why not f/1.2 by Sony?

Started May 14, 2013 | Discussions thread
TiagoReil
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Re: why not f/1.2 by Sony?
In reply to EinsteinsGhost, May 17, 2013

EinsteinsGhost wrote:

forpetessake wrote:

tomtom50 wrote:

forpetessake wrote:

You need to keep in mind that we are not talking about really fast apertures here. The f/1.2 on APS-C format is equivalent to just f/1.8 on FF -- hardly a superfast lens. Testing with focal reducers also seem to indicate there isn't a problem going up to f/1.0 on APS-C.

Can you cite a source? I had the opposite impression.

I'm not sure what you are asking about and what your impression is. It's a trivial math, equivalent lenses are scaled by the crop factor. The 50/1.8 lens on FF has the same properties as 34/1.2 lens on APS-C, provided all other things being equal. Focal reducers are a living demonstration of that.

Focus reducers are also concentrating light onto a smaller area, which should (theoretically at least) result in faster shutter speeds than would be achieved on FF.

The f-stop, however, is not an accurate representative of exposure (light transmission), and more so at larger apertures. Unfortunately, manufacturers get away with simply quoting f-stops. I wish t-stop would be included, as they do for cine lenses.

There was a resaon why in photography f number was more important than T number, and that for cine lenses T number was more important.

Actually, in theory, the t number and f number should be equal with a lens and coating that looses nothing of light. All lenses loose a bit, some loose more,cause of elements, goups (when 2 elements are sticked) and with coatings. IT is a balancing act. You add more elements, for instance a zoom, gives you more flexibility, you add coating, better contrast and less flare, but you loose a bit of light. Generally is not much, and actually primes loose in general less light than zooms, cause zooms generally have more elements.

In photography, before Autofocus, or even manual focus through the lens, people focused with distance and area in focus. The area in focus is given by the f number, not by the t number. On the other hand, lenses where simple, so the f number was close to the T number (from loose from elements and coating). So photographers learned to use f numbers and those where close to the T number to be acceptable. The magnitude that film got you was enough to make a bit of a correction when printing (add a bit of light) and thats it.

Cine lenses needed to be precise on the T number cause as you change them, you need to be exactly on the same exposure for a different reel. IF not, on screen, one after the other, a very small change was perceived and ruined the film. Thats why cine lenses are categorized that way. Cause for Movies, it is more important to have the exposure perfect than to have the focus perfect.

Now, with digital sensors, and the phenomenon we are describing here, the difference from f to t number is bigger because of limitations on digital sensors. Only on fast apertures.

All this that I mention about lenses is well known for a long time, and people that work in this field knows it pretty well. Only people new to this are surprise to realize that actually f is not a value for lighg, but as an aprozimation, it used to work pretty well. (like one would say certain scientific theories, though not exact, are good cause for day to day calculations give results good enough, ex, newtons formula of gravity). The problem is that digital sensors ruined this for fast lenses. Different manufacturers do different things. Some cook the files. Jpgs and even raws, others limit the lenses, others are more open to this. But this is a very well known problem.

Also, a lens is considered fast cause you can get a very fast shutter speed with it, not because the DoF is shallow. Those people that say that a fast lens is not fast on an aps-c camera have it wrong. It is still a fast lens (ignoring the sensor problem) it has less shallow depth of field than the same lens on a FF camera, but it is as fast in terms of light.

Finally, can we kill this!!!! and stop miss informing? The reason sony doesn't offer a faster than 1.8 lens for crop sensor is that it is not worth it, that the cost would be too big for a small return in light, and they consider their customers are mostly consumers, amateurs, advance amateurs (at least for crop sensor, FF is a bit different, but still there is a limit there, 1.4, they think the same there, not worth it to get that extra f1.2). Accept it , and keep on taking pictures.

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