Why no depth of field preview button?

When I first received my NEX-6 I thought that one of the things I was going to have to live without was depth of field preview. On a DSLR DOF preview is accessed by a button which causes the lens to stop down. After experimenting with manual focus I realised that on the NEX the lens is normally stopped down which means that the camera is normally showing DOF preview. Even better, because of the electronic view finder the image is always bright.

This behaviour does have consequenses. The phase detect auto-focus won't work at small apertures and contrast detect is going to work at its best at wide apertures which means that the camera has to open the aperture while it is focussing. You can prove this by going into aperture priority mode, setting the aperture to f22 and then half pressing the shutter button, watch the brightness of the image change on the display. If you look carefully (use a long lens for this) you can see the depth of field change in the viewfinder during focussing.

The other consequence is for the viewfinder which also works best at wider apertures. In order to get a bright image in the viewfinder at f22 the effective shutter speed has to be much longer than at f4 so the frame rate of the viewfinder drops dramatically, just like it does in low light.

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