D90 Focus Problem? Please help!

Started Apr 5, 2012 | Discussions thread
Forum MemberPosts: 78
Re: Please Validate--Live View vs. Viewfinder Test AF Results
In reply to SteveL54, Apr 10, 2012

I need to make it more clear in my instructions that the example photos I post are 100% center crops.

Oops. I honestly don't even know what "100% center crops" means.

If your posted photos are the full image then the camera may have been closer than recommended if that sheet with the target is 8 1/2 x 11 inch or A4.

The camera should be at a distance 50 times or so the focal length of the tested lens. So camera with a 50mm lens should be around 2.5 meters away. My example was run at 64X for a zoom set at 24mm which comes out to about 1.5 meters. The exact distance ratio isn't critical but it should be greater than 30X FL unless you are specifically testing AF accuracy at close range. (PDAF focusing errors are more common near minimum focus distance of a lens.)

Yes, my posted photos are the full image. I used my 24-70 lens, and my focal length was 50mm. So, I need to either back up to 2.5 meters and use 50mm or back up to 1.5 meters and use 24mm focal length. Right? I left everything taped to the wall so that I could repeat the test, if needed.

The target line pattern needs to be just big enough to fill the AF focus point's area of sensitivity. This is typically twice the dimensions of the AF point's marked bracket in the VF.

So, if I have the center focus point selected, I need to back up or zoom out so that the target line pattern becomes a much smaller part of the overall picture--as you said, approximately twice the dimension of the center AF point's bracket in the VF. I made sure that the pattern filled the entire AF area (all AF points) in the VF except for the outermost left and right one. Oops.

It's always helpful to have someone report back like you did to see how my instructions are interpreted or understood. I'm too familiar with the procedure so I may not emphasize something enough to those trying it out for the first time.

I'm happy to be a guinea pig. I thought that your directions were very clear. I just didn't understand the "100% center crops" terminology, so I framed my photos to look like those you posted. My bad. Also, any thought to making the test pattern available as a downloadable pdf from the test link? It took me a bit to figure out how to do it in word. I'd be happy to send you mine if you don't still have yours.

It would still help if we could see your photos at full size, since we might see an issue that isn't apparent at the partial magnification currently available.

Since my photos were uploaded directly from the SD card, they are considered "full size," right? Or did I do this wrong?

I'll repeat the test and repost the full images. I don't know how to properly do the "100% center crop" thing, but if someone can walk me through that, I'll do that, too.


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