What has happened to this glass?

[PaulFurman]

I've heard of doing (something?) like that to lenses to create low contrast for shooting very high contrast scenes and not blowing out highlights or losing things in the shadows. Hmm, maybe I'm thinking of B&W film processing actually... some reflective macro subjects can be very difficult in the sun.

[Harold Gough]

It might have been an attempt to emulate some of the performance of e.g. :

http://www.adaptall-2.com/lenses/51A.html

Harold

[DaveW]

How does that differ from the Nikon defocus lens Harold?

http://shashinki.com/review/nikon/135DC/index.htm

http://www.stacken.kth.se/~maxz/defocuscontrol/

According to Ken Rockwell the original Nikon defocus control lens was introduced in 1990, ten years after the Tamron however.

Canon also do a similar lens:-

http://www.the-digital-picture.com/Revi ... eview.aspx

DaveW

[Harold Gough]

Dave, I have no idea about Nikon lenses but I do have the Tamron. At the time it was made, the Tamron's exact function was claimed to be unique.

I note that the defocus ring of the Nikon is about one third back from the front: on the Tamron, only the aperture ring is between the soft focus ring and the rear mount. The closest focus is similar.

There seems to be two completely different soft focus mechanisms. For the Nikon "The 135DC lens can create a soft effect by having the Defocus ring set at aperture smaller than the actual aperture of the lens". This might suggest that diffraction is used but your second link says it is spherical abberation. The Tamron has special "high order spherical abberation" giving "optimum amount of flare". Thus, spherical aberration by two different mechanisms.

I have now located the handbook for the Tamron lens.

From your first link "By using the soft effect created by the 135DC lens, there is still a sweet spot of sharp focus in the photo" seems similar to what is claimed for the Tamron: "diffused focus effects with a sharp core".

The Tamron (handbook) tell how soft focus can be in the background or foreground * and that the latter increases the resolving power (due to a narrower density distribution) in the foreground. It invokes a gamma curve to say why softness varies with exposure. It gives several pages on effects, including "impressionistic". This contrasts with (from your second link) "At first glance the effect was not clearly visible since it is rather subtle".

* The Nikon has "Front" and "Rear" settings which set out to soften background of foreground, but, apparently, in a much less sophisticated/versatile way.

Unlike the other two lenses, the Canon is not sharp, when wide open, (a feature of Canon portrait lenses e.g. the 85mm) with the soft focus off, and is reported to have considerable chromatic abberation.

Harold