Flash Adapter for LS Lenses

[bill baker]

I've recently been revisiting the issue of using flash with my 67II system. Seems to me that what's needed, short of redesigning the focal plane shutter for a faster basic sync, is a way to sync the LS lenses with the TTL flash circuitry in the body. All that's really needed is a way to key the flash trip to the leaf shutter while still passing along the quench signal from the body to the flash. Not that hard, right? How about taking the basic Pentax 5-pin to hotshoe adapter and adding a jack for a PC cord to the LS, patching in the shutter trip from the LS cord for the trip signal pin of the 5-wire input? Wouldn't that work? And make the module a screw-in unit that would mate to the body using the 5-pin socket with a hotshoe on top, about the height of the prism. That would give you the equivalent of a high speed flash sync hotshoe (when using a LS lens) with something close to the form of a standard 35mm SLR hotshoe, just offset a tad. Beats the hell out of having a heavy flash mounted on the moment arm of the Pentax grip. Besides, I hate that @#%! grip, with or without a flash mounted.

 

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Obviously, the TTL calculation would be based on the 1/8th shutter speed of the focal plane shutter, but if I understand how TTL works the body calculates a quench signal based on the reflected flash metered through the lens and doesn't particularly care how long the shutter is open once it's metered the flash pulse. If the 67II had flash compensation built in then I could see that the TTL calculation would need an accurate shutter reading, but if I understand the 67II specs right it's only calculating an overall correct exposure, not balancing between flash and ambient illumination.

[milton_barrocas]

Great minds think alike. I have come across this idea myself some time

ago and will give it a try soon. The concept is simple enough just

have the leaf shutter provide the sync and the camera provide the

quench.This is simple to try by splicing a couple of sync cords there

are many ways to identify which signals are which in the sync cords

(my setup is a custom radio remote so I already figured out all the

signals). I have no doubt that this will work fine in the usual mode

of leaf shutter operation with maybe a slight hit in accuracy (its not

likely that the P67II flash meter is that accurate anyway). In the

special mode of leaf shutter operation this might not work because the

metering systems usually integrate the signal coming from the metering

cell. This means that background noise accumulates with time so if the

meter is just sitting there waiting for you to release the leaf

shutter eventually the background noise will build up and cause the

quench signal to trip. FYI here's the P67II hot shoe pinout and some

signal info tidbits. It's too bad you don't like the handle, I rather

like it.

 

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o <--data link

O <-- trigger

o o <-- ready

^

|--------------quench

 

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trigger - active low

quench - active high

ready - active high

 

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Here is what typical triger,quech waveforms look like

 

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trigger ------- ----------------

|_____________|

 

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quench ------- -------------------------

|_______|

 

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Interestingly the quench signal is the reverse polarity from many

other flash systems such as Nikon. A simple inverter can be built to

adapt a Nikon TTL flash to a Pentax camera. I have more info if anyone

is interested. It is very nice to know all the details of the flash

system. It has allowed me to build many TTL radio remotes very

inexpensively and to have just one type of flash for several camera

systems by buildind adapters. I've been wanting to add the leasf

shutter functinality for a while and always been quite certain it

would work.

[milton_barrocas1]

OK the diagrams didnt pan out. What the hell happen? I guess I need

more text drawing skills.

[bill baker]

Milton, clearly you're up on the technical details. I'm not clear on

why a shutter delay might cause the TTL meter to generate a premature

quench signal, though.

[milton_barrocas]

I was talking about a long shutter delay, remember that with the

90F2.8LS the leaf shutter can be fired independently of the camera so

the leaf shutter delay can be much longer than normal depending on

when you decide to trip it. Funny you replied now because I'm going to

shoot some slides up at yosemite this weekend and will report my

findings next week with some more details, again I'm quite sure it

will give me good exposures but we'll soon see...

(for this weekend since I'm traveling light I will be using a nikon

SB25. I made a shoe adapter machined out of ABS plastic (about 1 cubic

inch) that adapts the pentax shoe to a nikon. This adapter has a tiny

switch that allows me to turn off the sync signal from the camera. The

SB25 lets me fire the flash through the PC sync connector in TTL mode.

So I connect the adapter to the P67II's flash handle then I mount the

flash to the adapter. Then I connect a PC sync cord from the LS sync

directly to the SB25 flash's PC sync. I flick the sync switch on the

adapter to OFF and away we go... my setup is more complex because I'm

not only adding LS TTL functionality but also using a nikon flash,

there's no way I'm going to buy a wimpy AF330FTZ flash when I already

own a beautiful and expensive SB25, it just isn't right)

[milton_barrocas]

Great news. I got perfect TTL flash exposures. Even with the leaf

shutter in special mode to my surprise. It's a miracle the special

shutter mode even works (I only tried it with a coupe of seconds

delay). Actually to use the special mode you have to pull the TIME

switch and when you fire the camera's shutter the camera actually

turns off the flash indicator (indicating flash is no longer being

used??) but the TTL quench is still there after the leaf shutter fires

the flash.

Bill if you are interested in adding this functionality to your setup

let me know and I can suggest something or I can even build you

something at cost. I guess the first thing one has to decide is what

kind of flash and where the flash will be. For me I use an SB25 flash

on occasions when I must travel light or shoot fast. To mount the

flash I use the flash handle with the adapter. I'm afraid that this is

the best way to mount a flash (I got tired of messing around with

flash brackets and such) also I suspect the AE prism locking mechanism

is not strong enough to support any substantial flash. For high power

strobes I use a radio setup with a transmitter that again mounts on

the flash handle (I've been thinking of building a transmitter small

enough to fit and be supported by the 5-pin connector) The other

option which I don't use is the flash cable to use the flash off

camera. The last option is probably the easiest but of limited use.

[pascal_niklaus]

I am interested in building a slave flash unit that works with TTL and

would like to build one myself, something like Nikon's SU-4. However,

I have not yet found how exactly the Nikon hot shoe signals work.

According to your posting, Milton, you know more about this and have

even build a radio controlled one.

 

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I am experienced in electronics, so that is not the problem.

 

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I would be very happy if you could provide me with infos on the hot

shoe signals and how the communication works during TTL flash...

 

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Best regards,

 

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Pascal