How to use the "A" mode on a speedlight? (Not TTL)

[jason_fung]

How to use the "A" mode on a speedlight? (Not TTL)

Note: this is not a discussion of TTL metering because the cameras that I am using are not compatible with TTL. But they do have an

automatic mode that I would like to try and utilize.

 

Set aperture and ISO to the speed desired.

The speedlight will tell you the effective range that the speedlight will be usable at full power.

 

How does the speedlight meter?

(1) meter the whole scene, like with a centre weighted metering.

(2) use some kind of special matrix metering that can disregard backgrounds?

(3) use the IR to measure the distance and after some computer calculations, give an output for that distance. (Similar to using guide

number calculations)

 

Practical scenario:

Speedlight on a manual camera. Speedlight settings are dialed in to mimic those on the camera.

Situation A: speedlight is pointed directly at the subject. the neutral background is close to the subject.

Situation B: speedlight is pointed directly at the subject against a dark night sky. IE. the background is infinitely distant from the subject

 

Analysis of situation "A" and "B":

In situation A, metering systems (1), (2) and (3) will function equally well. However in situation B, the environment will trick the meter in

metering (1); and a successful meter in metering systems (2) and (3)

 

Situation C: the subject is next to a white wall. The background is irrelevant. The speedlight sensor is directed at the subject. The light

is pointed towards the wall. The intention is to use the wall as a bounce flash. If the metering system is based on (3), this situation will

not work. (2) will be fine. (1) might be tricky depending on the neutrality of the surrounding environment.

 

EXTRA: Situation D: (unlikely to occur because manual control and a light meter would be more appropriate in my opinion) light on a light

stand off camera to provide a highlight on the side. The meter is pointing at the subject. Can the automatic function provide sufficient

light (provided it is positioned correctly)? I realize this will be impossible with a shoot through umbrella or soft box. But possibly

conceivable with a reflective umbrella.

 

I thank you for taking the time to read this and indulge in my geekier question. Your contribution is greatly appreciated.

[frank_skomial]

<p>Flash light sensor angle of reading is usually narrower than the flash light spread.<br>

Could help if we know what flash you have?<br>

The A mode is usually simple measurement of returning light and cutting of the light output when subject brightness reaches desired setting.</p>

 

[jason_fung]

<p>I'm using a Metz 58-AF, but I subsequently sold all the compatible Canon gear that it was made for. Now I use it on a variety of manual film cameras from every format and size. I thought the principals would be similar across brands; regardless of Metz 58-AF, Canon 580EX or Nikon SB900. Is this not the case? </p>

<p>I.E. Is the automatic function different between the 430EX and 580EX? </p>

[bebu_lamar]

<p>The flash meters close to your #1 but with an angle of about 40 degrees regardless of the angle of coverage of the reflector which you can zoom. The sensor is not zoom. There is no matrix metering or IR distance measurement of any kind. <br>

Situation A is good and B would overexpose your shot. In situation C if the sensor is pointed at the subject and the flash head tilted to light the wall then it works fine. <br>

Situation D the flash provide sufficient illumination but only viewing from its position so it would not work. </p>

[rodeo_joe1]

<blockquote>"How does the speedlight meter?"</blockquote>

<p>It's effectively a centre-weighted average meter. There's no distance measurement involved, no IR beams, no multi-segment metering or anything like that. Nearly all speedlights that offer Auto-Aperture mode have a small (2-3mm diameter) recessed light sensor at the front. This usually has a lens incorporated to narrow its acceptance angle, and obviously must be kept unobscured and pointed at the subject in order to work properly.</p>

<p>The sensor simply integrates the amount of light reflected back from the subject over time, and shuts off the flash when sufficient light has been detected. The threshold of detection varies according to the ISO speed and aperture set on the flash, and with more recent digital systems those two parameters are read direct from the camera.</p>

<p>IME, AA mode works extremely well with Nikon and Metz designed flashes, and seems to cope well with different types of scene. Other makes can give more variable results, but on the whole they do work fairly well. The flash can be bounced or diffused and modifiers can be used; all as long as the sensor points at the subject and is reasonably close to the camera position. Some systems (notably Toshiba, Sunpak and Metz) offered an auxilliary remote AA sensor that could be fitted to the camera hotshoe while the flash itself was some distance away.</p>

<p>AA mode can even be used to control fill flash in full sunlight, but this does require a bit of experimentation and adjustment to find settings that give the desired result. Sensor circuit designs vary between flash makes/models and some are more immune to ambient light than others.</p>

[rodeo_joe1]

<p>Shown below are 3 different flashguns with their Auto-Aperture sensors circled. Lighting was provided by a bounced Metz 60 CT-4 in (guess what?) Auto Aperture mode. No exposure compensation or adjustment applied.</p><div></div>

[pete_s.]

<p>In addition also keep in mind that the ISO and aperture settings on the flash can be set differently than the camera.</p>

<p>For instance, if you set the flash to ISO200 and f/4, you can have the camera at ISO400 and f/5.6 which is the same exposure. But if you are overexposing you could adjust by stopping down the camera but still keep the flash the same.</p>

<p>Also the autothyristor setting doesn't rely on preflashes and because of that you can also meter the actual exposure with a flash meter.</p>

<p>Last but not least I wanted to add clarify that the sensor and the flash head can be pointed in different directions. It was somewhat mentioned above but if you for instance shoot with a reflective umbrella you could point the flash body and light sensor towards the subject and have the flash head rotated 180 degrees pointing into the umbrella.</p>

<p>PS. If you can also mix auto thyristor flash with manual flash if you have several flash units. For instance having one flash off camera in manual mode, triggered with an optical slave and one flash on the camera in auto thyristor mode.</p>