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04-17-2012, 10:48 AM   #16
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In addition to the A-pin you must short the Data pin also, the one closest to the screw drive. Most lens mounts will do that for you, but some with plastic or painted mounts and some narrow M42 lenses may not short it. If both A and data pins are shorted, then the camera will show the F-stop at what it thinks is wide open. The remaining pins on the lens provide a binary pattern to communicate the max and min aperture (actually the closed down f-stop e.g. f16 or f22 and the range in 1/2 stops). If all pins are shorted, the camera believes that you put a F1.2 - F22 lens.

If you leave the f stop at f1.2 (lens wide open) you can use all the P-TTL functions correctly. For M42 lenses, where the aperture blades stay closed as you spot down the lens, this will work fine for any aperture set on the lens (just keep the camera at f1.2). The camera will measure the flash "wide open" and then will calibrate, but since the lens is already stooped down, there will be no additional correction.

For K or M lenses is a lot trickier outside of the wide open aperture. The camera will stop down the lens if you dial a different f-stop on the body, but the aperture level on the lens closes the blades linearly to their diameter, not the area which will cause serious exposure miscalculations.

04-18-2012, 04:12 AM   #17
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Ignoring the hacks to try and fool p-TTL, there are two ways to get reliable flash with any lens, the first is to purchase a flash with its own sensor and use auto flash (just like the old days) there are many flashes out there that can do this, including some older Pentax flashes and Nikon flashes ( my BIL uses a nikon flash in auto mode with his Leica) or get the AF540FGZ which has a built in sensor

The second way is to get an *istD this camera can support TTL and p-TTL flash, so when you put a non A lens on the camera it defaults to TTL. It is one reason I kept my *istD note the *istD is also capable of operating in high speed sync mode using the body flash as the master, later cameras could only do this with an external flash.

It's too bad this functionality was not carried forward on later top of the line models
04-18-2012, 05:52 AM   #18
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Yep, I really don't understand why Pentax could not alter their DSLR behavior to allow P-TTL with legacy lenses...

With those "F--" lenses, it would only take moving the P-TTL pre-flash after the aperture close-down to allow full P-TTL support (just like we do with our homemade tricks)...
04-18-2012, 07:36 AM   #19
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QuoteOriginally posted by dlacouture Quote
Yep, I really don't understand why Pentax could not alter their DSLR behavior to allow P-TTL with legacy lenses...

With those "F--" lenses, it would only take moving the P-TTL pre-flash after the aperture close-down to allow full P-TTL support (just like we do with our homemade tricks)...
If the lens is stopped down, there is not enough light in most situations for the meter to work properly. The flash would have to discharge at much higher power for the pre flash and it will be recharging when the actual photo is taken (or it will introduce several seconds of shutter lag). With TTL (as in the old film and D, DS, DS2 *ist) the camera measures the light during the exposure and shuts the flash OFF when it thinks there was enough light.

04-18-2012, 08:01 AM   #20
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Well, from my own tests playing with m42 lenses and the tinfoil trick, the current pre-flash power level handles small apertures quite nicely...
I've even played with wireless a bit, and it worked quite nicely!

OK, maybe if you hit f/22 with a far subject, then you'll be in trouble and the flash exposure can be a little off, but for everyday use it would be perfectly all right... Way better than the current situation!
04-18-2012, 09:00 AM   #21
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Possibilities include:
- disabling the lens' aperture lever somehow
- figuring out how to best encode the lens' data pins to minimize exposure error.

I think Lowell knows how to do that - he certainly has the data needed to figure it out.
04-18-2012, 09:42 AM   #22
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Lowell published some great info on exposure errors for K & M lenses in a few posts one of which is in:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/122264-how-mak...ml#post1262456

Here is data from a lens Lowell modified so the camera thought it was an A type 50:2-16; as you can see the errors are large.


I think maybe the actual error could be made smaller by telling the camera the lens had different minimum and maximum f-numbers. I think (but have not shown) that having the camera think this was an f:1.2 - 32 lens the curves in Lowell's graph would flatten and there'd be less total error over a reasonable aperture range like from minimum to f:10 (or vis-versa). Unfortunately I don't have a K or M type lens to experiment with (maybe I'll buy one to try.)

Last edited by newarts; 04-18-2012 at 09:50 AM.
04-18-2012, 10:37 AM   #23
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Here is a link with the bit-patterns of the 5 pins that communicate min/max aperture Features and Operation of the Ka Mount

The camera will operate correctly on max and min f-stops with "doctored" K and M lenses. Anything in between will generate an error because the aperture level movements are linearly proportional to the DIAMETER of the iris, while on A-Lenses they are linearly proportional to the AREA of the iris.

It is fairly straightforward to calculate the error. Given the range of f-stops, calculate the percent of stopping down in EV units. Apply that to the diameter of the fully open iris ("Focal Length" / "Wide open f-stop") and you get the actual diameter of the lens at the camera's f-stop setting. The real f-stop (and the error) will be "Focal Length" / "Computed diameter".

This will hold true for all lenses with the same min/max f-stops regardless of focal length.

04-18-2012, 11:18 AM   #24
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QuoteOriginally posted by demp10 Quote
Here is a link with the bit-patterns of the 5 pins that communicate min/max aperture Features and Operation of the Ka Mount

The camera will operate correctly on max and min f-stops with "doctored" K and M lenses. Anything in between will generate an error because the aperture level movements are linearly proportional to the DIAMETER of the iris, while on A-Lenses they are linearly proportional to the AREA of the iris.
One might think that to be the case but Lowell's data shows exposure is incorrect at the largest f-number; perhaps this is due to the calibration factor used for the linear aperture mechanism ie f16 (old style) not equal to f16 new(style). His data implies the exposure curve is headed towards correct exposure at maybe f20-22.
04-18-2012, 11:45 AM   #25
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QuoteOriginally posted by newarts Quote
Lowell published some great info on exposure errors for K & M lenses in a few posts one of which is in:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/122264-how-mak...ml#post1262456

Here is data from a lens Lowell modified so the camera thought it was an A type 50:2-16; as you can see the errors are large.


I think maybe the actual error could be made smaller by telling the camera the lens had different minimum and maximum f-numbers. I think (but have not shown) that having the camera think this was an f:1.2 - 32 lens the curves in Lowell's graph would flatten and there'd be less total error over a reasonable aperture range like from minimum to f:10 (or vis-versa). Unfortunately I don't have a K or M type lens to experiment with (maybe I'll buy one to try.)
do you want my xr rikenon to try?

after all it already has the A pin, and the F2-F22 pin pattern drilled into the mount
04-18-2012, 12:18 PM   #26
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QuoteOriginally posted by Lowell Goudge Quote
do you want my xr rikenon to try?

after all it already has the A pin, and the F2-F22 pin pattern drilled into the mount
Darn! I already ordered an old lens to try!

Thanks anyhow.

Dave

EDIT After more thinking, I'm sure the relative error can be made smaller by diddling with the lens info pins, but probably not small enough to make it worthwhile. We'll see. At least I'll finally have a K type lens.

Last edited by newarts; 04-18-2012 at 03:30 PM.
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