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Re: [OM] Sharp vs contrast was Cheap, sharp Tamron 500/8 mirror in UK

Subject: Re: [OM] Sharp vs contrast was Cheap, sharp Tamron 500/8 mirror in UK
From: Moose <olymoose@xxxxxxxxxxxxx>
Date: Tue, 19 Aug 2003 13:30:19 -0700
It's more complicated than that. Others have posted in some detail about MTF curves to explain it all. In the simplest terms, your assumptions that the density of the center of a black or white line is the same as the density at the edge and that the densities of the black and white lines are the same with thin (high lpmm) as with thick lines is not true of real life films. What happens at the edge between black and white is not a simple, 100hange in zero distance, but a more or less gradual change, depending on the film. The way current film chemistry and the physics of light interact, the very highest resolution films have rather gradual boundaries. As a result, very thin lines never get anywhere near pure black and white and the resolution is between rather slight graduations of gray. Films with sharper edges tend not to resolve as fine lines, but those they do resolve are more sharply differentiated in density

Thus it is the combination of the ability to distinguish between finely spaced complete changes in brightness and the ability to delineate a density edge sharply (called acutance, in film) that are the 2 major determinants of apparent sharpness. Notice that I didn't use the word 'contrast', the overall density range between the brightest and darkest parts of the image, in the prior sentence. Contrast is a function in perceived sharpness, but a film may have high contrast and low acutance, vice-versa, and everywhere in between.

It turns out that wave functions are the best mathematical way to quantify these factors and the Modulation Transfer Function, MTF is a common way of defining these characteristics in components of imaging systems, such as lenses and films. Nonetheless, there are lenses with so-so MTF that are widely considered to be great performers in real world picture taking and vice-versa and films whose published specs don't seem to match results from use. Measuring MTF at a couple of frequencies doesn't seem to tell the whole story.

Moose

Johan Malmström wrote:

This is my opinion.

To get high contrast something must be sharp.
<snip>
Am I all wrong here?

Well, not 'all', just oversimplified.

So my question is: How could a lens yield sharp pictures with low contrast?

How low is low? What else is in the image chain? What do you see as sharp and how is that different than how I see? MTF is a combination of the performance of all the links, atmosphere, angle of lighting, lens, film/sensor, method of development, scanner, enlarger lens, paper, ink, etc., etc. A lens with really low contrast can't make a really sharp image. A film with really low accutance can't make a really sharp image. A lens with relatively low contrast and relatively good resolving power combined with a film with good contrast, good acutance and relatively low resolving power, may result in a really sharp looking (and that's what we are talking about here, there is no absolute measurement for sharpness!) 4x6 or 5x7, but look unsharp at 11x14 or larger.

If that isn't enough, oe can get the best possible combination of all the above factors and misfocus slightly, use too slow a shutter speed.......................... and it all goes down the tubes. It's a wonder we get any usable images at all! ;-) Congratulations to all of us for doing the impossible! :-)

Sorry it isn't as simple as you would like, but I think that's why photography is such a fascinating thing for most of us, and why it is as much art as science in so many ways.




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