Some help for the newbies

Ricky91t

Just saw this over on TP,How to calculate focal length needed on a lens to capture a specific subject,In theory i think it's about right.

The squirrel shot match's up with one i took before IIRC

Anyway here the link

And the quote

(Mods: I'm hoping this might be suitable material for the Tutorials section.)

In my line of business I often get people asking how big a lens (ie what focal length) they'll need for a specfic task. It's actually quite easy to work out if you know - or can estimate - how big the subject is, and how far away it will be. Here's how.

The basic equation is:

Focal Length = 0.75 x Sensor Size x Subject Distance / Subject Size

Here are some examples. In each case I'm using a Canon 350D which has a sensor measuring 22.2 x 14.8mm.


Let's suppose we want to photograph a giraffe, like this.



The big giraffe is about 4m tall, say, and we expect to be able to get within about 30m of it. The sensor dimension which matters here is the 22.2mm because we're shooting the giraffe in portrait orientation - the height of the giraffe aligns with the long dimension of the sensor.

So the focal length we need is 0.75 x 22.2 x 30 / 4 = 125mm.


Or suppose we want to shoot a squirrel, like this:



The squirrel is about 50cm long, say, including its tail, and we expect to be able to get within about 10m of it. Again the sensor dimension that matters is 22.2mm, because the length of the squirrel aligns with the long dimension of the sensor.

So the focal length we need is 0.75 x 22.2 x 10 / 0.5 = 333mm.


And a third example: photographing the moon.



The moon is 3,474 km in diameter and its average distance is about 380,000 km. This time the sensor dimension which matters is 14.8mm because we want to fit the diameter of the moon into the short dimension of the sensor.

So the focal length we need is 0.75 x 14.8 x 380,000 / 3,474 = 1,214mm.


Hope this helps. By the way, the experts amongst you will have spotted that the factor of 0.75 in the equation is an arbitrary one to help give you a decent composition - you don't want your giraffe / squirrel / moon to completely fill the frame, and this factor is there to give your subject bit of room to breathe in the frame.