YongNuo YN-560 flash - update

I did a few more tests to follow up on my initial impressions.

1. Optical slave: Infrared not visible light.

I discovered that the YN-560's optical slave detects infrared light not visible light.  This is useful if we want to avoid any contribution from the triggering flash, by covering it with an infrared filter such as the SG-3IR.

In the image above, I used the popup to trigger the YN-560 but covered the popup with an the SG-3IR, yet the YN-560 triggered successfully.

2. Sync speed:

An interesting discovery is that the YN-560 can sync at 1/3 stop higher than the natural sync speed, whether on-camera or on either optical sync mode, with only a very slight reduction in power.

Baseline shot at normal sync speed: ISO 200, f/32, 1/250, YN-560 at full power, on-camera:

Test shot: same as above at 1/320, YN-560 on-camera:

Test shot: ISO 200, f/32, 1/320, YN-560 on S1 mode, popup flash at Manual mode 1/128 with SG-3IR infrared filter:

Test shot: ISO 200, f/32, 1/320, YN-560 on S2 mode, popup flash at TTL mode -3 FEC with SG-3IR infrared filter:

Note: in the latter two shots, the white balance is correct because the camera is aware that flash is being used, and Nikon's CLS corrects white balance for flash use.  In the first two shots, the camera acted as though it was not aware that a flash was fired, therefore there was no white balance correction.

Another interesting discovery is that when the YN-560 is on-camera, the camera does not limit the shutter speed to 1/250.  This allows trick shots like the one above.

In the shot above, I used the YN-560 on-camera to trigger the SB-800 on SU-4 (optical slave) mode. The flash appears invisible on the SB-800's flash head although it lights up my face, because the flash is blocked by the rear curtain but its light on my face is not.  The flash unit itself is not obscured by the rear curtain however because it is illuminated by the ambient light from the sun.
 
3. Flash duration:
The published specifications for the YN-560 state that at full power, the flash duration is 1/200.  That is incredibly slow for a hotshoe flash (the SB-800 claims a flash duration of 1/800 at full power).  If the 1/200 flash duration is true, then at a sync speed of 1/250, you wouldn't get full power - instead you would get 1/3 of a stop less power (i.e., around 16.67% less power).

To test flash duration, I used a point and shoot camera, the Casio Exilim EX-V7 (which has no mechanical shutter and can theoretically sync at any speed), and compared the output of the YN-560 at speeds from 1/125 to 1/800 (the highest shutter speed on my point and shoot). My point and shoot only has TTL flash, therefore I used the YN-560's digital optical slave (which ignores preflashes).  I set the Casio's flash to -2 FEC and covered it with an SG-3IR.

a. Baseline shot (showing ambient "deleted")  ISO 64, f/10.6, 1/125.

b. YN-560 @ full power, ISO 64, f/10.6, 1/125:

c. same as above, at 1/200:

d. same as above at 1/640:

e. same as above, at 1/800:

Conclusion: the fastest shutter speed where there is no noticeable decrease in flash exposure at full power is 1/640.  At 1/800 there is a very slight reduction in flash exposure, presumably from clipping of flash duration.  Speeds higher than 1/800 were not tested.

4. Power:

There had been conflicting results regarding the YN-560's power.  Earlier reports found the power to be similar to an SB-600.  However, it appears there is now a consensus that the power is similar to an SB-800:

• Following a test of a production model, Speedlights.net found the power to be the same as a YN-460 II, which is greater than an SB-600 and almost identical to an SB-900, with an approximate GN of 33 meters at 35mm zoom and ISO 100.
• Strobist Flickr group member Kurbster found the YN-560 to yield an aperture of f/32 at a distance of 1 meter at ISO 100, implying a GN of 32 meters. 

These updated results are consistent with my first impressions of the YN-560, where I thought the output was similar to an SB-800.

5. TTL Accuracy with CLS AWL

As previously posted, it is possible to synchronize the output of the YN-560 with a commander flash and a slave flash operating using Nikon's CLS Advanced Wireless Lighting.  One question is whether TTL flash exposure remains accurate when the YN-560 is contributing light.  It would seem that it the answer should obviously be no because the YN-560 doesn't fire during the TTL preflashes, therefore its output is not taken into account in the TTL flash metering.  However, I was hoping that after the TTL preflashes fire, it specifies the flash exposure instead of flash output, and thus it would be possible for the YN-560 to avoid skewing the TTL accuracy.

To test whether TTL accuracy is affected by the YN-560's contribution, I fired an SB-800 on-camera and used it to trigger a YN-560 in a dual flash bracket.  For the baseline shot, I lit a subject with just the SB-800 on TTL mode.  I then did a test shot with the YN-560 contributing light and compared the two exposures for similarity.

Baseline shot: f/11, 1/250, ISO 200, SB-800 on TTL (0 FEC).

Comparison shot: f/11, 1/250, ISO 200, SB-800 on TTL (0 FEC) and YN-560 at 1/16 power.

Evidently, TTL accuracy is affected by the YN-560.

6. Other observations:
- My unit didn't come with a sto-fen type diffuser.  That's ok. I very rarely use them anyway.  Come to think of it, I don't recall ever using one other than for testing purposes because diffusion doesn't soften light.
- I agree with other reviewers that the pilot light (test light) is harder to press than the other buttons but doesn't take THAT much effort.  It's just stiff, that's all.
- The sound button (also functions as the power save button) occasionally doesn't respond.