A 14 MP Fuji Finepix S2940 digital (that i used today) 250 bucks..
Will probably not get you a razor sharp, full frame, photo of a jet a mile away doing 600 MPH. But . . . you can improve your chances if you crank the ISO up to 400 and the shutter speed up to 1/1000 or faster. Play with it at various ISO speeds until the images get too grainy for your taste. Most newer digitals will shoot fine at 400 ISO some will go even higher. Once you have the highest ISO you can use and still avoid too much grain, see how fast a shutter you can use and still shoot at an aperture of F8 or larger. (Most good digitals have a shutter priority setting.) Turn it to shutter priority then set at 1/1000 and see what aperture it chooses. If you get something higher than F8 you can up the shutter speed unit the aperture is at F8. Other than that all you can do is try panning as you shot. Follow the airplane for a good long ways before you squeeze the shutter and try to follow thru at the same speed.
Then get some good photo editing software, crop, sharpen and adjust luminescence, clarity, tint and sharpness and you will have a pretty decent photo.
I just bought Adobe Light table and am learning how to use it. Send me a couple of your best and I will see if I can improve them.
Good Luck!
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No huevos no pollo.