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Re: [OM] Re: bokeh&blades

Subject: Re: [OM] Re: bokeh&blades
From: Joel Wilcox <jowilcox@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 07 May 2003 07:20:36 -0500
At 09:07 AM 5/7/2003 +0100, you wrote:

Hi,

> The 75-150 is a gentle little lens which seems to do nothing badly.

Yes, mine's been sitting in the cupboard unused for several years, but I've been playing around with it again recently. Took a couple of shots to compare it to the 90mm/f2 and I'm very curious. One thing I noticed really quickly is that the colour rendition seems far less punchy. I was looking at a red brick wall with a tree in front of it, both lit by a low sun, with a great blue sky behind it. The 90mm/f2 being the impressionist it is, gave a Velvia look, whereas the 75-150 was a bit pale.

I noted this somewhat in C.H.'s photos too. The 75-150 seemed to have marginally the lowest contrast of all his photos with the D60. The lens has a reputation for being the least sharp and least contrasty at the long end.

I thought C.H.'s example acquitted itself very well though. Absolutely nothing shabby about it.

There must be huge sample variation with this lens, not the least because of its sheer age. My example I had always thought was pretty good. I had a little repair done to loosen up focusing and we discovered it had fungus on one element (I hope from a previous life -- I picked it up in a pawn shop). It is much sharper now! There are supposedly lots of examples of the older 75-150's with element separation. That will spoil contrast too!

It's quite possible to make a lens with a large number of blades that has bad bokeh. Usually, the blades are a bit rounded off, to make the diaphragm a bit more round. Straight blades would give regular octangles. Rounding the blades in the other direction would give stars :-)

The key article on bokeh that was referenced recently shows an example with triangles. I interpreted that to mean that you can get some really weird artifacts from unusual diaphragms, not that the diaphragm itself is the sole cause of good or bad bokeh. Or at least that's what I'm saying now that C.H. has sort of demonstrated that the blur characteristics must be more due to the lens formulation than a raw count of aperture blades. :^)

Joel W.


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