Wider Glass Better ??

[james_frater]

Here is a question I am hoping someone will answer for me.

 

In recent threads about lenses it has reminded me of something.

 

I have two new Sigma lenses 20-40 2.8 EX DG with a 82mm diameter,

and a 28-70 2.8 EX DF which is 72mm.

 

I see the Minolta 28-70 2.8 has a diameter of only 62mm

 

What should one be looking for, because 20-40 is a massive lense

with glass at diameter 82mm and makes me look like one of those

Pro's (As I am only a serious amature at the moment)

 

Wider glass give better results ?????

 

What's people take, because my default Minolta D 28-100 is only

52mm. Compared to these massive Sigma lenses.

[andrew robertson]

You should be looking at the entrance pupil size, not the maximum diameter of the front

glass.

 

I would suggest reading any one of the hundreds of primers on optics to become more

enlightened.

 

Food for thought, though - My Canon 20-35 f/3.5-4.5 USM has a front glass that's 56mm

in diameter, and my Voigtlander 21mm f/4 has a front glass that's 32mm across, despite

the fact that they have similar angle of view (at the zoom's widest setting) and apertures.

The Voigtlander is by far the best of the two.

 

There are so many variables at play in a lens that it's impossible to decide which is better

by measuring the front element.

[bill_thorlin]

Andrew is absolutely right - it is what the LENS produces that matters. Just for once size does not matter so do not feel intimidated if your equipment is small - it is what it can do and what you can do with it that matters.

[ilkka]

There are a few principles of optical design that affect all lenses and while there can be some variation between designs, as a general rule they tend to apply. These include: Faster lenses need larger diameter front elements (4/300 mm has smaller front lens than 2.8/300). Longer lenses have larger front elements (4/200 has smaller front lens than 4/500). In retrofocus designs, wider lenses have larger front elements (15mm Leica R lens has much bigger front element than 15 mm Voigtlander Heliar for the M system). Larger image circle requires larger lens (Pentax 4/55mm lens that covers the 6x7cm format has much larger filter diameter than a 2/50mm normal lens for 24x36mm format; the 4/55 is also a retrofocus design adding to its size).

 

These facts help explain some of your findings. 2.8/20-40 is much wider lens than 2.8/28-70 and is also strong retrofocus design. Thus it has wider front element. Your 28-100 is a slower lens than the 2.8/28-70 zooms, thus making do a with a smaller diameter front lens. The two 2.8/28-70 lenses should have roughly similar front lens diameters and filter sizes, but because of different optical designs and manufacturers' preferences for specific filter sizes they end up with different diameters.

 

Obviously the manufacturers try to minimise the size and weight of their lenses, making them more portable and also saving money in the manufacturing process (high quality glass and grinding are expensive). So there must be a reason why they use larger elements. But if you think a bigger lens must always be better, try removing the bottom of a large pickled cucumber jar and use that for a lens. You will quickly see that size alone does not decide the quality.

[yi_fay]

Most Sigma EX-series lenses have filter mount much larger than front lens element. With 82mm thread, the glass may be just 50-60mm in diameter. The advantage is to avoid vignetting when mounted with filter.

[Uncle Goose]

I got my but cracked after reading you Pro comment. Those so called pro's always running around with the big chunks of glass and in 99.9% of the times they don't even need it since they take photo's of a subject that is like 1 or 2 meter from them. You don't need a super zoom for that. It seems that it's more to boost there ego or to compensate their phallus (interesting comparison IMHO) much in the same way some people buy supper expensive cars to impress women. Think about that.

 

No, Wider glass doesn't mean better result, it's all in the build and the quality of the lens and not the amount of glass you put into it.

[dennis_fallon1]

> I see the Minolta 28-70 2.8 has a diameter of only 62mm

 

Which lens is that ?

 

The 28-70/2.8 (G) has 72mm filter threads

The 28-75/2.8 has 67mm filter threads.

 

Look at lens reviews/tests to judge lens performance, not specs. Look at specs to know what size filters to buy.

 

- Dennis

[peter_goodman]

One additional point to be made:

 

Obviously the larger the lens, the BRIGHTER the image, but not necessarily the BETTER the image. But another point is contrast. This is achieved partially by the coating put onto the lens. If the coating is made one way, the image will be affected by what ever that coating is. Another affect in contrast however is that some lenses use a plastic element and the contrast changes a bit. This because of different properties in the plastic. If you have a high quality lens, it will transmit light cleaner, finer and faster. If you have a lower quality lens, the opposite is true. Larger lenses will give you a brighter image because more light is passing through. But if the lens elements are of a lesser quality, that bright image will be of lesser contrast, and can at times actually cause a washout. I wouldn�t worry too much about lens size as so much the quality of the lens. Hate to say this, but you get what you pay for.