Sigma C21EN0291 User Manual

Normally, the lens’ aperture is always fully opened, except at the time the 
picture is actually taken, for looking through the viewfinder under maximum 
brightness conditions.   

 
 
However, by pressing the depth-of-field preview 
button, the aperture can be closed down to the 
selected shooting aperture before the picture is 
taken, enabling you to check depth of field 
through the viewfinder before shooting. 
 
 

 
 

When you set the focus for a particular subject, there is an area in front of and 
behind your subject, which will also be in focus. This is called the 
Depth-of-Field. 

 

Depth of Field will increase when 
 

i.e. When you photograph the subject under the same conditions but change the 
aperture value from F8 to F22, F22 will show more area in focus (more depth of field). 
 

i.e. When you photograph the subject under the same conditions, changing only focal 
length from 50mm to 28mm, the 28mm lens will show more area in focus (more depth 
of field). 
 

When you photograph the subject with same focal length lens and aperture value but 
change the camera-to-subject distance, the subject at the greater distance shows 
more area in focus (more depth of field) compared to subject at the shorter distance.