Normal is defined as conforming to a standard, the usual, the typical or the expected. When it comes to photography, normal most often refers to the standard focal length lens on a camera. A normal lens sees about the same angle of view as the human eye. Let’s delve into what normal means and why it’s important.
A “normal” lens is one whose focal length is the diagonal of the sensor of the camera. The sensor size is commonly known as the format. A full frame DSLR sensor is 36mm by 24mm. A cropped or APS-C sensor is typically 22.7mm by 15.1mm in size.Normal for a full frame sensor is 43.27mm. This gets rounded to 50mm or 55mm. The normal focal length for a cropped sensor is 27.3mm rounded to 28mm.
Wide angle or telephoto?
This depends on the normal focal length. Lenses that are shorter than normal are wide angles while lenses longer than normal are telephotos. For example a 28mm (normal for cropped sensor cameras) on a full frame camera is a wide angle. Put a 50mm (normal for a full frame) on a cropped sensor camera and it is a telephoto.
Depth of field
Why this is important is directly related to the lens’ depth of field.
I know, just another term from the world of “photo-speak.” Here’s what it means. Depth of field is the area that is acceptably sharp in front and behind the actual point of focus at a given aperture. Aperture controls depth of field. The smaller the aperture the greater the area of focus in front and behind where the lens is actually focused. Typically, these areas are one third in front of the point of focus and two thirds behind it. Beyond those distances, the image looks soft.
Depth of field is tied to focal length. The shorter the focal length the greater the depth of field for the lens. Conversely, the longer the lens the less will be sharp in front or behind the point of focus. Cropped sensor cameras have inherently greater depth of field than their full frame brethren. Which is a better choice? Neither one. They are different. Each one offers advantages and disadvantages.
Cropped sensor cameras are lighter, have greater depth of field and can offer less expensive lens choices. The smaller the sensor is the smaller the lens can be to cover it. Full frame cameras have greater resolution, better ISO performance due to larger photo sites, less depth of focus allowing easier creation of bokeh or out of focus backgrounds. Below, the photograph of Alexa was made with a Sigma 85mm f/1.4 lens on a Canon 5D Mark II full frame camera. ISO: 100 1/125 f/2.8
Focus fall off is readily apparent in the enlarged section below. Alexa’s left eye is the point of focus set on the lens. At f/2.8, the shallow depth of field has her right eye quite soft while the background shows round, out of focus highlights also known as bokeh.
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