[yusuf]

It focuses the full frame's light and compresses it into a smaller image circle, so you do get a faster lens. With a dumb adapter the light is wasted as there is no sensor to read the light at the outer edges.

Right, but it won't make shutter speed faster for the same aperture as carrlangas asked.

[carrrlangas]

Right, but it won't make shutter speed faster for the same aperture as carrlangas asked.

Why not? Yo have more light because you are taking full advantage of the FF image circle of the lens but hitting the same area as before (APS-C). That´s why the lens shows as f/1.0 in the first place. My question was if the real life value was a comple stop or a half or 2/3 of a stop due to light lost.

[kenafein]

Right, but it won't make shutter speed faster for the same aperture as carrlangas asked.

If you use the same shutter and aperture that you would use with a glass-less adapter, then it will be overexposed. So you would have to increase the shutter speed to get the same exposure. The T-stop value isn't necessarily 1 whole stop though. Some people have reported it being a little less, depending on the native aperture of the lens.

[carrrlangas]

The T-stop value isn't necessarily 1 whole stop though. Some people have reported it being a little less, depending on the native aperture of the lens.

That is what I wanted to know. So, it is a little less depending the native maximum aperture of the lens and how much vignette the lens produces wide open. Thanks

EDIT: this seems good info on the adapter:
http://www.dpreview.com/forums/post/51499289

After a week playing with Zhongyi Lens Turbo and using a number of different lenses I came up with this list of pros and cons. Maybe it will be helpful for somebody when making a decision.

Pros:

almost 1 stop gain is real, except for cases when a lens has a heavy vignetting
a slight increase in central resolution. I didn't notice any increase with very sharp lenses, but some increase in soft ones when fully open
you get (almost) FOV of the FF lenses back, no need to do multiplication, wide angle is back
it's shorter than a dumb adapter
it provides a good seal and protection from dust
Cons:

the biggest problem so far is the softening of the corners. Many FF lenses are soft in the corners already, and focal reducer makes it even worse than on FF camera. Some FF lenses that worked great with a crop sensor showed very noticeable corner softening. In real life shooting, it may or may not be an issue. I noticed most affected lenses are on the short end: 24-28mm. So I wouldn't recommend focal reducer for wide landscape shots, chances are the corners will be visibly soft. The longer lenses behave better and in real life shooting the corners are OOF anyway, so you wouldn't notice it.
the blue spot caused by the reflections in the adapter, which mostly affects smaller apertures and longer FLs. After a week of shooting I've found only a handful of pictures that had to a various degree visible blue spot in the center.
increased vignetting: I've only seen strong vignetting on 24mm lenses when fully open. The vignetting goes away at longer FLs and smaller apertures. I took hundreds of pictures by now and I don't think it's a big problem with the lenses I have.
the gain in speed might be smaller than you think it is. Many zooms have variable aperture. For example, when you take a picture at 24mm without a focal reducer, the lens can be f/3.5, but when you take the same picture with the same zoom with the same FOV at 33mm the lens can be f/4. So you get 1.48x increase in speed, not the 2x you might expect.
focal reducer is heavier than a dumb adapter.
two adapters I tried had very tight mount on the lens side, I'm afraid if I leave adapter on a lens for a prolonged time it can bind and will be difficult to remove.

[yusuf]

Tempting

any comment on this? from the same post at dpr

We were spoiled using cropped NEX sensor, where only 'bad' lenses showed noticeable corner softening. Now with focal reducer we have to face this issue. Though I must say the Nikkors I've been using were pretty good even in the corners, but FF Pentaxes turned out to have terrible corners. I know many people waiting for Pentax to release FF body, that will be quite an eye-opener when they try the lenses everybody considers great now.

[carrrlangas]

any comment on this? from the same post at dpr

Yes, I read that. It actually sounds logical. The A24/2.8 works great on the K-5 (better than DA*16-50 @ 24mm according to very basic test samples I took wide open) but must take into account the most compromised part of the lens is not being used. Wide angle lenses designing is difficult without the ade of modern ED and AL elements (more so 2 decades ago). Also, the Lens Turbo seems to degrade corner perfomance even farther.

[pinholecam]

If you want small sized lenses, I would suggest RF lenses.
They need not be very expensive.
Look for the Voigtlander lenses (available new), most don't cost that much (esp when the Pen-F are not that cheap anymore too these days).
The slower Leicas as well as some Canon/Jap brand RF lenses are not that expensive as well.

There is no cheaper option than to just use your existing Pentax lenses of course.

[rawr]

Since the Sigma 19mm, 30 mm and 60mm f2.8 primes for NEX are all optically excellent and cheap, as well as compact, I'm surprised no one is mentioning them here. They may not be MF, but you can use them that way if you wish
I already have the 19mm and the 30mm, and the 60mm is on its way...

[pinholecam]

Tempting

any comment on this? from the same post at dpr

Personally, I'd say its not true , or at the least lens dependent and user expectations.
Thats from my usage of a 5D with quite a few Pentax lenses. (from UWA to short tele )
I've not seen anything glaringly bad for the lenses on FF.

[jogiba]

Thanks. Regarding f-stop, say if you mount the 50mm with common mechanical adapter and wide open metering is 1/125 @ f/1.4, when you mount the lens with the lens turbo, is the metering 1/250 @f/1.0 ? I mean, do you really gain a stop?

I don't know if you will get a full stop but I use a Celestron Focal Reducer on my telescopes that converts my 2350mm F10 into a 1480mm F6.3.

You Tube

You Tube

Lens Turbo review :
http://gtbarnes.com/2013/05/16/nex-lens-turbo-speed-booster-initial-impressions-samples/

[kenafein]

Since the Sigma 19mm, 30 mm and 60mm f2.8 primes for NEX are all optically excellent and cheap, as well as compact, I'm surprised no one is mentioning them here. They may not be MF, but you can use them that way if you wish
I already have the 19mm and the 30mm, and the 60mm is on its way...

I have the 19 and the 30, and I am quite happy with them. Getting the 30 was perhaps the best $99 I ever spent on photography. They don't have aperture rings or distance scales, so they are limiting in a way. I really wish that Sony would add a distance indicator on the HUD display. I won't be buying the 60, since I have the 50/1.8; but it looks like a good one.

[kenafein]

I don't know if you will get a full stop but I use a Celestron Focal Reducer on my telescopes that converts my 2350mm F10 into a 1480mm F6.3.



Pentax K-7 with Celestron 9.25 and F6.3 focal reducer (1480mm F6.3) in 1536x1024 *** HD video mode - YouTube

Lens Turbo review :
NEX “Lens Turbo” Speed Booster Clone Initial Impressions & Samples | Gary Barnes

You are an optics junkie. What don't you have? That looks like a fun setup.

[kenafein]

FYI the price of the focal reducers is down to 166, from Roxen, on eBay.

[yusuf]

FYI the price of the focal reducers is down to 166, from Roxen, on eBay.

Nice, I am all set to get one, just looking at some reviews before I order.

Some mixed reviews here Zhongyi Lens Turbo first impressions - FM Forums

Comparision with SB here http://www.mirrorlessworld.com/?p=6265

[kenafein]

Nice, I am all set to get one, just looking at some reviews before I order.

Some mixed reviews here Zhongyi Lens Turbo first impressions - FM Forums

Comparision with SB here Lens Turbo vs. Speed Booster

I just bought one. I was on the fence too, but Metabones is stalling. Who knows when they'll have one for K-mount(or FD, or AI, or MD) and it will be $400. I think it will work quite well for the excellent portraits I've seen from you in the past, but it might be an issue for landscapes. At $166, shipped, I can live with some limitations. I did read that first link of yours, previously, and it did give me pause;but I've been pretty happy with the samples I've seen from several sources. When Metabones' version comes out, I might sell the lens turbo and get one. I doubt I'll lose more than $20-40. What I'd really like to see is Metabones make one universal adapter. Say a 38mm adapter, with modular adapters to fit several mounts.