Originally Posted by MIkhail
Thanks for answering.
So do I get it right that if the a7 does not do automatic cropping you would get true 50mm except with dark corners?
As I understand it the Sony full frame camera software automatically recognizes that a crop sensor Sony lens has been mounted and automatically switches to crop sensor mode. But this can also be turned off in the camera's settings if you want to. This is how Nikon full frame cameras work.
You may be confused about what people mean when they talk about a 50mm crop sensor lens being like a 75mm lens. This lens is always 50mm lens no matter what camera it is mounted on. This never changes. But if mounted on a camera with a smaller sensor (one with a crop sensor - which it was designed for) only the middle of its image gets recorded because that sensor is smaller. This makes the final image look like it was taken by a 75mm lens (in this case) as it seems to give a closer view just like a "true" 75mm lens would on a full frame camera.
But lenses designed for crop sensor cameras are not designed to "throw" an image circle big enough to cover a full frame sized sensor (which is why they tend to be smaller than full frame lenses which usually have bigger optics inside them). So if you mount it on a Sony full frame camera you will get a black area around the image recorded on the full frame sensor (called vignetting) or the image has to be cropped to get rid of this. Usually this can be done automatically inside the camera by its firmware which discards the black part of the image which did not receive any light from the lens. Or you can turn this feature off in firmware and do it manually afterwards using photo shop etc.