View Full Version : Focus technique for lenses working with 4K format.
04-07-2007, 06:43 AM
I can’t figure out what to use to make sharp focusing easier when shooting in 4K.
Forgive me for this question, but is the stepping up the aperture, the only way to gain depth of focus? I am pretty sure good focus puller knows more then one trick.
Also I can’t get around this plain of focus thing. Projectors lenses are corrected so plain of focus is almost flat and if not then the screen have to be bit curved, correct?
Wide angle lenses in particular have spherical plain of focus around the lenses. Are there wide angle and not so wide angle lenses that do have flat plain of focus?
I know RED will have some focus assist, but even the good quality lenses are so touchy to focus ring movement that I can’t imagine how I will work with it. Nose will be in focus and ears will be out of focus?
04-07-2007, 07:18 AM
I know RED will have some focus assist, but even the good quality lenses are so touchy to focus ring movement that I canít imagine how I will work with it. Nose will be in focus and ears will be out of focus?
What kind of lenses are you talking about? Using still camera lenses or cinema lenses? If you referring to still camera, then you've found out why they are not designed for cinema use. The ring movement is so small that you can't smoothly focus when tracking movement, etc. That's why when you use a Cooke or even a broadcast lens, you will find that it takes a longer roll of the focus ring to focus the same distance. Frankly, I don't know how people are going to use still lenses on the RED and get a good product. It's hard enough to get and keep a good focus in HD when you are shooting ENG style (especially sports) with a broadcast lens.
It sounds like your other issue is too shallow a depth of field. Starting around 40MM is where I found, I have to start worrying about keeping my subject in focus. However, you have to have a fairly long lens wide-open to have nose to ear depth of field blur (At 70MM close to my subject I can get that.) That is one of two reasons most people try to shoot at 5.6f--one being the sweet f stop for most lenses, and two being shallow DOF without being too shallow.
I hope that makes sense.
04-07-2007, 07:31 AM
I canít imagine how I will work with it. Nose will be in focus and ears will be out of focus?
Only the eyes need to be sharp. Working fairly wide you may only get one eye sharp!
04-07-2007, 07:33 AM
That is one of two reasons most people try to shoot at 5.6f--one being the sweet f stop for most lenses, and two being shallow DOF without being too shallow.
Very unusual to have the budget to light to T5.6. Most naritive is shot a T2.8 or wider.
04-07-2007, 09:35 AM
Yet I feel Red's usable ISO may be 1- 1 1/2 stops faster then its rated 320-380 iso under "certain conditions". Emergencys and budgets may dictate iso as its virtually noiseless at these levels so long as there is some light in the deep blacks (zone 1-2.5) (im guessing here)...
LOL so much for my Genny rental....
04-07-2007, 03:00 PM
to gain depth of focus? I
I think your answers have been addressed to a degree in this thread. May I respectfully suggest you check out the difference between Depth of Field and Depth of Focus.
Its a useful to have an understanding of this and how the two can relate.
04-08-2007, 08:31 AM
Guys, thanks a lot!
Yes, I am talking the cine lenses here.
Mechanical focus control should not be an issue here, we can always pick the higher gear ratio to get a wider stroke.
Maththew, I didn’t know that ~40mm is the good attention point for DOF
4K format gives us capability to capture more details and the easiest way to do it is to go wide angle, especially in 2.35:1 screen format
Turnover, good reading, thanks for pointing it out.
Now if someone has experience with the plain of focus.
Is it spherical or flat?
Since 4K format lends itself for high resolution shots more then film, we will have natural tendency to pack more details in to the scene.
More details we want to show, more crucial the focus is, in all meanings of this word.
Imagine two situations.
One we want to show action in front of richly sculptured historic building, 200 yards/meters wide.
Second, we are inside of church or castle, with great decorative elements and the action is at the middle of the chambers.
Since we are shooting in 4K, why not to use it to show the beauty in the details.
In the first example, since we are outside, I will pick the bright sunny day and step up the aperture to 8 or 11 and DOF should be good enough to show the action and the building behind, even if it is 200 meters wide.
Will T8 or T11 bring the chromatic aberration to the unacceptable level?
Even the best lenses have chromatic aberrations, not suitable for 4K
See my recent lenses test.
Also, what lenses will keep the plain of focus flat?
Are there special lenses for this?
In the second example we are stuck, action is 10’ from the camera and chambers are 100X400 and on top of it the ornaments and the ceiling are so beautiful, just perfect to use 4K resolving power to show it.
Should we use MOCO and have two shots combined, far and close?
Is there any other way to do it without spending $$$$ to light the scene?