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Originally posted by black_fimo

Hi,
I was wondering if it is normal to change the f-stop number during the shooting of a scene, or do you keep it the same for all the coverage(so same for close-ups and wide shots). Also, does the f-stop determine the depth of field(shallow/deep focus). So in that case would that mean that the depth of field would stay the same throughout the whole scene as well?
Thanks. I hope I was clear in what I meant, i'm completely new to all this.

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Let me try to tackle these questions one at a time.

First, yes, the aperture DOES help determine the depth-of-field. A wider aperture gives you less DoF and a smaller aperture gives you greater DoF. Focal length also is a factor in DoF. A longer lens (higher millimeter) gives you less DoF and a wider lens gives you less DoF. Except in Anamorphic, DoF (perceived focus) works this way.... wherever you set focus, let's say at six feet, the perceived distance of focus is roughly 1/3 the distance in front of the plane of focus and 2/3 behind it. That distance ( 1/3 in front and 2/3 behind plane of focus) changes as aperture and focal length change. There are tables and "devices" which help professional Focus Pullers know how much wiggle room they have when pulling focus. ( ie http://www.filmtools.com/kehidecaakak.html )

As far as changing your T or F-stop during a scene, it isn't normally something a DP wants to do. Generally speaking, if the master is lit for a T4 (or F4), then the rest of the coverage will be shot at a 4 as well no matter which lens is used. Naturally, this does affect DoF from shot to shot, but you're not likely to want extremely shallow DoF in a wide master shot on a 24mm anyway. But if you're shooting a CU on a 75mm or 150mm, then you're likely trying to get shallow DoF, which the lens helps with anyway.

It's a bit easier to control the various factors on an interior set when artificial lighting is used. Day exteriors are more difficult but with ND (neutral density) filters on the lens (to force you to open the aperture) and a lot of light to match the fill lighting on your subjects with the sun's light on the background, you can achieve shallower DoF.

Most DPs try to keep the aperture consistent throughout a scene but there might be minor variations. It's not likely that a master would be shot at a T8 and closeups at T1.3. The lighting changes that would require would be extraordinarily time consuming and cost prohibitive and frankly, unnecessary unless some very specific effect was trying to be accomplished. DPs also find that lens "perform" better in the middle of their range, meaning that if the wide end is a 1.3 and the other end is a T22, the lens will photograph better in the middle (ie, T4 or T5.6) than at either of the extremes, so most DPs will try to light for that stop (knowing, of course, which film stock they will be using and it's ASA).

Brian Dzyak
Cameraman/Author
IATSE Local 600, SOC
http://www.whatireallywanttodo.com
http://www.realfilmcareer.com

Thanks for the answer.
I was also wondering what exactly is the point of using a deep focus shot?
Because as everything looks naturally in focus in a wide shot anyway, due to the lenses being used, what is the purpose of a deep focus shot? I personally find it hard to tell the difference between a deep focus shot and something that is simply just a wide shot.

quote:

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Originally posted by black_fimo

Thanks for the answer.
I was also wondering what exactly is the point of using a deep focus shot?
Because as everything looks naturally in focus in a wide shot anyway, due to the lenses being used, what is the purpose of a deep focus shot? I personally find it hard to tell the difference between a deep focus shot and something that is simply just a wide shot.

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Typically, in a wide shot, the point of it is to show the viewer a lot o information, such as "this is what the city looks like." The more your picture has in focus, the more information the audience can get from it.

In addition to shot choice and angles and lighting, selective focus (on one object in frame) draws attention to something very specific that the Director is using to help tell a story. For instance, if you saw a baseball game being played in the foreground or background, but focus was ONLY sharp on a small sad child, then we might get the idea that the story is about that child and our attention would necessitate trying to understand why we are meant to focus our attention on that specific character at that time.

However, by using a greater DoF, the audience is asked to look at more things in the frame and presumably, the Director wants the audience to see everything that is in focus and learn from that.

There are instances when standard lenses and lighting setups do not allow for TWO planes of focus, such as a character in the extreme foreground in focus (perhaps on a hospital bed) and another character in the background (a visitor). To achieve that the camera could potentially have a very wide lens to accommodate the two characters in frame and a lot of light can be used to get the aperture to something deep, like a T22 in order to increase DoF. Some shots in the movie Citizen Kane were achieved like this.

But a different way to do this is to use a slant lens http://www.panavision.com/product_detail.php?imgid=113&id=88&cat=196 or a split diopter. Using these types of tools allows you to bend light to the point where two specific planes are in focus without extreme lighting or lens choice.

Brian Dzyak
Cameraman/Author
IATSE Local 600, SOC
http://www.whatireallywanttodo.com
http://www.realfilmcareer.com