why not f/1.2 by Sony?

Started May 14, 2013 | Discussions thread
DimitriDV Regular Member • Posts: 361
Re: that's not a fast lens

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 .


Dammit, most of the posts in this thread got me super confused, as indeed my understanding (and web research) was that a "fast" lens stays as fast even with different sensor dimensions...

If I want a fast lens it is indeed mainly for the above mentioned characteristics (speed); DoF is nice to play with in some circumstances, useful if shallow in some cases, annoying in other cases.

Low light performance is the main reason I would like faster lenses, and I was also asking myself why there aren't faster than f1.8 for E-mount, reason why I decided to get a manual Canon FD 50mm f1.4 instead of getting the E-mount 50mm f1.8; at equal speed I would have probably paid the extra money to also get AF. If I have to get a fast lens for low light circumstances I might as well get the fastest I can (with an eye on the price).

Anyway I'm not an expert, so to stay clear, what I am understanding is that (copied from the web):

"To be correct you'd have to say that a 30mm f/1.4 mounted on a 1.5x crop camera had a 45mm equivalent field of view, an f/2.1 equivalent DOF-aperture and an f/1.4 equivalent exposure-aperture."

Let me know if this is incorrect.

I do not enter in the merit if the Nex line is meant for professionals or amateurs; they did not stick such a label on them, so that's up to each customer's decision if they can make with it what they need. And each customer is entitled of asking to Sony faster lenses if they require so, or influence the market in their little and let the market decide.

I would also be grateful to people if they could include a few additional words on why you "lose a stop here/there anyway" stating why (so I can at least google the keywords), as that may teach something new to me and fellow amateurs. Also, taking in consideration only one aspect of a comparison only confuses (like the DoF/light gathered thing above). Thanks!

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