How to find out?

 

With the coming of digital still cameras, the photographs are comparing lenses to the film camera ones, and use the focal length of the 35mm film cameras as reference. Thus, because there is a great number or digital sensors, the focal length of digital cameras lenses indicated doesn't really let the customer to know if it is a wide angle lens or a normal one.

It is the same for the micro video cameras.

 

  It should be more rational to specify the lenses with their horizontal angular field of view. So, instead to say a 35mm, we should say a 54° lens.

The hereunder tables give the angular fields of view of different lenses.

See also the Excel table angular_field.xls

diagonal angle draw in the diagonal direction

horizontal angle draw in the horizontal width of the landscape frame.

vertical angle draw in the vertical height

 

For measuring field of view:

Set a marked rule facing the camera. The figures must be large enough to be readable: 15mm or 20mm high.

Tilt the camera depending on the field angle to determine: horizontal, vertical, or diagonal.

Read  the two last marks at each end

Measure  the distance D from the camera to the rule.

Subtract  the smaller mark to the greater to get the field width W.

 

Calculate  the angle of field A with the formula:

  A = 2 x ATN ((W/2)/D)

or with this one:

  A = 180 x W / pi / D             pi = 3.14116

ATN is the mathematical function "arc tangent" If you don't have a scientific calculator machine, you can use the Excel table: angular_field.xls

If you can't calculate, send me your measures, and I will calculate them for you.

Graphic determination

 

Draw the width X

From the center of X, draw the distance D

Link the ends of segments D and X

Measure the angle with a protractor

 

When drawing this graph with 10cm for 1m, the accuracy is of 1°

 

            35mm  FILM CAMERAS 

                              
     dimensions  of  film         aspect ratio

width       height      diagonal       

36 mm     24 mm     43 mm           1,5                                                   
focal     ....................angle....................  mm        diagonal      horizontal      vertical                                                 
  21          92°             81°             59°     24          84°             74°             53°     28          75°             65°             46°    35          63°             54°             38°    40          57°             48°             33°    50          47°             40°             27°    70          34°             29°             19°    85          29°             24°             16° 105          23°             19°             13°
135          18°             15°             10°
150          16°             14°               9°
200          12,3°          10,3°            6,9°
210          11,8°            9,8°            6,5°
300            8,2°            6,9°            4,6°
400            6,2°            5,2°            3,4°
500            5,0°            4,1°            2,7°
600            4,1°            3,4°            2,3°

    DIGITAL  CAMERAS  sensor 1/1,8

      pixels: 2048x1536 (3Mpix)

                  or 2272x1704 (4Mpix)

 

      dimensions  of  sensor         aspect ratio 

 width          height       diagonal        

 7,18mm    5,38 mm    8,93 mm        1,33  

  

focal     ....................angle....................  mm        diagonal      horizontal      vertical   

   5,6            77°              65°               51°     7,0            65°              54°               42° 
   7,8            60°              49°               38°  
   8,0            59°              48°               37°     9,8            49°              40°               31°  13,7            36°              29°               22° 
 16,6            30°              24°               18° 
 21               24°              19°               15°
 28               18°              15°               11°
 30               17°              14°               10°
 32               16°              13°               10°

It must be noted that the digital cameras have seldom very wide angle lenses.

 

Product information of digital cameras

 

The product informations of digital cameras almost always describe the lenses of P&S digital cameras as equivalent focal distance of 35 mm film cameras. This equivalence is done by comparing the diagonal field angle between the 35 mm film and the digital camera lens. But if the focal distances are equivalent, the image aspect ratios are different, thus the horizontal and vertical fields are slightly different.

For example the horizontal angular field is 54° in the 3:2 or 1.5 aspect ratio and is 52° in 4:3 or 1.33 aspect ratio for a 35 mm focal length of 24x36 mm film camera or  digital  equivalent.

 

Equivalent focal distance of SLR cameras

 

The camera makers and the photographs have found practical to express the ratio of the image converter sizes. The image converters are the 24 x 36 sensitive area and the digital sensors. The usual reference being the 26 x 36 film image which image aspect ratio is 3:2 or 1.5, the ratio is calculated with digital sensors. It express the ratio between focal length to get an equivalent field of view and it is called "focal length multiplier".

Attention: Do not confuse the focal length multiplier and the aspect ratio!

 

As example let see the sensor of digital SLR Pentax K10D which size is 25,1 x 17,6 mm but which effective pixels area is 23,5 x 15,7 mm. the diagonal of this effective area is 28,26 mm to compare to the diagonal of 24 x 36 film which is 43,27 mm. the ratio 43.27 / 28.26 gives the focal length multiplier 1,5.

Thus a lens 35mm focal length  mounted on the K10D body will produce an image with the same field of view as a 50 mm lens on a 35 mm film camera.