Why do most UWA's start with 16/17mm? Huge difference 17<>20mm? UV filters question

3 questions :

1 Why do most UWA lenses start with 16/17mm, strange that Canon doesn't make a 12-24 like Sigma or Tokino for example on FF. Maybe it's impossible to keep the IQ.
2 My widest mm's will be 20 2.8 soon, will it be a lot of difference with 17mm for pics inside buildings ? It's better than my 35mm anyway and rather have f2.8 than 17-40 f4.
3 UV filters block UV light, can anyone post a link of a pic what the negative effect of UV light can be in your pic as I avoid using UV filters, mostly only the hood or nothing or CPL filter ?

Answers to the best of my knowledge:

1) Lens design is about optical compromise throughout the entire process. Zooms are even more of a challenge. Chances are 16/17mm is probably the easiest to design around. I imagine the design needed to keep corners sharp at wider would be immensely complex. Reviews of the 12mm zooms on ff usually end up saying the corners are crazy soft. (I mean, remember that article about the Nikon 6mm going around a few months ago, it was huge and cost $150k!). There are rumours canon is going to come out with a 12-24mm soon, to rival the Nikon version which is supposed to be crazy sharp (the nikon, that is).

2) Yes. The difference in FoV is a lot more noticeable on the wide end per unit length than on the long end.

3) In digital UV filters are pointless. The sensors have built in filters to reduce UV anyways. The only use they have these days is for protection, but they also introduce vignetting, increase flare (loss of contrast) and reduce IQ. Frankly a hood is a much better option.

Thanks, where could I see a comparison between 17 and 20 mm ?
Cause this lens I want to buy is 20mm, I don't know if it's wide enough for buildings inside (urbex), the other option is 17-40 f4L (24 1.4 is not really UWA).
Here I can put 17mm left and 20mm right but the pics are a bit small in the box http://tamron-usa.com …cal-length-comparison.php

Concerning UV filters : yes I only use them at the sea (as I live at the coast). Could the water/sea salt damage my 70-200 II glass?
Not that I take pics in the sea, but the glass gets easily wet if you are even close to the sea.

CanonYouCan wrote in post #14659469
Thanks, where could I see a comparison between 17 and 20 mm ?
Cause this lens I want to buy is 20mm, I don't know if it's wide enough for buildings inside (urbex), the other option is 17-40 f4L (24 1.4 is not really UWA).
Here I can put 17mm left and 20mm right but the pics are a bit small in the box http://tamron-usa.com …cal-length-comparison.php

Concerning UV filters : yes I only use them at the sea (as I live at the coast). Could the water/sea salt damage my 70-200 II glass?
Not that I take pics in the sea, but the glass gets easily wet if you are even close to the sea.

http://www.the-digital-picture.com …L-II-USM-Lens-Review.aspx

Shows the difference between 16mm and 20mm.

Can't really say whether 20mm is wide enough.... the 17-40mm does give you that flexibility tho.

Re UV filters, if you're using it all the time around/in salt water, then yea, that's a case to use a UV filter.

I don't use filters on any of my lenses, the cons far outway the pros for me *shrugs*