Understanding Flash Zoom

[BlakBird]

So I was really off on this one. I thought when it says 105mm zoom that means how far the flash is traveling. According to articles I'm reading up on, the higher the number the smaller or tighter the beam of light that's flashed. Would this be accurate?

I have a YN565EX and I was having some practice with it at a graduation for a friend. I must have had it one too high or something because in a couple of shots you can see some "scrunchy" faces in the background of the person I was focusing on (the faces are kind of humorous actually). But based on what I'm reading, in order to reduce the power of the flash you would take it down from 1/4 to 1/8th or even higher..yes?

 

[tirediron]

The 'zoom distance' refers to the beam spread; in other words, when you see 105mm, the flash is trying to optimize it's pattern/spread for a camera fitted with a 105mm lens. If it is set to 24mm, then it will be putting out a much wider spread in order to illuminate the much wider field of view of the wide angle lens. If you're seeing 'background' behind the subject, then yes, reduce the power output from 1/4 to 1/8, or even 1/16.

 

[BlakBird]

The 'zoom distance' refers to the beam spread; in other words, when you see 105mm, the flash is trying to optimize it's pattern/spread for a camera fitted with a 105mm lens. If it is set to 24mm, then it will be putting out a much wider spread in order to illuminate the much wider field of view of the wide angle lens. If you're seeing 'background' behind the subject, then yes, reduce the power output from 1/4 to 1/8, or even 1/16.

So if I'm taking a portrait shot at a reasonably close distance (6-8 feet away) I want something closer to 105mm?

 

[Derrel]

The zoom head numbers refer to the width of the beam's spread or "coverage angle" for focal length, as expressed for a full-frame sensor or 35mm film camera--at least with "most" flash units. Some of the newer, higher-technology flash units automatically sense if the camera shoots full-field OR is it shoots crop-field (APS-C or 1.5x or 1.6x 'crop frame'). If you are using a wide-angle lens, ands want a wide beam of light sent out, you would set the flash to a "wide" setting, like say the 24mm zoom position.

With a telephoto lens, you can set the flash to a "TELE" zoom angle, like say 85mm or 105mm, and the beam of light is narrow, and will go pretty far with decent power.

It's not really how FAR you are away--it's matching the angle of your lens's field of view to the flash's zoom head.

Best thing to do is to test it out at night, against the side of a large, light-colore building; you can literally SEE what the beam spread adjustments "DO" by running some personalized, real-world tests that way.

 

[BlakBird]

The zoom head numbers refer to the width of the beam's spread or "coverage angle" for focal length, as expressed for a full-frame sensor or 35mm film camera--at least with "most" flash units. Some of the newer, higher-technology flash units automatically sense if the camera shoots full-field OR is it shoots crop-field (APS-C or 1.5x or 1.6x 'crop frame'). If you are using a wide-angle lens, ands want a wide beam of light sent out, you would set the flash to a "wide" setting, like say the 24mm zoom position.

With a telephoto lens, you can set the flash to a "TELE" zoom angle, like say 85mm or 105mm, and the beam of light is narrow, and will go pretty far with decent power.

It's not really how FAR you are away--it's matching the angle of your lens's field of view to the flash's zoom head.

Best thing to do is to test it out at night, against the side of a large, light-colore building; you can literally SEE what the beam spread adjustments "DO" by running some personalized, real-world tests that way.

Would the same test work in my bedroom at night? Our neighbors are over the top sensitive.

 

[TCampbell]

So I was really off on this one. I thought when it says 105mm zoom that means how far the flash is traveling. According to articles I'm reading up on, the higher the number the smaller or tighter the beam of light that's flashed. Would this be accurate?

I have a YN565EX and I was having some practice with it at a graduation for a friend. I must have had it one too high or something because in a couple of shots you can see some "scrunchy" faces in the background of the person I was focusing on (the faces are kind of humorous actually). But based on what I'm reading, in order to reduce the power of the flash you would take it down from 1/4 to 1/8th or even higher..yes?

When you buy a studio flash, they rate the flash power in watt-seconds... really just a representation of the amount of energy that the light might be able to provide. But there's more to it.

Suppose you have a flashlight. You point it at a target such as a white wall and notice how bright the spot is. Now remove the bulb and batteries and direct wire the bulb -- it's just not inside the flashlight anymore. Technically we know that it's the SAME battery and SAME light bulb. It provides the SAME "total" amount of light. But if you look at the wall it will be much less bright.

What's happening here is all about the reflector. When it was inside the flashlight, the reflector was focusing the beam to a concentrated area instead of allowing what was technically the same amount of light travel in every direction rather than all going the same direction. Focusing the beam of light is a really big deal.

This is exactly what the zoom head on your flash is doing.

Suppose your flash emits exactly 1000 photons of light when it fires (yes, clearly I'm making this up to make this easy to understand.)

Let's suppose you have a full-frame camera and a 100mm lens. The angle of view (when measured diagonally) is 24.4º (I looked it up using this website: http://www.tawbaware.com/maxlyons/calc.htm ).

Let's suppose your subject is 10' away from the camera and flash. The "dimensional" field of view is an area that measures 4' 4" diagonally (4.33')

That means the number of square feet inside that circle (area = pi X radius^2) is half of 4.33 (2.165'). Square that to get 4.69, then multiply by pi (3.14) to get 14.7 square feet. That's the "area" of the circle large enough to cover the field of view we can see in our camera with the 100mm lens. The flash needs to be able to light up this area.

Since we presumed our light emits 1000 photons (a ridiculous number but it makes the math easier) then that means we can divide our 1000 photons across the 14.7 square feet and we discover that assuming even illumination of light, each square foot will be illuminated by 68 photons.

You can think of those 68 photons as a measure of light. The unit does not matter. But lets just presume whatever we named the unit of measure... we have 68 of those "units" of brightness.

Now lets go to a 50mm lens and do the same thing.

The angle of view (diagonally) for a 50mm lens is 46.8º (notice it's not actually double, but it is close.)

The dimensional field of view at 10' is now close to 8' 8" (8.66') diagonally.

The area of an 8.66' circle is now 58.9 square feet (nearly 59). Notice that this is enormously larger than the our original circle when using a 100mm lens.

Now lets divide our 1000 photons to spread them equally amongst each square foot of our image circle. 1000 ÷ 59 = 16.9 (about 17 photons per square foot).

That's actually ONE QUARTER of the light per square foot. Again... same light bulb, but we're going to change the reflector to allow it disperse over a wider area.

As you increase actual distance that the light must travel from the source, the beam spreads out. It spreads out following the "inverse square" law. That laws says that each time the distance increases by a factor equal to the square root of 2 then the amount of light reaching each square foot at that new distance will be HALF. The square root of 2 is very close to 1.4 and for simplicity just use 1.4 as the value. That means if we changed our subject distance to 14' instead of 10' (10 X 1.4) then exactly HALF the number of photons would be illuminating each square foot of our subject. Using the 50mm lens example, it would be 8.5 photons per square foot in stead of 17 per square foot.

Pause for a moment.

You do NOT need to go through all this math to do flash photography, but it DOES help to have a grounding in how it all works because you'll realize that you can actually do flash photography strictly by the numbers rather than by trial and error.

Speedlite flashes simplify a lot of this by using a system called the "guide number". The guide number is a measure of how far the flash can properly illuminate a subject for a good exposure. But this would depend on the camera exposure settings. So they use a baseline exposure value of ISO 100 (that's reasonable) and they use f/1.0. Alarm bells might be sounding because if you check your camera bag, I'll wager you do not actually own an f/1.0 lens. That's ok. The REASON they use f/1.0 is because it makes the math VERY easy. Just divide the "guide number" by the f-stop you ACTUALLY plan to use (yes.. it really is just that) and that's your new distance.

Every time you change the reflector, however, you change the guide number of the flash. That's because the guide number is not a measure of how much light the bulb can emit... it's a measure of how much light will actually be delivered to your subject and THAT depends on how tightly we focus the beam of light. You would have to know the guide number of the flash for each focal length (btw, this is something you can calculate using either an incident meter that can do flash metering OR just by using your camera (but that's another thread.))

Back to your original question... the "zoom" is a measure of an angle and NOT a measure of a distance.

If the flash head is "zoomed" to 100mm it means that assuming you are pointing the flash directly ahead and not using it to bounce off a ceiling, wall, or putting it through type of light modifier (soft boxes, reflectors, etc.) THEN it will illuminate a circularly area large enough that the field of view as seen through the camera using a 100mm would be completely illuminated by the flash.

If, however, you decided to put a 50mm lens on the camera but zoom the flash head to the 100mm position and took a photo you would notice heavy light fall-off around the edges of the frame because the beam is focused so tightly that it's only able to illuminate the center area.

 

[WayneF]

So I was really off on this one. I thought when it says 105mm zoom that means how far the flash is traveling. According to articles I'm reading up on, the higher the number the smaller or tighter the beam of light that's flashed. Would this be accurate?

I have a YN565EX and I was having some practice with it at a graduation for a friend. I must have had it one too high or something because in a couple of shots you can see some "scrunchy" faces in the background of the person I was focusing on (the faces are kind of humorous actually). But based on what I'm reading, in order to reduce the power of the flash you would take it down from 1/4 to 1/8th or even higher..yes?

I don't know what scrunchy means, but I doubt that was the flash zoom.

The flash head normally zooms automatically to follow the lens zoom. For direct flash, the idea is that they should match.

If you zoom your lens to 85 mm for a tight shot, it has a smaller field of view (than wide angle at 24mm lens).

Some flashes don't even zoom at all (camera internal flash for example), and you could always use a wider flash with the narrow lens... you just waste flash power outside the view of the lens - the lens cannot see it.
Or the flash can zoom to match the lens, which concentrates all of its power into a smaller area more matching the lens view. More light, more efficient.

So flashes automatically zoom to track the lens. If your Youngnuo is not zooming, manually zoom it to one step less than 24mm, which is the Auto position, and it will. Then don't worry about it.

 

[BlakBird]

I don't know what scrunchy means, but I doubt that was the flash zoom.

Imagine you're having the most amazing dream in mid slumber and some a**hole comes into your room with strobe lights from a rave party. Imagine what your face may look like after that happened. That's a "omg I starred directly into this dude's flash I'm blind!" look.

 

[WayneF]

Imagine you're having the most amazing dream in mid slumber and some a**hole comes into your room with strobe lights from a rave party. Imagine what your face may look like after that happened. That's a "omg I starred directly into this dude's flash I'm blind!" look.

Well, the flash zoom was no factor in doing that. Zoom just makes the flash more efficient, capable of brighter, but then the automatic TTL just backs it off to lower power, to give the same picture exposure. And this flash picture looks reasonably exposed.

Normally flash is too quick to allow human reaction time to blink. People do blink, but after the shutter is closed and done.

But things like the Nikon Commander are particularly bad about causing blinking, which is virtually guaranteed, because they do a lot of flashing before the shutter opens.

Normal hot shoe TTL Preflash can be an exception for a few unusually sensitive people, but most people have slower reactions. Saying, usually normal hot shoe TTL is not a blinking problem (but a few people are exceptions).

This seems much more than a regular blink though, it also includes the facial expression, mouth, etc, so it seems like there must have been more going on here that we don't know about. Looks like it must have started before the shutter button was pressed? I am skeptical that normal TTL preflash offers that much opportunity for reaction time. And he did not do it in the other picture? So something more is happening. But it was not flash zoom.

To prevent blinking, the Nikon solution is the FV Lock option (on camera models with the commander). This does the preflash and commander signals early, gets the blinking over with, and then the subsequent shutter has no problem with blinking.

 

[BlakBird]

"This seems much more than a regular blink though, it also includes the facial expression, mouth, etc, so it seems like there must have been more going on here that we don't know about. Looks like it must have started before the shutter button was pressed?"

Haha, actually I was on rapid fire during that time. So it's possible I caught a blink after that one because he was dumb enough to be looking directly at it.

 

[Designer]

Would the same test work in my bedroom at night? Our neighbors are over the top sensitive.

You have to have a blank, fairly neutral wall, and then enough distance to properly test the width of the flash "hot spot". A large room would be a help.

Or if your neighbors go out for the evening, use the garage door with you standing in the driveway.

 

[BlakBird]

Would the same test work in my bedroom at night? Our neighbors are over the top sensitive.

You have to have a blank, fairly neutral wall, and then enough distance to properly test the width of the flash "hot spot". A large room would be a help.

Or if your neighbors go out for the evening, use the garage door with you standing in the driveway.

Yea I tried in my bed room last night against a blank wall but I don't think the distance was quite far enough. It wasn't an obvious difference bewtween 24-80.