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A poll about Auto ISO action with hot shoe TTL flash
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<blockquote data-quote="WayneF" data-source="post: 536358" data-attributes="member: 12496"><p><strong>Thoughts on Auto ISO action with the current Group C camera models</strong></p><p></p><p>These group C camera models will limit Auto ISO with TTL hot shoe flash to 2 stops maximum ISO increase, or to 4x Minimum ISO, presumably usually to ISO 400.</p><p></p><p>A fully powered speedlight (Nikon SB-800 or Yongnuo YN565 class) has 24mm guide numbers of about</p><p>Full power GN 98 x2 for ISO 400 is GN 196</p><p>1/128 power GN 8.5 x2 for ISO 400 is GN 17</p><p></p><p>So these TTL flash can operate over a seven stop range of power level, which at 24mm and this ISO 400 situation is from GN 17 to GN 196. Which describes very many common situations, and TTL at these extremes might range from f/16 at 1 foot (at 1/128 power to stop motion in water drop splashes), to f/2.8 at 70 feet (a ball game?). There are charts in the Nikon flash manuals describing these ranges. Longer zooms could approach up to 2x this 24mm GN, which both the water drops and the ball game would surely use.</p><p></p><p>But exceptions exist, due to limitations of maximum and minimum flash power level capability. </p><p> Suppose we try f/4 at 24mm at ISO 400 (and I did try it).</p><p>If at 6 feet, this is GN 24, and it uses the ISO 400, at very low power.</p><p>If at 3 feet, this is GN 12, which the flash at 24mm cannot do, even at 1/128 power. So it used ISO 200, out of necessity.</p><p></p><p>Suppose we try f/11 at 22 feet. This is GN 242 at ISO 400, but the flash at 24 mm can only do GN 196. So it used ISO 1400, out of necessity.</p><p></p><p>Both the Nikon and the Yongnuo flashes see this same action (deviating from ISO 400 when necessary). The camera controls TTL, but I am unaware how the camera determines maximum power capability of the specific flash. It must be part of CLS communication?</p><p></p><p>Top camera models also show the current Auto ISO value in the viewfinder right beside where it says Auto ISO. But in this situation (Group C TTL flash), it always continues to say ISO 400, even if conditions do switch it to ISO 200 or ISO 1400. It cannot meter and switch until the TTL preflash, so there is no time to affect the viewfinder. But the rear LCD result does show Auto ISO in red.</p><p></p><p>Group A cameras (TTL flash at Minimum ISO) also do this too, always have. </p><p></p><p>And Group B (TTL flash at high ISO) also must do it. I don't have a Group B camera, but obviously Group B cannot do much flash at ISO 3200. Because ISO 3200 GN is 5.67 times IS0 100 GN, or GN 98 x 5.67 = GN 555 at ISO 3200, therefore 10 feet would require f/55.5. This does not happen. I assume the same algorithm is used in all models.</p><p></p><p>So ISO 3200 is obviously reduced at last instant. Which then ISO also obviously affects the metered ambient exposure, underexposing it. However, it is lighted by flash, we might not notice much. So, there are unknown things happening, but my guess is in much less degree at ISO 400, with a more reasonable seven stop range.</p></blockquote><p></p>
[QUOTE="WayneF, post: 536358, member: 12496"] [B]Thoughts on Auto ISO action with the current Group C camera models[/B] These group C camera models will limit Auto ISO with TTL hot shoe flash to 2 stops maximum ISO increase, or to 4x Minimum ISO, presumably usually to ISO 400. A fully powered speedlight (Nikon SB-800 or Yongnuo YN565 class) has 24mm guide numbers of about Full power GN 98 x2 for ISO 400 is GN 196 1/128 power GN 8.5 x2 for ISO 400 is GN 17 So these TTL flash can operate over a seven stop range of power level, which at 24mm and this ISO 400 situation is from GN 17 to GN 196. Which describes very many common situations, and TTL at these extremes might range from f/16 at 1 foot (at 1/128 power to stop motion in water drop splashes), to f/2.8 at 70 feet (a ball game?). There are charts in the Nikon flash manuals describing these ranges. Longer zooms could approach up to 2x this 24mm GN, which both the water drops and the ball game would surely use. But exceptions exist, due to limitations of maximum and minimum flash power level capability. Suppose we try f/4 at 24mm at ISO 400 (and I did try it). If at 6 feet, this is GN 24, and it uses the ISO 400, at very low power. If at 3 feet, this is GN 12, which the flash at 24mm cannot do, even at 1/128 power. So it used ISO 200, out of necessity. Suppose we try f/11 at 22 feet. This is GN 242 at ISO 400, but the flash at 24 mm can only do GN 196. So it used ISO 1400, out of necessity. Both the Nikon and the Yongnuo flashes see this same action (deviating from ISO 400 when necessary). The camera controls TTL, but I am unaware how the camera determines maximum power capability of the specific flash. It must be part of CLS communication? Top camera models also show the current Auto ISO value in the viewfinder right beside where it says Auto ISO. But in this situation (Group C TTL flash), it always continues to say ISO 400, even if conditions do switch it to ISO 200 or ISO 1400. It cannot meter and switch until the TTL preflash, so there is no time to affect the viewfinder. But the rear LCD result does show Auto ISO in red. Group A cameras (TTL flash at Minimum ISO) also do this too, always have. And Group B (TTL flash at high ISO) also must do it. I don't have a Group B camera, but obviously Group B cannot do much flash at ISO 3200. Because ISO 3200 GN is 5.67 times IS0 100 GN, or GN 98 x 5.67 = GN 555 at ISO 3200, therefore 10 feet would require f/55.5. This does not happen. I assume the same algorithm is used in all models. So ISO 3200 is obviously reduced at last instant. Which then ISO also obviously affects the metered ambient exposure, underexposing it. However, it is lighted by flash, we might not notice much. So, there are unknown things happening, but my guess is in much less degree at ISO 400, with a more reasonable seven stop range. [/QUOTE]
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A poll about Auto ISO action with hot shoe TTL flash
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