Focal length

What is the deal with the focal length ?  What is the difference between 18-55mm and 55-200mm lens ? What is zoom ? Is the focal length of my point-and-shoot camera and DSLR directly comparable ?

Short story:

• So DSLR or point-and-shoot camera
• smaller focal length = wider angle = greater field of view
• larger focal length = narrower field of view = things look zoomed-in

• Don't compare "zoom" in cameras, or just the focal lengths. Compare their field-of-view = 35 mm equivalent focal length.

 Long story:
 
The pictures were taken using Nikon D40 (referred to as D40) and Panasonic Lumix DMC-ZS7 (referred to as ZS7).

On SLR's the lenses can be changed and we generally don't talk of zoom we talk of focal length of the lens being used.
On point-and-shoot cameras the lens is fixed and they are advertised with a zoom and not generally with the focal length.

For the D40 I have two different lenses. One has a focal length range of 18-55 mm and the other one 55-200 mm. The 55-200 mm lens is shown below.

 Fig.1. The 55-200 mm focal length lens for the D40.

Using the two lenses I can take pictures at any focal length in the range 18 mm to 200 mm.The focal length changes the field-of-view (FOV) or angle-of-view of the picture you get, or the scene you see in your LCD or the view-finder, or the magnification or the zoom. I will just talk of the FOV or angle-or-view and leave the magnification and zoom.

Simply put, smaller the focal length, more is the FOV = you get a wider FOV. Larger the focal length, smaller is the FOV.Consider the four pictures shown below. All were shot using the D40 mounted on a tripod. So the position of the camera was (approximately) the same for all the pictures. The focal lengths for these four pictures were (in the order: upper-left - upper right - lower right - lower left) 18mm 55mm 100mm 200mm.

Fig. 2. Pictures taken at different focal lengths using the D40. The camera was mounted on the tripod so the position of the camera was (approximately) the same for all the pictures.

The picture taken at 18mm has a much wider view than the one taken at 200mm. However at 200mm the camera has zoomed-in on the bicycles. So, you want the bicycles along with the trees and the parking lot = you want a wide angle = use a shorter focal length. If you want just the bicycles use a larger focal length (or move close to the bicycles).

Now onto the point-and-shoot camera.

The zoom is basically the maximum focal length divided by the minimum focal length. On the lens of my ZS7 the focal length range is mentioned as 4.1 - 49.2. This corresponds to 49.2/4.1 = 12 i.e. "12x Optical Zoom". The picture below shows the front of the ZS7 where the focal length is circled in red.

Fig.3. The front of the ZS7 showing the focal length range of 4.1 - 49.2 thus the 12x optical zoom.

The focal length cannot be compared across cameras directly. This is because if I put the 55-200 mm lens for my D40 on another camera then the FOV might not be the same. The way lenses are compared (in terms of their focal lengths) is by looking at what FOV they will provided if they were mounted on a 35mm camera (basically mounted on the same camera, the 35mm camera is the standard everything is compared to).

For my D40 the "crop factor" is 1.5. This means that if I put the 55-200 mm lens on the 35 mm camera then the FOV I will observe will be the same as that provided by a lens of focal length range of 55-200 mm x 1.5 = 82.5-300 mm made for the 35mm camera. So a focal length of 200 mm for my lens for the D40 is equivalent to 300 mm on a 35 mm camera.

Now for the ZS6 the "35 mm equivalent focal length" is mentioned in its specifications as 25-300 mm. Similar as above, what this means is that if this lens on the ZS7 (4.1-49.2) were put on a 35mm camera then the FOV it will provide will be the same as a 25-300 mm lens made for the 35mm camera.

So a focal length of 200 mm on my D40, and a focal length of 49.2 mm on the ZS7 are both equivalent to 300 mm on a 35 mm camera. Thus a picture taken at a focal length of 200 mm using the D40 and one taken at 49.2 mm using the ZS7 should have the same FOV. The picture below shows a comparison. The picture on the left was taken using the D40 at 200 mm and the one on the right was taken using the ZS7 at 49.2 mm.

O yes I did make a mistake. With the ZS7 I was taking the picture using the 4:3 aspect ratio, for comparison with the D40 I should have switched to 3:2 but I forgot. But you do get the idea of the field of view comparison (compare horizontally).

Fig. 4. Field of view comparison for D40 (200 mm =  35 mm equivalent of 300 mm) and ZS7 (49.2 mm = 35 mm equivalent of 300 mm)

• So DSLR or point-and-shoot cameras
• smaller focal length = wider angle = greater field of view
• larger focal length = narrower field of view = things look zoomed-in

• Don't compare "zoom" in cameras, or just the focal lengths. Compare their field-of-view = 35 mm equivalent focal length.