ETTL and under exposure

[A_S]

Hi Another quick one from me.

Testing my YN600EX-RT 2 at 105mm on ETTL i noticed my aperture number blinking, indicating under exposure. Take the photo and it comes out under exposed (@ 1/250s, ISO1600). Interestingly at 24mm same settings apertunre still binks exposure fine.

Put flash into manual, shoot at 100% power - white out same settings.

Shoot at half power, dito.

Shoot at 1/64 ditto

Shoot at 1/128 power and its still slightly over exposed.

I am not sure I get whats happening in ETTL? Surely the point of ETTL is that it adjusts flash power to ensure accurate exposure? Its clear from the manual tests that the flash has more than enough power to over expose on the same settings at 1/128th power - so why is ETTL under exposed by around 2-3 stops? (flash exposure compensation not set and even at 3 stops over exposed the photo is still under exposed)

confused.com

 

[TimHughes]

Definitely sounds odd. Which camera? What mode? What aperture?

 

[wiltw]

What camera model? Early on in Canon dSLR history, eTTL was known for underexposing shots (compared to the density of a frame exposed with non-flash automated exposure with ambient light) Then suddenly, with no explanation by Canon, their flash exposures started to be same density as their ambient light AE. Your results at -2EV or -3EV are quite dramatically underexposed, even compared to early in Canon dSLR history, however.

 

[A_S]

Camera is EOS R5 - Lens EF24-105 F4L - shutter priority to keep the flash sync speed at 1/250. the same behaviour was seen on 2 different flashes, both Yongnuo YN600EX-RT 2s fully charged batteries (Same batteries swapped into second flash)

I tried it on Mechanical shutter and that seemed to solve the problem - then changed it back to electronic first curtain and all was fine. Really wierd. So maybe it was a sync issue?

 

[Phil V]

Camera is EOS R5 - Lens EF24-105 F4L - shutter priority to keep the flash sync speed at 1/250. the same behaviour was seen on 2 different flashes, both Yongnuo YN600EX-RT 2s fully charged batteries (Same batteries swapped into second flash)

I tried it on Mechanical shutter and that seemed to solve the problem - then changed it back to electronic first curtain and all was fine. Really wierd. So maybe it was a sync issue?

Don’t use shutter priority when using flash as your primary light source. *

In doing this you’re telling the camera you want to control something that is illogical. (There isn’t enough ambient light for your auto exposure to get an adequate exposure).

That said, I’d have to run the experiment to see whether using the camera properly allows ETTL to work correctly with your parameters.

For info though, the Aperture flashing is exactly as expected, there clearly isn’t enough ambient for iso 1600, 1/250 and f4. But it’s irrelevant because your exposure isn’t reliant on the ambient light.

*Flash exposure:

Whenever you use flash you need to understand that you are balancing 2 separate exposures, the ambient and the flash.

When using flash as the primary light source, use either P, the green box or indeed M, which means either you leave decisions to the camera (ok) or you take control of all parameters (better).

The camera uses a complex algorithm to ‘guess’ what you’re trying to achieve when you use one of the auto or semi auto modes with flash.

But the basis of it’s decision making is that if you have chosen TV and a particular shutter speed, that the shutter speed is important to you, and that the flash has been switched on then you want to use it as ‘fill flash’.

When using flash as the primary light source my go to settings are M 1/160, ISO400 and f5.6 (and w mirrorless, that exp simulation is switched off)

The settings change from that as necessary to get the shot.

For flash as ‘fill’ I’d set the camera (usually using AV or TV) and the other settings to get the ‘correct’ exposure. Then ettl should be able to add fill to help with shadows. But importantly for fill to do its thing, the ambient exposure has to be correct first.

 

[A_S]

Thanks I wasn't aware that TV was out the question on Flash - I guess I wrongly presumed that Sync speed (1/250) should be matched and that that then any under exposure would be corrected by the flash power - i.e flash deduces what power it needs and solves for the blinking aperture number.

Lets say I put the camera in manual and set it at sync speed 1/250, ISO1600 F4 for same shot in the same location with the same light - would I still get the same problem? or would the camera compensate as its algorithm understands manual better?

 

[Phil V]

Thanks I wasn't aware that TV was out the question on Flash - I guess I wrongly presumed that Sync speed (1/250) should be matched and that that then any under exposure would be corrected by the flash power - i.e flash deduces what power it needs and solves for the blinking aperture number.

Lets say I put the camera in manual and set it at sync speed 1/250, ISO1600 F4 for same shot in the same location with the same light - would I still get the same problem? or would the camera compensate as its algorithm understands manual better?

It’s not that TV is ‘out of the question’ it’s understanding what your camera is trying to deduce from your choices.
Because you’ve made choices, it’s not trying to correct your mistakes (as you assume) it’s assuming that you want a shot that’s underexposed by x stops and that the flash should be 1/2 stop under that or thereabouts (to fill)

Yes if you use manual, the flash will just do its thing to create a ‘correct’ exposure because it knows that you know what you’re doing.

And if you use P, it’ll create a good exposure because it knows you’re leaving the decision up to it.

You don’t need 1600iso presuming you’re indoors, your speedlight has plenty of power for iso 400 and f4 or smaller.

If you google NEVEC (I think) you’ll find some really detailed descriptions of how ETTL tries to 2nd guess your intentions and precisely when and where it decides what the ‘correct’ balance should be.

However, I got half way through trying to understand it and deduced that it’s easier to work out the ratios yourself by using M and ettl where 90% of the time you get a perfect exposure with no tweaking.

But to reiterate the important point. It is always about balancing the 2 exposures. Whether you’re trying to take a snap in the kitchen, use a full blown studio setup, or overpowering the sun, or just a kiss of fill flash. It’s always the balance.

 

[A_S]

Thats perfect, really enlightening Phil. I'll do some tests once I get home tomorrow. It will be interesting to see if dialing down the ISO on manual and keeping the same settings on manual comes back with a correct exposure - as I did wonder why I was having to shoot at such a high ISO on TV.

 

[Phil V]

Thats perfect, really enlightening Phil. I'll do some tests once I get home tomorrow. It will be interesting to see if dialing down the ISO on manual and keeping the same settings on manual comes back with a correct exposure - as I did wonder why I was having to shoot at such a high ISO on TV.

In ‘all’ modes your viewfinder camera meter is metering the ambient. And it’ll expect you to either centre the meter or choose not to because ‘artistic choice’.

The ETTL flash metering is a separate function and starts to happen when you press the shutter button (or if using manual flash - never happens in camera)

Ergo - you are balancing 2 exposures. Do not think that is irrelevant; it’s the core piece of information and if you ‘get it’ flash photography becomes simple, and if you fight it, or ignore it, flash photography will appear to be very difficult, or ‘rubbish’ or impossible to get right.

 

[Phil V]

So if you’re in a normal domestic setting, you look at what the ambient light is doing to your subject and you decide:
Is that enough light for a picture?
Is it ‘good’ light for my picture?

If the answer is yes to both of those, centre your meter and take your shot.

If the answer is no to either of them, you then decide whether you want any of that ambient to take part in your image?

if the answer is yes, you have to choose a shutter speed and iso that will give that chosen exposure, whether it’s a little for atmosphere or a lot for the main light.

If the answer is no; set a low iso and whatever SS and aperture you need for your chosen picture, switch on the flash and choose where to point it, how else you want to modify it to create a pleasing light. Either using ETTL or judging/ measuring the flash output yourself.

 

[A_S]

Thanks - I think I get it.

Expose for the ambient conditons - to take it to extremes - if i need a 1 second exposure (hand held) and ISO 5000 to get correct exposure - set the camera to that, then the flash exposure element will do the rest and ensure correct exposure. i.e compensating for the 1 second hand held

 

[Phil V]

Thanks - I think I get it.

Expose for the ambient conditons - to take it to extremes - if i need a 1 second exposure (hand held) and ISO 5000 to get correct exposure - set the camera to that, then the flash exposure element will do the rest and ensure correct exposure. i.e compensating for the 1 second hand held

NO see my post above!
Is the ambient light enough for the picture you want to make?
Is the ambient light suitable for the picture you want to make.

If the light is crap (streetlight) and the background is unimportant to you, you can just shoot at 1/100 and ISO100.

If the ambient includes the Northerm lights and needs a multi second exposure, then you would choose to expose for that.

It’s an artistic decision re how you wish to balance the 2 exposures.

 

[Phil V]

You’d only do that if you wanted that ambient light.

You can choose to ignore it!

TWO Different exposures

One of them can be 20 stops underexposed if that’s what you want.

 

[A_S]

ok let me have a play I am obviously mis understanding!

Lets say I wanted to take a photo of a drop of water in a very dark enviroment. to catch the drop i need 1/x000 of a second which is way to fast for the ambient light. I do 2 things

I set the shutter at the speed i need to capture the water droplet and the flash second exposure will then account for the under exposure because i need to shoot at such a fast shutter speed.

ignoring aperture and ISO to keep it simple.

edit: obviously using high speed sync

 

[TimHughes]

ok let me have a play I am obviously mis understanding!

Lets say I wanted to take a photo of a drop of water in a very dark enviroment. to catch the drop i need 1/x000 of a second which is way to fast for the ambient light. I do 2 things

I set the shutter at the speed i need to capture the water droplet and the flash second exposure will then account for the under exposure because i need to shoot at such a fast shutter speed.

ignoring aperture and ISO to keep it simple.

In this case you can illuminate only with flash. The camera shutter speed is not important as the scene is only lit but flash. What is important is the flash duration. Details for that will be in the flash manual.

 

[Gav.]

Sorry to drop in..

Use the flash to freeze the action.

Hope you don't mind me dropping an image?

1/200 - f14 - ISO400 - Manual flash power to suit


Drop more Milk by Gavin Wickham, on Flickr

 

[Phil V]

Lets say I wanted to take a photo of a drop of water in a very dark enviroment. to catch the drop i need 1/x000 of a second which is way to fast for the ambient light. I do 2 things

As above, you can set a shutter speed of 1/100 and without flash your image would be pure black.

Add flash and not only is your image perfectly exposed, but the flash duration of 1/10,000 has frozen the droplet.

The balance is 0 from the ambient and 1 from the flash.

 

[A_S]

Thanks all - I'm getting there I'll go away and read a bit more on google - but at least I have a bit more of a framework to go and test stuff once I am home

 

[Phil V]

Thanks all - I'm getting there I'll go away and read a bit more on google - but at least I have a bit more of a framework to go and test stuff once I am home

There's nothing more to read on google.

Everything you need to know is in this thread.

It looks like you don't want to believe us

 

[Phil V]

Thanks all - I'm getting there I'll go away and read a bit more on google - but at least I have a bit more of a framework to go and test stuff once I am home

In your original scenario, the ambient was underexposed
And the flash was underexposed by further (probably - but as it didn’t register that’s only an educated guess)
Then you held the same underexposed ambient and used manual flash that was overexposed.

In @Gav. s perfect example, the ambient was underexposed (massively)
But the flash exposure is exposed perfectly.
That gives us a photograph that’s perfectly lit, perfectly exposed, perfectly timed.
But if we saw what his viewfinder meter said, it would have told us just that it’ll be underexposed.

That’s the balance of the 2 exposures.

 

[wiltw]

Don’t use shutter priority when using flash as your primary light source. *

In doing this you’re telling the camera you want to control something that is illogical. (There isn’t enough ambient light for your auto exposure to get an adequate exposure).

That said, I’d have to run the experiment to see whether using the camera properly allows ETTL to work correctly with your parameters.

For info though, the Aperture flashing is exactly as expected, there clearly isn’t enough ambient for iso 1600, 1/250 and f4. But it’s irrelevant because your exposure isn’t reliant on the ambient light.

*Flash exposure:

Whenever you use flash you need to understand that you are balancing 2 separate exposures, the ambient and the flash.

When using flash as the primary light source, use either P, the green box or indeed M, which means either you leave decisions to the camera (ok) or you take control of all parameters (better).

The camera uses a complex algorithm to ‘guess’ what you’re trying to achieve when you use one of the auto or semi auto modes with flash.

But the basis of it’s decision making is that if you have chosen TV and a particular shutter speed, that the shutter speed is important to you, and that the flash has been switched on then you want to use it as ‘fill flash’.

When using flash as the primary light source my go to settings are M 1/160, ISO400 and f5.6 (and w mirrorless, that exp simulation is switched off)

The settings change from that as necessary to get the shot.

For flash as ‘fill’ I’d set the camera (usually using AV or TV) and the other settings to get the ‘correct’ exposure. Then ettl should be able to add fill to help with shadows. But importantly for fill to do its thing, the ambient exposure has to be correct first.

Phil, unless Canon altered the logic for eTTL in mirrorless (vs. dSLR) there is no differentiation in the Canon logic of 'flash for fill' vs. 'flash for primary illumination'...there is only a SINGLE logic I did extensive testing of Canon dSLR flash logic over a decade ago proving that flash intensity did NOT vary based upon which of the the various shooting modes (P vs. Av vs. Tv vs. M) was being used during eTTL flash exposure. In my testing, I found that the amount of flash emitted DID NOT VARY, switching the camera between modes.

There can be variation in flash intensity at certain EV levels of ambient light. In light levels below EV13 and varying dependent upon specific EV level, NEVEC (Negative Evaluative Exposure Compensation) begins to kick in and pulls ambient-to- fill ratios out of whack, regardless of exposure automation mode! The amount of alteration is EV-specific, so it can be challenging for a pro to even know in advance how the ambient-to-fill ratio will be altered, without first measuring ambient EV level and then also consulting a table! NEVEC existence has been around since the days of the EOS film line, as reflected in the label of this chart. Note that Av mode behavior is identical to Tv mode behavior, in this characteristic. Notice also that the degree of variation is ISO-dependent! Canon knows how to screw with pros' heads, since they would want to control fill ratios (whereas the typical snapshooter is hardly aware of the significance of fill ratio control.

Additionally Canon screws with our heads by the inclusion of AFR (Automatic Flash Reduction), also EV-dependent and NOT exposure automation mode dependency. AFR is not camera mode dependent, it is solely based on ambient EV level. Some Canon models can disable AFR, but not all. I compiled this table many years ago.

Unfortunately, the demise of certain photography forums is cause of the disappearance of this combined insight in the photographic knowledge base.

 

[Phil V]

Phil, unless Canon altered the logic for eTTL in mirrorless (vs. dSLR) there is no differentiation in the Canon logic of 'flash for fill' vs. 'flash for primary illumination'...there is only a SINGLE logic I did extensive testing of Canon dSLR flash logic over a decade ago proving that flash intensity did NOT vary based upon which of the the various shooting modes (P vs. Av vs. Tv vs. M) was being used during eTTL flash exposure. In my testing, I found that the amount of flash emitted DID NOT VARY, switching the camera between modes.

There can be variation in flash intensity at certain EV levels of ambient light. In light levels below EV13 and varying dependent upon specific EV level, NEVEC (Negative Evaluative Exposure Compensation) begins to kick in and pulls ambient-to- fill ratios out of whack, regardless of exposure automation mode! The amount of alteration is EV-specific, so it can be challenging for a pro to even know in advance how the ambient-to-fill ratio will be altered, without first measuring ambient EV level and then also consulting a table! NEVEC existence has been around since the days of the EOS film line, as reflected in the label of this chart. Note that Av mode behavior is identical to Tv mode behavior, in this characteristic. Notice also that the degree of variation is ISO-dependent! Canon knows how to screw with pros' heads, since they would want to control fill ratios (whereas the typical snapshooter is hardly aware of the significance of fill ratio control.

Additionally Canon screws with our heads by the inclusion of AFR (Automatic Flash Reductinon), also EV-dependent and NOT exposure automation mode dependency. AFR. It is not camera mode dependant, it is strictly baed on ambient EV. And SOME Canon models can disable AFR, but not all. I compiled this table many years ago.

Unfortunately, the demise of certain photography forums is cause of the disappearance of this combined insight in the photographic knowledge base.

So - as per the original issue, NEVEC is programmed for less than what the ambient is set at. So if the ambient is 3 stops under, the flash will be roughly 4 stops under.

Which is exactly what I've been saying.

I didn't have the tables, but knew that I'd red it somewhere.

 

[wiltw]

So - as per the original issue, NEVEC is programmed for less than what the ambient is set at. So if the ambient is 3 stops under, the flash will be roughly 4 stops under.

Which is exactly what I've been saying.

I didn't have the tables, but knew that I'd red it somewhere.

The key point for all is that flash reduction 'for fill' is

1. not dependent alone on Tv vs. AV (they act the same!), although NEVEC does treat both differentnly from P or M
2. dependent upon ISO and ambient light level
3. and there is no deliberately selectable 'fill mode' per se vs 'primary illumination mode' (but there is hidden behavior not well documented, as discussed in #1 and #2(

So 'fill mode' or NEVEC or AFR would not explain OP issue of -2EV or -3EV underexposure...most Canon shooters have tons of snapshots using eTTL flash which is the primary source of illumination, correctly exposed. Why the R5 for the OP would behave any different is the puzzler.

 

[Phil V]

The key point for all is that flash reduction 'for fill' is

1. not dependent alone on Tv vs. AV (they act the same!), although NEVEC does treat both differentnly from P or M
2. dependent upon ISO and ambient light level
3. and there is no deliberately selectable 'fill mode' per se vs 'primary illumination mode' (but there is hidden behavior not well documented, as discussed in #1 and #2(

So 'fill mode' or NEVEC or AFR would not explain OP issue of -2EV or -3EV underexposure...most Canon shooters have tons of snapshots using eTTL flash which is the primary source of illumination, correctly exposed. Why the R5 for the OP would behave any different is the puzzler.

The op had set the camera in TV to underexpose (aperture flashing) believing that the flash would fire and automatically compensate.

But it didn’t, I believe it fired in line with your table at approx 1 stop less than the TV choice. But I may be wrong and it might be a malfunction.

When I shoot w flash in ETTL and the camera in M, I’m generally underexposing the ambient by a stop or slightly more, usually ETTL nails it, but sometimes I need 1/2 stop of FEC.

I’m now trying to remember whether I’ve ever used ETTL in a studio type situation where the ambient would be grossly underexposed.

 

[wiltw]

The op had set the camera in TV to underexpose (aperture flashing) believing that the flash would fire and automatically compensate.

But it didn’t, I believe it fired in line with your table at approx 1 stop less than the TV choice. But I may be wrong and it might be a malfunction.

When I shoot w flash in ETTL and the camera in M, I’m generally underexposing the ambient by a stop or slightly more, usually ETTL nails it, but sometimes I need 1/2 stop of FEC.

I’m now trying to remember whether I’ve ever used ETTL in a studio type situation where the ambient would be grossly underexposed.

My speculation, not knowing fully what the R5 can or cannot do, is that OP started by trying flash while using the electronic shutter. For a long time now, flash could only be fired on mechanical shutter, although there is one camera (maybe more?) that can fire flash while using electronic shutter. OP did mention trying mechanical shutter with flash, and that things were working,..but then "changed it back to electronic first curtain and all was fine.", which would seem to indicate R5 is one of those cameras that CAN fire flash with electronic shutter...why this is a puzzler.

I certainly have shot with eTTL in conditions with minimal ambient light, and trying to capture any ambient would have required too slow of a shutter speed to handhold the camera, and the flash exposure came out just fine.

 

[TimHughes]

My speculation, not knowing fully what the R5 can or cannot do, is that OP started by trying flash while using the electronic shutter. .

The R5 does not trigger flash when using electronic shutter.

 

[wiltw]

The R5 does not trigger flash when using electronic shutter.

And THAT explains the severe initial underexposure...the flash was NOT triggered to output any light.
Now we also need an explanation for why OP says the flash continued to operate after he returned to electronic shutter?!...a mistaken perception by OP?

 

[TimHughes]

And THAT explains the severe initial underexposure...the flash was NOT triggered to output any light.
Now we also need an explanation for why OP says the flash continued to operate after he returned to electronic shutter?!...a mistaken perception by OP?

I recall he said electronic first curtain, that does work with flash on the R5. I.e. for flash you need either mechanical shutter or electronic first curtain. Electronic shutter does not work.

 

[wiltw]

I recall he said electronic first curtain, that does work with flash on the R5. I.e. for flash you need either mechanical shutter or electronic first curtain. Electronic shutter does not work.

Looking up information that explains EFCS...

"Many recent cameras feature an Electronic Front-Curtain Shutter (EFCS) or Electronic First Shutter Curtain (EFSC). This is a mix of a mechanical and an electronic shutter. All Canon cameras with Live View since the EOS 40D (released in 2007) have this option, enabled by default as Silent LV Shooting. When EFCS is activated, the mechanical shutter is initially fully open (so light is reaching the sensor, which enables Live View). To take a picture, the exposure is started electronically, but it's ended by the mechanical shutter (second curtain) closing.​

​

This has a number of benefits. It means that although the system isn't completely silent, it's not as noisy as using the fully mechanical shutter. Using the electronic shutter to start the exposure means the camera is very responsive, and shutter shock is avoided. (Camera shake caused by the second curtain is not recorded because this curtain ends the exposure.) EFCS can also enable a faster flash sync speed than using either the mechanical shutter or electronic shutter. On the EOS R3, for instance, the flash sync speed rises to 1/250 sec."​

So, apparently, the involvement of the mechanical mechanism permits EFCS to trigger electronic flash, although a pure electronic shutter mode will not permit flash use today on Canon mirrorless.

 

[A_S]

Cheers guys a lot to catch up on since I last visited so I'll probably wait till the weekend and have a read

 

[TimHughes]

Cheers guys a lot to catch up on since I last visited so I'll probably wait till the weekend and have a read

TLDR - the initial problem was trying to use flash in electronic shutter mode. The flash does not fire in electronic shutter mode on the R5.

 

[A_S]

TLDR - the initial problem was trying to use flash in electronic shutter mode. The flash does not fire in electronic shutter mode on the R5.

Cheers - FYI (another TLA..) If i said electronic i meant Electronic first curtain - which is fine and does fire . I switched from electronic first curtain to mechanical and back in the original scenario

 

[TimHughes]

Cheers - FYI (another TLA..) If i said electronic i meant Electronic first curtain - which is fine and does fire . I switched from electronic first curtain to mechanical and back in the original scenario

Hmm then we are back to the mystery! Flash and ETTL normally works fine in electronic first curtain.