Which is it please? Thanks in advance. YA Sinclair
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Which is it please? Thanks in advance. YA Sinclair
The image circle is smaller in diameter at larger apertures (like f/5.6).
In addition, there will also be vignetting due to the lens barrel when you use the lens wide open i.e., at or near its largest aperture. This can be easily seen by removing the groundglass from your camera and squinting at the lens opening from the back. At or near the center, the aperture will look round but when you look at it from the corners, the circle will take on the shape of an American football due to the lens barrel cutting into the circle. DJ
If you examine the specifications of various lens manufacturers, you will see that the coverage (diameter of the image circle with even illumination and focus at infinity) is calculated with an aperture about 2-3 f stops smaller than the maximum (biggest). An exception to this would be with macro formulas which are generally calculated at closer than infinity. Anyway, you are apt to experience vignetting more at small apertures, this because the edges of the image circle will become more sharply defined as the lens is stopped down. This is why it is so important to check the corners of the ground glass at the taking aperture. If you are using a camera with a fresnel screen, you may find it even harder to see the corners with some focal lengths. If this is a constant problem for you, I would suggest the type of ground glass that has the corners cut. You would simply view the image at taking aperture, sighting through the corners to the lens. If you see the aperture, you won't experience vignetting.