Technical lens question.

[Ducky]

1. Simply put, a lens is a collection of glass surfaces and shapes that gather light and direct it to a film or digi surface.

2. Lenses are refered to as sharp, soft, hard, contrasty etc.

My question is: what happens inside a lens to cause these results and can they be specificaly designed to be contrasty, soft etc.?

I know what the terms mean, I wonder what happens to light in a lens to get differing results.

 

[Ducky]

"Light converter", that's a good term, I'll remember that.
Thanks.

 

[andyr]

A "perfect" lens will direct all the light that originates at a certain point in the plane of focus (i.e. on the subject) onto a single point on the image (film/sensor) plane.

A number of factors affect how well this can be achieved.

Different wavelengths of light (colours) are refracted in different amounts depending on the glass used. This is why a simple single-element lens will give colour fringing; or in B&W will be not sharp, since different colours originating from the same point on the subject end up at different points on the image, "smearing" the subject out. This is compensated for by using multi-element lens designs, with different glass for different elements, so that variations in the refractive index of one element can be compensated by variations in another element that have the opposite effect.

Light is not only bent (refracted) at a glass/air interface. It is also reflected and scattered. Scattered light may end up being scattered elsewhere in the optical system and ending up unfocussed on the image, resulting in flare. This is obvious when a bright light source is in the image; but may be a reason for low contrast even when not bright source is present, since scattered light will result in higher light levels in the black areas of the image than there should be.

The accuracy with which the lens is ground will affect sharpness, since if light that travels form the same point on the subject along different paths through the lens does not end up at the same point on the image then the lens will be less sharp.

Like most engineering problems there are trade-offs and no perfect solution. for example a well-corrected (for wavelength/colour variations) lens may require many elements and many glass/air boundaries (or more generally, boundaries between materials with different refractive indices). This may cause additional flare and lower contrast through greater scattering. On the other hand a lens with fewer boundaries may have better contrast but poorer sharpness due to chromatic abberation (different olours ending up at slightly different points on the image).

So there are tradeoffs between different performance characteristics (and price) that optical designers make.

Andrew

 

[Ducky]

Andrew,
Believe it or not, I I think understood what you said. As I understand it a lens can actualy be designed to give certain effects if that is the designers intend. A precision lens, as in a microscope, may design out all unwanted elements (at a price) while a camera lens will consider cost as well as results.
I think I'm close and I appreciate the explainations.
I'm sure if I googled enough I'd find this but you guys are so clear sometimes and this way others can read if interested.
Thanks.

 

[Roger Hicks]

There is also the point that there's a limit to how far you can predict the exact performance of a lens. There's often a particular 'look' that you discover only when you actually build the lens.

Source: Zeiss lens designers.

Camera lenses are 'precision' lenses too! You can't design out all unwanted elements, because there's always something you could improve. For example, it's easier to build sharp, contrasty, flat-field slow lenses than sharp, contrasty, flat-field fast ones. Or it's easier to build big, heavy, sharp, fast lenses than small, light, sharp, fast lenses.

Cheers,

R.

 

[Ezzie]

Nice thread, well put Andrew.

 

[andyr]

To summarize: a lens design is an engineering compromise. There are many different aspects of performance (sharpness, contrast, bokeh, size, cost etc) and they have conflicting requirements so not all can be optimum in one lens. The lens designer chooses what trade-offs to make - i.e. hat aspects of performance to prioritise and which are less important given the intended application and market (and how to "balance" between them).

I agree with Roger tht you can only go so far with theory and simulation, but ultimately you have to build the lens to assess the more subjective "feel". This is probably the result of two factors: (1) limitations in the analysis/design software; and (2) limitations in our understanding of human perception, and how a particular combination of attributes will be perceived by different observers.

Andrew