What's the significance of lens construction?

[Wild Wassa]

Why are there so many elements in a lens and why are they in groups? ... and following on from Franglais's first reply.

In a nut shell, ... but I'm happy to delve deeply into the theory if you like.

Compound lenses are necessary because single elements can not be highly corrected of aberrations.

Groups of elements consist of positive elements (convex lenses) and negetive elements (concave) which neutralize many of each other's aberrations, combining to produce an almost perfect positive lens.

These aberrations are only corrected for a limited range of object to image ratios. With the lens you are looking at, the Canon EFS 17-85mm f1:4-5.6, as you zoom the lens you change the subject to image ratio. The aperture not only changes the amount of light reaching the image plane, it also has a function called, being an effective aperture that helps to correct aberrations or unfortunately induce aberrations.

When the lens is at 17mm a minimum aperture of f4 will adequately reduce aberrations but when the lens is being zoomed out to 85mm if remaining at f4, aberrations will occur. By reducing the maximum aperture towards the higher focal lengths to f5.6 those aberrations are then reduced.

You might like to see this lens test programme for the EFS 17-85mm f1:4-5.6 showing the Nyquist Frequency which gives the maximum theoretical resolution of the lens at the different zoomed focal lengths in relation to the different apertures over the entire field of coverage. In this test you can set varying focal lengths and f-numbers and scan the field of coverage, to find the optimum settings for the lens. The test will give a relationship to sharpness over coverage in line pairs and shows changes in chromatic aberration and brightness fall off.

Set to 35mm at f5.6 the lens is close to optimum, from the centre of the image out to the edges of the field of coverage. At 17mm at the edges of coverage ... well check the test and find the most effective aperture.

The test is not forgiving on this lens, nor should it be ... I have the EFS 17-85mm f1:4-5.6 and I'm delighted with it. I think it is a good lens. The test showed me the weeknesses to avoid.

http://www.dpreview.com/lensreviews/..._c16/page3.asp

You will need Adobe's Flash Player installed, downloads can be done from the test page if you don't have Flashplayer. Also allow some time for the test page to load ... it was almost a minute and a half.

Warren.