I thought I’d just quickly publish measurements I made with my Yongnuo YN560-IV (Amazon, B&H) on-camera flash (speedlite), triggered with a Yongnuo RF-603 II radio trigger (Amazon, B&H).
All measurements were made with a lightmeter at a distance of 20 feet. The shutter was at 1/50 and the ISO was at 3200.
| f-stop reading at different zoom levels | ||||
| Power | 24mm | 28mm | 35mm | 50mm |
| 1 | 40.1 | 40.2 | 45 | 51.1 |
| 1/2 | 28.2 | 32 | 35 | 35.2 |
| 1/4 | 22 | 22 | 35 | 35.2 |
| 1/8 | 16 | 16.2 | 22.7 | 25.2 |
| 1/16 | 11.2 | 11.1 | 18 | 18.3 |
| 1/32 | 8 | 8.2 | 12.7 | 14 |
| 1/64 | 0.7 | 0.7 | 0.7 | 0.7 |
| 1/128 | 0.7 | 0.7 | 0.7 | 0.63 |
| f-stop reading at different zoom levels | |||
| Power | 70mm | 80mm | 105mm |
| 1 | 57 | 57 | 64 |
| 1/2 | 40.1 | 40 | 45.2 |
| 1/4 | 28 | 28.2 | 32.1 |
| 1/8 | 20 | 22 | 25 |
| 1/16 | 16 | 16 | 18.1 |
| 1/32 | 11 | 11 | 12.7 |
| 1/64 | 8 | 9 | 9 |
| 1/128 | 0.7 | 0.7 | 0.7 |
A few notes:
- The maximum you get at 20 feet is f/64 at 105mm zoom at ISO 3200. At ISO 100, it would translate to an f-stop of f/11. This gives us an effective guide number of 220 feet. The rated guide number is 190 feet (58m), so I guess we are within the margin of error.
- f/0.7 readings might be inaccurate due to the lower range of the light meter. Basically 1/128 is pretty much useless at 20 feet, which I’ve seen in practice as well. A little closer though (inverse square law!) and it makes a big difference.
- When using a softbox, it’s best to keep the zoom to its widest level, which is 24mm. So with a softbox you might be able to squeeze out f/11-f/20, depending on the diffusion used. I use the Photoflex LiteDome XS kit (Amazon, B&H).
- Conclusion: The guide number published by Yongnuo is accurate. There are many reports on the Internet to the contrary so I wanted to test this for myself.
I’m not a heavy user, but let me know if you have any questions.
[recnative1]
