Ultra wide angle lens the difference is only whether distortion is present or not. However, the shorter the focal length, the more difficult it is to implement rectilinear correction. In fisheye lenses, the visual angle is close to or more than 180 degrees in at least one direction. For example, a "diagonal fish eye" would have a viewing angle of at least 180 degrees within the diagonals of the frame. A "circular fisheye" would represent the image in the form of a circle. Rectilinear ultra-wide angle lenses are used in photography and cinematography sometimes to achieve three-dimensional perspective distortion instead of simply two-dimensional barrel
P-90 rather than subtract the signal between the coils; thus, the impedance can be lower, avoiding overwinding to obtain sufficient output. There is less signal cancellation. Sonically these sit somewhere between a true P-90 and a stacked pickup. They don't chime but they aren't nasal or lifeless either. They are ideal candidates for overdriven sounds where loss of sparkle is less important than avoiding hum amplification. These type of pickups have the same appearance of P-90s. The new generation Zero-Hum P-90 Hx by Kinman strives to have a P-90 sound and appearance, but is far more complex, with 202 individual parts.