All you mathematicians, cropping considering focal length?

[Persian-Rice]

I don't like to do math so maybe someone can help.

I have an image taken at 105 mm, now I want to crop it, but I want to crop it so it looks like it was shot at 365mm or 450mm.

Ok so why am I asking this dumb question? I took photo's at the Canadian GP at an effective 105mm(a75, is that correct? or is that including digital zoom? I didnt use digital zoom) last year, I will be taking my good equipment to the same event this year. I have the same seat, so I wanted to know if my glass is long enough or if I need to save and get something else before the race this year.

Is there a formula that will tell me how many pixels to cut off or what percentage to get rid of?

Thanks

[Persian-Rice]

am i complicating everything? or do I just make it 1/3 of the size?

[kawter2]

woudn't you crop the image by 29%???

[kawter2]

am i complicating everything? or do I just make it 1/3 of the size?


that is what i was thinking

[pfogle]

Hi P-R, is this what you were after?

If I understand it right, you shot last year with an A75 at full zoom which is equivalent to 105mm on 35mm film? Now you want to get a crop equiv to around 400mm.

If you're shooting on a 10D (1.6 crop factor) you'll need roughly 250-300mm glass.

cheers
Phil

[pfogle]

ps if you've got a 200mm, that's equiv. to 320, so you'd crop 20 percent to go from 320 to 400mm

hope that's right!
cheers
Phil

[hmhm]

Is there a formula that will tell me how many pixels to cut off or what percentage to get rid of?

To simulate the angle of view of a lens with double the focal length, you should crop by halving the image horizontally and halving it vertically. Note that this yields 1/4 the number of pixels.

If your original A75 images were at 105mm "35mm equivalent" and 2048x1536, and you wanted to show what, say, a 420mm "35mm equivalent" angle of view would look like, you could crop your A75 images down to 512 x 384, i.e. crop down to 1/4 the width and 1/4 the height. Note that this represents 1/16 of the area of the entire original image, and 1/16 of the pixel count. Also note that your A75 has a 4/3 aspect ratio, which is a little "squarer" than a 1DII or 35mm film, so an exact comparison isn't technically possible.
-harry

[griff2]

Persian Rice wrote:
am i complicating everything? or do I just make it 1/3 of the size?
You're nearly right. An effective 105mm is ~ 2x 50mm which is considered 1x. Forgetting crop factor of digital cameras, since we are solely looking at converting an effective 105mm to an effective 450mm you will need to zoom into the image by 9/2, i.e. each linear dimension will have to be the square root(2)/3 x each linear dimension. See included picture.
http://www.griff2.co.uk/pictures/picture.gif