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09-04-2009, 01:28 PM   #1
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Zoom on a flash

I have no idea what it's for and what advantage/purpose it serves. can anyone explain??

09-04-2009, 01:33 PM   #2
Ash
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Hi there and welcome.
The flash's zoom function focuses the beam of light to the angle desired, mainly to serve as a power conserving measure, but also can be used creatively to close the angle of light being emitted by the flash.

On a P-TTL flash with this feature, this is adjusted automatically according to the focal length of the lens attached to the camera. So a 50mm lens will tell the flash to hone the beam of light to cover the area that would be captured by a 50mm lens. A wider angle lens will require a broader light source and so the flash's zoom will adjust for that.

Given 2 subjects the same distance away from the camera, it would take a broader light (and therefore more power from the flash) to brighten the whole area of a 20mm field of view compared to lighting up the equivalent subject for a 50mm capture, which would need less power to light up, because the zoom focuses the beam just on that required area.

Hope I've explained it OK for you.
09-04-2009, 03:20 PM   #3
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QuoteOriginally posted by Ash Quote
On a P-TTL flash with this feature, this is adjusted automatically according to the focal length of the lens attached to the camera. So a 50mm lens will tell the flash to hone the beam of light to cover the area that would be captured by a 50mm lens.
Doesn't have to be a P-TTL flash. I have a couple of older flashes that provide auto zoom on the K10D and K20D bodies.

Thank you
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09-05-2009, 06:34 PM   #4
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Yeah, what Ash said. And it's a significant difference: at 85mm, the Pentax AF540FGZ has a guide number of 54 (that's where the 540 comes from), but at its least-focused setting (covering the same field of view as a 16mm lens on a Pentax dSLR like the K-7) the guide number is only 32. That's almost a three-fold difference in raw power.

03-16-2011, 10:08 AM   #5
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So is there a correlation between the guide number and zoom distance? I'm just trying to grasp the basics of this. When I shoot with my flashes, I either shoot manual and adjust by site (1/1, 1/2, 1/4 etc...) or I use the Pttl from an on board device. I'm trying to understand where the zoom fits in.
03-16-2011, 10:23 AM   #6
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QuoteOriginally posted by Deiberson Quote
So is there a correlation between the guide number and zoom distance? I'm just trying to grasp the basics of this. When I shoot with my flashes, I either shoot manual and adjust by site (1/1, 1/2, 1/4 etc...) or I use the Pttl from an on board device. I'm trying to understand where the zoom fits in.
The listed guide number is usually measured at full zoom for marketing reasons to make the flash look more powerful. When you zoom the flash head to the wide setting, the flash spreads the light out more, and the guide number goes down.
03-16-2011, 10:26 AM   #7
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right now, I understand the Zoom to adjust the beam of light. If I want the light to be spread out more...I drop the zoom down to 24mm. If I want a narrow hard beam, I'm bump up to 85mm. I could then still increase the intensity on manual by going from 1/4 to 1/1. Would this in theory be correct?

03-16-2011, 10:53 AM   #8
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QuoteOriginally posted by Deiberson Quote
right now, I understand the Zoom to adjust the beam of light. If I want the light to be spread out more...I drop the zoom down to 24mm. If I want a narrow hard beam, I'm bump up to 85mm. I could then still increase the intensity on manual by going from 1/4 to 1/1. Would this in theory be correct?
Right, although in terms of "hard light" vs "soft light", zoom won't make much of a difference for direct flash.

What it will make a difference for is: If you are in a situation where you cannot achieve proper exposure at max power and the flash is "zoomed out", you may be able to achieve proper exposure when the flash is "zoomed in". Of course, the zoom needs to be equal to or wider than the lens focal length (if on-camera) if you require full coverage of the frame.

The other is that, for an equal exposure scenario, if the flash is zoomed in as far as possible without vignetting, recycle times will be lower because you can drop the power.

There are tricks that can be performed (such as feathering) with zoomed flashes that do not cover the frame.

An extreme example of flash zoom is the Better Beamer - this focuses a flash beam so it only covers an area similar to the FOV of a 200-300mm lens or so, but allows for MUCH more light in that area. I've used it for taking pictures of my undergrad school's pepband from across the whole hockey rink (I have season tickets in the townie seats.) - even a max-zoomed 540 could not emit enough light for such a situation.
03-16-2011, 02:37 PM   #9
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QuoteOriginally posted by Deiberson Quote
So is there a correlation between the guide number and zoom distance? I'm just trying to grasp the basics of this. When I shoot with my flashes, I either shoot manual and adjust by site (1/1, 1/2, 1/4 etc...) or I use the Pttl from an on board device. I'm trying to understand where the zoom fits in.
I've got a chart where I've gathered the published guide numbers for various Pentax and Pentax-compatible flashes at each available focal length setting. (Unlike a zoom lens, flashes generally have discrete steps.)

Guide Numbers - Pentax P-TTL Flash Comparison

QuoteOriginally posted by Deiberson Quote
right now, I understand the Zoom to adjust the beam of light. If I want the light to be spread out more...I drop the zoom down to 24mm. If I want a narrow hard beam, I'm bump up to 85mm. I could then still increase the intensity on manual by going from 1/4 to 1/1. Would this in theory be correct?
Yes. Many Metz flashes have a feature where the flash automatically stays one step wider than strictly required in order to provide more diffuse light. Higher-end Nikon flashes (sadly, for Nikon's camera system, of course, not ours) offer different "patterns", from very focused to extra-broad.

But Entropy is also right in saying that the effect isn't extremely dramatic. If I have time later I'll post some pictures demonstrating.
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