D lens data screws up TTL BL direct flash

Started Dec 4, 2012 | Discussions thread
OP WFulton Senior Member • Posts: 2,695

_sem_ wrote:

You can try one more exercise... With the aperture wide open and a scene with objects at different distances, scan the whole focus distance range with as small steps as possible, as if you were doing a "focus stack". Every once in a while try to find the plane of focus and record the distance (tape measure) and image filename. Then make a chart of recorded distances and see if there are any irregularities (aside for the rough steps).

I fail to understand how that is any different than my existing chart,  which moved the camera to 10, 15, 20 feet, recorded distance, and compared to D lens data.

Some lenses (I have the 12-24 and 16-85) are nothing but gross irregularities. Not even approximately correct.

Others mostly give up reporting distance, and report infinity, which is a good thing, it disables this flaw (ignores distance override of TTL metering).  14-24 for example quits after 5 feet.

And some are wrong, but not hugely so.  Some are wrong the other way, causing no problem (similar to infinity).  Others are wrong enough to cause flash irregularities however.

I do not question why the lens is wrong.  It seems an impossible task anyway, lenses and cameras do not measure or know distance.

I do question why the TTL BL flash system would use these worthless numbers anyway.

I want the option to turn it off.   TTL flash mode will do that, but it is not provided now except on the one SB-910 flash model.  System is TTL BL, which does this.

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