Achieving shallow depth of focus with video for a film like look

shiner · #1 (**permalink**) 12-26-2007, 11:42 AM

Not easy to do with a consumer camcorder but here's a few tips

You'll need a few manual controls on the camera

Step 1. Get in as close as you can to the main point of interest (and focus) in the frame leaving the maximum distance between your point of interest and the background

Step 2. Change setting to manual focus and focus on main point of interest in the shot

Step 3. Leave your cameras auto exposure on and put ND filter to maximum, if enough light to shoot with, set cameras exposure to manual, if not enough light, change ND filter to next lowest and exposure and try again till you have enough light to shoot with.

You're now ready to shoot

And here's a little video using these tips http://www.digitalgloss.com/ShallowFocus.wmv

Z Cheema · #2 (**permalink**) 12-26-2007, 09:42 PM

Step 1, you can also use a longer zoom to reduce DOF

step 3, you can also use the shutter speed to balance the open iris.

Mark W · #3 (**permalink**) 12-27-2007, 04:07 AM

Step 1 - a if you habitually use a wa convertor, remove it for shallow dof shots.

If you must have shallow dof get the camcorder with the biggest ccds you can afford - the larger the ccd the shallower the dof you can get.

I recently went from a cam with .333inch ccds to .5 inch - I am sure the better optics helped too but the difference was very clear.

shiner · #4 (**permalink**) 12-27-2007, 09:15 AM

Quote:

Originally Posted by Z Cheema

Step 1, you can also use a longer zoom to reduce DOF

It shouldn't matter what end of the zoom you are using. If you frame the shot the same, the depth of field should remain the same. Here's a comparison of framegrabs at different ends of the zoom

sspeiper · #5 (**permalink**) 01-08-2008, 04:04 PM

The two images taken at different focal lengths seems rather odd then.

Normally, a longer focal length and a wide open aperture gives you a shallow DOF. Short focal length and stopped right down gives a greater depth of field.

The longer lens tend to 'bunch things up' somewhat too, everything appears to be in the same plane.

Teaching you to suck eggs aren't I? I'll get me coat...

Mark W · #6 (**permalink**) 01-08-2008, 04:45 PM

In that perfect world where I am never broke and everyone wears cardigans and is nice to eachother all the time I would have me one of these and be able to film with pin sharp dof.

P+S TECHNIK | Member of 35Digital

The Guru · #7 (**permalink**) 01-08-2008, 06:31 PM

What Shiner's written is absolutely right. However most people have difficulty accepting it. I know at my seminars there's always one prosumer who says "I don't care about the theory, I know that I have less depth of field at the telephoto end of my zoom" and then I have to spend ages telling him why he's wrong.

Firstly, a lens is only sharp at the precise point of focus. A few millimeters either side and the unsharpness begins. The question is: at what point does "a bit unsharp" become "out of focus?" Which is like asking "how long is a piece of string?". So we say that once the point becomes larger than "X" it's unsharp. We come up with this figure depending on the number of pixels, HD or SD transmission and so on. This is important. Look at a picture on a 3" screen and it all looks sharp, look at a 42" plasma and it all looks out of focus! In other words, Depth Of field is variable, depending on all sorts of circumstances.

What we're calling Depth Of Field is normally thought of as the distance in front of the focussed mark plus the distance behind the focussed mark which "appears" to be still in focus. The main three criteria which affect video DoF are the aperture, sensor size, and distance.
So, let's take an interview as an example. A 25mm lens, at F2.8 focussed on 10 feet has a theoretical Dof (for Video) of four feet. To get the same size image from a 50mm lens, you need to be 20 feet away and, at f2.8 it has a DoF of...4 feet.

However, due to the foreshortening effect of a lens at the long end of the zoom, the focus appears to "snap" in and out a lot quicker than at the short end which gives an impression of a smaller DoF. By having a "smaller" area of background, this too can give the impression of a smaller DoF. Also an unsharp object (such as a plant) which is far in the background with a wide lens is "bigger" due to the compression of a telephoto and, therefore, more noticeably present and unsharp.

Try a few tests and you'll see what we mean.

MOSH · #8 (**permalink**) 01-09-2008, 02:47 PM

so in a nutshell...

longer focal lengths technically have the same DoF, just a stronger visual impact

(in my understanding of the subject)

The Guru · #9 (**permalink**) 01-14-2008, 11:00 AM

Yeah, providing that the subject is the same size in the frame. In other words a head and shoulders shot at f2.8 will have the same depth of field with a wide angle or with a telephoto, all other things being equal.
What the background does, is another thing entirely.

Marc Peters · #10 (**permalink**) 01-14-2008, 11:23 AM

Shiner / Guru --> Any objections to me taking your ever so wise words and turning this into a nice little guide?