Guide Number Help

I just got a 430ex to go with my 350d. I’ve tried to do as much studying before I start asking stupid questions so now I would like to have someone confirm that my logic here is correct.

My goal is to figure out the range of my flash in feet so I know how far away something can be and still be properly exposed.

If I take my Nifty Fifty lens (50mm f/1.8) and set my ISO to 100 and my aperture to 8.0, I take the guide number for 80mm (since 50mm on my 1.6 crop equals 80mm) which is 40 and divide it by 8.0 and multiply it by 3.3 to convert to feet and I get 16.5ft. That seems reasonable.

But where I start to question my logic is when I take this example to the extreme. If I change my ISO to 1600 I get a guide number of 160 because the guide number increases by a factor of 1.4x each time I double my film speed. Take 160 divided by my widest aperture of 1.8 and multiplied by 3.3 and I get 296ft or about 99 yards. Does this really mean that I can take a properly exposed photo of a subject at the other end of a football field with virtually no ambient light? That seems overly optimistic to me.

Do the guide numbers really work like they theoretically should, or am I way off on something here?

The 430EX only understands the actual focal length of the lens on your camera, not the "35mm equivalent field of view". Thus, it will always be projecting a beam that's wider than necessary for lenses on an APS-C camera such as your 350D. The 580X, on the other hand, has been taught by Canon to understand the body's format - at least for the latest body models.

For direct use of a flash unit (directly aimed at the subject and no "diffusers" or other modifiers being used), a properly calculated guide number should work over the range of ISO settings and apertures that your camera has.

The above is assuming manual use of the camera and flash, of course. When in ETTL mode, there's no way of knowing for sure how the camera/flash combination is going to figure out and implement exposure settings.

If you could experiment a little and report your results to the forum, I am sure it would be interesting to quite a few folks.

If used on a compatable Canon DSLR, or with the proper dedicated cord, I think the Canon 430EX will take the sensor size (crop factor) into consideration (just like the Canon 580EX) for optimal coverage (not wider than actually needed).

This specification link seems to indicate this.

It says, "Image Size Zoom Control: With cameras compatible with auto zoom, it zooms automatically to match the camera's image size".

Possibly this is not exactly what Canon means, but it sure seems to be the case.

The wording for the 580EX is a bit different it indicates:

"Automatic adjustment for lens conversion factor with EOS-1D Mark II and EOS 20D (camera requires firmware upgrade)"

This seems more difinitive.

http://www.usa.canon.c​om …egoryid=141&mod​elid=11923

Here is the link for the Canon 580EX specs.

http://www.usa.canon.c​om …egoryid=141&mod​elid=10514

Update:

I just found this on the "Features" list for the Canon 430EX at B&H it indicates:

"Optimum Flash Coverage - Automatically adjusts the flash coverage to adjust for differences in chip size on digital cameras"

See:

http://www.bhphotovide​o.com …SA&addedTroughT​ype=search

Enjoy! Lon

have a look in your manual - it has a table for aperture/feet
Page34 in this manual
http://home.comcast.ne​t/~snapit/430EX_Manual​.pdf

On the LCD of this flash, it tells you the expected range of the flash based on your setup. I've tried different focal lengths, with different apertures, and different ISO's, but I can never get the expected range to get higher than 60ft. Theoretically, if I have a 105mm lens with f/1.0 and ISO 100, I should be able to reach 141.1 feet with a GN of 43. I don't have a f/1.0 lens, but the example in my original post shows that I should have a range of 296ft with a 50mm lens at f/1.8 and ISO 1600 but I'm not getting that.

I suppose my original question had less to do with sensor size/crop factor and more to do with is it really possible to get a 296ft reach with this flash? Not that I would need that far, but that is what my math is telling me. If my math/assumptions are wrong, where am I off?

mabas9395 wrote:
I. If I change my ISO to 1600 I get a guide number of 160 because the guide number increases by a factor of 1.4x each time I double my film speed. Take 160 divided by my widest aperture of 1.8 and multiplied by 3.3 and I get 296ft or about 99 yards. Does this really mean that I can take a properly exposed photo of a subject at the other end of a football field with virtually no ambient light? That seems overly optimistic to me.

Do the guide numbers really work like they theoretically should, or am I way off on something here?

I don't own the 430, so I will start by assuming that it decreases the angle of coversage suitably to fit 1.6crop frame, and the resulting intensity modifieds accordingly. Your calculations about ISO are correct. ISO100 ---> ISO1600 is 1:4 on the GN conversion. Will the light reach that far, NO. A normal GN computation assumes typical reflectivity in the settting due to overhead white ceiling and nearby walls. Outside, both are missing so the GN from the manufacturer is an overly optimistic number, as you surmise.