Crop a 18mm shot to 300mm ?

Discussion in 'Beyond the Basics' started by prodigy2k7, Mar 22, 2009.

  1. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    Is this possible? To use some sort of software to crop an image produced by a wide angle (like 18mm) and convert it to produce the same shot as if you shot it from a 300mm lens (of course HUGE loss quality)?

    What i mean is for example:
    Lets say you go to the race track (drag strip), you stand at the finish line with a 50mm lens, take a snap shot of the starting line, go home and load it into software, zoom in and crop it until the car at the starting line is in frame, and it will tell you the appropriate focal length needed to produce that shot.

    So you can kind of guess what focal length you need before buying it and your 600mm prime turns out to be too short or too long for what you wanted... You know what I mean?
     
  2. Dwig

    Dwig TPF Noob!

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    I'm not aware of any software that would directly display the equivalent FL from a crop though it would be possible provided the camera recorded the actual FL in the metadata (most do).

    On the other hand, its a rather easy calculation if you:

    1. know the original FL (of course)
    2. know the original resolution in pixels
    3. crop in a good bitmap editor (e.g. Photoshop, ...) maintaining the same aspect ratio any crop without any resampling.
    4. find the resulting resolution in pixels.

    You would simply divide #2 by #4 and multiply the results by #1.

    In Photoshop, simply record the image width in pixels as found in any of several controls such as Image Size (don't make any changes, just record the width in pixels). You would then select the Crop tool and set it to a size in inches or mm (NOT pixels) that matches the aspect ration of your camers (most DSLRs are 2:3 so setting 2" and 3" or 4" and 6" will do the job). DO NOT set a ppi value. Now crop the image however you want and then check the resulting image width, again in pixels. If the original image was taken with a 50mm lens, has a width of 3000 pixels, and your crop is 500 pixels wide then:

    3000px / 500px = 6
    6 * 50mm = 300mm. the resulting equivalent FL

    Alternately, if you were contemplating the purchase of, say, a 400mm lens and you have a 50mm lens to make test shots with you could, with Photoshop at least, set the crop tool to a size in pixels, again without specifying a ppi value, that is equal to 1/8th (50/400 = 1/8) of the camera's resolution. Again if the camera is delivers 2000x3000 pixel images you would set the Crop tool to "250 px" and "375 px". The resulting crop would simulate a 400mm lens on that camera, at least in terms of field of view.

    Not all bitmap editors have as controlable Crop tool as Photoshop. The later approach is not likely to be available in simpler applications. The first approach may also be more difficult if you can't force the crop to a specific aspect ration. You would have to eyeball the crop to attempt to match the correct shape.
     
  3. epp_b

    epp_b No longer a newbie, moving up!

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    Heh... no, sorry, it doesn't work that way. First of all, you'll have unprecedented crap resolution after cropping. Second, it still wouldn't be the same because different focal lengths create different effects on perspective, depth of field, distortion, etc.

    Here's a shot at 18mm (28mm equivalent) from a 6mp original:

    [​IMG]

    300 / 18 = 16.6...
    3008 / 16.6... ~= 180
    2000 / 16.6... ~= 120

    Here's a 100% crop from the from the centre of the original photo at 180x120, roughly equivalent to what 300mm would capture.

    [​IMG]

    Even from the highest available resolution DSLR (Nikon D3X) at 24mp, the crop would would be 365x240:

    6048 / 16.6... ~= 365
    4032 / 16.6... ~= 240

    Still a piddly image.
     
  4. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    hmm k thanks :)
     
  5. Dwig

    Dwig TPF Noob!

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    1: Its true that the resolution would be "crap" if you are cropping a large amount, like the "make an 18mm simulate a 300mm" from the OP's title line.

    2: Perspective is not affected by focal length, other than indirectly by influencing the photographer to shoot from a different position. It is purely a matter of the relationship of the lens-to-subject and lens-to-background distances. The cropped image would have EXACTLY the same perspective.

    3: True, the DOF would be different if you use the same marked f/stop for the cropped shot with the short lens and the uncropped long lens shot. The cropped shot would show greater DOF though the difference would likely be lost in the haze of the lower resolution.

    4: Distortion is not a factor of focal length directly. No two lenses of differing optical design will have identical barrel or pincushion distortion. Its impossible to generalize as to whether there would be a visible difference or not. You'd have to specify exactly which two lenses were used in order to say.
     
    Last edited: Mar 23, 2009

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