Hello!

Perhaps, my question is too theoretic. May be, it does not refer to the beginners section. Nevertheless, I put it here as I am a beginner.

I would like to know how to evaluate the object’s size on the camera’s sensor roughly. As I understand, the object’s size on the sensor depends from the object’s original size, the distance between the camera and the object and the focal length.

When I asked question how to determine minimum shutter speed for moving objects in this forum (

https://www.pentaxforums.com/forums/pentax-beginners-corner-q/165146-how-calc...g-objects.html), I received very good answers and links to interesting web-pages. According to one page, the object’s size on the sensor is reduced by the factor R = F/D, where F is the focal length and D is the distance between the camera and the object. I rearranged the formula, did some simplifications and formulated such a formula:

Objects size on sensor (in mm) = Objects original size (in m) * focal length (in mm) / distance between camera and object (in m)

I tried to carry out such a calculation for the high building or tree:

- the objects is 20 m tall (high),

- the distance between the camera and the object is 25 m,

- the focal length is 28 mm (widest angle for my standard kit lens).

So the calculation is the following: 20 * 28 / 25 = 22,4 mm

I have Pentax K-x which has APS-C (23.6 x 15.8 mm) sensor. If the calculation is correct, the objects size on the sensor exceed the sensor’s length (15.8 mm) but it is under the width of the sensor (23.6 mm). So I could take the picture by holding camera vertically. And the object’s size will take 95% of the width of the sensor and the photograph, in turn.

In these calculations I have not taken into account the crop factor. As I understand, it does not affect the object’s size on the sensor. It only determines whether the object’s size exceeds the dimensions of the sensor or not.

I would like to know whether this formula is correct for rough calculations. Such calculations could help to determine which lenses (for example, 18-55 or 50-200) take before going out to do shooting

.

Best regards

Alberts