Strange results using flash with my camera

[northlondon43]

Hi everyone

I am having problems with my Samsung NX11 camera, which I always put in manual mode, when I use it either with my internal a-ttl flash (GN11) or external Samsung flash (GN42) in a-ttl mode.


Problem 1 – scenario: taking a picture of a subject indoors at night, some ambient light from room table lights.

Settings: shutter 1/80, ISO 400, flash distance 2-3m, focal length 50mm, tripod mounted

I then take a series of photos varying the aperture between f5.6 and f22. My expectation is that whilst the subject exposure should remain the same (due to a-ttl) the background will darken as I close the aperture (less ambient light captured). What actually happens is that the whole picture darkens as I close the aperture. Why?

Problem 2 – scenario: taking a picture of a subject indoors at night, no ambient light.

Settings: Aperture f8, ISO 400, flash distance 2-3m, focal length 50mm, tripod mounted

This time I take a series of photos varying the shutter speed between 1/60 and 1/3. My expectation is that, as there is no ambient light, changing the shutter speed should have no effect on the subject exposure. What actually happens is that the whole picture darkens as I reduce the shutter speed. Why a) does the shutter speed have any effect if there is no ambient light b) assuming there’s an explanation for a) does reducing shutter speed reduce exposure when the theory says it should increase the exposure?

As a result of the first problem Samsung agreed to service the camera; I was told they replaced the PCB. However, on testing the camera on its return, Problem 1 remains. I discovered Problem 2 last night after deciding to test the flash in other situations so no idea if it would have reacted the same way before the Samsung service.

Thanks in anticipation

Adrian

[Black Pearl]

The a-TTL will switch off when the camera is used in manual mode so the flash is giving the same amount of light on every exposure.

[northlondon43]

Hi Robin. That's certainly not my understanding of how a ttl works.

[Neelly]

Hi Adrian I have an NX11 that I have never used the flash on but after looking at the manual and playing about with it I'm not sure there is attl on it with manual settings. I certainly get the same results as your problem 1 Wider aperture lighter picture which makes sense if it just a standard flash. I notice "smart flash" only works in scene modes.

[inkista]

Problem 1 – scenario: taking a picture of a subject indoors at night, some ambient light from room table lights.

Settings: shutter 1/80, ISO 400, flash distance 2-3m, focal length 50mm, tripod mounted

I then take a series of photos varying the aperture between f5.6 and f22. My expectation is that whilst the subject exposure should remain the same (due to a-ttl) the background will darken as I close the aperture (less ambient light captured). What actually happens is that the whole picture darkens as I close the aperture. Why?

Because there's an upper limit to how much light the flash can put out. Going to f/22 is going to go far beyond it. For every stop you close down the aperture, or stop you lower the iso, you have to double the light output. 1/80, iso 400, 2-3m, at f/5.6, you're probably pretty close to needing 1/2 or 1/4 power, and it cannot go past full power (1).

Remember, aperture and iso affect BOTH the ambient and the flash. And your speedlight is only powered by AA batteries.

Problem 2 – scenario: taking a picture of a subject indoors at night, no ambient light.

Settings: Aperture f8, ISO 400, flash distance 2-3m, focal length 50mm, tripod mounted

This time I take a series of photos varying the shutter speed between 1/60 and 1/3. My expectation is that, as there is no ambient light, changing the shutter speed should have no effect on the subject exposure. What actually happens is that the whole picture darkens as I reduce the shutter speed. Why a) does the shutter speed have any effect if there is no ambient light

Because there IS ambient light. No ambient light would mean you're shooting in a pitch black room.

b) assuming there’s an explanation for a) does reducing shutter speed reduce exposure when the theory says it should increase the exposure?

If by "reducing" you mean "making shorter", yes, it will reduce the exposure. It should not increase it unless you make the shutter speed interval longer. And, as I said, it's because you have some ambient light in the situation.

Increasing the shutter speed (making it longer) will not increase the flash portion of the illumination. But it will increase the ambient.

[northlondon43]

Thanks Kathy, plenty of food for thought though I'm not sure I entirely understand your reply to problem 1:

I think you're basically saying that the flash can't cope with the settings I used. Whilst I can see that might happen with the internal flash (GN11) surely the external flash (GN42) could cope? I'll have to read up about the 1/2 or 1/4 power reference as I don't understand how that's calculated (but thanks anyway).

Problem 2: I was shooting more or less in a pitch black room, i.e. night time, no light through the window, perhaps some ambient light from a table light in the next room....put it this way you couldnt see anything in the room where I was shooting with the naked eye straight away.
Even if there was some ambient light it doesnt explain (to my beginners mind) why the whole picture (as opposed to the background) darkens when I reduce the shutter speed. By reducing I mean going from 1/60th to 1/3rd shutter speed. As the latter is slower it should increase the ambient light yet the overall exposure (not just the subject) was darker at 1/3rd than 1/60th!!

Adrian

[inkista]

I think you're basically saying that the flash can't cope with the settings I used. Whilst I can see that might happen with the internal flash (GN11) surely the external flash (GN42) could cope? I'll have to read up about the 1/2 or 1/4 power reference as I don't understand how that's calculated (but thanks anyway).

The 1/2 or 1/4 power are just relative to the full power output of the flash. Most flashes, when set into manual (non-TTL) mode, can be controlled by discrete power settings by powers of two, so we can match to stops in iso/aperture/shutter speed. TTL masks these settings from you by setting the power output level automatically, based on the metering of a pre-flash burst of a known brightness.

Yes, I was saying your flash can't cope with your settings, and yes, there is an upper limit to how much light a flash, whether external or pop-up can put out. Let's look at the guide numbers as a clue.

A flash's guide number is given as a distance, assuming that you're shooting at iso 100. You then divide this distance by the f-number of the aperture you're using to see how far the flash will throw the light when you're shooting at full power. I think this may be the part of the puzzle you're missing.

Also. Manufacturers tend to fudge guide numbers. They usually measure it at the highest zoom/narrowest spread, which throws the light farther. I think only Nikon does it reasonably by specifying the zoom of the flash is set to 35mm. For example, Canon's 430EX II is supposed to have a guide number of 43m but that's zoomed out to 105mm; the measured guide number zoomed to 35mm, is only 31m.

So, lets assume your GN42 is actually GN32.

Take 32, divide it by 22 (because you're using an aperture of f/22), and your flash, zoomed to around 35mm, is probably only going to be able to spit the light out 1.45 meters. And light falls off by the inverse square law (i.e., rapidly).

This is what takes a lot of newcomers to flash by surprise: speedlights are, indeed, underpowered. Using them well is often about nursing every spare erg out of them that you can, and conserving power. Yes, you may still need high iso settings and wider apertures. And now you know why your pop-up flash really can't reach much beyond a meter or two.

Even if there was some ambient light it doesnt explain (to my beginners mind) why the whole picture (as opposed to the background) darkens when I reduce the shutter speed. By reducing I mean going from 1/60th to 1/3rd shutter speed. As the latter is slower it should increase the ambient light yet the overall exposure (not just the subject) was darker at 1/3rd than 1/60th!!

Yup, that doesn't make sense. So now, I'd say go back to the photos, and look at the EXIF information to see what else might have changed (iso, aperture). And also, did you switch the shooting mode on the camera?

I nave no idea how the Samsungs work, but Canons and Nikons typically change the automatic/TTL flash behavior based on the shooting mode on the camera. In full Manual mode, whatever you've set on the flash and on the camera goes, and you decide the ambient/flash balance. In aperture & shutter priority, however, the assumption is that the flash is to be used for fill. Exposure settings typically go to what would be used for ambient, and a small amount of flash is flicked out to fill in the shadows, and to bring the subject up to acceptable levels. In P or full Auto modes, however, the general assumption is that the flash is supposed to be the main source of illumination, and the ambient/flash balance flips the other way. The background can be as dark as it wants, and the subject is all that's metered and exposed for.

[northlondon43]

Hi Kathy

Firstly I’d just like to say thanks for being so patient and providing an illuminating (no pun probably intended) answer to problem 1 as it’s encouraged me to do so further research....

My external flash operates between 28mm and 105mm. The guide number range is 26-42 so, given that I was taking the pics with a focal length of 55mm, your suggestion of working with GN32 seems reasonable. However I was working with an ISO of 400, not 100. This suggests I should be increasing the GN by a factor of 2, hence a working GN of 64. GN64 divided by f22 is approx 3 meters. Thinking back to taking the pictures I was actually at a maximum of two meters (not sure it was even that), so the flash should have been able to cope. Saying that, the GN32 is a guesstimate which does rather affect the outcome!

That aside, I did say that I took a number of pics between f5.6 and f22. I’ve attached two pics (subject bronze vase), one at f5.6 and one at f13; you’ll notice how the whole picture, not just the subject has darkened considerably. Using a GN64 again my flash should easily be able to cope as the max subject to distance range at f13 is now nearly 5 meters.

Problem 2 – Yep I’ve already checked the EXIF data and nothing changed between shots bar the shutter speed including the shooting mode. I ran the same trial last night with exactly the same result...weird. I’ve attached two pics (water jug) at 1/60th and 1/3rd for that as well.

Apparently problem 2 is being referred to a Samsung engineer as the service centre are scratching their heads as well!



Adrian

[northlondon43]

correction - the focal length of 55mm I gave hadn't been converted to 35mm format ... the equivalent is roughly 80mm. The flash manual states that the GN at 80mm is 40. So shooting atf22 gives me a max subject distance of just under 4 meters

[inkista]

First off, what's the EXIF information on your four test shots? It looks like it's been stripped out. I know you said what you were using, but sometimes the EXIF can show you weren't using the settings you thought you were.

And does the flash manual specifically state that it can identify and compensate for sensor size in the camera? I'm assuming that the flash can ONLY be used on NX cameras, so the chances of it switching from a full-frame/film mode to an APS-C crop one is highly unlikely. It's probably already calibrated with the crop factor worked in, because, as I said, the manufacturers like to boost the guide numbers as high as they can.

The focal length you're using, and how far from the fresnel lens on the front the flash's bulb is placed to cover the FoV are two separate things. Just because it looks like you're using an 80mm lens, doesn't necessarily mean the flash head was zoomed to the 80mm setting. Your flash's zoom settings could actually match for 1.5x crop sensors. You'd need to test by manually zooming the flash to see.

And one more question: did you have a modifier (say a stofen or something) on the flash? Because all modifiers eat light.

[northlondon43]

Because I had to reduce the file size of the pics to upload onto here the EXIF data isn't available online. However I've just spent about 30 mins looking at the extended EXIF data between each set of pics and I can't see any difference bar the aperture or shutter speed settings I used.

I assume the flash can only be used on the NX series as it is Samsung NX specific. The manual gives focal distances for both APS-C and 35mm conversions, eg. minimum distance is 18mm APS-C and 28mm 35mm converted (GN26), max distance is 70mm APS-C and 105mm 35 mm converted (GN42). Various inbetween focal distances are also given.

Your penultimate paragraph doesn't mean anything to me (I am new to this) sorry.

Nothing on the flash (modifier ...whatever that is)

Either way....with the vase pictures, why does the vase (subject) also darken when I close the aperture given that I'm only 2 meters max away from it when I take the pic?

[inkista]

My personal opinion (and it's ok to disagree) based on my experience is that guide numbers are just a guide. And what the manufacturers say a guide number is is not necessarily accurate enough for strict by-the-numbers work, any more than the 1/focal_length rule for shutter speed is. It's a heuristic, not an algorithm.

Flash zooming is not lens zooming. The focal length of your lens translates to a given field of view for the image. The longer your lens, the narrower the field of view; the shorter the lens, the wider the field of view. Your flash's light can be spread out, or focused to be more narrow, and the narrower the beam is, the farther it can travel. The spread is controlled by the flash simply sliding the bulb in the head nearer or farther away from the front of the head. The nearer it is, the wider the spread and the shorter the beam, the farther back into the head it is, the narrower and more focused the beam becomes. When the flash is "zoomed" to 50mm, it then is giving a spread that covers the field of view of a 50mm lens.

As I said, because the NX gear is not going to be used on anything else, the 50mm zoom setting (and flash guide number) is probably for using a 50mm lens. Because the flash looks more powerful if they give it the guide number bump with the crop factor. You're trying to add the crop factor on AGAIN, and I think it's already been accounted for.

As for why your second shot is darker across the frame, my guess is that you're probably at or near full-power in the f/5.6 shot, and by the f/13 shot you've got beyond it, so the TTL can't adjust the power to keep the pot lit identically to the f/5.6 shot. Moving the aperture from f/5.6 to f/13 is decreasing the aperture by 2.3 stops. Which means you've reduced the light by a factor of 2^2.3 => 5. So the flash now needs to put out 5x the amount of light it did for the f/5.6 shot to illuminate the pot identically.

Also, aperture and iso affect both flash and ambient light. Shutter speed is the only independent control, since it affects only ambient and not flash. BUT. You also have to keep in mind that you didn't separate the light sources on the pot. It's being lit by both flash and ambient. Unless the pot was a black silhouette black in a shot you took without flash, the ambient is also hitting it, and the effect in the image is going to be the combination of the ambient+flash.

Assuming you've got enough separation and your flash is properly flagged off, etc. so that your subject is lit only by the flash, and your background only by ambient, then, if you want to adjust the ambient but not the flash level, just change your shutter speed. If you want to adjust the flash but not the ambient, then adjust your flash power output, and compensate the same amount with the iso and aperture. In TTL modes, the camera is automatically adjusting the flash power, so changes to the iso, aperture, shutter speed will be transparent in terms of how illuminated the subject is--as long as you're within the power output range of the flash.

A modifier is basically something you put in front of the flash to change the quality of the light that comes out. The diffuser panel on some flashes is one example. A Stofen Omnibounce (which is what I was referring to) is a small tupperware-like piece of plastic you can pop on the head that will throw the light out in all directions, which will help it bounce off all the walls/ceiling, and will considerably soften the light from a flash in an indoors shot. Outside with nothing to bounce off, though, you're just throwing power away when you use one. Most diffusers will eat up a stop or two of light, as will bouncing.

[northlondon43]

Cheers Kathy though it doesn't make sense to me that my flash is nearly at full power at f5.6, only 2 meters from a subject...if that was the case what would be the point on spending £180 on an external flash lol? I must admit your latter point about the ambient also illuminating the subject is interesting though I'm still confused.....surely the point of TTL is to keep the exposure of the subject the same (within the limit of the flash) no matter what you do to adjust the ambient (whether via ISO, aperture or shutter speed)?

[inkista]

Cheers Kathy though it doesn't make sense to me that my flash is nearly at full power at f5.6, only 2 meters from a subject...

What what if I state it as "being above 1/5 of the flash's full power"? 1/2 or 1/4 power would still put you past the max. output of the flash by f/13. Each power level of the flash is a stop. 2.3 stops down from full power (which is where you have to be to be at the edge of max. power on the f/5.6 shot) is 1/3rd of a stop below 1/4 power. What happened with the f/8 and f/11 shots?

In my experience, f/22 is a big ask of any speedlight.

if that was the case what would be the point on spending £180 on an external flash lol?

To get a light that goes off camera. To get a light that can tilt and swivel and not be on-axis. To get a flash you can bounce. But as I said up front, you need to accustom yourself to thinking of this as a small battery-powered flash that can only put out a finite amount of light, because that's what it is.

I must admit your latter point about the ambient also illuminating the subject is interesting though I'm still confused.....surely the point of TTL is to keep the exposure of the subject the same (within the limit of the flash) no matter what you do to adjust the ambient (whether via ISO, aperture or shutter speed)?

Yes, as I said, that would be what would happen--if you stayed within the power output limits of the flash. Your background/ambient level might vary, though.

You also have to understand that TTL is a metering-based scheme. Just as your metering is rarely 100% accurate, TTL settings are rarely accurate. Which is why flash exposure compensation was created.

I would recommend trying everything out in Manual and seeing what settings changes do then. It might be easier to wrap your brain around what's going on by explicitly setting your flash to blast at full power, 1/2 power, 1/4 power, etc.