General Lens Specifications Question

Photo Joe

Sometimes I feel like such a mooch asking all these questions, but thus far everyone's been very kind and polite while helping me out. Thanks to all you for that.

So here are my questions: I've been doing some lens researching/shopping lately, but I tend to get a little lost when it comes to the technical specifications list. Sure focal range, aperture, IS, USM etc are all fine for me. But I have no idea what it means when a lens lists "X elements in Y groups." What's the significance of this?

Along those lines what's the significance of the number of aperture/iris blades? In general do you get a larger aperture with more blades?

And lastly, can I calculate the maximum magnification of a lens? Most macro lenses list it easy to see since its a big seller, but a lot of general or telephoto lenses don't have the information easily accessible.

Thanks yet again!

darryl

Photo Joe wrote:

Along those lines what's the significance of the number of aperture/iris blades? In general do you get a larger aperture with more blades?

Oooh ooh, I know this one: bokeh

LiquidAir

Photo Joe wrote:

Sometimes I feel like such a mooch asking all these questions, but thus far everyone's been very kind and polite while helping me out. Thanks to all you for that.

So here are my questions: I've been doing some lens researching/shopping lately, but I tend to get a little lost when it comes to the technical specifications list. Sure focal range, aperture, IS, USM etc are all fine for me. But I have no idea what it means when a lens lists "X elements in Y groups." What's the significance of this?

Along those lines what's the significance of the number of aperture/iris blades? In general do you get a larger aperture with more blades?

And lastly, can I calculate the maximum magnification of a lens? Most macro lenses list it easy to see since its a big seller, but a lot of general or telephoto lenses don't have the information easily accessible.

Thanks yet again!

Elements in groups:
People who are really in to lens design seem to get something from this but I am not sure what.

Aperture blades:
The number (and shape) of the aperture blades affects the bokeh and the shape of out of focus highlights. If you look at this thread you can tell how many blades the aperture has by counting the points on the star. If the aperture has an even number of blades, the star will have the same number. If the aperture has an odd number of blades, the star will have twice as many. 10 points typically means 5 blades. 8 points means 8 blades. Apertures with curved blades often don't make a star at all. For the quality of the bokeh typically more blades are better and curved blades are best though like most things in lenses there are execeptions.

Magnification:
[focal length]/([closest focus] - [focal length]) works for simple lenses, but most camera lenses aren't simple (see elements and groups). That said, for most lenses it gets you at least in the ballpark. Heres a (made up) samples: 100mm lens, 600mm closest focus:

100mm / (600mm - 100mm) = 0.2

Photo Joe

So anyone know the significance of elements in groups? Still curious about this one.

ziggy53

Photo Joe wrote:

So anyone know the significance of elements in groups? Still curious about this one.

There is no simple answer.

Zoom lenses tend to have more elements and more groups than prime lenses. Other than that, there is no direct correlation between the number of elements and how they are grouped and the quality or close focus or most other useful measures.

One reason to list elements and groups is to differentiate between similar products. Another is to try to convince prospective buyers that a lot of lens complexity must equate to quality, which it does not.

More valuable qualities might include the use of specialized glass formulations and/or aspherical elements. Constant aperture, internal focus, ring-type ultra-hyper-super-sonic focus motors, and such might be other ways to differentiate lenses and justify costs.

To put it another way, don't worry about the number of elements in a lens or how they are grouped. It mostly just doesn't matter.

pyry

Photo Joe wrote:

So anyone know the significance of elements in groups? Still curious about this one.

You can use those numbers for a drinking games with other gear heads. Drink when you find a lens with more than 20 elements, try to guess the zoom range from it, take a sip for every aspherical surface in your bag of lenses...
'cos thats about all you can do with that information frankly.

There is always some loss of light to reflection at every surface, which would mean proneness to flare and loss of contrast - but this is actually much more a function of how good the anti-reflection coatings are.

Here's a summary of what I have in my bag:
Canon 17-55/2.8 IS - workhorse, love it to bits, sharp as it gets - 19 elements, 12 groups.
Canon 100-400 L IS - nice and sharp lens for birding, astro, landscaping even - 17 elements in 14 groups.
Sigma 10-20 EX - Trusty wide zoom - 14 in 10.
Canon 85/1.8 - a longish prime, sharp and fast - 9 in 7.
Sigma 30/1.4 - a fast normal prime, sharp if you hit the dof - 7 in 7.

I you see any other pattern in there than zooms vs. primes - I'll buy you a beer. And just mess that pattern up the 500/4 L IS has 17 elements. And remember the 28-300 L IS when you drink, 23 elements

pathfinder

Elements and groups relates to a certain extent, to how many air-glass interfaces there are and the fewer the better all things being equal. Because there is some reflection and scatter at each air-glass interface, which decreases contrast and sharpness..

But all things are not really equal anymore due to the sophistication of modern lens design and fabrication. Aspherical lens design and High refractive index glass have altered older optical design criteria, so that elements and groups mean less than they used to years ago.

Generally, primes will have fewer elements and groups than zooms, and this is one ( of many ) reasons that GOOD primes are usually equal to or better than good zooms. This was more true 40-50 years ago, before the use of computers to calculate ray diagrams in lenses. Look at some of Leica's and Zeiss' lenses from the 30's and 40s.