Retra Strobes, TTL & ISO with a Sony

Hi,

I have a Sony a7r4 and I've mostly used video lights in the past but I have recently purchased a pair of Retra strobes and an UW Technics TTL converter. I've been doing some bench testing and have some questions that I'm hoping others can help with.

First, I tested the TTL functionality by setting the strobes to TTL and then, with the camera in manual mode (as I always use), I varied shutter speed, f-stop, etc. one at a time and checked the exposure of the resulting pictures to make sure the strobe power was adjusted to keep it mostly consistent. It actually worked very well and I used the histogram to verify the exposure.

I also tested the strobe in manual mode and varied the strobe power to get a picture with a "good" histogram when camera settings were changed. 

So, here are some questions:

1) Strobe vs Camera Flash Exposure Compensation in TTL Mode

If I want to increase or decrease the flash exposure, does it matter whether I do that in camera or on the flash? Let's assume I want to increase exposure. I would think the "downside" to doing that on the flash is that it requires more power which means fewer total flashes and longer recycle time. If I do it in the camera, I think it would basically do the same thing by telling the flash to increase power output?

2) ISO in TTL Mode

When I use my video lights, I control the f-stop and shutter speed for whatever I'm trying to photograph and then use Auto ISO but with a fixed maximum.

How does Auto ISO work with a flash (or does it)? Does it simply use the max ISO or does it somehow calculate ISO during the TTL process? I know this is a newbie question. I'm also assuming that most people using a flash set a specific ISO?

3) Strobe Mode

I've read the UW Technics & Retra documents, but I wanted to verify something that I've tested but I'm not sure I completely understand it. If I set the UW Technics rotary switch to TTL for Retra but then decide I want to shoot some photos in Manual mode on the strobe. Since the camera case is already sealed shut, I can't change the rotary switch so I think there are two ways to do this:

     (1) - Change the strobe to M mode and then adjust the power accordingly. I'm assuming the camera will still send the TTL signals to the strobe but it will just ignore it?

    (2) - I've read in the documents that you can set the Sony camera flash mode to "Wireless Flash" which I believe disables (?) the TTL signal information and then you would adjust the power on the strobe? Would you ALSO need to put the strobe in M mode?

Thanks,

- brett

1 hour ago, TmxDiver said:

3) Strobe Mode

I've read the UW Technics & Retra documents, but I wanted to verify something that I've tested but I'm not sure I completely understand it. If I set the UW Technics rotary switch to TTL for Retra but then decide I want to shoot some photos in Manual mode on the strobe. Since the camera case is already sealed shut, I can't change the rotary switch so I think there are two ways to do this:

     (1) - Change the strobe to M mode and then adjust the power accordingly. I'm assuming the camera will still send the TTL signals to the strobe but it will just ignore it?

    (2) - I've read in the documents that you can set the Sony camera flash mode to "Wireless Flash" which I believe disables (?) the TTL signal information and then you would adjust the power on the strobe? Would you ALSO need to put the strobe in M mode? 

You have two ways of doing this. One is to set the camera to WL flash mode and the strobe to manual ("ON") - this will disable the pre-flash on the converter and set the strobe to fire on first pulse. The other is to leave the camera in TTL and set the strobe to "Smart SL" mode (you need to load this mode into U1 or U2 bank using the phone app beforehand), then fire the camera once - the strobe will learn the pre-flash pattern and subsequently fire only on the last flash.

1 hour ago, TmxDiver said:

1) Strobe vs Camera Flash Exposure Compensation in TTL Mode

If I want to increase or decrease the flash exposure, does it matter whether I do that in camera or on the flash? Let's assume I want to increase exposure. I would think the "downside" to doing that on the flash is that it requires more power which means fewer total flashes and longer recycle time. If I do it in the camera, I think it would basically do the same thing by telling the flash to increase power output?

2) ISO in TTL Mode

When I use my video lights, I control the f-stop and shutter speed for whatever I'm trying to photograph and then use Auto ISO but with a fixed maximum.

How does Auto ISO work with a flash (or does it)? Does it simply use the max ISO or does it somehow calculate ISO during the TTL process? I know this is a newbie question. I'm also assuming that most people using a flash set a specific ISO?

Thanks,

- brett

The issue with using exposure compensation with optically triggered flash is that the flash doesn't know how long the main pulse will be until it arrives.   It needs to measure pulse length then use a lookup table to decide how long to fire for and it may not get this right.  See this post:

View archived post in: Sea&Sea YS-D3 doesn't work with UWT TTL trigger?

If you do the compensation in camera - the camera calculates how much longer or shorter the pulse needs to be and fires for that length of time.  The other plus for doing compensation in camera is you only need to do it once, not on both strobes.    The power is the same no matter how you decide to do it - it is set by pulse duration.  So if you want more flash power it fires the strobe for longer.

I don't know how auto ISO works with flash and it might vary between camera models. I would not recommend it.  If the camera decides the ISO - you don't know what ISO it will choose from exposure to exposure.  At the very least it might vary the ambient to flash ratio enough to be a problem, for example is the ISO went way up for some reason it might be enough that you are not getting a black BG any more with your chosen shutter and aperture. 

7 minutes ago, ChrisRoss said:

The issue with using exposure compensation with optically triggered flash is that the flash doesn't know how long the main pulse will be until it arrives.   It needs to measure pulse length then use a lookup table to decide how long to fire for and it may not get this right.  See this post:

View archived post in: Sea&Sea YS-D3 doesn't work with UWT TTL trigger?

If you do the compensation in camera - the camera calculates how much longer or shorter the pulse needs to be and fires for that length of time.  The other plus for doing compensation in camera is you only need to do it once, not on both strobes.    The power is the same no matter how you decide to do it - it is set by pulse duration.  So if you want more flash power it fires the strobe for longer.

I don't know how auto ISO works with flash and it might vary between camera models. I would not recommend it.  If the camera decides the ISO - you don't know what ISO it will choose from exposure to exposure.  At the very least it might vary the ambient to flash ratio enough to be a problem, for example is the ISO went way up for some reason it might be enough that you are not getting a black BG any more with your chosen shutter and aperture. 

Perfect, thanks Chris. Much appreciated.

So, I think the summary for "best practices" would be:

1) Set the ISO (along with f-stop and shutter speed)

2) Use in-camera flash exposure compensation

Thanks,

- brett

39 minutes ago, Barmaglot said:

You have two ways of doing this. One is to set the camera to WL flash mode and the strobe to manual ("ON") - this will disable the pre-flash on the converter and set the strobe to fire on first pulse. The other is to leave the camera in TTL and set the strobe to "Smart SL" mode (you need to load this mode into U1 or U2 bank using the phone app beforehand), then fire the camera once - the strobe will learn the pre-flash pattern and subsequently fire only on the last flash.

Perfect, thanks!

One quick question: In "Smart SL mode" do you need to fire the camera once each time you turn on the strobe(s) or does it learn it permanently? Does that question make sense?

- brett

17 minutes ago, TmxDiver said:

One quick question: In "Smart SL mode" do you need to fire the camera once each time you turn on the strobe(s) or does it learn it permanently? Does that question make sense?

Each time you turn on the strobes. Note that using this mode adds a slight delay to your shots and also restricts burst mode usage. I have the UWT converter and Retra Pro strobes and use them in manual mode (WL + ON) 99% of the time. Smart SL is for when the converter's batteries run out mid-dive and I have to switch to the pop-up flash (I use a Sony A6300, so that's an option).

1 hour ago, Barmaglot said:

Each time you turn on the strobes. Note that using this mode adds a slight delay to your shots and also restricts burst mode usage. I have the UWT converter and Retra Pro strobes and use them in manual mode (WL + ON) 99% of the time. Smart SL is for when the converter's batteries run out mid-dive and I have to switch to the pop-up flash (I use a Sony A6300, so that's an option).

Again, very helpful! Thanks. 

Given my "newness" to strobes, I will probably

1) Start with TTL (manually set ISO along with f-stop & shutter speed per the other discussion and use in-camera flash compensation) and then

2) Start testing manual with manual mode with  (WL + ON) as you do.

One (maybe) last question on #2 above - Given that you aren't using TTL, when you adjust exposure compensation, do you do it in camera or individually change each strobe?

Thanks again,

- brett

35 minutes ago, TmxDiver said:

One (maybe) last question on #2 above - Given that you aren't using TTL, when you adjust exposure compensation, do you do it in camera or individually change each strobe?

Thanks again,

- brett

If you in manual on the strobes they give the same duration flash every time so exposure compensation is not a thing.  If I understand the option for leaving the camera in TTL all that procedure does is get the strobe to ignore the first flash and trigger on the second and power output is adjusted as you would with the strobes in manual.

I don't use exposure compensation; I shoot full manual. When I'm on wide angle, I eyeball the local conditions after splashing down, then set up a baseline - ISO 100 if it's bright and clear, ISO 400 or higher if it's a bit dim (early morning or sunset dive), f/8-f/11, 1/60-1/125 shutter speed, half to quarter power on the strobes, then find a spot and take a test shot. If the foreground is blown, I reduce strobe power, if the water is too dark, I reduce shutter speed or open the aperture a bit, or increase ISO. With a bit of practice, I get my baseline either on the first shot, or within two or three. Once the baseline is established, I swim around looking for opportunities, and eyeballing the distance, I adjust strobe power up or down; it's really not that complicated. If I need to take a specialty shot, like a sunball, then I go to the appropriate settings (such as 1/160, f/22, ISO 100, or something around 1/1000 f/11 and HSS) then go back to 'normal' settings for that dive.

For macro, the process is similar, but I generally use 1/160s shutter speed and f/16-f/22 aperture range. Same as for wide angle, I take a few test shots at the beginning of the dive, then adjust as needed. If I want to shoot shallow depth of field, like for example separating an ornate ghost pipefish from its background, then I dial the strobes down to minimum and sometimes use HSS as well.

I used to use TTL with SeaFrogs ST-100 Pro strobes and pop-up flash, and it worked reasonably well, but once I got the Retras, I switched to full manual and never looked back. It really isn't that complicated to use and produces much better results. On a recent dive my UWT trigger ran out of battery, and I didn't have Smart SL loaded into the strobes, which forced me to use TTL with the pop-up flash, and it was super annoying. I still got some of the shots that I wanted, but doing this without manual control was just pissing me off:

49 minutes ago, Barmaglot said:

I don't use exposure compensation; I shoot full manual. When I'm on wide angle, I eyeball the local conditions after splashing down, then set up a baseline - ISO 100 if it's bright and clear, ISO 400 or higher if it's a bit dim (early morning or sunset dive), f/8-f/11, 1/60-1/125 shutter speed, half to quarter power on the strobes, then find a spot and take a test shot. If the foreground is blown, I reduce strobe power, if the water is too dark, I reduce shutter speed or open the aperture a bit, or increase ISO. With a bit of practice, I get my baseline either on the first shot, or within two or three. Once the baseline is established, I swim around looking for opportunities, and eyeballing the distance, I adjust strobe power up or down; it's really not that complicated. If I need to take a specialty shot, like a sunball, then I go to the appropriate settings (such as 1/160, f/22, ISO 100, or something around 1/1000 f/11 and HSS) then go back to 'normal' settings for that dive.

For macro, the process is similar, but I generally use 1/160s shutter speed and f/16-f/22 aperture range. Same as for wide angle, I take a few test shots at the beginning of the dive, then adjust as needed. If I want to shoot shallow depth of field, like for example separating an ornate ghost pipefish from its background, then I dial the strobes down to minimum and sometimes use HSS as well.

I used to use TTL with SeaFrogs ST-100 Pro strobes and pop-up flash, and it worked reasonably well, but once I got the Retras, I switched to full manual and never looked back. It really isn't that complicated to use and produces much better results. On a recent dive my UWT trigger ran out of battery, and I didn't have Smart SL loaded into the strobes, which forced me to use TTL with the pop-up flash, and it was super annoying. I still got some of the shots that I wanted, but doing this without manual control was just pissing me off:

 

Agree, It's not that complicated, I briefly dabbled with TTL but very quickly changed over to manual.  The thing with manual is it's repeatable.  If you stay the same distance from subject you'll get the same exposure, TTL probably often is repeatable, but if the framing or subject changes a bit the camera might tweak the flash exposure up or down.  

8 hours ago, ChrisRoss said:

Agree, It's not that complicated

I'm with Chris and Barmaglot. I was a big TTL enthusiast pre-Retra days and had a TTL system that worked well with either (back in the day) Heinrich Weikamp TTL converter or, later on, a UWT trigger on the Z240s.

But I find with the Retras that Manual is dead easy and, as the guys say, repeatable. I operate pretty much as Bamaglot suggests with a quick baseline setup once I'm down on the bottom and settled - then adjust as necessary. 

13 hours ago, ChrisRoss said:

If you in manual on the strobes they give the same duration flash every time so exposure compensation is not a thing.  If I understand the option for leaving the camera in TTL all that procedure does is get the strobe to ignore the first flash and trigger on the second and power output is adjusted as you would with the strobes in manual.

Thanks, Chris. This discussion has been very helpful. I plan to write up a blog post and summarize it all so that others can benefit.

In terms of your statement above, can I then conclude the following in terms of light intensity & pulse duration:

1) Manual Mode - constant pulse duration. In camera flash compensation setting doesn't change anything. Strobe power varies the intensity of the light but not the duration of the flash.

2) TTL Mode - flash pulse duration controlled by TTL logic in the camera. In camera flash compensation increases or decreases the pulse duration. On board flash power knob increases or decreases flash intensity but not duration.

Is that correct?

Thanks,

- brett

14 minutes ago, TmxDiver said:

1) Manual Mode - constant pulse duration. In camera flash compensation setting doesn't change anything. Strobe power varies the intensity of the light but not the duration of the flash. 

No, completely incorrect. Strobe power is modulated by pulse duration; peak intensity is always the same. Higher power settings make the flash bulb burn for longer, using more of the capacitors' charge. Read this thread for in-depth info:

View archived post in: Flash duration?

17 minutes ago, TmxDiver said:

2) TTL Mode - flash pulse duration controlled by TTL logic in the camera. In camera flash compensation increases or decreases the pulse duration. On board flash power knob increases or decreases flash intensity but not duration.

Not quite - in TTL mode, the camera/trigger first fires a pre-flash at a reduced power setting, the camera evaluates the resulting exposure based on the configured metering settings (it can be center spot, custom spot, wide, etc) and flash compensation setting, then calculates and fires a pulse of a duration that it considers to be correct for proper exposure, and the strobes trigger with a power setting based on the duration of that pulse - specific details, like what trigger pulse duration results in what flash power setting depend on the exact strobe model, hence the need to set the strobe model in use on the rotary dial. The power knob on the strobes themselves applies a modifier on top of that.

There is a third option - you can set the camera to WL mode and the strobes to TTL, and in this configuration you can use the flash compensation setting in camera as centralized strobe power control, with -3 corresponding to minimum power and +3 corresponding to maximum power. I've never used this myself in the field, but it is mentioned in the UWT trigger manual.

A flash tube's output is fixed. All you control is how long it burns for. This is true whether in manual or TTL.

In TTL, the feedback circuit is controlled by the camera metering the return of the pre flash, and then selecting an appropriate length (as its algorithm determines)  of flash duration to light the scene. In-camera flash compensation adjustments may (or may not) affect these exposures depending on the algorithm. 

Dialing in compensation on the strobe is the best way to ensure that any required TTL exposure is modified to suit a requirement. I think the option of using in camera flash exposure compensation is not necessarily a function available with all circuits/cameras or flash combinations...

I find it easier for sure to dial down the strobe (especially with the Retras)  rather than, usually, having to push/turn two buttons/dials to get in-camera exposure compensation.  

6 hours ago, TmxDiver said:

Thanks, Chris. This discussion has been very helpful. I plan to write up a blog post and summarize it all so that others can benefit.

In terms of your statement above, can I then conclude the following in terms of light intensity & pulse duration:

1) Manual Mode - constant pulse duration. In camera flash compensation setting doesn't change anything. Strobe power varies the intensity of the light but not the duration of the flash.

2) TTL Mode - flash pulse duration controlled by TTL logic in the camera. In camera flash compensation increases or decreases the pulse duration. On board flash power knob increases or decreases flash intensity but not duration.

Is that correct?

Thanks,

- brett

As others have said, not quite - the pulse duration varies as you change power in manual, but on a specific power setting the pulse is the same every time, no matter the pulse length from the trigger.  Correct that flash compensation in camera has no impact. 

In TTL the camera decides on the pulse duration based upon the pre-flash reading.  Flash exposure compensation increases or decreases the pulse length that the camera has calculated to increase or decrease exposure due to light from the strobe.  It's easy for the camera, it calculates a new pulse length and proceeds to produce it.

The problem for the strobe is that it doesn't know anything about what the camera has calculated it just receives a pulse of light.  The strobe then needs to read that and respond with a pulse of the same length.  But if you want to dial in compensation on the strobe it needs to read the whole pulse and add or subtract from the pulse as required to implement that compensation.  I have seen quite a few reports of TTL compensation having little impact when dialled in on the strobe.  

All of these calculations happen very quickly however the strobe pulse is finite in length and you need enough time while the shutter is open to read the pulse calculate a new pulse length and emit that.  That's all fine until you start increasing flash power.  A Z330 as I recall has a 1/250 fullpower pulse, so is on the whole time the shutter is open with a 1/250 flash sync speed.  Circular flash tubes like the Retra have longer pulse lengths.  Doesn't leave much time to read a pulse and then repeat it.  This may be why wide angle TTL doesn't work so well as you tend to be at high powers??

All that aside I would suggest try compensation on the strobe and the camera and see how it works, in theory the the camera should work better and can be adjusted without moving your hands from the handles - the degree of difficulty will vary from camera to camera.  Of just bite the bullet and dive straight into manual flash.