Does the general problem of focus shift with Dagors also apply to Gold Dot Dagors? If this focuing problem does exist, I would appreicate comments as to the extent and nature of focus shifts with Dagor Gold Dot lenses.

-- Robert J. Triffin (RJTRIFFIN@RCN.COM), December 17, 2001

Answers

Sorry, I know nothing about Dagors, but with a header like that I had to click on your post. For a second there I thought I had landed at the unmoderated forum at photo.net!

-- Terry (tcdvorak@aol.com), December 17, 2001.

I did not mean to be provacative with my header, and I appologize for the mis-spelling in my posting.

Robert J. Triffin RJTRIFFIN@RCN.COM

-- Robert J. Triffin (RJTRIFFIN@RCN.COM), December 17, 2001.

I have this vague recollection that Wisner addresses focus shift in longer Dagors in the Q&A section of his site at WWW.Wisner.COM. Check the historical Q&A section on his webpage. You can do a "find" on "Dagor" once locate the correct page. I could be wrong about focus shift, but I know that he speaks about Dagors.

-- neil poulsen (neil.fg@att.net), December 17, 2001.

I have a 12" Gold Dot Dagor, and I haven't noticed a problem, but maybe it depends on the subject distance to some extent. Under what conditions is the focus shift said to occur, if it is?

-- David Goldfarb (dgoldfarb@barnard.edu), December 17, 2001.

Dagors are notorious for focus shift, which is the result of uncorrected zonal spherical aberration. As far as I have heard the gold dot version was supposedly a coated version of the dagor (however, there has been some claim that dagors and artars were redesigned every now and then to take advantage of new glass types etc., but for the most part, this seems unsubstantiated). A Dagor has only 4 air-glass surfaces and the benefits of coatings are minimal. Assuming the design is similar to the Dagor (which would be my guess), I would expect the gold dot Dagors to show focus shift as well. Keep in mind that focus shift is partly an optical ilusion based upon how we tend to focus. It is caused by uncorrected zonal spherical aberrations, which means that rays from the center of the lens are brought to focus at a different point than rays from the periphery of the lens surface. This means that there is a range of focal lengths forming a sort of tubular range of focus. This is also the reason that Dagors are said to provide a pleasing glow around highlights. When the lens is wide open, light from the center and periphery compose the image points. As the lens is stopped down, rays from the center dominate. If you judge focus by the amount of contrast, you are likely to experience focus shift as you stop down. Cheers, DJ.

-- N Dhananjay (dhananjay-nayakankuppam@uiowa.edu), December 17, 2001.

Interesting. I usually recheck focus when I stop down if possible, so maybe I'm correcting it without realizing that I am doing it.

-- David Goldfarb (dgoldfarb@barnard.edu), December 17, 2001.