Understanding flash/body communication exposure

I was never a big flash user. I've traveled the country and primarily shot street photography, outdoors and landscape. Now I'm learning flash, and trying to understand inside and out how the camera and flash talk to each other in determining exposure. So when I shoot without a flash, I determine what my exposure should be and I take the picture. With a flash in the mix, does the camera make any adjustments to exposure? So of you shot f2.8 se 60ss and iso 800, what does the camera do if anything at all when te flash is attached? Does it change metering etc?

Frodge wrote in post #16723911
I was never a big flash user. I've traveled the country and primarily shot street photography, outdoors and landscape. Now I'm learning flash, and trying to understand inside and out how the camera and flash talk to each other in determining exposure. So when I shoot without a flash, I determine what my exposure should be and I take the picture. With a flash in the mix, does the camera make any adjustments to exposure? So of you shot f2.8 se 60ss and iso 800, what does the camera do if anything at all when te flash is attached? Does it change metering etc?

There's been a guide to the EOS flash system available online for years.

And the Cliff Notes version from POTN - Actually the Curtis Notes version from POTN
https://photography-on-the.net/forum/showthre​ad.php?t=171657

Frodge wrote in post #16723911
With a flash in the mix, does the camera make any adjustments to exposure? So of you shot f2.8 se 60ss and iso 800, what does the camera do if anything at all when te flash is attached? Does it change metering etc?

Those Tutorials listed above are great but here is a very simplified version to get you started:

In the case of Canon, the camera really does nothing. All it will do is limit the shutter speed to 1/200 or 1/250 (max sync speed) and show you a "Flash Ready" Icon in the viewfinder. It hardly knows there is a flash attached and will tend to treat it as "Fill Flash" only.

It is best to shoot with the camera in Manual mode and use the camera's exposure meter to underexpose the scene at least 1 or 2 stops, and then let the flash supply the correct amount of light. You can use Manual Mode or ETTL on the flash but there is no meter that tells you how much light is going to be used.

Bounce flash looks the best and of course Auto ETTL flash is the easiest. If the photo is too dark or light, you can dial in some - or + Flash Exposure Compensation (FEC).

With Manual flash you must change the flash power any time you change aperture, ISO or shooting distance.

I agree with msowsun. The best way to shoot flash is in Manual mode. ETTL-II works about 90% of the time. Make your own ISO choice, too. In most modes, Canon bodies will default to ISO 400 with the flash enabled. That is not always the best choice.

This is really weird, but P and Green Box actually do a better job of selecting ISO than AUTO-ISO does when shooting flash. Auto-ISO is stuck at 400 with flash. At least P & GB attempt to get it right.

The flash fires twice, if you watch carefully you can see it. The first flash is the pre-flash, which is at a set level. When this flash fires the camera meters how much light the flash put out vs how much the meter thinks it needs to make the exposure correct. The second flash occurs when the shutter opens to provide the light to make the exposure.

Thanks for the help guys! Have some weekend reading!

GaryD wrote in post #16724730
I agree with msowsun. The best way to shoot flash is in Manual mode. ETTL-II works about 90% of the time. Make your own ISO choice, too. In most modes, Canon bodies will default to ISO 400 with the flash enabled. That is not always the best choice.

This is really weird, but P and Green Box actually do a better job of selecting ISO than AUTO-ISO does when shooting flash. Auto-ISO is stuck at 400 with flash. At least P & GB attempt to get it right.

Yes I use M most of the time with the flash and I have my set button configured for FEC so I can quickly adjust the amount of flash if it is over/under.

Another thing with flash is HSS. I finally learned the value of that late last year, we were out on a bright sunny day in an apple orchard, very challenging due to the harsh contrast, and HSS allowed me to get a nice fill while still keeping the overall feel of a bright sunny day.

Keyan wrote in post #16724789
Yes I use M most of the time with the flash and I have my set button configured for FEC so I can quickly adjust the amount of flash if it is over/under.

Another thing with flash is HSS. I finally learned the value of that late last year, we were out on a bright sunny day in an apple orchard, very challenging due to the harsh contrast, and HSS allowed me to get a nice fill while still keeping the overall feel of a bright sunny day.

Agreed. On both counts.

OK, the previous posts have gotten you started in understanding flash. Now comes the 'fun' (right?!) part...the reality of the nitty gritty dirty details.

IN THEORY the camera behaves no differently with flash on vs. flash off, but Canon designed in two 'features' into their system which make that an untruth in certain situations. If you conducted tests, you MIGHT or you MIGHT NOT stumble into these particular situations, though!

Canon has characteristics called NEVEC, and AFR, which either

1) alter the amount of ambient light exposure, or
2) alter the amount of flash emitted light.

These happen within a certain range of ambient light brightness. Folks who lived with EOS cameras in the 1990s had visibility to NEVEC, but folks who only got their first EOS in the digital decade are blissfully unaware that their camera is changing their flash:ambient contrast ratio when shooting flash fill shots when shooting under EV7-13 conditions! A pro who shoots both 1Dx and 5DIII on the same job can turn on/off NEVEC and AFR on one camera but not the other, so his contrast ratio flash fill results are inconsistent from camera to the other if he wants to have control of fixed contrast ratio on one camera. NEVEC takes effect within EV7-10, AFR takes effect within EV10-13. Outside that range (EV7-13), the flash:ambient contrast ratio change does NOT happen! NEVEC and AFR programming are tied into particular low levels of light (and tied in to the EV level correspondence to that level of light). It cares not about the specific shutter speed, it cares about the level of light.
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For your fundamental understanding of ETTL flash, ignore NEVEC and AFR. But to understand why your camera might seem to deviate in some situations, keep both in mind as reasons why you camera might seem to act oddly under certain circumstances!

https://photography-on-the.net …p?p=16679534&po​stcount=29
https://photography-on-the.net …p?p=15698377&po​stcount=31