Originally posted by bull drinkwater you are missing my question. do the new lenses have the 1.26 crop factor with the 645d that the vintage lenses have?
Focal length is focal length, and image circle is image circle. These are related but separate attributes, but they are facts not affected by the size of the image the camera detects behind them.
A 55mm lens magnifies scenery elements the same size on the sensor as any other 55 lens. But a 55mm lens for a small-format camera might only have a 2-inch image circle, and leave the corners dark, if the sensor is bigger than that. The 55mm lenses for the 645 have 3-1/2” image circles, and the 55mm lenses for the Pentax 67 has a 4” image circle. But inside the circle, the scenery is magnified the same amount. That’s why lenses designed for larger formats work on smaller formats, but not necessarily the other way around.
So, a smaller sensor merely cuts off a part of the scene, rather that magnifying it differently. That’s why it’s called a crop factor. We can speak of “equivalent” focal lengths—those focal lengths that provide the same field of view for different formats. But for a larger format to have the same field of view, it needs greater magnification to fill the larger frame with the same scene. Greater magnification requires a longer focal length. So, 75mm on the 645 film frame is
equivalent to 55mm on the 645 digital sensor, because it magnifies the same scene a sufficiently greater amount to fill the larger frame.
The 25mm focal length is less than half the diameter of the 645 digital frame, and a third the diameter of the film frame. That means it has to have an image circle at least three times its focal length to cover film. That is rather extreme, and it’s why that lens has such a bulbous front element. But it also means the light is approaching the sensor at a shallow angle, and any image spilling off the sensor has a tendency to light up the mirror box, which reflects around and lowers contrast. That’s why they limited the image circle mechanically in the later version.
And I bet Ed Hurst’s fears are well-founded—I bet there is other frame-limiting masking inside the lens in addition to the tighter lens hood.
At one-third of the film frame diameter, 25mm is about equivalent to 14mm in a small-format 24x36 camera. Those lenses have bulbous front elements, too. But the question is whether it will light the corners of the larger frame.
There is another question, too. And that is whether the lens performs well enough outside the digital frame. That may also be why they mechanically limited the image circle.
Rick “sensors don’t change focal length” Denney