There was a post over at DPR that showed some ADI vs TTL shots.. they didn't really make any sense. I took some shots with my beercan just to review what was done before. I've always set ADi for my lenses that have 8 contacts and TTL for lenses that have only 5. I believed that the camera would automatically switch to TTL when it had to but I was making the menu change anyway. These were shot with my 36am flash. in the first the camera was set to ttl but was bounced from the ceiling too. this is just so you can see the scene. what I wanted was the clock because the glass and face is so reflective it always messed with the flash if it was in the picture.



This shot was with TTL turned on and I added no compensation just a .3 stop on the ambient slide that I always leave set. This is what I expect to get from TTL. the hot spot on the clock face knocks the flash back and it underexposes. usually I just add flash comp and then it exposes properly.



This is the troubling shot. remember this is a beercan and is not supposed to be ADI compatible but this is what I expect from a D lens that is exposed for distance instead of reflectance. can anyone explain whats going on? I used pattern metering for all these and the same settings as above. the only change was switching to ADI in the menus. I tried some other subjects and got the same results 100% of the time as long as there was something significantly reflective in the scene and taking up a large portion of the scene. every single time the TTL would under expose with these settings and setting to ADI solved it.

In a way, all AF lenses are (D) lenses. Under normal conditions, the camera---any Minolta AF SLR or DSLR---will know about the focus distance. The trick is in the words "normal conditions." It means: AF shaft coupled and any focus limiter, if present, disabled or, if enabled, not preventing the lens from focusing to infinity. Under these conditions, any AF lens basically is a (D) lens.

Unlike conventional AF lenses, a real AF (D) lens will transmit focus distance information also when the AF is disconnected or unable to focus to infinity due to a focus limiter, and with greater accuracy. That's all. With a conventional AF lens, the cameras will derive the focus distance information indirectly, from the lens type and the number of revolutions of the AF shaft, with infinity as the starting point. With an AF (D) lens, the lens itself will encode the distance information directly (which basically is what you can read out from the distance scale but with greater accuracy) and send it to the camera.

-- Olaf

On a non-D lens, how does it know how many revolutions relate to distance? Is it from the lens ID/database or does it work it out some other way?

The reason I ask is that I am curious as to the accuracy of this information with 3rd party lenses that notoriously re-use IDs that Minolta/Sony have already allocated.

So when the camera is set to ADI and a non D lens is attached the camera doesn't fall fall back on TTL at all but tries to use the information it has to make ADI work and thats why theres diferent result with each setting.

Does this also explain why Minolta cameras always rotate out to infinity when turned off so the distance measuring can be zeroed ?

So if the lens has the right chip (original Minolta) wouldn't the distance be plenty accurate without the encoder for flash ? What do they gain with the encoder? seems the non D lenses should be able to calculate the distance at least accurate enough to be within the DOF on a subject. isn't that good enough especially for fill flash outdoors, seems to be the only time I use direct flash anyway at least without rear sync.

I wouldn't be surprised if using direct flash with a (D) lens and a (D) flash that the ADI flash strength calculation is used by the camera preferentially regardless of whether TTL is selected on the menu, TTL seems too be the weak link in flash in the digital world, goodness knows why, it's not as if it's some new concept, it has been a real boon to flash photography for decades, I don't know why exactly they just didn't refine TTL for the digital age better, that would have made more sense to me, maybe it's to do with cutting out third party manufacturers, or making life difficult for them, hard to say.
The only difference I heard of between a (D) lens and a non (D) lens was the (D) lens had a focus encoder built in that relayed very accurate focus distance info back to the camera. That's all well and good, but it's still fraught with peril, if, for a number of reasons the AF system didn't focus at the correct distance, poor light, front or back focus etc. then there will still be an exposure error. With light decreasing by the square of the distance (twice as far = four times fainter), it's extraordinarily easy too get errors in flash strength/performance.
Greg

Greg, While I agree that TTL wasn't broken, the opportunity to get flash right with ADI is compelling. How many times have you had a flash metering catch the wrong part of the foreground and expose for that, leaving the focused on subject in the back too dark? Of vice versa. If the camera knows that your subject is 32 feet away, it can ignore that 10 foot away foreground item that'a highly reflective and instead meter for the distance.

A good idea, even if it does complicate matters even more!

Dusty

Yes, it's very easy to fool the system when using direct flash, one has to be very careful with composition indeed, with any near objects in the frame the the camera is stiffed with a choice of loose loose whatever it does, either the far object is exposed ok with the near one suffering the neutron bomb effect, or the near one exposed ok and the far one lost in the murk, there isn't normally any escape from the inverse square law whether using ADI or TTL.
Bounce TTL of course would be the best if there are near and far objects as usually most things are at a similar distance from the ceiling and thus stand more of a chance of being evenly lit, but that can bring additional difficulties if you need too use an f/stop that is deep enough to bring near and far objects/subjects into focus...then you start to need a fair bit of power from the flash gun.
I did notice one time though when shooting along the length of long table with guests on both sides (and backing up further away not being an option, nor was ceiling bounce) that the system can fooled by the relatively wide-ish angle of lens zoom into setting the zoom on the flash to match the lens angle of view, (the focus was about 2/3 down the length of the table), that flash zoom setting was not going to work as the nearby people were going too be over exposed and the far ones way under exposed, (a photo confirmed this), it was possible though to overcome that too a degree I later discovered, by overriding the flash zoom manually and zooming it to 85mm-ish even though the lens was at something like 20mm, this allowed the exposure system much more latitude to expose the far end with the punch of the zoom flash and the nearby subjects getting lit by the spray coming off the edges, can work ok with a little bit of experimenting, but far above the capabilities of the system to do by itself.
Also on the weekend (against my better judgement), I decided to do a little investigating of the mysterious realm of taking direct flash photos of shiny/reflective things that occupy most of the frame. My first subject was a china cabinet that had a glass front, a mirror back internally, glass shelves and crystal whatnots on the shelves. I had the edge of the cabinet door (one of them) near the edge of the VF frame so that almost the entire frame was glass and crystal and mirror, and too make things as tough as possible I used ISO200 at f8. The A100 + F56 actually performed well above my expectations either with ADI or TTL including the AEL button on some for ambi balance, (standard flash sync.) most exposures were reasonable with a couple slightly overexposed, the only exposure that was quite overexposed (maybe two stops), was when I tried HSS flash.
I then moved the tripod over in front of the old grandfather clock and did a re-run, which included adjusting the tripod height until the face could reflect squarely back into the camera, as soon as I got the camera square on I instantly got a near black frame, just the rim of the face slightly visible and the brilliant reflected spot (on TTL), nothing else visible. Thinking that ADI would fix it was wrong...it was just about the same again, maybe because by this stage with so many photos of reflective things the system had finally caved.
I did manage to get it back though just by shifting the tripod a foot to the side and taking a couple of photos zoomed back out a tad...and presto back too normal again.
I'm not sure what conclusions one can actually draw from any of this, except too say, try too avoid strong reflections with direct flash as much as possible...because the only way I overcame the clockface reflection was to revert to M mode both on the camera and flash and adjusting settings accordingly to get a decent exposure.
I was also left wondering if the flash didn't spot the "other" flash and send it a signal to stop shining in "my" face which it then picked up from the reflection too shut itself off.
Greg