Archived posting to the Leica Users Group, 2014/02/20
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The Sony a7r focuses at approximately the the f-stop it is set at; it opens what looks like maybe a half stop more than the shooting aperture. You can look at the front of the camera and see the iris opening just a bit when it's focusing. It does not open all the way to wide open (f/2.8 for the 35mm FE) for easier focusing and brighter viewing. So, the a7r is unlike any modern SLR I am aware of. The typical SLR stays wide open for viewing and focusing, and then stops down for the shot. With an electronic viewfinder, the viewfinder is just as bright no matter how closed down the iris is, as long as the exposure (iris, shutter and ISO) is on the mark. As such, half the reason to have the iris open (viewing) is eliminated. The other reason SLRs have the iris open when focusing is to have the DOF very shallow, so it's easier to focus. However, this raises the issue of focus shifting when the iris shuts down. Sony gave Zeiss one more variable to forget about (relatively). Since the focusing is being done at close to the shooting aperture, Zeiss is free to allow focus shifting as needed to meet other design parameters. This is another example of the extent to which the optics and electronics are being designed as a package. So, one irony of the Sony is that while it's able to use legacy glass, it is also going to have optics that are more tied to the camera than in the past. As such, I think Zeiss will be able to produce sharper lenses for less money than could be done when they had to worry about focus shift issues. Note that there is an interesting possible future solution to the vibration -- have the iris do it all. It can be both iris and leaf shutter. Some compact cameras do this. However, you'd lose legacy glass compatibility and some very high speeds. I would not be surprised to see some lenses with a leaf shutter option. They are the smoothest shutters. Paul www.PaulRoark.com On Thu, Feb 20, 2014 at 10:19 PM, Frank Filippone <red735i at verizon.net>wrote: > Paul says it does stop down before it AF, Sonny says it stays open during > AF... Specifically with the same 35mm FE lens > > ???? I am confused..... > > Frank Filippone > Red735i at verizon.net > > > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information >