why not f/1.2 by Sony?

Started May 14, 2013 | Discussions thread
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In reply to franzel, May 16, 2013

franzel wrote:

tko wrote:

Don't know why people call this a fast lens.

F1.2 35MM = F1.8 50MM (there is a reason the OP picked those numbers.)

There is a single reason why a lens with a wide max aperture is called fast - it allows for a higher - faster - shutter speed at the same ISO, to help avoid camera shake or freeze fast moving objects .
It also provides a brighter finder image for SLR cameras, and helps to focus more precisely .

Shallow DOF is only a side effect, and can be used as a photographic effect if desired, but is not the reason 'fast' lenses were being introduced originally . 
In general photography, and outside of the amateur enthusiast realm , DOF is merely one of many aspects that need to be controlled by the photographer, and a creative tool only in very view applications .

A f1.2 lens will always be a f1.2 lens; the single purpose of this f-number is to determine the resulting shutter speed at a certain ISO value.

This result will always be the same, no matter the focal length, format, sensor or film; ISO values are (supposedly ) absolute figures and already include sensor/film specs, and (actual, not equivalent) focal length is part of the f-number calculation .

DOF comparisons for different sensor formats can be made, like focal length comparisons, but that's not part of the technical lens specifications at all .

Also, film/sensor size based DOF calculations are a lot more complex than comparative angle of view calculations; the latter gives you the 'crop-factor', which can be applied to a certain focal length to simplify lens/format comparisons, but the crop factor (or rather FL and format size) is only one part of the DOF measurement .

...ISO is rather irrelevant -- what matters is how much light falls on the sensor and how efficient the sensor is.  All the ISO setting does is adjust the brightness of the LCD playback and OOC jpg.  It's only effect on noise is that for many sensors (not all), higher ISOs have less read noise (the additional noise added by the sensor and supporting hardware) than lower ISOs.  In other words, ISO 400 pushed 2 stops is more noisy than ISO 1600.

So, f/1.2 on 1.5x puts the same total amount of light on the sensor for a given shutter speed as f/1.8 on FF, which will result in the same noise for equally efficient sensors.

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