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17 – Focal Length

1 – MMs

Lenses have two types of MMs: diameter and focal length.

Let's look at each one.

MMS & Diameter

The front of a lens has a diameter, such as 67mm.

You need to know the diameter of the lens if you're buying a filter.

The easiest way to determine the diameter is to look at the inside surface of the lens cap.

The diameter is usually printed there.

MMS & Focal Length

Technically, focal length is the optical distance from the front of the lens to the sensor.

Practically, focal length is how much you can see of a scene.

Wide Angle

A wide-angle focal length "sees" a lot from side-to-side.


A telephoto focal length magnifies a distant part of a scene and brings it closer.

Fixed Focal Length v. Zooms

Some lenses have only a single focal length, a fixed focal length.

Many photographers have a 50mm lens because the lens lets in lots of light, and has less depth-of-field.

The lens only has one focal length, 50mm.

In contrast, zoom lenses have many focal lengths.

For example, an 18mm to 200mm zoom lens has all of the focal lengths between 18mm to 200mm.

2 – Focal Length Examples


18mm - Wide


50mm - Normal (Not Wide or Telephoto)


70mm - Telephoto


450mm - Telephoto

3 – Odds & Ends


Lens cleaning is described in the Camera section.


Clear filters are described in the Camera section.

Polarizing filters are described in the Light Contrast section.

4 – Lens Factor

Lens focal lengths are based on 35mm film.

Because most sensors are smaller than film, they crop the image.

They "see" less.

They're more telephoto.

On Nikon digital SLRs, lenses are 1.5 times more telephoto, compared to a film camera.

On Canon digital SLRs, lenses are 1.6 times more telephoto, compared to a film camera.

1.5 and 1.6 are called the lens factors.

You multiply the focal length of your film lens by one of these lens factors to get the focal length of the lens on most digital SLRs.

Full Frame Cameras

A few cameras have full-frame sensors (FX).

They're the same size as 35mm film.

Therefore, the focal length of a film lens is the same on these full-frame cameras.

5 – Flare

In your viewfinder, flare may be:

• A whitish haze.

• Geometric shapes.

Flare is created when you aim your camera toward a bright light source, such as the sun.

The light enters the lens and bounces around inside.

This causes the haze and geometric shapes.

Good Flare


Flare can be good for a photograph if it adds romanticism, impressionism, fantasy, and so forth.

Flare is often used when doing a portrait.

Bad Flare


If flare doesn't add anything to your photograph, use a lens hood or use your hand to cast a shadow of the light source on your lens.

The following techniques will reduce flare, but less noticeably than the above methods.

1) Remove filters from the lens.

2) Use a fixed focal length lens instead of a zoom.

A zoom generally has more glass surfaces, which may cause more flare, than a fixed focal length lens.

5) Use a smaller aperture.

6) Make sure your lens is clean.

6 – Shape & Background

Most photographers use focal length in only one way:

They zoom back-and-forth to compose their photographs.

That's great.

But, you can use focal length in two other ways.

You can change the shape of the subject.

And, you can move the background further away and closer.


In the first pair of photographs, compare how the leaves in the lower left corner, and the broken branch in the middle, look in each photograph.


18mm - Wide Angle

Click Photograph to Enlarge


70mm - Telephoto

Click Photograph to Enlarge

Note how the leaves and branch seem to be sticking out more, in the left photograph.

This is because a wide-angle focal length was used.

In the right photograph, using a telephoto focal length, the leaves and branch are closer together.

The photograph is flatter.


Below, the same fallen tree was photographed from the side.

In the left photograph, the road is further away.

This is because a wide-angle focal length was used.

In the right photograph, the road is closer because a telephoto focal length was used.


18mm - Wide Angle

Click Photograph to Enlarge


70mm - Telephoto

Click Photograph to Enlarge

For another example, go to More from Your Zoom.