Working out focal length

[mrbez]

Hi Guys,

How can we work out what focal length a lens would provide?

Ie, if I take a photo that has been taken with a 17-50, how could I know where to crop that to see what it would like if taken with a 200mm?

I am trying to figure out if a 70-200 is something that I would really want.

I am intrigued by street photography but don't think I have big enough cojones yet to use a smaller lens, so to start out I would like to try a 70-200. If I have a pic taken at 50mm, how could I crop it to see what it would be like with a 200mm? Also, if anybody can help, how far back could I stand with a 70-200 in order to get a portrait?

Thanks.

 

[tdodd]

It's simple maths based on the ratio between the focal length you shot at and the focal length you want to simulate. e.g. if you shot at 50mm and want to see what a 200mm lens would give you then you need to crop out a section that is 50/200 = 1/4 of the width and 1/4 of the height.

For the 40D, with its 3888 x 2592 pixel images you'd want to crop out an area of (3888/4) x (2592/4) = 972 x 648 pixels.

To turn that into a simple formula....

Cropped pixel dimension = original pixel dimension * focal length used / focal length desired.

Repeat for horizontal and vertical dimensions.

 

[fitzer]

Is THIS the sort of thing you're after?

 

[mrbez]

....

Does that mean that the final image with the 200mm lens would look like a 972 x 648 crop on the 40D original?

Like this:

Or do I need to crop 972 x 648 off the top and also down the side?

Thanks.

 

[tdodd]

I mean you crop a 972x648 rectangle from anywhere within the image you like.... EDIT : Just as you've shown in your edited reply above.

That will give you the framing of a lens with a focal length 4X longer than the one you used.

Of course, if you crop like that then your IQ goes tits up, but you'll get a feel for the extra magnification offered by the longer lens.