TTL / TTL-BL and Metering Modes: What's your take?

Started Jul 27, 2014 | Discussions thread
WFulton Senior Member • Posts: 2,695
Re: Nikon TTL vs TTL-BL flash modes

fotowbert wrote:

I can confirm what WFulton said about Nikon's FV lock. When used it changes the flash behavior of my D80 SB-600 combination.

I know no reason why FV Lock ought to change TTL mode exposure, unless the camera aim is moved.   But it does.  It is not a big change for TTL mode, but is a slight change.  The manual does tell us the diameter of the center spot FV TTL meters, which we don't know if it is the same or a different diameter without it.  I cannot imagine why it should be different though?  But there is some change.

But TTL BL mode direct flash can be a big change, and is explainable in some part.  TTL BL direct flash is governed by D lens distance, and if the metered TTL exposure is greater than the calculated Guide Number exposure (for the D lens distance), then the GN distance overrides, and the TTL exposure is limited, to prevent obvious overexposure, according to the direct GN distance (assuming D lens is accurate, which is not often very true for zooms).  Lots of ifs and buts.     We know we use FV Lock to prevent the subject from blinking at the preflash, but the manual says the purpose is so we can get an exposure reading, and then move the camera aim for the picture, and of course D lens distance would have no meaning then.  So it seems reasonable that FV Lock has to ignore D lens and its possible safeguards.

The SB-600 is on of the special ones, with its own overt TTL BL or TTL menu.  The only Nikon flash still in production with that menu is the SB-910.  All the others, and third party too, all default to TTL BL mode (unless Spot metering is selected, which switches all flashes to TTL mode).  So direct flash is going to see this FV Lock thing more, on most flash models now, because the Nikon system is a TTL BL system.  And FV Lock switches out the D lens distance.

This Spot metering solution (to regain TTL) is confusing.    D lens too, since many zooms are often badly wrong.  All this is more reasons for flash to vary.    I do  think TTL BL vs TTL ought to be a camera menu, in all cameras, affecting all flash, since it is the camera system that does the metering.  The flash just flashes as it is told to do.

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