Manual focus lens focus indicator

Started May 11, 2012 | Discussions thread
Vlad S
Senior MemberPosts: 3,073Gear list
Split prism is impractical on µ4/3
In reply to Marvin Doering, May 12, 2012

Marvin Doering wrote:

I agree with the comment on the split image focus screen but I'll go you one better. The split image screen on a 45 degree angle. That was available on some of the older Nikon SLRs and I found it worked better than the traditional split image screens.

Split prism is impractical on micro 4/3 for 3 reasons:

1. Split prism focusing needs an optical device (prism) that would only detect light from one edge of the lens. Such a device can't be placed in front of the whole center, but in principle, it can be placed in front of some of the pixels. Fuji has patented such arrangement for on-sensor phase detect, and Nikon must be using something like that in their 1 system. Nevertheless, to form a detailed part of an image, as opposed to a just a line as sufficient for PDAF, one would need a lot of such pixels, meaning that the "picture taking" pixels would suffer.

2. The sensitivity of such device depends on the diameter of the aperture. It's just like a regular rangefinder, but instead of the two windows it uses two edges of the lens. Because the micro 4/3 f numbers correspond to the aperture diameters that are only 1/2 of that on full frame SLR, the accuracy would be correspondingly low.

3. Because the size of the aperture is important for the accuracy, SLR keep the aperture open, and only stop it down during the shot. If we use a manual lens on a ยต4/3 camera we would have to stop down the lens by hand.


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