The Nuts and Bolts of Off Camera Flash – Part 3, TTL wireless


CLICK HERE FOR PART 2 – Manual Flash

Ok, now that we’ve talked about getting your strobe off camera, and triggering it manually, lets talk about the other “main” option for firing it – wireless TTL

In simplest terms it means that your camera and flash “talk” to each other to automatically determine the proper exposure.  Just like the automatic metering modes for your camera (where it calculates the exposure based on reading the light of the scene) TTL flash does the same thing – lets your camera automatically calculate the amount of flash needed for a scene.  Generally the way this is done is that the camera fires a “pre flash”, a small burst from the flash to “test” the scene, which is read and exposure/amount of flash is calculated.  The shutter is then opened, and the actual exposure is taken with the flash firing to the degree calculated by the pre-flash.

Now bear in mind that although wireless TTL and manual flash both achieve the same end result (firing your flash off camera, they are very different beasts.  Remember all that hardware we talked about for firing your flash manually?  (pocketwizards, cables, ebay triggers etc…?)  None of that will work for TTL.  Remember that all those devices are doing is carying a “FIRE” pulse to the flash.  they are essentially “dumb”.   TTL requires actual communication between the flash and the camera.

The good news is that if you have a modern dslr and “system” strobe (meaning the manufacturere’s dedicated strobe, designed to work with TTL)  you may already be able to do wireless TTL with no additional hardware.

Now as mentioned, modern TTL implementations rely on a preflash to meter the scene, and the the flash fires based on this meting calculation.  When the flash is on-camera this is no problem, since the strobe can “talk” to the camera directly (notice the several other pins on the foot of the “system” flash vs. the manual flash?  those are used for ttl communication)

<- TTL flash foot vs. manual flash foot.

But what happens when we want to take that TTL flash off camera?  Somehow it needs to be able to recieve the metering information to “know” how to set it’s output for the exposure.  Most current camera makers solve this problem by using an optical TTL signal.  In other words, the camera/master uses a series of quick flashes that are “read” by an optical sensor on the strobe, and these tell it the metering/TTL information.  There is a “master” device, usually an onboard flash either built-in or mounted on camera which sends out the preflash communication, and the wireless flashes are the “slaves” that read the flashed signal given by the master and pop accordingly.

This method of communication is actually quite sophisticated.  Most of the current optical TTL systems can control multiple groups of flashes at independent power levels, and power output/ EV compensation can be set directly from the master (it transmits this information to the slaves).  In practice what this means is that you can set flash power level & ratios directly from your camera/master device without having to adjust each strobe manually.  Cool huh?

In terms of a master: Nikon, Olympus, and Pentax all have DSLRS that allow you to use the built-in (pop-up) flash as a master to control slaved TTL strobes, while canon requires you to actually have a strobe mounted on camera (or use the ST-E2, which is a dedicated wireless TTL controller) to control your slaves.   I’m not going to go into the actual setup and configuration of each TTL system, as that is *way* beyond the scope of this article.  Suffice to say, RTFM 🙂  In short, you switch your strobes into slave mode, your camera/master into “master” mode, and then are able to control your slaves from the master, setting ratios and such.  The master tells the slaves how to fire based on the preflash metering information, and Bob’s your uncle…

Now given the advantages of wireless TTL flash, you might be wondering why not always use it and forget about manual?  There are a couple of big downsides to TTL flash:

1 – it is proprietary. The pre-flash ttl protocols that each manufacturer uses to communicate between master and slaves are specific to that manufacture.  What this means is that you are locked in to using that manufacturer’s strobes that support it’s TTL protocols.  Want to use 3rd party? out of luck.  want to mix in studio lights? No dice…  (actually it is possible to do this, but we’re not going to talk about it here 🙂

2 – (and this is the biggee) line of sight (LOS) is required. Since the TTL information is sent optically (by superqick pre-flashes), the camera and strobe have to be able to “see” each other in order to “talk”.   In practice what this means is that you are limited as to where you can place your strobes off camera (since they have to be able to see the signal flashes) and also that these systems become, shall we say, less than reliable in bright ambient conditions or outdoors (since all the ambient light makes it difficult for the sensor to see the signal flashes).   This is a generally “game breaking” limitation for working pros in the field who need to be able to depend on their strobes to fire every time without fail?  (remember I said that rock-solid reliablility is the reason to pay $200 for a pocketwizard over a $20 ebay trigger?  same deal here.)

Enter the RadioPopper…

Much in the same way that a pocket wizard or ebay trigger acts as a “bridge” for the “fire” signal in manual flash setups, the radiopoppers act as a “Bridge” for the visual signal flashes required for TTL communication.

Basically they way radiopoppers work is you have a transmitter and receiver – the transmitter sits on top of your “commander” unit (either an on camera strobe or dedicated commander unit – st-e2 or su800) and “reads” the magnetic pulses created by the master’s signal flashes (apparently the signal flashes generate a magnetic field around the flash head as well).  The transmitter then sends this information via an RF signal, much like a PW (but in this case it is more than just a “fire” signal) to the receiver which has a tiny little LED light inside.  The reciever decodes the RF signal and uses it’s little IR LED to replicate the same signal flashes right in front of the sensor, giving the strobe the exact same optical TTL signal it would have gotten from the commander unit regardless of how far away it is (or around a corner etc…)!  It’s a rather clever setup.  The fact that it is simply tranlating the signals from the master means that it is not system specific – the RadioPoppers will work with both Canon and Nikon’s TTL systems (other systems are not “officially”  supported, but reports have confirmed at least basic functionality with Sony/Minolta and Pentax).

The beauty of Radiopoppers is that since they are essentially just an RF “bridge” for the system’s optical TTL signal, they support all the TTL functionality that the system does, inclusing HSS (High Speed Sync), without the limitations of line-of-sight or range that the optical system imposes.    I personally have a set of radiopoppers, and after using them for a few weeks, I have no idea how I ever managed without them.   Even when I am setting my flash power manually, the ability to do wireless HSS without the limitations of optical TTL is a beautiful thing.  Plus for a lazy slob like me, the ability to control levels/set ratios without walking to each flash is just worth it’s weight in gold.   I really can’t say enough great things about the radiopoppers.

The only downside to the radiopoppers is that they only do TTL.  In other words if you just want to trigger manual flashes or studio lights, the radiopoppers are useless (since they are just an RF bridge for the optical signal) you still need pocketwizards or the like.

Anyway, in conclusion, while off camera TTL is somewhat complex, the hardware needed for it is quite simple – in general it is just a matter of having a compatible master and slave unit – either the onboard flash or hotshoe mounted, along with line of sight to trigger the slave flashes.  If youare limited by the shortcomigns of the optical signaling system, Radiopoppers will give you RF reliablility/capablility while preserving the TTL functionality of the system, so in essence you have a choice between using the built in capabilities of the system or radiopoppers, and that’s about it!  Wireless TTL is a very useful feature (albeit with drawbacks) and in my opinion belongs in the repitoire of every strobist! (If it’s good enough for Joe McNally, it’s good enough for me 🙂

Stay tuned for the 4th and final part, when we talk about the little oddities of flash triggering, notably sync speed and ways to “cheat” it!

Continue to part 4 – syncing and misc. topics!

Radiopoppers working with Pentax! (preliminary testing)

After eagerly following the saga of the Radiopoppers, from their vaporware inception to their current production I finally have a pair in my grubby little hands.  Oh happy day 🙂

(you want a review?  Here’s my review: radiopoppers rock my face 437 ways from sunday.  Get some.)

Now I got them to use with my newly acquired Canon 5D kit, but as it happens I still have some Pentax gear as well.  Officially Radiopoppers do not support Pentax PTTL, but I figured what harm could it do to test.

I mounted the P1 receiver on a Pentax 540FGZ flash.  Flash was set to wireless pttl slave (SL1).  Note that on the 540 the sensor you need to position the bead over is the lower right corner of the face, as you look at the flash *not* the round part in the middle (that’s just the AF assist light)

I set the K10d onboard flash to wireless controller mode, and covered the actual flash itself to ensure no light was coming out and triggering a false positive.   I fired off a few shots, and sure enough no slave firing – good!

Then I turned on the P1 transmitter, and placed it on top of the k10d.  The popup actually seemed to support it’s weight, and the transmitters fit rested nicely against the top of the eyecup.  I think it would be mountable with a little piece of velcro on top of the pop up flash!

The big moment – I fired a shot and sure enough, the slave popped!  Exposure looked correct too (roughly, just by glancing at the LCD.   I ran through a few apertures from 2.8 to 8 and the flash exposure seemed to remain consistent, indicating that metering info is working!  I could visibly see the difference in light output as the flash popped at different apertures as well.
So it seems that at least the basic functionality of the P1s works with Pentax’s PTTL wireless sytem.  it actually fires the flash, and ttl metering appears to work.   Bear in mind however, that this is far from a scientific test and YMMV.  Further testing will be needed 🙂