Equation to determine focal length

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Adam Cross

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Hi, I am wondering perhaps quite a specific question. I need to know which lens is required for a certain job. Could anyone please tell me how to calculate which lens is required to photograph a subject at x distance away which is y metres wide using the sensor resolution of the Canon 500D and to ensure the subject (a puffin) within the frame is greater than 16 square pixels?

Is there an equation to calculate the required focal length given all the above parameters?

Man ythanks.
 
Thankyou Paul for your help. However, is there an equation specific to pixel size. For example, which focal length is required for a working distance of 700m photographing an area 200m wide and ensuring that the subject (a puffin at height 0.28m and width 0.15m, area ~0.042 m2) within this area is greater than 16square pixels? Again, assuming a sensor size of 22.3 x 14.9 mm (Canon 500d).

Apologies if I have not understood the previous reply
 
Somewhere in the camera manual will almost certainly be a specification relating to pixel dimension of the sensor - say 10,00 pixels high. Should be relatively simple mathematics to calculate the pixel size of a penguin on your sensor.
 
If you want to photograph an area of the scene that is 200m wide and your puffin is just 0.15m wide then you could fit 200/0.15 = 1,333 puffins across the width of the frame. Since your sensor is 4,752 pixels wide each puffin would be 4,752/1,333 = 3.56 pixels wide. Call it 4. Since the puffin is twice as tall as it is wide you would have one whole bird covered by approx 4x8 = 32 pixels. If you round down to 3x6 then you get 18 pixels, so either way that would seem to meet you needs.

To swap the formula posted earlier around in order to find the required focal length it should be....

Focal length = (Sensor Size x Subject Distance) / Subject Size

In your example your "subject" is the whole field of view of 200m rather than each individual puffin.

That gives Focal Length = 22.3 x 700 / 200 = 78mm.

Armed with that knowledge you can easily scale up or down for different subject distances or focal lengths. e.g. halving the subject distance will halve the field of view and double the size of each bird, resulting in four times as many pixels per bird. Doubling the focal length will do the same thing.
 
Don't know What phone you got if you got I phone or simular you can get a app..
 
Yes I would be interested in the app too for an htc
 
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