Hi Kel, you didn't explicitly say why you weren't happy with the results but I assume it's because you weren't able to balance the background
exposure with the subjects? Flash is not my strong point but a few comments...
I assume by "on camera flash" you are referring to the pop up flash? When you pop up the flash in
aperture-priority mode, the fastest
shutter-speed (s/s) will be limited to your flash sync speed which looks to be 1/200 here. So if a scene is metered and the ambient
light gives a s/s of 1/500 (as in image #2), the camera (with flash up) will only allow a max of 1/200 so the scene will be over-exposed (by a bit over one
stop in this case). I think this is what happened in your first image here - the subjects are brighter because the scene is over-exposed by the ambient
light, I don't think the actual
light from the flash has had any impact on the image (there are no flash shadows for instance). Popup flashes are not particularly powerful and have a maximum range (for 'correct'
exposure) of only a few metres at low ISOs.
If you had selected an
aperture so that the metered
shutter speed was 1/200 or slower, then when you popped open the flash the s/s would have stayed the same, so the metered area would still be 'correctly' exposed (excluding any flash influence). So based on image #2, 1/500 @ f/4.2 is the same
exposure (approx) as 1/125 @ f/8. However stopping down from f/4 to f/8 halves your effective flash distance so you may now need to get closer to your subject for the flash to be effective. (Note that some cameras have a setting for the minimum s/s with the flash on so the s/s speed won't drop below that setting - default 1/60 ?).
Exif shows that you used spot
metering, where did you take the reading from? If you metered on the subject the reading would give you the 'correct'
exposure for that spot using the ambient
light. If the
exposure settings don't change when you pop up the flash (e.g. stayed at 1/125 @ f/8) then I don't think the flash would add any
light (assuming TTL) because the camera already sees the subject at the correct
exposure. (Not completely sure of this but seems reasonable to me). Something else to keep in mind if spot
metering off skin is that the reading for 'average' Caucasian skin will be around a
stop under-exposed so you need to dial in some +ve
exposure compensation.
So, what to do?
Personally I'd probably just rely on available
light in this situation.
Exposure in image #2 is probably as good as you'll get for the subjects in that location. If the girl had lifted her head a bit and turned her face a bit away from the horse's head those shadows on the right side of her face may have been reduced. I'd get the best
exposure of the subject I could without blowing out too much else and then sort it out in post.
Weather here looks overcast with the sun poking through a bit. If that's the case you might be able to wait for the sun to 'go behind' a cloud and then shoot with the sun (behind a cloud) behind you. This may result in flat lighting so subjects 'lose their shape' a bit, but it means the subject and the background will have the same
exposure.
If you have a
light coloured wall nearby (shed/stables etc) you could perhaps position your subjects so that the wall acts as a reflector pushing some
light back into shadow areas. If not a neutral colour watch out for colour casts though.
If you want to use fill flash you will need to set your ambient
exposure so that the
shutter speed is 1/200 or lower. In daylight this often means that you require a narrow
aperture which then reduces the effectiveness of your flash. You can increase the
ISO to increase flash effectiveness but this then makes it more difficult to keep the
shutter speed down to control the ambient. Fun, yes?
What you could perhaps try is
metering in matrix mode. This will take the background into account when giving you a reading. You could then use the flash compensation settings (+ve or -ve) to get the subject to your desired
exposure. Obviously you need to be within the flashes effective range. It might be easier to put the flash in manual mode - that way if the flash is set at full power (1/1) and is still not providing enough
light you would know that you need to move closer to your subject.
Some photographers like to under-expose the background by a
stop or two so that the subject has a bit more 'pop'. To do this you dial in some -ve
exposure compensation and then get your (flash-lit) subjects back to correct
exposure by dialing in the same amount of +ve flash-compensation. However I'm not sure how effective this will be with the power limitations of the popup flash.
If you are looking to use flash regularly it might be worth investing in a speedlight (if you don't have one already). They have a lot more power than the popup flash, they (most) can be angled to give bounce flash (although not very useful outdoors), and they can also be used off camera (with triggers) to provide more creative opportunities.
Anyhoo, hope I'm not completely wrong here and some of it made some sense...
Cheers.