Not intended to support or refute anyones argument. 1/3 stop is approximately 26% variation. 1/6 stop is approximately 12.2% variation. 1/10 stop is approximately 7% variation.
If you said you could visually ascertain (just looking at slides on a lightbox, no densitometer) less than 10% variation in exposure, I'd be sceptical. That still permits about 7% error, which is a huge tolerance by the standards of modern equipment. My guess is that any error less than 5% would be indiscernable to the unassisted eye. What I can't cast judgement on is the accuracy of apertures set from the body with KA lenses and bodies (pre *ist). When in manual mode they are reputed to have poor consistency. The *ist uses a different protocol and could be better. Future Pentax lenses may use motorised diaphragms (it was rumoured to be a capability of the new mount) which would be still more accurate. Setting apertures from the lens is acceptably accurate and, as mentioned in this thread, in Av mode the camera does a final recalculation of exposure after the diaphragm stops down to compensate for any error. Last, a comment and a question about Velvia. A lot of time has passes since the original Velvia came out, but I remember very clearly that it was targetted to professionals shooting for publication. Its high Dmin was meant to hold better highlight detail than existing transparency films when translated to the output of offset presses, and it was never meant to look its best for lightbox display, and certainly never when projected. If you expose this film for those two purposes you will be overexposing it (which probably explains the common preference for ISO40 rating). A good Velvia tranny looks dense next to any other brand and type of transparency. My question is: has the intended usage of Velvia changed in recent years? regards, Anthony Farr
