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Trying to get any hard formula so according to that calculation for different focal length i place object accordingly. Eg.

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    \$\begingroup\$ Do you mean a formula to calculate the distance at which the subject must be positioned in order to appear the same size in your composition, across different focal lengths? \$\endgroup\$
    – osullic
    Sep 15, 2022 at 9:29

3 Answers 3

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If you swap to a lens with focal length twice longer, you have to stand back twice farther, to duplicate the same field of view. Or (concerned about the nit-pickers) move the subject back twice farther from camera, same math.

Replace twice with any factor, such as 1.3x or 0.5x.

Technically, this is same field of view at the focused subject distance. Other distances, such as the further background will have a changed field of view, due to the different angle of view of the focal length.

For example, if the distant background is a distraction in a portrait picture, you can use a longer lens and stand back appropriately further to see the same portrait field of view, but with a reduced background area, and then shifting the camera (or subject) slightly sideways can choose to include the best part of that background.

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As you know: A camera working at a specific subject distance with a specific lens focal length will image a specific field.

As an example: 35mm FF format 24mm by 36mm with a 50mm lens mounted, 5 units subject distance (may be meters, feet, inches millimeters.

Vertical angle of view 27.0⁰ 2.4 units vertical field

Horizontal angle of view 39.6⁰ 3.6 units horizontal field

Formula using EXCEL computes angle of view

=((ATAN((24/2/50)))*180/PI())*2

=((ATAN((36/2/50)))*180/PI())*2

Formula for field of view in this case 5 is camera to subject distance

=(TAN(27.0/2/180*PI()))*2 * 5

=(TAN(39.6/2/180*PI()))*2 * 5

Note: Makes no difference which unit is entered can be millimeters or feet etc.

You asked, I think, if you change to a different focal length lens, can you make an adjustment and maintain the same field of view?

Answer, you can compute a revised camera to subject distance that replicates the original field of view.

This is a simple ratio problem. Suppose we replace the 50mm lens with a 25mm lens. The ratio is 25/50 = 0.5

Now multiply original subject distance by this ratio thus 5 X 0.5 = 2.5 Place the camera with the 25mm lens mounted at distance 2.5 and the field of view will be the same as before.

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You can't...

You can change the distance proportionally so that the subject occupies the same area of the image, as described by Wayne. But that will not change the FoV/AFoV, which is an intrinsic characteristic of the lens' focal length (the resulting recorded FoV/AFoV is further influenced by sensor size/crop factor).

So, while the subject may be recorded the same size, what is recorded in the foreground, and more significantly in the background, will be quite different when using significantly different focal lengths.

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  • \$\begingroup\$ Angle of View is an angular measurement. Field of View is a linear measurement of the width, diagonal, or height (usually width) of the plane parallel the to camera's imaging plane at either a specified distance from the camera or at the current focus distance. Diagram \$\endgroup\$
    – Michael C
    Sep 21, 2022 at 3:48
  • \$\begingroup\$ Actually, it is field of view and angular field of view... they are the same thing except that FoV is at a specified distance. princetoninstruments.com/learn/camera-fundamentals/… \$\endgroup\$ Sep 21, 2022 at 14:07
  • \$\begingroup\$ AFoV is the same as AoV in that both are angular measurements measured from the point of the camera. On the other hand, FoV, as described in your own link, is an areal measurement of a flat plane at a specific distance from the camera. Of course any rectangular portion of a flat plane has an area that is dependent upon the linear measurements of its two axes. \$\endgroup\$
    – Michael C
    Sep 25, 2022 at 13:16

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