I keep hearing that the a7's sensor is optimized for use at higher ISOs. Is this really true or are you able to get silky smooth "noiseless" images from it at the lower settings, as I would hope?

There are four different "A7" cameras. In the context of modern full-frame cameras and including top-of-the-range $6k competitors:

A7 - Slightly below average ISO

A7R - Slightly above average ISO

A7S - Significantly better ISO than any other camera

A7 mk2 - About average ISO performance.

 

So if when you said A7's you meant A7S, yes it is very much optimised for low-light. Some have commented it's low megapixel rating leads to a pleasingly large "grain" when grain does eventually appear.

 

A "League table" of the best low-light cameras

http://www.dxomark.com/Cameras/Ratings/Sports

Stills vs Nikon's best

http://cameratimes.org/sony-a7s-vs-nikon-d4s-vs-nikon-df-vs-canon-eos-6d-high-iso-comparison/

Video vs Canon’s 2nd best

http://youtu.be/Y76zjaNiFhc

Sony demo, smoking fish

http://youtu.be/XgbUgNiHfXM

Amateur, moonlit beach

http://youtu.be/LOJyBmDvp5c

 

Many people only look for noise and grain levels when comparing high ISO. Look also for loss of colour information (e.g. a "flatter" look) when comparing/considering the max ISO you'd be comfortable shooting at.

I mean just the plain vanilla Alpha 7. In other words I suppose I'm just trying to find its "sweet spot" ISO-wise.

While the A7 has better ISO performance than any crop camera, it's nothing special compared to other full-frame models.

Other than being full-frame it's not optimised for ISO at all (it's more of an all-rounder).

 

See test images at various ISOs here: http://www.imaging-resource.com/camera-reviews/sony/a7/sony-a7A7.HTM

I mean just the plain vanilla Alpha 7. In other words I suppose I'm just trying to find its "sweet spot" ISO-wise.

 

With my A7, I go up to 6400 ISO sometimes, but I try to stay at a max of 1600 ISO .

In a night shot, the result at 6400 ISO is still good, very few noise, but you can see an important "smoothering" of the picture. Not disturbing at all to use on a screen, but I won't print it. Some tries I've made at 12800 ISO show only a small difference (not really more noise, but more smoothering and it starts to look more like a painting than a photo).

 

So, all in all, I try not to go higher than 3200 ISO, or, better 1600 ISO (the result is astonishing and really suitable for large printings)

 

My own personal chart :

 

1600 ISO : no problem at all, result is excellent

3200 ISO : result is very good, printing in small size is OK

6400 ISO : only if no other possibility or to use on a screen only

12800 or more : don't use

As others have said, the regular A7 produces very clean & smooth low ISO images, yes. High ISO is good but fairly middle of the road for FF and 24MP. Its a great all round sensor/output.

I keep hearing that the a7's sensor is optimized for use at higher ISOs. Is this really true or are you able to get silky smooth "noiseless" images from it at the lower settings, as I would hope?

The A7s has a true native ISO of 3200.

It is very clean way above that.

I routinely shoot poorly lit Video at ISO 6400

but have gotten very good images at ISO 100