What am I doing wrong

[benstanley]

<p>I'm trying to shoot some fully manual shots with flash using guide numbers etc, but usually seem to get a 1-2

stop overexposure and can't figure out why. In P/Av/Tv, and even M (with flash set to auto) I get a

normally exposed flash shot.</p><p>However when I shoot in M with manual flash it goes pear shaped and I can't

figure out what I'm doing wrong. </p><p>- Canon 60D + 17-40/4L + 430EXII on camera.</p><p>- Shooting

in M at 1/250th & f/4 to remove ambient (which would require ~1 second), and ISO set to 100</p><p>- 430EXII

in manual<br></p><p>- Custom function enabled to automatically adjust zoom for sensor size</p><p>- Lens set to

24mm, 430EXII set to 24mm, crop factor icon visible on flash lcd screen<br></p><p>- 430EXII set to 1/8</p><p>- no

exposure comp or flash exposure comp set<br></p><p><br></p><p>According to my charts at 24mm and 1/8 the

430EXII's ISO100 GN is 8.8. So at f4 that gives me a range of 2.2 meters. Yet when I stand at that

range I blow everything by 1-2 stops. In addition, the range on the flashes back lcd says 3.0m at that

setting so its not marrying up with what I'm calculating so I must be doing something wrong? If I then

change *only* the flashes mode back to ETTL it comes out correct. If I step back to a bit over 3 meters

back in manual mode, its definitely a lot better but perhaps still a touch overdone, although there is a lot of

white in the scene (I can't find a large enough neutral test surface in my house!)<br></p><p>Am I using the GN's

wrong - does the crop factor affect the GN table in the manual so I shouldn't be using the table in the

manual? Or am I doing something even more dumb?<br></p><p><br></p><p><br></p><p><br></p>

[Mattman944]

<p><em>does the crop factor affect the GN table in the manual so I shouldn't be using the table in the manual?</em><br>

Yes, but you can still use the table if you calculate the equivalent focal length. 24 x 1.6 (or whatever), gives a GN of 11 meters. </p>

<p>Guide numbers assume an average amount of reflectance from nearby objects. If you are in a small area with light colored walls your reflectance will be very high and will cause the effective GN to be higher. Conversly, if you are outside with nothing to reflect, the effective GN will be lower.</p>

<p>The GN is only a <em>guide</em>. When more accurate exposure methods like a flash meter or camera histogram are available, they should be used. -MattM</p>

[benstanley]

<p>Thanks. I think Canon should have mentioned the x1.6 thing in the manual to make it clearer then! I haven't been able to find many references explaining this online. I guess hardly anybody tries to put the flash in manual these days anyway...</p>

[nathangardner]

<p>do you have a sample shot? ISO 100 is generally pretty low for flash photography unless you are compensating by a strong flash or wide aperture. And at ISO 100 it makes it hard to expose the parts of the photo unaffected by flash, for example if you were outdoors on a cloudy day or in the shade using ISO 100 and flash, you could easily get a properly exposed subject (from the flash) and a very underexposed or black background because the ambient light was not enough to expose the sensor at ISO 100. Again a sample image would be the best way to assess the situation.</p>

[john_tran14]

<p>Specification says GN is 43m at 105mm zoom but what is the GN at 24mm or 24X1.6 mm (if the flash knows your camera has cropped sensor?)</p>

[benstanley]

<p>John, GN at 24mm in the manual is 8.8 (in meters) at *1/8* power. But going on what Matthew said, I need to multiply that by 1.6 which gives me ~3.5 meters range at f/4. I tried this out and the exposure now looks somewhere in the region of correct :) So problem sorted!<br>

I know the GN's are a guide, but I wanted to be able to dial in something fairly close from doing the maths in my head and then fine tune from there, rather than being way out each time and probably needing to take an extra shot to get it right.<br>

Nathan, underexposure was not the problem here :) A ex430 2 meters from a subject is not going to struggle :)<br>

John</p>

[nathangardner]

<p>sorry, I read that completely wrong. If everything is overexposed, then either your flash is too close, too powerful, or aperture is too wide, but I'm just stating the obvious.</p>