Notice anything funny about the scenes?  They look somewhat similar, right?  Same three aliens, same ground texture, same red sky...

Would you believe that the two scenes are identical?

In fact, they are.  The two scenes are actually the same frame in a choreography, taken from two different cameras.  The only difference between the two (besides the camera position) is the focal length of the lens.  The first scene has a camera low to the ground with a very high focal length.  The second scene uses a camera high in the air with a low focal length.  You have just entered the wonderful world of focal lengths!

In photographic terms, the focal length of a lens is defined as the distance between the film and the optical center of the lens when the lens is focused on infinity.  The focal length of the lens on most adjustable cameras is marked in millimeters on the lens mount.  By adjusting the focal length on your camera, you affect the zoom.  The larger the focal length, the higher the amount of zoom.

Alternatively, Hash defines the focal length of a lens as determining "the amount of the scene the Camera can see left and right."  The amount of scene that the camera can see in inversely proportional to the focal length of the lens.  The smaller the focal length, the more scene will be visible.  Likewise, large focal lengths yield a smaller scene view.  In the two scenes above, the first scene used a focal length of 600mm, while the second scene used a focal length of 35mm.  Notice the difference in the amount of scene that is visible.

While at first the two definitions seem like they are saying two different things, they actually go hand in hand.  Here is an example using a focal length of 600mm:

Figure 1.1 shows the scene from the camera's point of view, while Figure 1.2 shows the entire scene from a top view.  The black arrows point to the aliens.  Notice the X-Z positions of the objects in the scene.  The camera is at (-1800,7000) and the farthest alien in the scene is at (-200,-2254).  That's a distance of 9391cm between them.  The distance between the camera and the foremost alien is 3979cm.  Looking at the scene on the left though, you probably wouldn't guess that the distances are so high, or that the distance between the first and last alien is over 5400cm.  Considering that each alien is only about 200cm tall, that would mean that you could lay 27 of them head-to-toe between the first and last alien.

Notice how the cone angle is quite narrow.  That follows with what the Hash definition of focal length says.  The higher the focal length, the lower the amount of scene that the camera can see.  Looking at the Figure 1.1, we can verify that we can't see very much scenery beyond the local area of the aliens.  Also notice that even though the camera is almost 4000cm away from the first alien (20 alien-lengths), we have a high enough zoom level that we can see him just fine.  This agrees with the photographic definition of focal length.

The next pair of images results from decreasing the focal length down to 300mm:

For figure 2.1, we have moved the camera closer to the first alien.  Since we decreased the focal length, we know that the amount of zoom has decreased also.  To compensate for this and to maintain the "framing" of the first alien, we had to bring the camera closer to the object of interest.  Figure 2.2 verifies this when compared with Figure 1.2.  Notice this time that the cone angle is wider, which also coincides with the lower focal length.  Sure enough, we can see more of the scenery around the aliens in Figure 2.1.  Notice however, that the two aliens behind the first alien appear to have moved farther back.  In reality, they are in the exact same positions as they were in Figure 1.1.  Compare the positions of the arrows in Figure 2.2 with Figure 1.2 and you will see that they point to the same spots.

This demonstrates how increasing the focal length compresses the depth of field.  This is similar to watching a track race during the Olympics.  As the runners come down the home stretch, the camera is pointed straight at them.  It looks like everyone is neck-in-neck.  Yet, when they show a side view, the runner in first place is visibly farther ahead than anyone else.  This is because the camera pointing down the home stretch is positioned far away and has a high focal length, "flattening" the depth of field.

The next scene illustrates this further:

Now are focal length is set to 135mm and the camera has moved in even closer to the first alien.  Notice that while he still remains relatively in the same place in the frame, his buddies now appear even farther back.  Figure 3.2 confirms that they actually haven't moved back, but our camera has moved in and the cone angle has become even wider.  Figure 3.1 shows that we can see even more scenery around the aliens.

Next we see the same shot, but with a 50mm focal length:

50mm approximates how the human eye sees.  In other words, our eyes have a focal length of about 50mm.  Again, while the first alien remains framed in generally the same spot, his friends now appear to be much much farther away.  We can now even see some mountain peaks on the left and right of the scene.  Figure 4.2 shows that the camera is now very close to the first alien, 870cm in fact.  That's less than 5 alien-lengths away compared with the 20 alien-lengths of the 600mm example.  Notice also that the cone angle has widened considerably.

For our last example, we decrease the focal length down to 35mm:

35mm should be a familiar number for you.  Many pocket cameras have 35mm lenses.  Have you ever noticed how when you get your film back from the processor that you took with your 35mm camera, the people that you were taking pictures of always look farther away than you remember them to be?  This is because the 35mm focal length of the camera elongates the depth of field more than your eye, which we said above has a focal length of about 50mm.  It's sort of like your side view mirror on your car: "Objects are closer than they appear."  Notice how far away the last alien is.  He is a mere speck of his former self, disappearing into the horizon.  He actually hasn't moved at all, as can be confirmed by Figure 5.2.  Notice also how much wider still the cone angle is at this point.

Study these two figures, and then go back and look at Figures 1.1 and 1.2 again.  Pretty big difference, eh?

You may have noticed something about the "look" of the action between the first example and the last example.  Look at the alien's arms and legs in Figure 5.1 and see how they seem to "swing out" towards you more than in Figure 1.1.  Again, this is a distortion caused by the different focal lengths and how they affect the elongation of the depth of field.

This is noteable because of the ever-important "Head Shot."  When taking a head shot of a subject, typically the photographer will move the camera far back from the subject and zoom in with a high focal length.  This flattens the depth of field and decreases the amount of distortion towards the camera.  The last thing you want is someone's nose stretching into the camera lens.  Below is an example of the same alien head shot, taken from two different cameras:

Figure 6.1 used a focal length of 50mm with the camera relatively close to the subject.  Figure 6.2 used a focal length of 210mm with the camera much farther away from the subject.  See the difference?  The first shot has much more distortion of the face, so much so that it even changes the overall shape of the head.  The second shot shows a much more accurate view of the alien's head, decreasing the distortion and showcasing the "roundness" of the alien cranium.

The distortion in Figure 6.1 also has the effect of making the alien appear more "sinister."  In Figure 6.2, he looks much friendlier.  This leads us to the next point, which is that you, as the Director, have absolute control over the appearance of your work.  Simply by adjusting the focal length, you can have a sinister-looking alien or a friendly-looking alien.  For example, in the case of a car coming at the camera, it might make more sense to have a smaller focal length, to enhance the effect as the car comes forward.  It's up to you.

So what does all this mean for you?  Hopefully, you are now stunned by the various cinematic possibilites that you can create simply by adjusting the focal length and camera position.  Just by altering these two things, you can drastically alter the framing of a scene.

A perfect example of this is what I call the "Spielberg Zoom."  This effect is created by starting with the camera very far away from the subject, but zoomed in on the subject using a high focal length.  The camera is then physically moved towards the subject while the focal length is decreased.  This has the effect of zooming in on the subject while at the same time making the scenery recede into the background.  This really focuses your attention on the subject, but in an eerie way.  Usually it is accompanied by swelling, scary music.  Rent out "Jaws" for an example of this in a Spielberg film.  To download a Quicktime example of the "Spielberg Zoom" done with Hash and our alien friends, click here.

This concludes "Fun With Focal Lengths!"

Copyright 1998 by Jim Sherwood