Film and depth of focus- how do they do it?

Many times in films I’ve seen a shot which normally occurs after the character in frame has received some sort of shock. Basically, the ‘star’ of the shot stays front and centre, but the background seems to draw back, as if the distance between the person and whatever is behind them is rapidly increasing.

My question is, how do they do that? Is it trick photography, or special lenses or what?

And while I’m on the subject, making films. If we assume that the the exposure time of a single frame on a camera is a standard fifteenth of a second (if that is the normal frames/second rate), am I right in assuming that the aperture of the lens must change when shooting in low light?

I turn to you, oh SDMB, for enlightenment!


  1. Zoom lens being zoomed from telephoto to wide angle.

  2. yes

From what I know, this effect is achieved through simultaneously pulling the camera away from the subject while focusing in tighter. Or, the camera moves in one direction swiftly and the operator focuses aggresively in the other direction. It seems it must be done very quickly for max effect.

I'm sure someone here has a better technical description, but i think that's the gist of it.

There is a term for it, but I can’t remember; the trick consists of zooming in while moving the camera backwards (or vice versa). It isn’t a depth-of-field effect (except in that zooming may incidentally alter the depth of field).

As far as I remember, the person in-shot stays pretty static, so it must be quite an art to effectively zoom in at the same speed as the camera is taken away.

Also, I’m going to dig out my telephoto lens when I get home tonight and have a proper look, but I don’t recall that zooming in make the background seem further away, doesn’t everything just get bigger?

You may sense a degree of scepticism here, but I just want to get it all straight in my head!

As for your second question, it depends on the effect desired. Most productions require a good amount of light for detail, and if one is to shoot in low light it can be tricky. Accomodating low light can achieved through special lenses or very senstive film stock (the more sensitive a film stock is, the ‘faster’ it is said to be).

I have not seen the movie, but Kubrick directed a movie called ‘Barry Lyndon’, which utilised special lenses and film stocks. The movie is set in 17th century Ireland, I believe, and Kubrick wanted to film scenes in candlelight, to create a sort of authentic historical experience. But he also wanted them to be immensely detailed, hence the special apparati.

btw, the standard frames per second is 24.

Nerrie - you can duplicate the effect without any camera at all:

Put two apples on the table, one in front of you and the other about a foot behind it. Bring your face really close to the front apple and it looks a great deal larger than the one behind.

Now walk six feet backwards and compare the size of the apples - they look much closer in size.

Here is a diagram that might help to explain it - as the camera moves backwards, the lens is zoomed so as to frame the subject consistently - the amount of background visible through the same virtual subject frame decreases as you move away.

The most memorable scene in which this is done is in JAWS when Cheif Brody (Roy Scheider) is sitting on the beach when someone screams “shark!” The shot begins with a shot of Brody zoomed in somewhat, and then the camera dollies in as the zoom is brought back to a pretty wide angle–the lens is close to Brody’s face. Great shot.

Another thing is what’s called forced perspective: in LORD OF THE RINGS, the first one, early on in a shot, Gandalf and Frodo are riding in a cart together side by side. Frodo appears very small while Gandalf appears huge. They made a lopsided cart in which Gandalf sat very close to the camera while Frodo sat several feet behind him, even though it looks like they’re side by side.

It’s called the Vertigo effect and was first used in Vertigo by Alfred Hitchcock.

Another great example of Forced Perspective is in Back to the Future 2, during the several scenes that take place in the tunnel. The tunnel was a set, and the lights on the side were attached to the walls closer and closer together as they went back, thus making the tunnel appear much longer than it actually was.

This is a little off the main question. In the movie Citizen Kane, director Orson Welles got great depth of focus by using a lot of light and then stopping down the camera aperature to get the correct exposure. When the lens aperature is wide open the depth of focus is small and when the aperature is closed way down the dept of focus is large.

He has scenes in a room where one actor is right up close and the other is across the room and both are in focus. The actors sweatted a lot because of the heat from the lights but he did get a great effect.

I suppose all distance is, as far as a film is concerned, is it looking smaller. I think my problem was that I was thinking of it all in 34-dimensions when film is effectively working in only two.

Erm, brain the size of a planet me!

I didn’t really mean plain old three dimensions at all . . .

IIRC (from a documentary) Spielberg pioneered this shot in Jaws. There’s a scene where Roy Scheider is sitting on the beach and suddenly realises there’s a shark in the water. The camera races towards him on a dolly, while moving from telephoto to wide angle (or the other way round, I can’t remember). His head stays the same size and in focus, while the background distorts. It’s a very effective technique in the context of “silent, horrified realization”.

jjimm, Hitchcock was the first. link 1 and link 2. Vertigo was released in 1958, well before Jaws’ 1975 release.

Film movies are still made at a standard 24 frames / second.

Digital movies may be made at 15, 24, or 30 frames / second.

You are confusing frame rate with shutter speed (exposure time). They are not the same thing at all.

A movie camera runs at 24fps, but that doesn’t mean that the exposure on each frame is 1/24th of a second. Such a long exposure time would make for very blurred images.

Further to my last post (sorry!)… I am not a movie camera expert, but I believe the “shutter” in a movie camera is actually a rotating disc that can be opened to varying degrees to give a set shutter speed.

E.g., at 24fps, with the disc open 180 degrees, effective shutter speed is 1/24 x 180/360 = 1/48 sec.

Narrowing the angle will give a smaller shutter speed, but you are more likely to get a “strobing” effect due to the lack of motion blur between frames.

I mighta known…