How is zoom and distance to subject related?

[dlwjiang]

If you have a 50mm view looking at something 50 meters away, and you extend that to 200mm, would that mean you have the equivalent field of view of using 50mm to look at the object 50/4 = 12.5 meters away? Is that the same as increasing the size of the object on the sensor/screen by 4. (so a 16x increase in surface area) ?

Is the image "compressed" in anyway? I always thought that "compression" was a subjective phenomenon due to taking away the things in the scene that can create a sense of scale.

[IABoomer]

You might want to check out this thread on perspective compression.

Perspective Compression

[nickbedford]

Using the field of view calculator you can see that it's not linear like you might surmise.

The further you zoom the lens, the more length it takes to decrease the field of view. Shifting 5mm at the wide angle drastically reduces the field of view whereas shifting 5mm at the long end only decreases the field of view marginally.

This is the diagonal field of view given by a 50-200mm lens on an APS-C sensor, for example, with 25mm increments.

[dlwjiang]

I see, now that you mention it I remember something like this, how the background changes but the foreground doesn't. Think it's a somewhat commonly used effect in movies to create a dramatic "realization".

[nickbedford]

Originally Posted by dlwjiang

I see, now that you mention it I remember something like this, how the background changes but the foreground doesn't. Think it's a somewhat commonly used effect in movies to create a dramatic "realization".

The one I remember is the Lord of the Rings scene where the black rider is about to ride up the forest road when Frodo and the Hobbits fall down the hill from the farm.

[sk66]

Focal distance and zoom are ONLY related in framing an image (and has been noted it's not linear).
"Compression" from zooming is a subjective impression, compression from using a longer focal length at a longer distance to get the same framing is not, it's a real change of perspective.

I wrote THIS POST on the subject a while ago.

[nickbedford]

Using the fantastic illustration software suite known as mspaint.exe I created this high quality diagram.



This is the reason why perspective changes then you frame an object the same whilst using different focal lengths.

A short focal length used when shooting the same subject framing means the relative distance between the back of an object and the front is large. In the diagram, the back of the circle is twice as far from the camera as the front.

A long focal length used when shooting the same subject framing means the relative distance between the back of an object and the front is small. In the diagram, the back of the circle is about 5-10% further than the front, making the perspective much flatter as relatively the two sides are very close to each other.

[AnonymousKiwi1]

That is a quality diagram. I am supremely impressed by it's awesomness.

It does explain the concept well, though. Good job.

[nickbedford]

Originally Posted by AnonymousKiwi1

That is a quality diagram. I am supremely impressed by it's awesomness.

It does explain the concept well, though. Good job.

Thanks, it took a while, and I had to do some pretty advanced vector manipulation to get it right, but in the end, I'm happy with the way the diagram came out.