[grhazelton]

Actually p-TTL is cheaper as it deletes the need for an extra sensor. P-TTL uses the cameras normal metering

As I recall the LX used the same sensor for both flash exposure and "normal" exposure. Cheaper? By a tiny amount per unit amortized over the production run for Pentax, but for the user quite expensive in terms of new equipment purchase.

[Lowell Goudge]

As I recall the LX used the same sensor for both flash exposure and "normal" exposure. Cheaper? By a tiny amount per unit amortized over the production run for Pentax, but for the user quite expensive in terms of new equipment purchase.

actually, as I recall, didn't they have a dual metering approach, one with normal exposure but also a second where if exposure changed during a shot, below normal sync speed, they used the flash sensor to change the open time to prevent over exposure?

[cyberjunkie]

I may be wrong, but i recall that there was a sensor reading the light off the film plane. It was used for TTL-flash metering.
I think it was a separate sensor.
From what i remember, Olympus was the first to introduce this kind of approach, but i guess the high-range OM cameras used a pattern on the shutter curtains.
I'm not 100% sure, but i didn't make search before posting. I prefer to learn from the next poster (with better memory than myself!).

cheers

Paolo

P.S.
I don't think that Pentax chose the new P-TTL system, just as a way to sell new flashes.
Probably it was just a simpler, no frills approach, perfect for digital cameras; on top of that, it was cheaper, as it didn't need a separate sensor.
TT-flash support could have been a nice addition to a real top-of-the-line camera (something like a new LX), which wasn't made at the time, and has not been made ever since.
Of course a similar addition could have been a costly one, both in terms of engineering and realization, and could have been a commercial harakiri (impairing the sale of the new line of high end P-TTL flashes).
On the other hand, top line cameras can give huge profits... so i'm not entirely sure of my point.
Whatever, a K10D on steroids was not made, and we'll never see another TTL-flash DSLR camera. There are slim chances for a new medium format body, if the current resurgence of analog photography will expand to a wider user base.
BTW, the bigger formats on 120 film, and all the large formats, are still very competitive with full frame digital bodies, even considering quality vs price (with regard to some particular photographic fields).

[grhazelton]

actually, as I recall, didn't they have a dual metering approach, one with normal exposure but also a second where if exposure changed during a shot, below normal sync speed, they used the flash sensor to change the open time to prevent over exposure?

Have a look at this: Pentax LX - How does its metering works The LX has a secondary mirror on the back of the main mirror. During viewing this secondary mirror flips down to send light from the semi-transparent center of the main mirror to the rear facing sensor in the base of the mirror box. The reading taken is displayed in the viewfinder and approximates what shutter speed will be actually used. When the mirror rises during exposure the sensor reads first the light reflected from the pattern on the first curtain, then the film. If the light changes during the open shutter time the sensor can increase or decrease the exposure.

As I understand it in flash mode the sensor reads ambient light off that patterned curtain, starts the flash, reads flash plus ambient during the exposure and quenches the flash when enough light, ambient plus flash, has been measured. It would seem if another camera's flash fires during the LX's open curtain flash the additional light is compensated for.

In use the system works very well. Additionally the viewfinder displays a flash ready light, a confirmation light indicating proper flash exposure, and switches from X synch to automatic exposure mode during recycling.

Only one sensor for both flash and ordinary exposure.

Automatic exposure sans flash goes to EV -6. I've taken moonlight shots with exposure times of several minutes which were properly exposed. Reciprocity failure is a real factor in such shots, so one "derates" the film per the maker's recommendation. Fireworks displays are very easy, even without being able to "chimp."

Quite a come down to a maximum auto exposure of only 30 seconds with our current DSLR's.

[Mark DeB]

The ist ds has the flash quench circuitry that supports TTL but only in M mode with M lens ( or -A lens not on "A"). And only with an external flash that supports TTL.

Hi everyone,

I realize this is an old thread but I only came across it today. I am a little confused by some of the information here, so let me report what I am seeing. I have a *ist DS2 and an AF280T flash. I do find that, much of the time, the flash appears to work properly when it is set to TTL AUTO and the camera is set to P, Av, or Tv, not just M (with the aperture ring set to A). In these cases, the flash goes off but does not discharge completely, and the exposure seems correct. So doesn't this mean that it is working in TTL mode? (I realize that wombat2go was describing the DS, not the DS2, but I thought they were similar.)

This is not to say that my experience with this combination has been trouble-free. Behavior has been inconsistent: sometimes the flash did discharge fully and the result was overexposed, but I subsequently cleaned the contacts and now everything seems to be working okay.

So it seems to me that the DS2 does support TTL with the AF280T in modes other than M. Any correction or clarification would be much appreciated.

Thanks,

Mark

[hks_kansei]

If it helps, my *istDS works perfectly fine with my little Pentax AF160 flash.

Not 100% what kind of system that flash uses, but it's one with the little "eye" on the front to read flash timing/power.
You focus then set aperture to the scale on the flash.

[Mark DeB]

Hi everyone,

I realize this is an old thread but I only came across it today. I am a little confused by some of the information here, so let me report what I am seeing. I have a *ist DS2 and an AF280T flash. I do find that, much of the time, the flash appears to work properly when it is set to TTL AUTO and the camera is set to P, Av, or Tv, not just M (with the aperture ring set to A). In these cases, the flash goes off but does not discharge completely, and the exposure seems correct. So doesn't this mean that it is working in TTL mode? (I realize that wombat2go was describing the DS, not the DS2, but I thought they were similar.)

This is not to say that my experience with this combination has been trouble-free. Behavior has been inconsistent: sometimes the flash did discharge fully and the result was overexposed, but I subsequently cleaned the contacts and now everything seems to be working okay.

So it seems to me that the DS2 does support TTL with the AF280T in modes other than M. Any correction or clarification would be much appreciated.

Thanks,

Mark

p.s. I am using A lenses, not DA.

[wombat2go]

Hi Mark,
I don't have an AF280T, so I just tested the ist ds with an -A lens using the AF360fgz( has p-ttl) and the AF500ttz ( no p-ttl)

With AF360 on, to get TTL function, the lens has to be moved off A in camera M mode. Otherwise the AF360 will stay in p-ttl. The iso has to be low ( I forgot how low) so I use iso 200

With AF500 on, and iso 200 selected, the flash went to TTL mode and exposed reasonably consistently from f/5.6 to f/16 with camera in M mode and lens both on A, or off A, and both shutter and aperture control are available.

AF500 also appears to work correctly in TTL in Av mode at various apertures, but the shutter is fixed at 180th, which is a limitation for the flexibility to use a combo of available light and fill flash.

In P mode, indoors, the camera selected wide open lens, the shutter reverted to 180th with the AF500 in TTL, and it exposed OK, but the fixed shutter and wide open lens is too much of a limitation.

Hope this helps, the test certainly helped me to cull some weak AA batteries from my aging supply of Ray-O-Vacs!