FEL, EV, exposition metering

[Destructor]

Gentlemen,

Sorry for my English first. I have read enormous number of articles describing camera exposition metering. Spot, average, centre-weighed etc. are among them. If flash is not used and complete scene is exposed under natural light, camera metering evaluates EV according to selected evaluation algorithm. In my camera (Canon EOS 550D) evaluation results are shown in a viewfinder under -2..+2 EV ruler at the bottom. I can reach +0EV simply switching a camera into manual mode, setting required aperture and varying shutter speed, for instance. The main idea is that I can see evaluation result in a viewfinder and correct EV number accordingly.

In case if flash is used, exposition divides into two parts: background (camera metering) and foreground (TTL II with pre-flash). Is there any way to see flash estimation results? For instance, I can try locking flash exposition value by pressing FEL button. Will flash estimation results be displayed somewhere in this case?

If I am willing to use flash in manual mode, how can I guess whether flash EV has been set correctly? Thank you in advance.

Regards,
Pavel

[arith]

Hi Pavel and welcome; I think maybe you have to take the photograph and look at the histogram. Somebody who uses flash all the time may be along soon to give better advice.

[herbert]

Hi Pavel,

Unfortunately Canon does not provide any metering feedback from the flash unit. This makes sense as the camera has to fire the flash in order to capture the light for metering. To get metering feedback would require continuous flash firing or a modelling light (a low power continuous light). The 320EX flash unit does have a modelling light but there is no flash metering feedback. Essentially the modelling light is adding to the ambient exposure and so cannot be differentiated from that. To separate the two requires metering with and without the flash. The modelling light is just useful for video or focusing in live view mode.

So you set the flash exposure compensation but then you have to trust that the metering will do exactly that. The best feedback is to look at the photo after it is taken and adjust the flash exposure compensation.

The feature that would be nice is a readout of the flash power used on the last eTTL exposure. This would allow you to use eTTL for a few test shots and then switch to manual mode with the same flash power. Canon do not provide this. Maybe in 10 years we may have that feature.

Alex

[Destructor]

Hi Alex,

I do appreciate your comprehensive answer. Please correct me if I am mistaken. The only method I have to use in order to estimate flash exposition is trial and error one. Right? Forgotten to say I have SpeedLite 430EXII flash. Thank you again.

Regards,
Pavel

[herbert]

The only method I have to use in order to estimate flash exposition is trial and error one. Right?

Yes.

If you had one then you could use a light meter at the position of the subject to determine the amount of light needed to correctly expose the subject. Without one then you just have to experiment.

Note that the Canon flash system works in basically two modes:

1. When there is a lot of light, for example outdoors during the day, it will use the flash for fill light. Here the flash will not contribute much to the scene and the flash exposure compensation will not affect the photo very much. It will only affect the objects close to the camera by adding a bit of light.

2. When there is not a lot of light, for example indoors or outside at night, it will use the flash for the main light. Here the flash will contribute a lot to the scene and the flash exposure compensation will affect the photo more.

Note that the Canon flash system is easy to use in situation 1 but not as easy in situation 2. This is because the camera will always meter the scene as if there were no flash and set the shutter speed/aperture accordingly. This can result in a bad choice of settings for situation 2, e.g. a long shutter speed causing subject blur.

In situation 2 where I would like to have a fast shutter speed to stop movement I find it useful to set the camera shutter and aperture using manual mode. I usually meter for ambient at about -2EV. I then leave the flash in eTTL mode since the subject to camera distance will be varying, e.g. as my subject or I walk around the room.

I would only recommend manual flash when the subject to camera distance is constant. In this case you will get repeatability between shots and can find a setting that is suitable.

Alex

[Colin Southern]

Unfortunately Canon does not provide any metering feedback from the flash unit.

Hi Alex,

They "sorta/kinda" do this with the 1D series cameras. We basically have 2 graphs in the viewfinder - the first shows ambient metering as per normal, while the 2nd shows where it will TRY to expose the flash zone relative to what it thinks will be a correct flash exposure.

So if you set FEC=+3 - and the flash has enough power - then that's what you get; if the flash doesn't have enough power then you just get a full dump.

Pavel should be able to set FEC on the 550D, but it won't show in the view finder (as far as I know) - but in all other respects it should be the same - but it's still going to be based on biasing whatever it finds with the preflash - not what it actually finds. But then again, this really isn't any different to normal metering in that you have to apply EC if the scene varies significantly from a medium gray.

[Colin Southern]

Hi Pavel,

You might find these previous threads informative when it comes to how metering really works ...

Custom Settings for 1D Mark III

What is the best method for learning how to expose photos correctly?

What does it mean to "meter off of ..."

[herbert]

Hi Alex,

They "sorta/kinda" do this with the 1D series cameras. We basically have 2 graphs in the viewfinder - the first shows ambient metering as per normal, while the 2nd shows where it will TRY to expose the flash zone relative to what it thinks will be a correct flash exposure.

Thanks for the info Colin. I knew the 1-series had a bigger viewfinder, I did not know what extra information they added to the LCD readouts.

My 7D only has a little icon to tell me that flash exposure compensation is set. If I use the dedicated function button to adjust this then the main exposure indicator in the viewfinder temporarily switches to the flash exposure setting allowing you to adjust it through the viewfinder. That is about as much as you get for feedback.

I'll keep hoping that the 1D mark IV prices come down when the 1Dx hits the streets. At the moment it is holding its price extremely well.

Alex

[William W]

eTTL, typically uses Evaluative Metering; I understand that the 550D does not have a Custom Function to allow the eTTL to use Average metering, but other EOS cameras do – and that will make a difference.

Also, (and this does applies to the 550D): the Flash’s functionality will vary dependent upon the Camera Mode (Tv Av P or M) which is used; this is as well as the camera’s metering taking into account EV of the ambient light of the scene.


If you want to really understand the metering and the functionality of Canon Flash I strongly recommend you purchase this book: Mastering Canon EOS Flash Photography; Guy, N.K.; Rocky Nook Inc, Santa Barbra California USA.

The book will give you great detail if that is what you require.



WW

[Colin Southern]

I'll keep hoping that the 1D mark IV prices come down when the 1Dx hits the streets. At the moment it is holding its price extremely well.

Sigh, if it EVER hits the street. I've got mine on order, but apparantly it's officially puched back until at least the end of April now. Heck, I'd be happy enough to just get my new 600EX-RTs & ST-E3-RT ... which ALSO should have been out now

[Colin Southern]

If you want to really understand the metering and the functionality of Canon Flash I strongly recommend you purchase this book: Mastering Canon EOS Flash Photography; Guy, N.K.; Rocky Nook Inc, Santa Barbra California USA.

+1

The author was kind enough to send me a courtesy copy, and I fully endorse Bill's comments about it. By the way, it's now available in the Amazon Kindle store, so one can be reading it within a few minutes if one wishes.

http://www.amazon.com/Mastering-Cano...2982120&sr=8-2

[Destructor]

Gentlemen,

I have never expected I will get so many comprehensive answers to problem I have faced with. I do appreciate your help. Thank you for all your inputs. Really

Regards,
Pavel

[William W]

отличный !

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