Follow Focus SD - HD - 2K - 4K

[Jimmy Jib]

Just occurred to me that focus might get incrementally trickier as you increase resolution. Could this be true? Focus never seemed to be an issue with SD, then HD came along and Focus became an issue. Focus assist, follow focus, peaking tools and all sorts of things came into the market to help attain precise focus. As resolution is increased, all the way up to 4K and beyond... does pulling focus become much harder?

[Landon D. Parks]

Focus becomes 'trickier' in several different cases.

 

But yes, while resolution is not really a factor in keeping things in focus, a high-resolution image that is out of focus is much more noticeable than a lower resolution image that is out of focus. The size of the screen showing the image counts too... A high res image on a huge cinema screen will show more focus issues than will an SD source shown on a TV or monitor.

[Chris Steel]

With celluloid the circle of confusion (one way of determining DoF) is effected mostly by the size of stock (8mm, 16mm, 35mm) and the size of projection (TV or Cinema). As the Film stock gets smaller, you need increasingly small circles of confusion to maintain equivalent DoF. As you project larger you also need smaller CoC to maintain equivalent DoF.

 

With digital, the size of stock gets a touch more complicated. The size of stock is broken down into sensor size and resolution. Having a CoC smaller than a pixel wont make a difference at projection but if your pixels are smaller then you'll need ever diminishing Circles of Confusion to keep it at a sub pixel size. Sticking with Super 35, a standard CoC for film projected on a cinema screen is between 0.0007" and 0.0005". An Alexa in 16:9 mode has a recommended CoC of 0.00032" , about half what most would consider acceptable on film.

 

Also, the difference from high resolution TVs and cinema Projection is relatively close (Compared with SDTV and Film projection). I'd use the same CoC for both TV and Projection as a touch of softness on a big screen is acceptable and that same image is plenty sharp on a TV. If I knew it was web only, I'd go for a more forgiving CoC like 0.001" as compression and phone screens get rid of any "Bonus Sharpness" from smaller CoC.

Let me know if i've gotten anything wrong or missed out something important, written in a bit of a rush.

[AJ Young]

Focus at 2K and beyond is literally splitting hairs. It really depends on screen size and viewer distance.

 

Steve Yedlin does a great break down of resolution: http://yedlin.net/ResDemo/

 

And he has a chart for screen size: http://www.yedlin.net/ViewingAngles_v004.pdf

 

In general, the bigger the screen, the more prevalent the buzzing of focus. The smaller the screen, the less noticeable buzzed focus will be. Case in point: Dunkirk and Intersellar in IMAX 70mm vs. 2K.