View Full Version : Depth of Focus
I'm not 100% sure if the back focus is dead on in our camera and I have a shoot tomorrow.
I'm a bit rusty on Depth of Focus so here is my question:
When you pull focus, are you effectively moving the Depth of Focus back and forth?
What I am asking is can you compensate for the port being out by adjusting the focus by eye (I know it's not the best way to set focus), or is it impossible to get a sharp image if the back focus is out.
I'm thinking (and hoping) it's the former.
Thanks in advance.
Finner
01-30-2008, 11:05 AM
No
If your back focus is off you are up shit creek.
chuck colburn
01-30-2008, 11:06 AM
You might be able to eye focus a prime (depending on the FFD error) but you will not be able to hold focus if you are using a zoom lens as a zoom lens.
Cooke S4 32mm. I don't need to "pull" focus as we are on a mo-con rig and we can stop and set focus points at will.
Mike Prevette
01-30-2008, 11:19 AM
what Chuck said.
Evin Grant
01-30-2008, 11:20 AM
If it is slightly out it should be fine. Any more and it might affect the sharpness because of the floating element design.
chuck colburn
01-30-2008, 11:21 AM
Just out of interest.... why don't you just adjust the FFD (flange focale depth) of the camera? I thought the new adjuster made this easy.
Thanks for the help.
Chuck,
Basically because I'm not sure if it is really out and if I might make things worse. I haven't adjusted back focus on a RED before. I know how to shim an Arri, but I had tools for that. You know, the thing that looks like a lens but has a clock like scale in the front. I forget what it is called.
I think I can get some time on set tomorrow, but all I have is a 17 HD monitor and an allen key. I guess I could get the sparks to put together a large make shift siemens star.
chuck colburn
01-30-2008, 11:38 AM
http://en.wikipedia.org/wiki/Image:Siemens_star.svg
Print up five of these and put one in the center and one in each corner. If you footage scale on the lens reads on at 3', 5', and 8' chances are real good that your camera is on (short of there being something wrong with the lens itself)
I take it that you don't have an EVF.
If only I had a printer.
Well, I managed to get in through RDP and send 5 copies to the printer and get the production manager to pick them up tomorrow morning.
We have a second focus puller joining tomorrow which is great. It's always nice to have a second opinion (and someone to share the blame).
chuck colburn
01-30-2008, 12:20 PM
As long as the shot will allow it and you can pry "Sparky" away from the craft services table or off the makeup girl, remember small stops are your friend.
I have a friend in the lighting department and we have 3 hours to set up the mo con rig :)
As much as I have screamed about doing tests before we shoot I am looking forward to this one. Such a controlled environment and so much time to shoot very little. We're going to do every pass once 3200 and once at 5600.
I wanted to test if we could remote start the cam through the RS232, but I know so little about that and if it is even enabled that I don't think we'll try.
I was out. It was 0.9M when the camera was 1 metre away. We found time to set up 5 stars and do a quick back focus. It took me a while to realise you have to twist the port back and forth to set it. Fairly easy, but we didn't feel it was completely true across the whole of the lens.
Rich Schaefer
02-01-2008, 07:26 AM
Irri, use a tape measure, and measure 3', 6', 12', 50' & infinity if they are sharp then you are good to go! You can adjust your camera's back focus with that lens but let's hope the lens is spot on! (decent chance with an S4).
Good Luck!
Rich
Thanks Rich
Our second camera arrived today so the first is going back to RED (for another reason). I've asked them to check the focus.
I tested the new one with a Zeiss which has recently been checked over by Arri and it seems fine. I'd still prefer a more accurate way of setting it though. Something you could lock into the port which has a digital read out of the distance.
Finner
02-01-2008, 09:56 AM
Irri
The back focus is fairly easy to see if it is out. Grab your widest lens(probably 18mm) throw it on your camera and check your measurements with that. You will be able to see quite quickly if it is out. I pulled off an old red PL mount in the middle of a shoot because it was out and had to add graphic tape to the shims to be able to set the collimation right and was able to do that and get it close in about half an hour. I would suggest before you send the other red back you throw a wide lens on it, open up the adjustable PL system and give a go at trying to set the collimation yourself. You will not risk anything and it will give you a good idea of how hard or easy it is to set.
cheers
Daren
I did set it. The camera is going back for a different reason.
I am sorry for being indiscrete, for what reason does the cam goes back to RED home ?
The sound board seems to be damaged. RED were very nice about it.