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06-22-2011, 10:39 AM - 1 Like   #1
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Automated flash systems (pttl) don't begin to use flash zoom potential

This little discusion is more for the new folks and enthusiasts that perhaps haven't explored all the possibilites of modern flash equipment.

I've found very little in books that adequately discuss the topic of flash zoom, and its creative possibilities. Thats why i decided to write this discussion. If i'm in error in any of my conclusions, or you are using flash zoom in other creative ways, please comment in this thread - i'm always ready to learn more.

What is it?
Flash (strobe, speedlight) ZOOM is a feature of some modern flashes that allows one to move the flash reflector relative to the light emitting tube. This feature results in one being able to choose between a narrow intense beam of light or a wider beam of light that covers more area but is less intense.

Why is it labelled in terms of lens Focal Length?
Matt DM's pttl website ( Feature Comparison - Pentax P-TTL Flash Comparison ) discusses how the light beam width at a specific flash zoom position, say 50 mm, corresponds with the lens field of view for that focal length (50mm). While illuminating the whole area covered by a camera lens is desirable in some cases; in many strobist situations, the flash is off camera by 45 degrees or more and the analogy of the flash reflector position to the camera lens is more confusing than clarifying.

In my picture below, the flash is facing the camera lens, so any comparison of the zoom feature to the lens is irrelevant as far as field of flash coverage.




How does PTTL treat lens zoom in an automated fashion?
The Metz 48 is my only automated TTL flash so i'll talk about it. When the flash is on the hot shoe, zooming from one FL to another results in the same FL showing up on the LCD window in direct flash mode. So the correlation of zooming lens or prime lens FL to flash zoom is useful to some degree in assuring the field of view is lit. But with the flash "off-camera" it makes no sense at all. I think most modern TTL flashes have a similar strategy.

Metz allows one in PTTL to use their 2-button push to alter the beam size of the flash "zoom" even in PTTL. But i suspect many users just let the flash system do its thing and don't think about zoom position.

What are the creative possibilities with manual adjustment of zoom?
In the picture above, i chose to use a narrow intense beam of flash lighting power behind the person and directed at the foot area. This resulted in a nice set of leg shadows stretching towards the camera. So you can use the narrowest beam of a flash, usually 85mm or 105mm, to cover a smaller area in your scene, or you can use the widest beam of the flash, usually about 28mm to help bleed off lighting power, i.e. to prevent hot spots in one's scene or perhaps within a softbox.

Motorized zooms are even showing up in relatively inexpensive manual flashes such as the new Yongnuo YN-560 with a 24mm to 105mm range. Lets consider the GN at each end of the that range: at 24mm its GN is 28 and at 105mm, its 49. If one uses the 18mm flip down screen, the GN is only 17. Thats a 3X variation of light intensity just by using the zoom/screen variations, not including manual fractional power variability. These numbers aren't that different from the Metz 48 in manual mode.

So don't imagine that the modern X-TTL systems are doing something brilliant with the zoom feature. As far as i know, these automated systems only optimize the dedicated flash zoom setting when the flash is on-camera. In fact, even on-camera, my Metz48 flash blanks out the zoom setting when the flash is tilted from the direct flash forward position, i.e. PTTL is no longer controlling that setting.


Last edited by philbaum; 06-22-2011 at 03:49 PM.
06-22-2011, 12:49 PM   #2
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great read for noob and experienced out there!
06-22-2011, 01:09 PM   #3
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Thanks for writing this. I'm a flash novice so this is helpful.
06-22-2011, 01:41 PM   #4
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Very timely indeed as I was just given my first off camera P-TTL flash by my children - a Tumax rebranded one.

06-22-2011, 02:01 PM   #5
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Thanks Phil, this is very helpful info for beginners like me.

One question regarding zoom head usage.

In situations where reduced flash power is needed, is it correct to widen flash zoom head? for example, with AF540FGZ, at 50mm zoom position, flash output at 1/64th is GN11 at ISO400, assuming ASA400 film. If one were to open up the aperture to say 2.8 or so for shallow DoF, with the main subject only 1.5 meters away. The flash would be too powerful to be left at 50mm zoom ? is changing the zoom head the correct way to reduce flash power in this instance or would it be better to fit a diffuser or some sort?
06-22-2011, 06:09 PM   #6
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QuoteOriginally posted by jackseh Quote
Thanks Phil, this is very helpful info for beginners like me.

One question regarding zoom head usage.

In situations where reduced flash power is needed, is it correct to widen flash zoom head? for example, with AF540FGZ, at 50mm zoom position, flash output at 1/64th is GN11 at ISO400, assuming ASA400 film. If one were to open up the aperture to say 2.8 or so for shallow DoF, with the main subject only 1.5 meters away. The flash would be too powerful to be left at 50mm zoom ? is changing the zoom head the correct way to reduce flash power in this instance or would it be better to fit a diffuser or some sort?
I don't claim to be a flash expert, so there's that . IMO, there is nothing wrong with widening the flash zoom to reduce power, provided it doesn't degrade the image you had in mind. If the subject were close to a background that you wanted to separate it from, then widening the flash might be undesirable in lighting up the background. Flash lighting falls off as the square of the distance from the flash, so the background may not be a problem. If widening the flash just means that the light energy is going to be spent outside the viewing area of the lens, who cares?

On the other hand, many dslrs and some flashes have FEC, flash exposure compensation in one or both devices that one can use. You can also use Auto or TTL with that flash to reduce exposure if the auto programming is more effective than the manual controls. One could take the flash off the camera and put more distance between it and the subject, bounce the flash off the celing or side walls to reduce power by 2 stops, use a diffuser as you suggested, lower the iso or get slower film (not too practical on the film), and i'm probably missing something.

Zoom is just another wrench in the toolbox IMO, to be used when its effective. Hope that helps.
06-23-2011, 01:12 AM   #7
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awesome post, i'd just like to add some of my own experiences in using the zoom function.

in the few times i've been forced to use my flash on-camera (*gasp*) i found that the zoom function was a great tool to use in terms of subject-background separation when bouncing the light off the roof, since you can use a tighter beam to light only your subject. here's an example of what im talking about:



spotted this rather humorous scene while shooting for one of the dances at my university. I wanted to light the girls as well as the pervy looking guys behind them so i set my zoom to form a fairly tight beam. you can see how the light falloff is affected by looking at the carpet, just enough spill from the flash to light the two guys, but tight enough to leave out the rest of the scene
06-23-2011, 05:30 AM   #8
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Nice post Phil!

06-24-2011, 04:38 PM   #9
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QuoteOriginally posted by adpo Quote
awesome post, i'd just like to add some of my own experiences in using the zoom function.

in the few times i've been forced to use my flash on-camera (*gasp*) i found that the zoom function was a great tool to use in terms of subject-background separation when bouncing the light off the roof, since you can use a tighter beam to light only your subject. here's an example of what im talking about:



spotted this rather humorous scene while shooting for one of the dances at my university. I wanted to light the girls as well as the pervy looking guys behind them so i set my zoom to form a fairly tight beam. you can see how the light falloff is affected by looking at the carpet, just enough spill from the flash to light the two guys, but tight enough to leave out the rest of the scene
adpo,
a good example about using the zoom function for ceiling bounce to create a more interesting picture!!!

Everyone else:
appreciate the comments on the information. I just had this feeling that there was more to zoom than most of us were using.
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