Third party lenses that fail to provide accurate distance information

<p>I was getting incorrect exposures with flash on my SB800 on a D7100 with a Tamron 17-50mm f/2.8 AF (original version, non-AF-S) then read in Thom Hogan's manual that this lens provides inaccurate distance information. This happened whether the flash(es) are on or off the camera. Aside from Nikon's 17-55 f/2.8, do any of the lenses in this class send accurate distance information to the camera?</p>

 

<p>The use of distance information to regulate flash exposures seems like a completely backward step to me. It's like reverting to the dark ages of Guide Number calculation; and it's as if TTL exposure control had never been invented!</p>

<p>Anyway, that aside. Distance information will only work properly for direct flash. With bounced or diffused flash it's an irrelevence. And if you're using direct flash, the SB-800's Auto-Aperture mode will work just as reliably as, or in some cases better than, i-TTL.</p>

<p>"This happened whether the flash(es) are on or off the camera." - As with bounced or diffused flash, off-camera flash definitely does <em>not</em> need distance information from the lens. What's important is the distance from flash to subject, and there's no way for the lens to know that or communicate it to the camera. For example: The camera and lens might be 20ft from the subject while the flash is only 10ft away. In such a case it's useless knowing the lens's focused distance. Same if the position is reversed and the flash is 20ft from the subject and the camera 10 ft away; the lens distance information is useless here as well. In both of those situations, using the built-in sensor of the flash in AA mode would be far better than relying on the lens-to-subject distance.</p>

<p>What does the EXIF say about the distance in those on-camera flash shots? Agree with Joe that for off-camera flash that information is meaningless.</p>

<p>I'd often wondered about just how useful the <strong>D</strong> in a D series lens was.....especially with regard to AF, but Non AF-D, lenses.</p>

<p>Does EXIF display focused distance?</p>

<p>I guess a timed pre-flash <em>could</em> determine flash to subject, via bounce, distance. Be very quick though..:-)</p>

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<p>Does EXIF display focused distance?</p>

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<p>Yes, but not all EXIF viewers display it. I use exiftool.</p>

<p>"I guess a timed pre-flash <em>could</em> determine flash to subject, via bounce, distance." - I hope that wasn't a serious suggestion Mike.</p>

<p>With light travelling about 1 foot per nanosecond, there's no (affordable) light sensor with a response time fast enough to measure the flash return distance with any useable degree of accuracy. But don't even suggest it. Otherwise Nikon's SB-1020 speedlight will cost about £20,000 - including cryogenic cooling adapter and quantum-tunneling processor backpack.</p>

 

<p>"<em>The use of distance information to regulate flash exposures seems like a completely backward step to me. It's like reverting to the dark ages of Guide Number calculation; and it's as if TTL exposure control had never been invented!"</em> - nonsense,</p>

<p>- resultant perhaps from insufficient understanding how D information enhances the CLS / iTTL system, if provided by the lens to a compatible camera.</p>

<p>The focus distance information provided by compatible lens is the most modern and refined technology to suport iTTL/Balanced flash mode, and the 3-D focus tracking.</p>

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<p>No RJ, it wasn't! I thought the..</p>

 

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<p>Be very quick though..:-)</p>

 

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<p>....might have given the truth away. It shouldn't effect the sync speed then!</p>

<p>Dieter, thank you. I'll take a look at that.</p>

<p>Hector, I read this morning that most zoom lenses give inaccurate D readings, Nikon's included.<br>

<a href="http://www.scantips.com/lights/ttlbl-d.html">http://www.scantips.com/lights/ttlbl-d.html</a><br>

Here are just a few examples of this guy's findings.</p>