I have seen many camera review sites illustrate the effect of using different focal lengths on the same photograph using frames. There are also similar illustrations to compare different sensors (full frame vs APS-H vs APS-C vs micro 4/3). I find this kind of visualization to be very useful in comparing the actual effect of the longer focal length\different sensor size. Of course the ability to do a wider focal length visualization would also be equally useful, but that's not possible.
Is there any ready-made software or plugin that enables this? If not, is there any simple technique to visualize the effect of longer focal lengths on an image (one would of course have to use the zoom information in the image EXIF data to accurately represent this)? All I can think of is to use some trigonometry to do the necessary calculations for the frame sizes for cropping.
Answer
If using GIMP (and probably any other image editing software), then you could use the selection tool and set the size of the crop frame to a required size and aspect ratio. This way, you can see the relative sensor size superimposed on your image while letting you panning it around.
Note that this technique is good only for smaller-than-the-actual sensor sizes, unless you "cheat" by stating the original was shot with a bigger sensor than actual.
UPDATE: Unfortunately, using GIMP, you cannot simply set the ratio of the size if the selection box to match the crop factor (or focal lengths ratio). As @Stan Rogers commented, you'll have to set the size in pixels based on the simple focal length ratio. Then you can move the selection box to the desired location and if you like, add a rectangular frame to the image itself, so you can compare several selection sizes.
In order to add the rectangle, use the "Edit" -> "Stroke Selection..." dialog. The default settings will stroke a solid rectangle on your image.
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