Unusual question

Chris Johnston

Chris Johnston

Veteran
This will seem like a strange one but , because I frequently do 2 and 3 cam shoots by myself so I just thought I'd ask.

Say that I'm shooting with a FS100 with a 50mm nikon set at f8. I do enhanced zoom to set the focus where they're currently standing. Hit record. Now I run over to the other camera and stay with that one because it's mostly close ups of stuff in there hands or stuff on a table that they are moving around, etc. My question is , how much can they move around (for and aft) before they start to move into an "out of focus" range on the first cam? Is there a chart/excel sheet etc that shows something like...

50mm (correctly focused) at f5.6 = 2 feet back and 1ft 10 inches forward? (just guessing at these numbers...)
50mm (correctly focused) at f8 = 3 feet back and 2ft 8 inches forward?
24mm (correctly focused) at f5.6 = 1.5 feet back and 1ft 2 inches forward?
etc etc etc for different lens at different f stops.

The reason is I'm scared of getting something out of focus while I'm NOT at the camera so I've been setting the f stop up higher (f11/f13 or so) and bumping the gain up so I can get good exposure but still making sure they stay in focus if they move.

Definately not the best way to do it but.....
Granted an 85mm at 1.4 is going to be slim but I'm mostly talking about two shots. (or medium shots..)
Hope I explained that correctly....
 
Chris Johnston said:
This will seem like a strange one but , because I frequently do 2 and 3 cam shoots by myself so I just thought I'd ask.

...
Click to expand...

Your living on the edge my friend ...whenever I do 2 camera shoots I need real eyes on both cameras. An assitant would give the best results but you say you do it often so my hats of to you :)
 
Everts said:
Your living on the edge my friend
Click to expand...
Not as dangerous as it seems.
For live events,
1 cam steady on the drummer (he's not moving, he's sitting)
1 cam steady wide on the stage (get the whole band)
1 cam I run for guitar / singer close ups. (and occasionally a 4th cam from behind the band so you can see the band interact with the crowd ,also static)

For the 2 cam TV stuff
1 cam on a wide 2 shot
1 cam I work for the close ups and small things in hands/on tables etc
for the TV stuff the second cam is close so I can easily do small pans if they move side to side.

I'd love to have an asst, but then somebody would have to pay them, right? (yes, I'm a cheap skate!)

PS, Thanks RX8.......exactly what I was looking for.
 
I put DOFMaster on my iPhone so I can use it in the field. Really nice to have. There are some other even more powerful apps for this as well, for both iOS and Android if you have a smartphone.
 
It might be helpful to know that while depth of field is solid math and the depth of field
tools are good stuff, figuring out the circle of confusion or the pixel size number is slightly subjective.

You might want to test that the circle of confusion number is giving you trustable results on the bench
before you go out into the world. One way to do this is to setup the camera with a high resolution monitor
and look at a flat test target (book cover, whatever) that is say 3m away from the lens on the dot.

Open the iris up to f/2.0 or however fast you can manage and make sure the test target is dead in focus.
Stop down to f/5.6 or something in the middle. Now use one of the apps or excel spreadsheets to generate
a depth of field prediction for 3 meter focus and f/5.6. With a second test target or book cover, verify that
things go out of focus where they should in front of the true focus and behind the true focus.

If you discover that the depth of field app/speadsheet is too optimistic (things go out of focus before the
app/spreadsheet predicts) you need to lower the circle of confusion number of bit. Try 10% smaller or
something like that. If the depth of field app is too pessimistic (things are still in focus when the app/spreadsheet
says they shouldn't), raise the circle of confusion number.

I know this sounds like a p.i.t.a. but you only have to do it once per type of camera (e.g. FS100) and per
video format (1080i/1080p/720p/whatever). You do not have to do it for each lens you own.
Also, if you do this experiment, tell us about it so we don't have to do it ourselves.
 
"P-Cam" is a great iOS app. Swiss army knife for DP's. Calculate DOF, FOV, data rates, storage requirements, etc.

..and while we're on the subject, "Helios" is another very useful app. It calculates the sun's position based on your location (GPS). so you can wave your phone in the air and see a virtual sun path superimposed on the screen, complete with time of day for each position. So you can accurately predict what time you need to start shooting that epic sunset shot and know exactly where the sun will be.
 
Dustin Moore said:
It might be helpful to know that while depth of field is solid math and the depth of field
tools are good stuff, figuring out the circle of confusion or the pixel size number is slightly subjective.

If you discover that the depth of field app/speadsheet is too optimistic (things go out of focus before the
app/spreadsheet predicts) you need to lower the circle of confusion number of bit. Try 10% smaller or
something like that. If the depth of field app is too pessimistic (things are still in focus when the app/spreadsheet
says they shouldn't), raise the circle of confusion number.



I know this sounds like a p.i.t.a. but you only have to do it once per type of camera (e.g. FS100) and per
video format (1080i/1080p/720p/whatever). You do not have to do it for each lens you own.
Also, if you do this experiment, tell us about it so we don't have to do it ourselves.
Click to expand...

I think I will do this when I get a chance and try to narrow it down. I set the DOFMASTER/m web page as a bookmark on my android and, as Rick said, VERY VERY usefull. (takes all the "eyeballin/guesswork" out of it) Since I shoot 1080P almost all the time and the web page is not really really specific (for android anyway) about format, (and coc size) I know it's still a little bit off, but still good enough for what I shoot most of the time. (I rarely find myself with a tape measure trying to figure out an 85mm F1.4 at 5 feet scenario) but making sure the focus is OK on clients logo on the wall 6 feet behind the 2 shot is important.

PS been using 0.02 mm as coc size in the calculations even though I think it's a little bigger? (or is 1/1250 more accurate?)
 
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