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Camera Focal Length (for example, in 3D scene rendering)

If you enjoy my free writings, please consider my $19.95 art how-to ebook, Art Head Start, for novice and intermediate artists.


 

Focal Length intro

We all know how important the position of your camera in a scene is. You may be less aware that your combination of Camera Focal Length plus Camera Distance from the subject can produce important and maybe unexpected results for:

  • Geometric perspective
  • Object relationships
  • Atmospheric effects
  • Reflection angles


Understanding Camera Focal Length

(This section is from my Art Head Start ebook)

35mm film size cameras are the standard in modern film-based photography. Most digital cameras are designed to mimic them. At this film frame or digital light sensor size, a Normal Lens has a focal length of 50mm. “Normal” means that the geometric perspective you get with such a lens is close to that of human vision, when the resulting photo is viewed at a normal distance.

The main idea when using lenses of different focal lengths is to move you and your camera virtually (optically) nearer or farther from your subject, without actually changing your physical position.

Camera lens focal lengths form a spectrum from Wide Angle through Normal to Telephoto. Photo pros may have many lenses to choose from, but a minimum set would be something like a 28mm Wide Angle, a 50mm Normal and a 150mm to 250mm Telephoto. A single “zoom lens” might also be available that can be adjusted throughout that range.

Besides changing the camera’s distance to the subject virtually with focal length, if you also change the camera’s physical distance from the subject, such that you maintain the same Point-of-View (POV), then the apparent effect of each focal length on the geometric perspective of the image will be more noticeable. Novice photographers often think this seeming geometric distortion with different focal lengths is a form of lens distortion, but it’s not.

For instance, if you take a panoramic photo, using an extremely wide angle lens, it will look distorted geometrically when viewed from a normal distance. But if you hold it close to your eyes and wrap it around your head, it will look perfectly normal in geometry as you turn your head. You can also superimpose images of 3 focal length lenses, taken from the same position,, and see that they don’t distort the image, but simply frame different sized portions of the scene

So actually, each focal length is just “cutting a rectangle” of a different size out of the available view, not distorting it. The other myth is that these geometric perspective effects are caused by the lens.

Changes to the camera location, as in my examples to the right, cause the perceived geometry changes. Even though changing the focal length is required to enable the change of camera distance while keeping the same POV, it is not the focal length causing the perspective changes. As you can see, I had to use slightly different camera heights, as usually happens when the camera is not perfectly level.

'Wide Angle' Focal Length of 28mm.

Same scene in each image. Only the focal length, camera distance and camera height were changed. The box is 4 Vue units tall, 4 units wide and 16 units deep.

'Standard' 50mm Focal Length simulates human vision.

'Long' or 'Telephoto' 200mm Focal Length.

What to remember

The combination of changing the camera to object distance and also the lens focal length, such that the same Point-of-View (POV) is maintained, creates changes in the look of the geometry of the image.

The apparent image geometry changes are caused by the difference in camera distance. Yet the changed distances, with the same POV, would not be possible without also changing the focal length, so you could be justified in saying that both focal length and camera distance are responsible for these effects.


Practical example from an actual Vue 5 Infinite project

I experimented with changing both the camera location and lens focal length in a recent Vue 5 Infinite project. The final image can be viewed larger for the full effect.

By the way, this image used a Photoshop image to create the terrain and 2 EcoSystems (sheep, right; trees, middle). The plowed ground, height leveling, gravel texture and ditch details were painted using the Terrain Generator tools.

Hope you’ll find some of my observations helpful.

Goals

I wanted to arrive at an optimum composition that this developing image was suggesting to me. It “wanted to become” a symmetrical, statically balanced, image with strong eye movements between the 2 human figure Centers-of-Interest, but with an unhappy disruptive visual element centered between them.

Notice that careful observation of the growing image, plus my inspiration from the Van Gough sketch, plus design training and personal artistic vision allowed me to design a very specific composition and visual elements in detail. This shows how important working from a sketch and a well-developed concept is. Here is where spirit meets engineering to establish concrete goals for an image - the “Criteria”.

Some of the criteria

  1. Keep the horizon line at the level of the figure’s heads - critical to viewer's eye movement and emotion developed by the visual barrier of trees.
  2. Show the 2 side ditches clearly for meaning and emotion.
  3. Keep a pleasing arrangement of sheep in relation to the female figure, without throwing the ecosystem dice many too more times (had to clear and re-populate SO many times, to get a pleasing arrangement of sheep models).
  4. Maintain a “rectilinear, biaxial symmetry, statically balanced” type of image composition. This composition type concentrates the viewer on the relationship of the figures.
  5. Show the best rotations for the two figures.
  6. Get the right reflective properties and proportions for the water areas.

Results

Final image with 70mm lens (View full size for full effect)...

70mm lens:

70mm lens for final image.

Camera lens focal length and distance to male subject in Vue units on the "Y" axis are shown.

225mm lens:

Same image with 225mm lens.

Here you see an extreme telephoto lens of 225mm. This satisfied items 1 and 2, and actually shows the side ditches better than the 70mm lens final version. But I didn’t like the relationship of the sheep to the female figure or the way the stand of trees seems to shrink away from the horizon.

To get the horizon line where I like it, the camera is tilted down a bit, it’s not perfectly level. I’ll wager that your camera is often tilted too. The reason that could matter to you is that as you move your camera back and forward, you have to also change its height. This affects scene reflection angles, like that of my water and it also changed my water’s ripple effect. There is quite a dramatic difference in the 3 water renders, but I didn’t change the water material at all.

The telephoto lens also shows a smaller section of sky, pretty much removing the subtle clouds. I had to adjust the fog & haze as well. These were much heavier, from the longer camera distance, and this changed the whole contrast of the scene, as well as the look of the horizon.

20mm lens:

Same scene with 20mm lens.

This is a very wide angle lens of 20mm focal length, almost a “fish-eye” or spherical lens. You may notice that this is close to the 28mm default focal length of Vue 5 Infinite – a focal length that is usually too low in my experience.

Although the 20 mm lens shows off the sky nicely, it caused a number of problems. For one thing, the sheep have moved so far in relation to the female figure that you’d swear it’s a different ecosystem population – but it’s the same one.

The water looks very different too. And the side ditches are party obscured. These two changes are from my having to lower the camera considerably, in order to keep the horizon line where the image needs it to be. Again, this is a common problem because you often don’t have the camera perfectly level. Therefore moving back and forward means also some up and down, to maintain the same POV.

Areas of the full scene which i wanted to stay outside of the camera viewport are now visible too, resulting in that disturbing triangular water shape you can see on the right. The whole image has a rounded look to it, and the trees look smaller, from this wide lens and close camera.

~ End ~


 

Wishing you a creative future!
   _jim coe
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