**Exposure**
  * http://www.calculator.org/calculate-online/photography/exposure.aspx

Typical one-half-stop f-number scale
^f/# |1.0 ^1.2 |1.4 ^1.7 |2 ^2.4 |2.8 ^3.3 |4 ^4.8 |5.6 ^6.7 |8 ^9.5 |11 ^13 |16 ^19 |22 |

How to calculate: 
<code>sqrt (2) ^ 3 = 2.8 Here we have f2.8

One stop down:
sqrt (2) ^ 2 = 2.0

Hence:
sqrt (2) ^ 1 = 1.4
sqrt (2) ^ 0 = 1
sqrt (2) ^ -1 = 0.7

an f number of 0.95 relates to a stop of -0.148
sqrt (2) ^ -0.148 = 0.95

Consider Leica 50mm f0.95 is 10,000 USD, a 0.3 stop slower, 
Voigtlander 50mm 1.1 just 1,000 USD, 
a 0.5 stop slower Pentax K 50mm f1.2 just 400 USD, 
and Pentax M 50mm f1.4 just 100 USD
</code>

If **aperture 1 stop larger** (f2 is 1 stop larger than f2.8), **shutter speed** can be **twice faster** (1/25s can be boost to 1/50s)

Thus, get a **35mm f2 prime** lens definitely gives **fast and better performance at night and a All Day walk around purpose** than **28-70mm f2.8** zoom lens.

**Fast and Light-weight is main advantage of Prime Lens**, comparing to fast expensive zoom alternatives.

**Alternative way to create "Large Aperture"**
  * Bokeh-panorama and Flickr: Ryan Brenizer's Photostream
    * http://www.ryanbrenizer.com/2011/05/brenizer-method-instructions/
    * http://www.flickr.com/photos/carpeicthus/page8/

====== Bokeh ======

  * https://www.zeiss.com/content/dam/Photography/new/pdf/en/cln_archiv/cln35_en_web_special_bokeh.pdf