Lens Calibration

Started Dec 15, 2007 | Discussions thread
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SteveL54 Senior Member • Posts: 2,694
Re: Lens Calibration

Bill Heiser wrote:

The document at focustestchart.com is very informative and helpful.
But I have a dumb question. What's the recommended method for
ensuring the camera is mounted at exactly 45 degrees on the tripod?

Don't worry about getting the angle exactly 45 degrees. I just eyeball the elevation angle.

It's more important to get lateral positioning and azimuth correct. If these are out of alignment a perfectly focused chart image will read front focus on one side of the chart and back focus on the other side of the chart. But even that alignment is not critcal. The best tool for ensuring lateral and azimuth alignment is the viewfinder grids. I always turn them on.

I'd post a link to a photo where I intentionally misaligned to the target but Pbase has been acting up for the past few hours.

Also, there's been mention here of AF fine tuning at close distances
not resulting in proper settings for focusing at infinity. What's
the recommended procedure for fine tuning for infinity focus?

I just use a good sized house well down the block. Any wide open field with a big well lit target at the other end will do. Just make sure it and other objects beside it more than fill the focus bracket indicator. If something closer impinges on the focus area the AF may adjust to it's distance instead.

Lastly, there's been a lot written about the 17-55, and how (when
it's wide open) it is sharp at the center but not at the edges. How
should one accomodate for this in the fine tuning process?

I don't have this lens but from what I've read there are some considerations to take into account when testing it.

I wouldn't rely on the chart to test this lens at it's short end. (under 24mm) The target area is a bit too small unless you are real close. You should do the overlay trick I describe in the last section of my ealier post. That will allow you to get further from the chart.

http://forums.dpreview.com/forums/read.asp?forum=1039&message=26037091

Also this lens exhibits field curvature below 24mm. The scales are out to the side and reading them could indicate a false front focus because of the way this lens's focus plane curves. Other lenses like my 18-70 would register a false back focus at it's short end because the field curves the other way.

If you what to test at 20mm or so, I'd line the camera directly over one of the side scales. Place the overlay sheet directly over that side scale. So when you take the photo of the chart (after auto focusing on the overlay) the scale runs directly through the center of the image. Field curvature does not affect the result with this approach

Steve

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