I stand up the back of a room on a riser and capture a speaker at podium + their presentation on the screen using a blackmagic production 4K. I estimate I am 8 metres from the subject.
Currently I'm using an f2.8 lens and adjusting it in davinci as the room is poorly lit.
Is there a way to gauge the depth of field that will be in focus if I use an f1.8 lens?
Thanks in advance
You will need an app or chart for the calculation. There are apps on both iOS and Android for this. Just search depth of field calculator.
With a 50mm lens shooting at f/1.8 you're depth of field will be very shallow; more so the closer your subject is to the camera, with your DOF becoming deeper if your subject moves away.
Some lenses have a built in Hyper-Focal chart on the top of the lens; showing what your "Reasonably considered in Focus area is at" which changes as you change your aperture.
If you need a less shallow depth of field, you can always stop down the lens. Also keep in mind; the 50mm 1.8 is not incredibly sharp at 1.8 to begin with. Most lenses have sweet spots in terms of tack sharp; typically about 1/3 into the f-stop range.
The only lenses that will give you pristine tack sharp focus at apertures like 1.8, are for example the Master Primes by Arri; running $30k per prime.
Hope this helps.
Going to f/1.8 will brighten up everything, the Screen and the Presenter, and reduce your DOF. With a less expensive Lens your going to be stopping down to get a sharper image; wasting time and money, and not accomplishing what you want.
What you want (I'm guessing) is for the Presenter to be better lit, presumably you can see the Screen (otherwise you have a lot of problems).
Trying to light the Presenter in that setting either would physically block the Screen or give odd side-lighting making them spooky looking (unless you wanted to try a ceiling mounted spotlight).
What you want to do is get a Square Filter Holder and a Hard Edge Graduated Neutral Density 0.6 Filter. Just slide the Filter to darken the Screen leaving the Presenter unaffected, then crank the ISO a touch to brighten the Screen back to normal.
Now the Screen is correctly exposed and the Presenter isn't hidden in the dark, everything is evened out.
This costs less than a new Lens (unless you are planning on buying the best possible Filters, that you may not need, or the cheapest 1.8 that you would not want).
Using a Depth of Field Calculator we can see that for a BMPC 4k with 8 meters to the subject, the near limit is 7.3m and the far limit is 8.85m. This means you'll be a little bit shy of having everything completely in focus, but it should be pretty close to acceptably in focus. If you can get a hair more distance it would buy you a little more space and might cover your full range.
