Given a prime lens of high quality, a doubling of the focal length can be simulated fairly well by cropping the image, then using the Photoshop ‘bicubic sharper’ imager resizing capability of PhotoShop. You do not get the reduced depth of field that would be created by the longer focal length, but in other respects, the end results can rival those obtained with an X2 extender, but without the loss of lens speed that these devices entail.