Balancing Ambient and Flash with Auto-FP mode

[michael_s10]

<p>From reading a number of articles on the web, it seems that Auto-FP mode allows one to shoot at shutter speeds above 1/200th, which should help reduce the amount of ambient light relative to the amount of flash.</p>

<p>However, it also appears that using auto-fp mode reduces the power of the flashes. </p>

<p>So, one reduces the ambient light, but also reduces the flash power. If they are reduced by the same amount, it would seem that the photos would more or less look the same. For example, if ambient loses a stop, and flash power loses a stop, well then, all things are even.</p>

<p>I must not understand this correctly then - what does one gain by Auto-FP mode?</p>

[jose_angel]

<p>The ability of e.g. to fill with flash using wide apertures under daylight. You can freeze the action (moving subject), to have a shallow DoF and a perfectly illuminated subject without overexposure.</p>

[Matt Laur]

<p>No, the flash power isn't reduced in terms of its contribution to the exposure - it's reduced in the sense that you can only get just so much out of it, because of how it discharges <em>throughout</em> the exposure. The net result is that you can't use it over the same distances that you would when it's in a more standard mode. Usually, this means working with the flash closer to your subject. If you have to be back from your subject, use the flash as an off-camera slave (the CLS controls can still tell the flash how to behave for fast sync).<br /><br />In essence: when you're in that mode, you've got the ability to shoot with <em>very</em> fast shutter speeds, but it's like having a not-as-powerful flash. But you can still use it very effectively, even up against quite bright ambient light, as long as you're thoughtful about positioning it. Note: don't forget you can use multiple Nikon speedlights as slaves, and buy back some of the power lost to having to use them in this mode.</p>

[frank_skomial]

<p>" If they are reduced by the same amount" - nothing is by the same amount.</p>

<p>Flash power loss depends on the shutter slot travel time, and also on the shutter fast speed. That is how many more consecutive adjucent flashes are needed, or how many times the vetical shutter frame distance is greater than the open narrow shutter slot width.</p>

<p>Since Auto FP is a remote CLS and multiple flashes system, you may need to place many FP flashes closer to the subject, and not on a camera that could be located farther away, and up to 20 meters, or about 60 feet, if SU-800 is used as a commander and line of sight is not obstructed.</p>

<p>"<em>what does one gain by Auto-FP mode?" -</em> one can use shutter speed faster than the mechanical camera max sync speed. This is needed for outdoor portraits with wide open apertures for shallow depth of field, and low ISO. Something that is not possible to achieve without neutral density filters, or special electronic shutters like one built in Nikon D70.</p>

[frank_skomial]

<p>Electronic flash causes avalanche of current causing light flow through xenon tube, and causing very short duration lihghting, usually much shorter than the time needed for open shutter slot traveling time across the sensor.</p>

<p>It is easy to shorten flash duration by use of a Thyristor that will cut of the xenon tube current flow, and will make flash duration shorter.</p>

<p>However, it is not easy to extend electronic flash duration, since the current rushes quickly (as an avalanche) through the xenon tube.</p>

<p>In order to make flash light appear as lasting longer, to cover antire time of shutter travel, multiple consecutive and adjucent smaller flashes are electronically generated, with help of a thyristor. Total duration of many smaller flash blasts is appopriately generated to cover selected fast shutter speed slot running across the sensor vertical dimension.</p>

<p>Total flash energy stored in a power capacitor is dispensed in smaller amounts, small enough to sustain and allow multiple blasts. Smaller energy flashes have much smaller range, and therefore, we talk about the lower power in FP mode.</p>

<p>The flash energy is actually not lost but dispensed into many smaller flashes lasting together longer, of much smaller energy for much shorter usable distance.</p>

[michael_s10]

<p>Well, I took the opportunity to run some very unscientific experiences with auto-fp, complete with 3 sb600's rigged to shoot through a medium chimera softbox.</p>

<p>I'm going to run some more representative tests tomorrow during fuller daylight (it was pretty late here when I tried), but near as I can tell, auto fp is not a great way to change the balance between ambient light and flash. I think I was probably looking for some law of physics to be violated (conservation of energy or something like that).</p>

<p>As mentioned by others, it does work well for getting a wider open aperture when ISO is at its effective minimum.</p>

<p>Thanks again for taking the time to explain!</p>

 

[Matt Laur]

<p>In what way were you out of balance? If the answer is that you're getting too much ambient light, then fast sync is <em>the</em> way to solve the problem. Higher shutter speed means less ambient light, period. Then just crank up the flash power, and you get the mix you want. The flash power (within the limits of what the flash units can emit) is independent of the shutter speed, thanks to fast sync. So ... it's a very good way to balance the two. You just have to take control of both things separately.</p>

[mark liddell]

So you can shoot at wider apertures, f/16 is not always ideal of portraits and weddings.

 

From tests the loss in flash power is more than the reduction in ambient light in FP mode making ND filters a better alternative as you are still able to shoot at the flash sync speed.

[Matt Laur]

<blockquote>

<p>From tests the loss in flash power is more than the reduction in ambient light</p>

</blockquote>

<p>Which tests, conducted how? Note the comments above about choosing your scenarios carefully. Work with the flash at a distance that doesn't give up a lot of power to the inverse square law. That's often best if it's off-camera (easy, these days, using CLS).<br /><br />With an ND filter, you <em>are</em> reducing the flash's power in exactly the same way you're reducing the ambient light. Which means you have to run the flash that much hotter in order to make a difference. Reduce it enough with ND, and you'll reach a point where you can 't overcompensate with flash power anyway. The choice of strategy depends on many variables.</p>

[michael_s10]

<p>Matt - </p>

<p>I'm going to rephrase my goal here a bit...</p>

<p>Consider a setup as follows:<br>

1. SB600 place off axis - 6 ft away from subject. This distance can't change, since moving the light will change the softness of the light.</p>

<p>2. SB600 at max power.</p>

<p>Let's say at F5.6 & 1/200th of a second, I can underexpose the background by one half stop, with the subject perfectly exposed.</p>

<p>If I use FP mode, can I underexpose the background even more (while retaining identical foreground exposure), without increasing the power on the flash (it's maxed out), or moving the flash (that would change the softness of the light)?</p>

<p> </p>

[jay_poel]

<blockquote>

<p>should help reduce the amount of ambient light relative to the amount of flash</p>

</blockquote>

<p>I think you are misunderstanding. The idea to FP is to allow the flash to fire above the sync speed to help balance the exposure (or overpower it if the flash is powerful enough and that is want you want to acheive). <br>

I am of the understand that the ambient light is what you (your camera) are exposing for and the flash is there to fill in the shadows. Taking shots of my kids outdoors almost always necitates (is that a word?) the use of my SB600 to allow me to get the shallow depth of field I want and to freeze the motion of the kids playing. Typically I am shooting at 1/500s or faster at f/2.8 or f/4, ISO 200 with the flash set to FP, BL (balanced), and iTTL mode. I adjust the EV compensation on the flash as required.</p>

<p> </p>

[Matt Laur]

<p>Michael: if the flash is already at maximum output, no, you're probably out of luck without having a second slave (or more). CLS does make that part easy, though.<br /><br />Using a different light modifier, the better to get the light closer, may also be a losing bet - since modifiers eat light. That would be a trial and error thing.</p>