Yet another question about ambient light ... sorry

[northlondon43]

Hi everyone

I'm determined to crack my inability to understand how to use my external flash on my Samsung NX11 camera.

Last night I was playing (practising!) in the hotel room where we were staying just taking pictures of my backpack on the floor trying to understand how shutter speed affects ambient light (evening time outside so room dark with room lights on). All the theory/books/videos I've seen clearly say that (as long as the shutter speed is below my sync speed) reducing the shutter speed = more ambient light.

I was in camera manual mode with the Samsung external flash on ATTL mode.

My set up was aperture f8, ISO 200, flash distance 2m, focal length around 50mm

I took 2 pictures with the shutter speed, with the above settings, at 1/125 and 1/15. My expectation was that the background in the former photo would be darker than the latter given the faster shutter speed. What happened was the 1/15 photo was darker which makes no sense to me. I tried the same thing again using f4.5 and again using f11 just to see if it made any difference. Ditto results. I stress that no other setting was changed.

What am I missing as I'm metaphorically tearing my hair out (I have sooo little left lol)

Ta

Adrian

[Carl in Louisiana]

Hey Adrian,

I found this site to be very helpful in using external flash and hope that it will help you just as much. Once your finished with 101 there is a 102. Good luck and I look forward to your photos.

http://strobist.blogspot.com/2006/03/lighting-101.html

[Andrew1]

When you change your shutter speed your flash in ATTL is changing power to give you a good exposure. To see an example of the effect your are after, shutter speed affects light, turn off the flash. Also in the shots you took originally, your background (if any) lighting should be different.

[northlondon43]

Cheers for the pointers guys but I don't think (apologies if mistaken) that they directly address my question.

Adrian

[Geoff F]

Admittedly I don't have that camera; but I was never really happy with my modern external flash, using variable TTL output, until I started using the camera with manual settings and letting the auto flash find it's own level. With a little flash output compensation.

Modern flash units work differently from the old fixed output flashes which could be effectively controlled by adjusting the shutter speed.

[Colin Southern]

Hi Adrian,

The ambient exposure at 1/15th should have been a LOT brighter, so as far as I can tell, one of two things must have happened:

1. The camera didn't take the shot at those settings (some kind of over-ride / safety-shift)?

2. The flash is entering the exposure in areas where you're expecting only ambient.

Since you've "caught it in the act" so to speak, how about posting the 2 shots - with EXIF Data intact - so we can do a "post-mortem"?

[Manfred M]

I don't know your camera or its operating modes, but getting down to basics; you are shooting your flash in a darkened room, with ambient light being supplied by artificial light in the room. If it is like the hotel rooms I've been in, that means it is pretty dark. I'd be surprised if you had more than 200W worth of light in the room.

Assuming your off-camera flash is around 50 W-s (that might be a bit low), and your flash fires at around 1/1000 sec; your flash is giving off 50/0.001 = 50,000 W during that very brief time it is on; and the ambient light in the room is little more than a rounding error. So long as you stay at or below the synch speed of your camera, the only light actually affecting your image will be that from the flash itself, regardless of what your camera's actual shutter speed is.

Bottom line; in the lighting conditions you are shooting under, varying the shutter speed is not going to have any significant impact on your exposure (assuming that your ISO and aperture are the same).

If you are using an automated mode, where your camera uses TTL exposure control, it will vary the light output from the flash to give you the "correct" exposure, so you are essentially letting the engineers that designed the exposure algorithms in your camera decide what is "correct" exposure.

If you are playing around., I suggest you set your camera on manual and see what adjusting the aperture and shutter speed does, so that you get a better feeling for how these two controls impact your exposure.

[northlondon43]

Will do Colin...though probably tmw now.

[Colin Southern]

Following on from Manfred's line -- try some manual exposures without the flash on to get the ambient portion at least 1/2 descent.

[northlondon43]

Thanks Manfred for the suggestion; I'll have a go over the next couple of days. p.s. if I didn't say, the flash is an external on camera type not an off camera type.

Colin - here are the two pics I was referring to (shot at f11 as opposed to f8 as I originally posted). Hopefully I've exported them correctly from LR4 so that the metadata is there as well.

Cheers

Adrian

[Colin Southern]

Hi Adrian,

I think the flash is simply compensating. In the 2nd shot where there's more ambient you can see a shadow heading back towards the camera from the far bench leg which isn't present in the first image -- so the ambient light is getting in, although I suspect not a lot of it.

Would be interesting to see a couple of shots of the same scene at the same settings but with flash turned off. Might also be interesting to repeat the same with a manual flash amount too so you can see how much the flash is contributing in a given scenario.

[Manfred M]

I think Colin is bang on.

I've never quite figured out why my flash acts the way it does at times when shooting in one of the automated modes. I find that when I can't get consistent and predictable lights with flash, I go back to 100% manual shooting with the flash (even when I am just using it as a fill light). If I use mutliple light sources, I will often look at them one at a time to see how they are contributing the the image (as per Colin's suggestion to shoot with just the ambient light) and put them all together in the end, once I've figured out the contributions of each light source.

The nice thing about digital is that it is really cheap and easy to do this. One of these days I'm going to get myself a laptop and shoot tethered just to make the testing easier on me.

[northlondon43]

Oo eck Colin I'm not sure what your answer means ref my question about why reducing the shutter speed has darkened the photo. Sorry but I'm an amateur with a big A.

[Colin Southern]

Oo eck Colin I'm not sure what your answer means ref my question about why reducing the shutter speed has darkened the photo. Sorry but I'm an amateur with a big A.

Hi Adrian,

Basically, I think the camera has thought of it like this ...

- "1/125th - next to no ambient light at that speed - will have to leave it all up to the flash" Mr. Flash - this is what you've been trained for - MAKE US PROUD! And the flash didn't disappoint.

- "1/15th - OK - in terms of ambient light, things are better - still not great mind you - but lets assume we're after a fill flash function now - so lets tone things back a bit so the ambient light can compete" - and so it's knocked the flash power back a couple of stops.

I still think the 2nd shot is more flash than ambient though.

Keep in mind that I've got no idea how your metering is designed to work - or what kind of spread your flash has. That's why I think adding a manual exposure at those two speeds without flash might just give you an indication of how much or how little ambient light you have entering the scene (that's certainly how I start to setup a mixed lighting scene in the Canon world anyway, because the flash can only add light - it can't take it away - so I need to know what effect the ambient light is having on the exposure first; sometimes I want to slow the shutter to let more light in to brighten a background like yours up -- other times I want to speed it up to reduce the ambient and thus saturate a sky more -- sometimes I have to slow it right down to get any exposure at all).

Does that help?

[William W]

I was in camera manual mode with the Samsung external flash on ATTL mode.

I note “A-TTL Mode” - I am confirming that you are using a Samsung Flash Unit? (as well as a Samsung Camera)

***

I took 2 pictures with the shutter speed, with the above settings, at 1/125 and 1/15. My expectation was that the background in the former photo would be darker than the latter given the faster shutter speed.

It is.

Look closely at the two samples.

Don’t compare the two images to EACH OTHER; but rather compare the DIFFERENCE of the intensity in the background and intensity of the foreground WITHIN each photo. Now establish which has the GREATER DIFFERENCE and which has the LESSER DIFFERENCE.

The image with the LESSER DIFFERENCE between the intensity of the background and the intensity of the foreground is the shot made at 1/15s.

This can be especially seen UNDERNEATH the bench seat: in the first image there is HARD shadow from the flash and NO ambient getting in there; in the second there is A LOT OF ambient getting in there to make a GREATER AMBIENT exposure compared to the image which was made at 1/125s.

***

What happened was the 1/15 photo was darker which makes no sense to me.

Yes exactly: but that is the exposure of the WHOLE SCENE - which you are now seeing as 'a darker photo'.

With Flash and Ambient combined there are always THREE Exposures to consider when interrogating the result:

• The Flash Exposure
• The Ambient Exposure
• The Combined Exposure

***

The reasons that the EXPOSURE of the WHOLE SCENE is different between the two photos could be, but is not limited to, one, or a combination of the following factors:

• It is very tight quarters (i.e. there is not much ‘background’ and ‘foreground’) the AUTOMATIC FLASH function was confused as to what was actually “foreground” - Note that the Framing between the two shots, is DIFFERENT the Flash was NOT pointed at exactly the same spot in each frame.
  
  
• The flash did not fully recycle – Note that both were shot within seconds of each other, this could account for less Flash Intensity in shot #2
  
  
• The (Samsung) Flash A-TTL has a relationship to the Focus Point / Subject Distance, perhaps assuming that the main Subject is at that Focus Point distance – Note the Plane of Sharp Focus in the shot at 1/125s is well BEHIND the bag; the Plane of Sharp Focus is about DEAD ON the bag, for the shot at 1/15s – this could account for the Foreground in Shot 1 being slightly over exposed by the Flash as the Auto TTL wanted Flash exposure at the Plane of Sharp Focus.
  
  
• The Samsung A-TTL function DOES interpret and factor the Ambient as part of the ‘overall’ (could be via the Shutter Speed?) to compute the final (flash) exposure - I note that from the VERY LITTLE I know about the Samsung A-TTL (Automatic Through The Lens) System, I do know that it is very different to the E-TTL (Evaluative Through The Lens) System of Canon (about which I know a lot more).

I have a diagram (which of course I cannot now find) describing the outline of the Samsung A-TTL functionality.
It implies that the A-TTL does indeed factor in the Ambient Reading, differently the Canon System and that is the reason I offered the idea that the A-TTL System might read the Shutter Speed into some of its calculations, even though the Camera is in ‘M’ Manual Mode.


***

To continue further testing -

I suggest that you use the Flash in ‘M’ Mode for these tests.

Also ensure it is fully recycled.

Also BEFORE taking any test shot for ‘Flash as Fill’ or ‘Dragging the Shutter’ it is imperative that Sample Shots of the Ambient are taken – preferably for ALL the Shutter Speeds for which the Flash is to be tested.

With the AMBIENT ONLY images recorded, you will have a base against which you can compare the EFFECTIVE Flash Exposure.


***

Here is a sample proof sheet, where the Flash was being tested to show Students several Functions (Flash as Foreground Fill; Rear Curtain Sync and Flash to Arrest Subject Motion)



NOTE that Frames 3 is ‘Ambient Exposure Only’ (for the Tight Shot of ET) and also Frames 4 and 13 are ‘Ambient Exposure Only’ for the Longer Shot.

WW

[William W]

Basically, I think the camera has thought of it like this ...
1. "1/125th - next to no ambient light at that speed - will have to leave it all up to the flash" Mr. Flash - this is what you've been trained for - MAKE US PROUD! And the flash didn't disappoint.
2. "1/15th - OK - in terms of ambient light, things are better - still not great mind you - but lets assume we're after a fill flash function now - so lets tone things back a bit so the ambient light can compete" - and so it's knocked the flash power back a couple of stops.

Hi Colin,

We were writing at the same time (well I went out for coffee, too.)

Yes I concur with your summation - except on a technical note - it is NOT the camera thinking: it is the Flash Thinking as the camera is in M Mode.

I believe from all the evidence I have had (with a Student with a Samsung Camera & Flash) that yours is a very good explanation the 'General Functionality' of the Samsung Flash when it is in 'A-TTL Mode'.

And that is why I am thinking that the A-TTL mode reckons both the Shutter Speed and the Ambient TTL in its computations, even though the Camera might be in ‘M’ Mode.

Of course it is always possible that the Camera might be taken out of ‘M’ Mode when an A-TTL Flash is attached and active – but that is far less likely, IMO.

WW

[Colin Southern]

Hi Colin,

We were writing at the same time (well I went out for coffee, too.)

Yes I concur with your summation - except on a technical note - it is NOT the camera thinking: it is the Flash Thinking as the camera is in M Mode.

I believe from all the evidence I have had (with a Student with a Samsung Camera & Flash) that yours is a very good explanation the 'General Functionality' of the Samsung Flash when it is in 'A-TTL Mode'.

And that is why I am thinking that the A-TTL mode reckons both the Shutter Speed and the Ambient TTL in its computations, even though the Camera might be in ‘M’ Mode.

Of course it is always possible that the Camera might be taken out of ‘M’ Mode when an A-TTL Flash is attached and active – but that is far less likely, IMO.

WW

Hi Bill,

You go out for a coffee? BS - more likely a Kiwi red wine

Not at all familiar with Samsung flash A-TTL mode to be honest (was really just thinking of Canon ETTL II behaviour when I wrote my bit). Are you saying it's kinda equivalent to Canon's EXT flash mode ("thyristor mode")?

[William W]

Here is an A/B illustrating most of what I wrote:


The TOP Image is the 1/125s Shot and the BOTTOM image is the 1/15s Shot
The Bottom Image has had the OVERALL EXPOSURE increased to the ‘same’ as the top image – the reference being the wall on the right and the bag.
(‘same’ means ‘good enough for the purposes of this illustration’)

Note at Point A, the 1/15s image is about 1 Stop LIGHTER: certainly there is MORE Ambient Exposure in this shot. (Now we understand that there are three stops difference between 1/15s and 1/125s – and this is why a REFERENCE “AMBIENT ONLY” Shot is required to compare the Relative effect of the Flash exposures.)

Note at point B – in the Flash Shadow – there is about 1 Stop Difference – the Ambient Light is exposing that area more, in the 1/15s Shot.

Note that at Point C, in then 1/15s shot, there is a definite HARD SHADOW from the rear left leg of the bench (Colin has mentioned this). This further confirms there is more Ambient exposure recorded in the 1/15s shot.

Note that at Point D, in the 1/15s Shot we are very close to the Plane of Sharp Focus - in the 1/125s Shot the Bag is Out of Focus – this shows the Camera was focussed at a different point, for each shot

Note that looking in line, vertically, the 1/15s shot has the camera moved slightly to camera left – this shows that On Camera Flash is NOT seeing (and metering) exactly the same Scene in each shot.

Note that the EXIF (at the originals) records shots taken on the same day at: (1/125s) – 18h:15m:29s and the (1/15s) - 18h:15m:56s – that being 27seconds between shots and certainly it is within the realm of possibility that the Flash did not fully recycle. (Which of course if NOT fully recycled, opens up further questions as to how the A-TTL does actually function)


WW

[William W]

Not at all familiar with Samsung flash A-TTL mode to be honest (was really just thinking of Canon ETTL II behaviour when I wrote my bit). Are you saying it's kinda equivalent to Canon's EXT flash mode ("thyristor mode")?

Yes, perhaps: could be, but more 'sophisticated'.

I reckon the Samsung Flash in A-TTL Mode is talking to the camera for sure and taking the Camera's TTL Ambient into account for the FLASH calculation . . .

I believe it highly likely that it is relevant that BOTH the Framing AND the Plane of Sharp Focus are different in each shot.

Regards,
Bill

[William W]

Hi Bill,

You go out for a coffee? BS - more likely a Kiwi red wine

Haha!

This is my latest find we had it with Pasta, last night – very nice:


But I am cooking Beef tonight so you just talked me into trying this >


. . . got to go and stoke the fire, CU later.