Lens Questions Regarding Sharpness

[iflynething]

I keep continuing to think about this:

1) If glass is transparent, how can one lens be sharper than another. I understand if it has nano coating, or ED glass, but how does it change, glass is just glass unless it's in a lens. Is it similar to comparing museum glass to just glass in a frame? I guess the 18-200 is the wal mart frame glass and a 300 f/2.8 is the museum glass?

2) When changing the aperture, how in the world does it make a lens sharper. Technically, and ONLY technically speaking and how I see it: Hold your hand up and make a circle with your index finger touching your thumb.....did you do it?
Ok now look right through it and start to close that hole? The only thing that changes is the amount that you can see in front of your hand. Applying this to a lens, if you are closing an opening (the aperture), how and why does this change lens sharpness.

I'm sure some other questions might pop up, but this is all I can think of at the moment.

~Michael~

 

[Derrel]

Optical glass can have different properties,based on the source materials used to make the glass. For example, crown and flint glass can be combined in a simple two-element "closeup filter" or "closeup lens" to make it an achromat, like the Canon 500D. Different types of lass have different refractive indices. Your example is kind of like, "Well, Gallo jug wine is wine, so all wine must be like Gallo wine."

Lens elements can be ground...they are ground....some of the newer moulded aspherical lenses are mould-poured...some Wikipedias articles might help you. Basically, asking how one lens can be sharper than another...imagine a camera with a single lens, a meniscus lens...adding a second, and then a third lens element (doublet, triplet) were the approaches early lens designers went with. By the time a fourth lens element was added, it was possible to correct a number of optical aberrations,and produce lenses with surprisingly good sharpness,contrast,and freedom from most optical defects, since the lens apertures were not all that "fast".

In the modern era, computer ray tracing has aided lens design a huge amount, and modern industrial methods allow companies to make lenses with very precise tolerances; the quality and precision of the lens barrel and its spacing and retaining devices, internal flocking of barrels and the sides of elements, the precision and perfection of element grinds and polishing,etc,etc...all those kinds of things can make one lens design sharper than another that is manufactured to a lower standard.

 

[Garbz]

The physics behind it relate to aberration. Glass doesn't bend all light equally. The effect is dependant on the properties of the glass, and the angles at which light enters and exits. Light that hits the extreme edges of a lens element bends different wavelengths at different angles compared to light that hits the centre of the lens at a right angle.

Now apertures work on the theory that the entire lens is used to render a point source of light. You can think of this as holding a magnifying glass up and then moving your head left to right (not the glass) to see subjects on the other side. If you cover up a lot of the magnifying glass leaving only the centre point open you can still see everything by moving your head left and right, only now you're only using the centre of the lens, and thus less aberrations are introduced. In that kind of rough envisioning if you stop down the aperture you're no longer using the edge of the glass which has far harder to correct issues than the dead centre.

If glass bent all light equally then you could get away with just a couple of elements, and as mentioned above by Derrel a lot of corrective elements are used to bring these differences under control. The sharpness of a lens is therefore a product of manufacturing accuracy and the great juggling act that is lens design. I.e. do you go for a sharper zoom lens with less distortion, or do you put corrective optics in to get distortion free zooming with the acceptance of reduced sharpness etc.

 

[iflynething]

Thanks for you information Derrell. It's such a complicated process to get great optical glass it seems and guess explain why lenes such as the Nikon 24 f/1.4 are so expensive.

~Michael~