Is Focal Length and Field of View linear?

i.e. if you double the focal length would the size or something in the image double?

That depends on what you mean by "size'".
The linear dimensions scale linearly; areas scale by the square of the FL.

A more detailed answer would interest me aswell

If you double the distance to a subject, double the focal length to keep the subject framed the same.

Here's a reason why this is something worth remembering:

Let's assume that you are taking a photo of some friends in a scene that has mountains in the background. You stand 20 feet from the people and view the scene. A 50mm lens will let you fill the frame with the group of people and some of the background quite nicely, so you take a shot. Then you realize that the mountains are rather small in the background.

Back up to to 40 feet (twice the distance) from the group of people and view the scene, you will see that the mountains are now larger relative to the people - twice the size they were before, in fact. However, the people are smaller in your viewfinder. You now need a 100mm lens to keep the people the same size as in the first image, but the mountains now appear twice the size that they were in the first shot.

Why is this? It's because the additional twenty feet that you put between yourself and the people is insignificant relative to the fifteen miles between your viewing spot and the mountains.

Great explanation SkipD. Now, I'd like to hijack this thread.

How about with macro lenses? Let's say a 50mm macro and a 100mm macro? How will the image change?

KirkS518 wrote in post #16414597
Great explanation SkipD. Now, I'd like to hijack this thread.

How about with macro lenses? Let's say a 50mm macro and a 100mm macro? How will the image change?

For the situation described by Skip? Not at all.

The only real difference between a macro lens and an ordinary lens is that the former has been designed to be able to focus on closer objects (and some stuff about flatter focal planes, or something like that).

Here's an interesting little bit of info about focal length (and it kinda follows the inverse square law)

As you double the focal length, the field of view is 1/4 the area (1/2 vertically and 1/2 horizontally)

So if you have a 200mm mounted, and want to see what a 400 would look like, the framing would be from the centre AF point to a corner.

No, it can't be. Focal length is theoretically infinite. Field of view (an angle, really) is theoretically a maximum of 360 degrees.

SkipD wrote in post #16414456
If you double the distance to a subject, double the focal length to keep the subject framed the same.

Here's a reason why this is something worth remembering:

Let's assume that you are taking a photo of some friends in a scene that has mountains in the background. You stand 20 feet from the people and view the scene. A 50mm lens will let you fill the frame with the group of people and some of the background quite nicely, so you take a shot. Then you realize that the mountains are rather small in the background.

Back up to to 40 feet (twice the distance) from the group of people and view the scene, you will see that the mountains are now larger relative to the people - twice the size they were before, in fact. However, the people are smaller in your viewfinder. You now need a 100mm lens to keep the people the same size as in the first image, but the mountains now appear twice the size that they were in the first shot.

Why is this? It's because the additional twenty feet that you put between yourself and the people is insignificant relative to the fifteen miles between your viewing spot and the mountains.

Good post. Thanks.

Abu Mahendra wrote in post #16416062
No, it can't be. Focal length is theoretically infinite. Field of view (an angle, really) is theoretically a maximum of 360 degrees.

Can focal length be negative? No.
That is the limiting value that equates to the maximum limit on FoV.
There is an inverse relationship between the two.

Abu Mahendra wrote in post #16416062
No, it can't be. Focal length is theoretically infinite. Field of view (an angle, really) is theoretically a maximum of 360 degrees.

A larger focal length corresponds to a smaller field of view. An infinite focal length would be zero field of view i.e. all light rays are parallell so your sensor of x mm would capture x mm of a mountain whatever distance that mountain might be (ignoring athmosphere and all other problems involved).

Abu Mahendra wrote in post #16416062
No, it can't be.

What question are you answering with this?

Abu Mahendra wrote in post #16416062
Focal length is theoretically infinite.

Since when?

Abu Mahendra wrote in post #16416062
Field of view (an angle, really) is theoretically a maximum of 360 degrees.

No conventional lens will come anywhere close to an angle of view of 360°. It's tough to get a lens to cover 180° and most that get close are "fish-eye" lenses.

Here is a graph of the relationship that I found through Google:
http://www.pentaxforum​s.com …length-vs-field-view.html

SkipD wrote in post #16416765
What question are you answering with this?

Since when?

No conventional lens will come anywhere close to an angle of view of 360°. It's tough to get a lens to cover 180° and most that get close are "fish-eye" lenses.

Yes - a flat sensor means that when the lens gets wider then the field of view will go towards 180 degrees - i.e. light rays going straight sideways - and not towards 360 degree.

Continuing furher than 180 degrees requires a very bulbous front lens element that can catch lightrays originating behind the sensor plane.

But all this is irrelevant. With matematical models you could implement raytracer software that can see 560 degrees or whatever you like but it isn't meaningful in the real world. When doing a panorama stitching, it's possible to rotate two, three or four turns and then stitch. But hat use is a panorama that repeats the same view multiple times?

So we just have to accept that mathematical formulas are valid even when physical limitations stops us from building such lenses. Our physics formulas covers speeds up to light speed even if we can't build a car that runs much more than some factor of the speed of sound.

KirkS518 wrote in post #16414597
Great explanation SkipD. Now, I'd like to hijack this thread.

How about with macro lenses? Let's say a 50mm macro and a 100mm macro? How will the image change?

Both a 50mm and a 100mm macro lens can capture your subject at 1:1, i.e a 5mm fly could project to 5mm on the sensor.

The difference - just as non-macro - is that the 100mm lens gives 1:1 reproduction at twice the distance from the fly. So you disturb the fly less. And - just as for normal photography - thr 100mm lens will give a narrower field of view so the background will be more magnified than when captured with the 50mm lens.