Reply to thread

Message

<blockquote data-quote="WayneF" data-source="post: 316412" data-attributes="member: 12496">I don't know.&nbsp; Hard question.&nbsp; <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile&nbsp; &nbsp; :)" loading="lazy" data-shortname=":)" />&nbsp; &nbsp; &nbsp; Focus distance is measured from the focal plane mark near the rear of the camera (where the film or sensor is). When and if the camera is ever concerned with flash distance (D-lens distance), this is the only number it knows. Several zoom lenses get it quite wrong though, so there are worse problems.&nbsp; <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile&nbsp; &nbsp; :)" loading="lazy" data-shortname=":)" />But light is measured from its source, which technically ought to be the flash tube, but I tend to think of it as the front lens of the speedlight.&nbsp;&nbsp; Comparing flashes seems OK if consistently done.&nbsp; I can see that softboxes and umbrellas are different though, front fabric is obviously not the source.&nbsp;I agree, I doubt it is quite that critical for the speedlght.&nbsp;&nbsp; One meter is about 40 inches, two inches is 5%.&nbsp;&nbsp; 2 or 3 meters seems much more reasonable for human subjects (for perspective), so an inch or two shouldn't matter much.The difference in 72 and 70 inches is 0.08 stops.</blockquote>

[QUOTE="WayneF, post: 316412, member: 12496"] I don't know. Hard question. :) Focus distance is measured from the focal plane mark near the rear of the camera (where the film or sensor is). When and if the camera is ever concerned with flash distance (D-lens distance), this is the only number it knows. Several zoom lenses get it quite wrong though, so there are worse problems. :) But light is measured from its source, which technically ought to be the flash tube, but I tend to think of it as the front lens of the speedlight. Comparing flashes seems OK if consistently done. I can see that softboxes and umbrellas are different though, front fabric is obviously not the source. I agree, I doubt it is quite that critical for the speedlght. One meter is about 40 inches, two inches is 5%. 2 or 3 meters seems much more reasonable for human subjects (for perspective), so an inch or two shouldn't matter much. The difference in 72 and 70 inches is 0.08 stops. [/QUOTE]

Verification