Field of View question

Roy C · 20 Nov 2010

When you shoot with say, 600mm on a 1.6 crop camera you have a FOV equal to 960mm from a full frame camera.
If you then crop 50% of the frame (say a 9mp crop from a 7D shot) is there a way of calculating the FOV that this crop is showing (I guess it is not as easy as 960 x 2).

StewartR · 21 Nov 2010

When you're using a telephoto lens rather than a wide angle, it is almost that easy.

However, if you're cropping a 9MP section out of an 18MP original image, you need to realise that the area (and pixel count) has been reduced by a factor of 2, but the linear dimensions have only been reduced by the square root of 2 which is 1.414. So the field of view is equivalent to 960 x 1.414 = 1350mm approximately.

This assumes you've maintained the same 3:2 aspect ratio. If your crop is a different shape to the original image, it gets more complicated.

Roy C · 22 Nov 2010

StewartR said:
When you're using a telephoto lens rather than a wide angle, it is almost that easy.

However, if you're cropping a 9MP section out of an 18MP original image, you need to realise that the area (and pixel count) has been reduced by a factor of 2, but the linear dimensions have only been reduced by the square root of 2 which is 1.414. So the field of view is equivalent to 960 x 1.414 = 1350mm approximately.

This assumes you've maintained the same 3:2 aspect ratio. If your crop is a different shape to the original image, it gets more complicated.

Thanks for that Stewart, it makes sense

as an example: a full frame from the 7D is 5184 pixels wide, if I crop (at 3:2) a part which is 2000 pixels wide then the FOV will increase by a factor of 2.59 (5184/2000). Is this right ?

StewartR · 22 Nov 2010

Roy C said:
a full frame from the 7D is 5184 pixels wide, if I crop (at 3:2) a part which is 2000 pixels wide then the FOV will increase by a factor of 2.59 (5184/2000). Is this right ?

Yes, if you're using a telephoto lens.

With wide angles the calculations aren't quite the same, but for any focal length over 50mm this will give an answer which is virtually exact for all practical purposes.