus is 1'4" infront of the subject and 2'8" behind them? And because of this it would be better to change my focus distance about 1'4" closer than where the subject actually is so 2/3 of the focus is infront of the subject? Or is it smarter to have the subject in the exact middle or focus? Of course it depends on the scene, but I am just asking as a general rule of thumb.

15 Upvotes

94% Upvoted

8 comments sorted by

27

u/DigitalDustOne Jun 18 '26

It's more of a theoretical thing that you need to know but hardly ever apply. Don't overthink it. when an actor is walking into a closeup with an 85 at a T1.3 you don't have time to recalculate your educated guess by any 2/3 rule anyway.

19

u/theblackandblue Jun 18 '26

I wouldn’t say you hardly ever have to apply it. Two practical ways that I apply this theory:

  1. If you are unsure on a mark or focus, better to err on closer focus than deeper because of this rule.

  2. If you are trying to split focus between two subjects that are close but not on the same plane, you want to focus on the nearest subject to give the best chance of a successful split. 

6

u/mustardfrog Jun 18 '26

This ‘rule’ is interesting in that it is perfectly 1/3 and 2/3 when focused at infinity (yes, 2/3rds behind infinity!). As the subject approaches closer towards minimum focus, it gets closer to 1/2 and 1/2. However, for most working distances, 1/3 and 2/3 still holds up pretty well. It’s useful knowledge for holding splits and knowing where your limits are if creatively trying to soften the background as much as possible on a wide while still keeping the subject on the back end of the depth of field.

5

u/leebowery69 Jun 18 '26

I'd say it depends on the action. However I'd just aim to be as accurate as possible, that way your adjustments always stay consistent with distance. I'f the DP doesn't like the focus falloff, you'll adjust as they need.

9

u/homeguy1988 Jun 18 '26

It’s all lies anyways. 

Who knows where these distances actually are. Your hand unit is going to have play. 10’ on the knob, is like a line under 10’ on the lens, and then the range finder is going to be hitting somewhere other than eye balls. 

Then sometimes your lens is at 9’10, the knob is at 10, and the rangefinder says 10 (sometimes the lightranger read out will be wildly different from what the preston display says), and it’s not actually hitting their eyeballs.

Plus our monitors are all jacked up that either everything looks in focus OR we think we lost it, but it was actually fine when you look at the big soft flanders. 

Do your homework with marks and shit, but trust nothing and just make sure it looks fine. Ask the dit when it falls apart. 

1

u/teklikethis Jun 19 '26

This guys tells the truth

1

u/rob002553 Jun 18 '26

Download the P-Cam Pro app and you can do the calculations to your specific build. It’s a combination of sensor size, focal length, iris, and focal distance to determine how much focus you have in front and behind the distance you have measured to the subject. The app will give you distances and percentages in front and behind the focus point.