TTL flash and camera metering question

peter_4059

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I recently added a Godox TT350o flash to my kit and I'm trying to get my head around how TTL works in conjunction with my EM10 MKII. I've been running some basic indoor tests to try to understand how the metering works when using the flash and I'm coming up with something I don't understand.

I've set the camera mode on M, ISO on Auto and dialled in an aperture and shutter speed such that it selects an ISO (320) which is above the minimum (200) and the exposure meter is reading zero which I believe means the shot will be properly exposed.

I then activate the built in flash. ISO drops on auto to minimum (200), I don't touch the aperture or shutter but now the meter shows -0.7.

If I attach the GODOX flash I get the same behaviour. I originally thought this might be a compatibility issue however the same thing happens with the on-board flash so I'm now wondering if there is another setting at play or this is just how the meter works with flash.

So my question is why does the camera drop the ISO to minimum when either flash is activated and show -0.7 on the meter, rather than choose an ISO where the meter remains at 0 indicating proper exposure?

Any help would be greatly appreciated.

Peter
 

wjiang

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It doesn't drop the ISO to minimum, it just drops the auto ISO to the minimum that the flash allows. If it gets really dark and you are trying to bounce flash the auto ISO will end up at something like 800 rather than 200 due to flash power limitations. The indicated exposure with TTL flash turned on is a bit of a lie - the ISO will be adjusted depending on the TTL pre-metering.

As for the inconsistency - it's an unfortunate issue with TTL - the camera has no way of knowing what the exposure will be until it fires the TTL pre-flash, the meter may just be inaccurate when it comes to adjusting for flash exposure. That's why for true control it's better to go to manual flash rather than TTL.
 

peter_4059

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Thanks for the reply. That makes sense. I thought the TTL pre-flash would fire and the meter take a reading when the shutter button was half pressed however it sounds like when the flash is active this only occurs when the actual shot is taken.
 

cdmicha

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As for the inconsistency - it's an unfortunate issue with TTL - the camera has no way of knowing what the exposure will be until it fires the TTL pre-flash, the meter may just be inaccurate when it comes to adjusting for flash exposure. That's why for true control it's better to go to manual flash rather than TTL.
When shooting with flash indoors, I'll typically set shutter speed, aperture, AND ISO- then take a few tests to see what power I end up using. Godox has a really handy TCM feature where you can switch over to manual shooting after a TTL reading, so its easy to change the ISO or flash power to your liking afterward. This also keeps your exposures about the same (barring changes in ambient lighting or subject distance). As a matter of fact, I used manual only flash for years and only since getting the Godox system with TCM have I used my TTL flashes more often, as it makes my setup time much faster. Set exposure manually, fire with TTL, TCM to see/set power setting, adjust ISO/power to my liking, and done- usually in only 1-2 shots.
 

thenextpage

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It doesn't drop the ISO to minimum, it just drops the auto ISO to the minimum that the flash allows. If it gets really dark and you are trying to bounce flash the auto ISO will end up at something like 800 rather than 200 due to flash power limitations. The indicated exposure with TTL flash turned on is a bit of a lie - the ISO will be adjusted depending on the TTL pre-metering.

As for the inconsistency - it's an unfortunate issue with TTL - the camera has no way of knowing what the exposure will be until it fires the TTL pre-flash, the meter may just be inaccurate when it comes to adjusting for flash exposure. That's why for true control it's better to go to manual flash rather than TTL.
Do you know if there’s a way to control manual flash power through the omd cameras? I often use the camera function to control flash compensation when shooting ttl, but haven’t found a way to control the flash power when set to manual
 

Michael Meissner

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Do you know if there’s a way to control manual flash power through the omd cameras? I often use the camera function to control flash compensation when shooting ttl, but haven’t found a way to control the flash power when set to manual
On my E-m5 mark I, I just go to the super control panel for flash mode (lightning bolt) to manual flash mode. Then using the SCP once again, the item below the flash mode is the power level when you are in manual flash mode. On the E-m5 mark I, I was able to set various power levels from full to 1/32 power.

Note, just to be clear, there are two manual modes, one where you preset the aperture, shutter speed, and usually also the ISO. The other is the mode for manual flash control where you fire the flash as a single pulse and control the power level.

And I imagine it only works for external flashes if the external flash supports both TTL and manual modes, and uses the Olympus/Panasonic control system. Some of the cheaper third party flashes don't have manual mode support.
 

exakta

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Followup to Michael's post above about two manual modes:

-- With the camera set to manual, the flash can still be set to TTL-Auto. In this case, you can set compensation on the camera and still get TTL exposure.

-- With the flash set to manual, you must set the camera to manual to get proper exposure.
 

thenextpage

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Followup to Michael's post above about two manual modes:

-- With the camera set to manual, the flash can still be set to TTL-Auto. In this case, you can set compensation on the camera and still get TTL exposure.

-- With the flash set to manual, you must set the camera to manual to get proper exposure.
Hi Exakta,
If you happen to know, I haven’t been able to figure out; is it possible to control the flash power through an omd body that is shooting manual, when the flash is set to manual power? I shoot with my omd in manual mode, but use ttl flash setting because I like to be able to adjust flash power through the camera body (which can be done in ttl mode via flash exposure compensation.
 

barry13

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Hi Exakta,
If you happen to know, I haven’t been able to figure out; is it possible to control the flash power through an omd body that is shooting manual, when the flash is set to manual power? I shoot with my omd in manual mode, but use ttl flash setting because I like to be able to adjust flash power through the camera body (which can be done in ttl mode via flash exposure compensation.
Hi, I doubt FC would work in full-manual flash mode, but you could try it.
Otherwise, Mu-43 bodies can control Mu-43 and 4/3 flashes through a flash menu on-camera, including remote (infrared) flashes. This does work in full-manual mode.
 

exakta

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is it possible to control the flash power through an omd body that is shooting manual, when the flash is set to manual power?
My E-M10 can't do it when I shoe mount my FL360L. The manual option is greyed out. It can control flash power of the built-in flash (why bother, the GN is only 8.2m!!!).

But with my FL360L off camera using RC, then I can control the power.

No idea about other bodies.
 

PakkyT

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I've set the camera mode on M, ISO on Auto
Don't have your model but I looked up the manual and according to that, your model does not support Auto ISO in manual mode and when you have Auto ISO selected and switch to "M" the camera will default to ISO 200. Was there a FW update that changed that behavior?

dialled in an aperture and shutter speed such that it selects an ISO (320) which is above the minimum (200) and the exposure meter is reading zero which I believe means the shot will be properly exposed
Normally the meter on your camera will always show you the ambient or background exposure, not the exposure with flash added because it can not know what your flash output will be ahead of time. So you dial in your settings and if the EV meter shows zero then you are set to properly expose your background without flash. I am still confused if you are in manual mode and see the ISO auto changing but you did state you were able to dial BOTH the shutter and aperture? Can you double check that?

I then activate the built in flash. ISO drops on auto to minimum (200), I don't touch the aperture or shutter but now the meter shows -0.7.
That would make sense because the meter measures the background it is currently looking at. If you switched to manual mode (or whatever your changed) and the camera now defaulted to ISO 200 per the user manual, then going from ISO 320 to 200 is dimming your exposure by 2/3rds stop, or -0.7 which is exactly what your camera is telling you.

Edit: A thought came to mind giving this some more time to cook in my brain. Are you sure you are not confusing your shutter speed with your ISO setting? I am wondering if you are actually in manual mode, your ISO is locked on ISO 200 and the numbers along the bottom are "320" Fx.x +/-EV and the 320 is actually your shutter speed. When you then put your flash on, your camera is automatically lowering your shutter speed from 1/320 to 1/200 because that is the max "X Sync" setting of your camera for flash use.

However that doesn't explain your exposure changing to -0.7 because going from 1/320 to 1/200 should make the image brighter. Perhaps you misread the display and it showed +0.7, which would make sense if only the shutter speed slowed down that much. ???
 
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Michael Meissner

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My E-M10 can't do it when I shoe mount my FL360L. The manual option is greyed out. It can control flash power of the built-in flash (why bother, the GN is only 8.2m!!!).

But with my FL360L off camera using RC, then I can control the power.

No idea about other bodies.
Yeah, I see the same thing now that I actually tried it with an external flash rather than the clip-on flash. I put on my FL-900R flash onto both the E-m5 mark I and E-m10 mark II, and I get the same behavior. If the flash is in TTL mode, you cannot select Manual flash mode. If I turn the flash mode to Manual, I still cannot set the power level from the camera. So, I went back to my E-5 and Stylus-1 with the FL-36R, and I got the same behavior.

I agree it should be possible to set a TTL flash to manual mode, and also set the power level from the camera. But it doesn't seem possible for a directly connected flash.

Sorry about that, I really thought it would work.
 

peter_4059

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Don't have your model but I looked up the manual and according to that, your model does not support Auto ISO in manual mode and when you have Auto ISO selected and switch to "M" the camera will default to ISO 200. Was there a FW update that changed that behavior?


Normally the meter on your camera will always show you the ambient or background exposure, not the exposure with flash added because it can not know what your flash output will be ahead of time. So you dial in your settings and if the EV meter shows zero then you are set to properly expose your background without flash. I am still confused if you are in manual mode and see the ISO auto changing but you did state you were able to dial BOTH the shutter and aperture? Can you double check that?

That would make sense because the meter measures the background it is currently looking at. If you switched to manual mode (or whatever your changed) and the camera now defaulted to ISO 200 per the user manual, then going from ISO 320 to 200 is dimming your exposure by 2/3rds stop, or -0.7 which is exactly what your camera is telling you.

Edit: A thought came to mind giving this some more time to cook in my brain. Are you sure you are not confusing your shutter speed with your ISO setting? I am wondering if you are actually in manual mode, your ISO is locked on ISO 200 and the numbers along the bottom are "320" Fx.x +/-EV and the 320 is actually your shutter speed. When you then put your flash on, your camera is automatically lowering your shutter speed from 1/320 to 1/200 because that is the max "X Sync" setting of your camera for flash use.

However that doesn't explain your exposure changing to -0.7 because going from 1/320 to 1/200 should make the image brighter. Perhaps you misread the display and it showed +0.7, which would make sense if only the shutter speed slowed down that much. ???
There is a menu item you can select "gear E"-ISO-Auto-All that lets you select Auto ISO in M mode. Definitely not confusing shutter speed and ISO in this case :).
 

PakkyT

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There is a menu item you can select "gear E"-ISO-Auto-All that lets you select Auto ISO in M mode. Definitely not confusing shutter speed and ISO in this case :).
Ah ok I see that now. So it must be that while you can use Auto-ISO in "M" it may be that when you attach a flash the camera overrides that and you have to pick an ISO and defaults to ISO 200. You can check that by using the super control panel (SCP) while in manual mode. With the flash off the camera see if ISO Auto is selectable. Then put the flash on and see if the "Auto" setting is now greyed out. I will have to try it on my E-M1 and check how it works on mine.

Anyway, if that is the case, then what I said above stands. The camera is metering for the scene as it sees it without flash. If set up so you have an EV of zero, then you are properly exposed. Add the flash and the ISO drops from ISO 320 to 200 and you lose 2/3 EV, so with the same shutter and aperture, showing an EV of -0.7 makes perfect sense.

Also as a point of note, when using the flash and you do want to meter the background correctly, it is better to use something like the Digital ESP metering (the one that averages the entire scene) rather then spot metering. Because you are going to use the flash to expose your subject so to control how much or little you will see of the background you want to more accurately be metering on the whole scene rather than, for example, the model standing right in front of the camera. Great, now I am going to have to test that out later and see if that is correct or I am making it up as I go. ;)
 
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