Tv Zoom Trick

[Bert45]

There is a thing they do on TV with a black and white photo (it's always b&w, I think, and the photo is usually old) where the subject of the photo in the foreground gets bigger as the camera seems to zoom in, while the background stays the same. Could you explain how they do that in simple (-ish) terms?

[TheChair]

It's common in many colour films too. Called a dolly zoom. The camera moves back on a dolly, while the lens is simultaneously zoomed in.

[TheChair]

... or vice versa :-)

[snags]

[Mamyalynne]

Some more examples here.

[jno]

invented by Hitchcock for Vertigo

https://youtu.be/G7YJkBcRWB8?t=26

Is that what you mean? With photos, they sometimes just photoshop them so part of them approaches you while the rest doesn't.

[Bert45]

No, that's not what I mean! Yes, I know about the dolly zoom, as used in Jaws. No, I'm talking about a black and white PHOTOGRAPH, a still picture taken perhaps 50 years ago, where it's as if they've cut out the subject of the photograph in the foreground and zoomed in on that, so it gets bigger and obscures parts of the background that was previously visible around the subject. It's device used on many real life history programmes where they can't have us looking at a still photo for 10 seconds while the voiceover carries on, so they create motion by zooming in on the foreground while the background stays the same. It's obviously done by a computer somehow, but how do they tell a computer what is the foreground and what isn't?

[THECORBYLOON]

They could use two layers, one for the foreground and one for the background, then apply the effect only to one of them.

[Bert45]

Hi, Corbyloon. How do they get two layers?

[THECORBYLOON]

I don't know the technicalities but I'm aware that with various programs, such as photoshop, images can be allocated to one of multiple layers and then manipulated.

It's like a pile of acetate sheets with an image on each and the "pile" makes up the whole composition.

Each one can be changed so if the program "knows" to change the focus on one of the layers, the others are not affected and the viewer sees an element coming into and out of focus.

[Bert45]

The image doesn't go out of focus, it just gets bigger. Surely you've seen this on TV? I could understand it if someone got a pair of scissors and cut around the image in the foreground, because a person would know what s/he wanted to achieve, but a human could not cut the image as perfectly as the computer does. I could imagine that a person could look at a greatly magnified image and 'draw' round the image with the mouse, clicking to tell the computer where to 'cut', so when the image was returned to normal size, any slight mistakes inn 'cutting' would be too small to see. But I doubt that that is the way it's done, it would take too long. Perhaps they tell the computer to look for contrast, the foreground being darker than the background, but I could see that not working where parts of the foreground might be lighter, or parts of the background could be darker, so the computer couldn't tell where the contrast was.

[THECORBYLOON]

Whether it's a change of focus or size, the software applies the effect on to the chosen layer(s) but how the software performs that, I have no idea.