An Unusual Focusing Aid

[xpatUSA]

I was taking a snap of some construction work (I'm closing in the veranda). About 10ft away, at a slight angle, was a wire screen. The camera has a true pentaprism optical viewfinder and no live view. There, before my very eyes, was obvious moiré which moved around as I adjusted the manual focus. So I took a shot with the moiré in the center and then, for good measure, with it also to the left and to the right.

It worked out well in that the RAW images were in focus and the fine mesh was visible in the final images.

Not surprisingly, the moiré was plainly evident in the embedded quarter-size preview JPEG but it was barely visible in the converted image.

Point of this post is that I was quite surprised to see moiré in an optical viewfinder!

[TonyW]

My understanding, which might be a bit flaky, is that moire patterns occur when a periodic pattern on the object produces a pattern on the sensor which is slightly out of tune with the pixel frequency, somewhat in analogy with the beats that occur between two musical notes slightly out of tune. In your case, could there be some pattern in the system, for example grinding marks on the mirror, which is normally not visible?

[xpatUSA]

My understanding, which might be a bit flaky, is that moire patterns occur when a periodic pattern on the object produces a pattern on the sensor which is slightly out of tune with the pixel frequency, somewhat in analogy with the beats that occur between two musical notes slightly out of tune.

Not flaky, Tony, you describe the well-known phenomenon very well.

In your case, could there be some pattern in the system, for example grinding marks on the mirror, which is normally not visible?

The only thing that springs to mind is something called the 'focusing screen' which is often of a type called 'Laser-matte' but I don't know whether that refers to a completely matte screen or a clear screen with a matte border. In any case, I view a matte surface as being un-patterned, so I'm still baffled. However, in photography, I've yet to find anything called what it actually is

Anyhow, here are a couple of cropped screen captures:

Low res. Preview:



Higher res. Output:



Duh . . .

[Inkanyezi]

Moire patterns in the viewfinder of an slr is not new, although using it as a focusing aid was not feasible in the early viewfinders, which did not have the repititive pattern of the fresnel lens in the same plane as the ground glass.

The clue is that there actually is a repetitive pattern in the focusing screen, i.e. fresnel lens that is regular. Also most "laser matte" or whatever designation the manufacturer wil call them (acute matte et al) might have such repeating patterns, and they can thus present moire when there is a slightly different pattern projected onto it. On modern screens that have the pattern in the plane of focus, it can be used for focusing.

I have also observed the same in the EVF and on the screen of my Panasonic and Olympus µ4/3 cameras and used it for focusing. Sometimes a shimmer may appear at an edge, serving the same purpose.

[xpatUSA]

Thanks Urban,

Clearly, I still have to learn about cameras