zoom lens/zoom flash sync

[Rusty Shackleford]

My guess is that there is some basic rule of mathematics that has eluded me on this, but: the 550EX flash works with either 35MM or digital cameras and the 'zoom' of the flash will automatically sync with the zoom of the lens (i.e. when the lens is zoomed to 75 the flash will zoom to 75mm so the area of flash matches the area covered by the lens). Soooo, since a lens set for say 50MM is effectively an 80MM on a digital camera wouldn't the flash zoom be off by the same ratio? Is my brain just shorted out on this?

[PacAce]

My guess is that there is some basic rule of mathematics that has eluded me on this, but: the 550EX flash works with either 35MM or digital cameras and the 'zoom' of the flash will automatically sync with the zoom of the lens (i.e. when the lens is zoomed to 75 the flash will zoom to 75mm so the area of flash matches the area covered by the lens). Soooo, since a lens set for say 50MM is effectively an 80MM on a digital camera wouldn't the flash zoom be off by the same ratio? Is my brain just shorted out on this?

No math is involved here nor is it required. The lens says to the camera, "I'm at 50mm". The camera tells the flash, "The lens is at 50mm". And the flash sets itself to 50mm (or the nearest zoom setting it can go to, erring on the wide side. Even if the camera effectively sees 80mm, the 50mm flash coverage will also cover the 80mm the camera sees.

[Phil Hall]

The 1.6x factor is really a crop and not an extender, the chip only records 62.5% of the full image. It is still a 50mm lense so the flash zomms to 50 mm.

[robertwgross]

Another way to think about it is this:

On a full-frame film camera, if the lens says "I'm at 50mm" and the flash zooms to 50mm, then it works, the light is spread out enough for the field of view, and it works.

On a digital with 1.6 factor, if the lens says "I'm at 50mm" and the flash zooms to 50mm, then it works, and the light is spread out a little farther than the digital field of view needs, but that is still OK.

The only thing to complain about are that there is a little light wasted. Better for the error that way than if the error went the other way, and the edges of the subject would be dark.

Maybe someday the communications will be better, and the lens says "I'm at 50mm" and the camera modifies that with "but there is a 1.6 factor", so the flash zooms to 80mm or so.

---Bob Gross---

[Phil Hall]

I had this problem when I first started using the D30. If you add a 1.4x or 2.0x extender the camera recognizes the new focal length of the lense. Surely the canon engineers could do this with the flash zoom, if it were necessary. Robert was right in the first part, all you do is "waste" some light.

[PacAce]

I had this problem when I first started using the D30. If you add a 1.4x or 2.0x extender the camera recognizes the new focal length of the lense. Surely the canon engineers could do this with the flash zoom, if it were necessary. Robert was right in the first part, all you do is "waste" some light.

My camera's in the shop right now so I can't test it but doesn't the flash zoom in to fixed zoom points on the flash and not conitnuously as the lens is zoomed in and out? If that's the case, then in really doesn't serve any purpose to take the camera's crop factor into account, does it?

[Rusty Shackleford]

That's right, the flash zooms in steps: 28, 35, 50, 70, 85, 105 so it's always at or wider than the lens. It seems that more than just 'a little light' would be lost, but I guess it must not be enough to be considered significant or the camera would be programmed to compensate the same as it does for a 1.4 extender. Keep in mind I didn't ask for myself...but for the benefit of others who were probably too embarrassed to ask such a question :wink: