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Diaphragm
Diaphragm lens opens wide enough for artistic use and low-light shooting. The diaphragm is electronically controlled and has 5 blades. At narrow values of the glare in hips turn into nuts.
Depth of field, sharpness
The quality of the image at open aperture will not allow using it as often as we would like, primarily due to the shallow depth of field. Various aberrations, which are usually observed with all lenses precisely at the widest aperture, will also dilute the company with a shallow depth of field.
Photos at wide open aperture of the Canon EF 50mm f1.8 II can truly be called artistic. Sharpness is present only in the center of the frame, so it makes no sense to use the open aperture of this glass everywhere. Narrowing the aperture down to f/4 adds sharpness, bringing it to, in fact, evenly distributed across the entire field of the frame.
Sharpness is definitely better than that of the same massive Canon EF 18-55m f / 3.5-4.5 IS II, but Soviet optics and optics of the GDR of the Soviet period, at the same cost, will give better picture quality. For example, for the same price of a used Canon 50mm f1.8 II, you can find an original used Carl Zeiss Jena Tessar 50mm f2.8 T. autofocus in which, as it were, but little sense from it.
At wide open the DOF is so thin that autofocus can only be used in Live View, and it is very easy to miss through the viewfinder. If you get used to it, you can get good pictures.
Vignetting
Corner vignetting is noticeable from wide open apertures up to f / 4. It can be corrected without problems in post-processing, or with a software vignetting corrector built into the camera.
Diffraction limit
The diffraction limit comes at f / 16, it is at this value that the "crumbling" of the picture is visually and clearly visible. It looks like a photograph taken on an old mobile phone, with a full range of color deviations and loss of sharpness.
Chromatic aberration
Chromatic aberration, its most common form, when green or blue halos appear around sharp drops in image contrast, is well developed at open aperture, but disappears completely by f5.6. In fact, it is enough to hold down the aperture down to f2.8 so that this effect is not so noticeable.
Coma
Drops significantly at f / 4, and a long soft contrast transition to the edge of the frame becomes identical to the center of the image at f / 5.6.
Distortion
Distortion is noticeable on a full-frame camera, and even then, provided there are straight lines in the immediate vicinity of the edge of the frame. If you have a cropped camera, then this should not bother you.
Boke
The background blur of the Canon EF 50mm f / 1.8 II is enough to separate it from the subject. The background is not completely blurred. On a narrowed aperture, we can easily observe pentagons of highlights in hips... Five-petal diaphragm cut corners mercilessly. Bokeh has a definite structure, very similar to torsion, like the Helios.
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