Flash settings. Do i understand them correctly?

I'm fairly new to DSLR's & asking because until now a lot of my images with flash have a subject in the foreground that's a bit too bright. So, i've been doing some research last few days and this is what i've come up with so far to try and understand the situation:

- Take a test shot

- If your overall image exposure seems correct (when using flash i usually use a semi auto mode like 'P' and so it should be) but the flash seems too strong in the foreground, then 1st try reducing the flash power. This will change the balance (ratio) of ambient (background) light and foreground (flash light). However, doing that (on my camera at least) also seems to reduce overall image exposure. You could try checking the shutter speed too (instead of reducing flash power). In theory, in 'P' mode, shutter speed is done for you by the camera (1/60th sec for flash), but if using other manual modes then make sure I'm using a slow enough shutter speed to capture enough ambient light to expose the image correctly (which reduces the need for flash power for the overall image)

- If after checking/adjusting flash power and/or shutter speed your image is then too dim (but your ambient to foreground light ratio is now correct), then you would increase your iso to compensate which lifts the camera's sensitivity to both the ambient and foreground light to give you correct overall image exposure (not sure if having 'auto iso' on or off is a good idea or not (it's currently 'on')).

Does that sound about right? Or is it somewhat wrong and needs correcting?

ps. I'm using a Nikon D3100 and a Speedlight (bounce)

Many thanks. I'd love to understand it all.

paul_b said:

- If your overall image exposure seems correct (when using flash i usually use a semi auto mode like 'P' and so it should be) but the flash seems too strong in the foreground, then 1st try reducing the flash power. This will change the balance (ratio) of ambient (background) light and foreground (flash light). However, doing that (on my camera at least) also seems to reduce overall image exposure. You could try checking the shutter speed too (instead of reducing flash power). In theory, in 'P' mode, shutter speed is done for you by the camera (1/60th sec for flash), but if using other manual modes then make sure I'm using a slow enough shutter speed to capture enough ambient light to expose the image correctly (which reduces the need for flash power for the overall image)

- If after checking/adjusting flash power and/or shutter speed your image is then too dim (but your ambient to foreground light ratio is now correct), then you would increase your iso to compensate which lifts the camera's sensitivity to both the ambient and foreground light to give you correct overall image exposure (not sure if having 'auto iso' on or off is a good idea or not (it's currently 'on')).

Click to expand...

At last, a good question about actually using flash, and actually in the flash forum! Hurray. (certainly not making fun of you, making fun of everyone else )

Are you using TTL flash, or manual flash? Which flash? (I'm asking TTL or TTL BL mode). Are you indoors or outside in brighter light? These are sort of different situations, so I'm holding back, not knowing yet what we are discussing.

It isn't clear if you refer to reducing flash power of manual flash mode, or if you mean using Exposure Compensation. I think you may mean Exposure Compensation? Because reducing flash power level would not reduce the ambient exposure. You said bounce, so surely indoors.

On Nikons, Exposure Compensation (the +/- button by shutter button) affects both ambient and TTL flash, reduces both equally, and makes overall picture darker.

Probably the better solution (for TTL flash) is to instead use Flash Compensation (see camera manual, but on the D3100, I think it is the same button that opens internal flash door (doesn't open if using hot shoe TTL flash).

Flash Compensation only affects relative TTL flash power level (like changing power level on a manual flash). TTL level is still relative to the ambient exposure though.
Exposure Compensation affects both ambient exposure and flash exposure.

(and all these ways add for the final flash value)

We could talk about P mode (good for fill in bright sun, less good indoors), or Auto ISO (generally bad news for flash, esp indoors), or about 1/60 second (does one size really fit all?), or about fill flash or main light flash, etc, but yes, the idea is to watch your first flash result (in a new situation), and then if necessary, you can correct it in the next shot to make it better, like you want it.

Higher ISO (relative to flash) would be for when the flash had insufficient power capacity otherwise.

My signature link might be of interest, particularly Fundamentals Part 4 (flash involves a double exposure).

Hello mate

Thanks for the reply.

Further information:

I'm using a Nikon SB-400, and i haven't touched the settings for it, so i assume it's on the default TTL setting (i'm not at home with my camera atm so cant check).

I don't use flash outdoors (perhaps i should), so all my flash images are from indoors. 'Auto iso' was always on (my reason was because i was always forgetting to check the iso levels from one day to the next, and i'd notice after shooting that they'd be different to what they should of been).

When i was explaining about reducing flash power, i was referring to reducing the flash compensation setting rather than the exposure compensation setting.

It's interesting you say don't use 'auto iso' indoors, because it's always on, in my camera. Should i turn it off then? Why is it bad news indoors?

Thanks for the link, i will have a read now.

I so want to understand it all

Then yes, Flash Compensation is how we "correct" automatic TTL flash, to be the flash level we desire in our results. Digital cameras make this trivial now, since we can see what we are doing while still at the scene, and can fix it easily, any way we wish.

The term TTL has a few meanings. In the menu, it means Through The Lens metering, generally means "whatever the flash system can do".
In the flash system, there is TTL and TTL BL modes, different modes and reactions.

On the SB-400, and SB-700, and the camera internal flash too, the menu that selects TTL is actually TTL BL mode, Nikons default balanced flash mode.
This means for fill flash outdoors in bright sun (to reduce the harsh shadows on faces, etc), this default TTL BL mode automatically does flash compensation to not blast and overexpose the subject (already metered for the ambient), and it is also a very pleasing and proper result, lighting-wise (really good stuff - actually very necessary for peoples faces). The EV number is not shown, but it does this automatic flash compensation.

But Indoors, as main light (not fill level), sometimes TTL BL can underexpose, and sometimes a little +EV flash compensation is necessary. That does seem the opposite of your complaint though, however you are using fill flash with high ISO. That should also reduce your flash level (some automatic flash compensation, but you can still control what it does with your own flash compensation - all sources of flash compensation add to a final result).

In contrast to TTL BL mode, the other actual mode called TTL (not on your menu) is not fill flash, it comes ahead strong with full metered flash value, regardless of any ambient or not, and if outdoors for fill, it is necessary to use near -2 EV Flash Compensation for fill level. Indoors however, it often has advantages, not underexposing as much (either way is just a starting point for our adjustments however).
Selecting camera Spot metering switches the flash metering from TTL BL to be TTL mode. The flash does NOT do Spot metering, Spot is only about metering ambient, but it does switch flash metering mode. Ambient bright enough to meter (high ISO probably is) will be affected by Spot metering (but the flash metering won't be affected by Spot).

The SB-400 is a little lower powered flash than some, and bounce flash at f/5 probably does often need near ISO 800 (depends on ceiling, and height). As fill flash though, its needs are maybe a stop or so less.


Indoors, we have two main general choices, to ignore and subdue any (incandescent) ambient light ( black picture without flash), or choose to include it, make use of it, and use the flash as fill.

The big problem with allowing the incandescent light is that it is orange, and some fluorescent light is green, etc. This causes color problems in the image.
Two solutions, keep the ambient light subdued, out of the picture (with low ISO and fast shutter speeds, black if no flash).
Or we can add color filters on the flash, to make the flash be the same color as ambient, so then we can use Incandescent White Balance, etc.
There is no White Balance choice that can deal with different colors of light in the same picture.

Allowing just a trace of the incandescent light can be a plus, a warming effect, people like it. But too much is orange.
At low ISO, 1/60 second normally does that small trace. High ISO though, the ambient becomes the main light, and the flash is fill.

The problem (or advantage) of Auto ISO is that indoors (in light less bright than outdoors), ISO WILL ALWAYS BE HIGH.
(Nikon has made different choices about how Auto ISO affects flash, but on D3100 age models, AUTO ISO only depends on ambient, and it will be high indoors, flash or not).
High ISO is not necessarily considered a plus, but indoors, the flash is always working into high ISO. And we see the orange ambient better then. High ISO is choosing to see it.

The 1/60 second with flash (in camera P or A modes) is not a metered choice. Reach up and turn the flash power off, and you will see it change to slower... dunno, depends on ISO, but with low ISO, very much less than 1/60 second in dim light. Turn flash back on, and it jumps to 1/60. This is just a MINIMUM SHUTTER SPEED WITH FLASH, because, well, because you are using flash instead, we don't need a slower shutter speed. This is not any intelligent choice by the metering system about the light requirement. It is just something that usually works OK, usually causing no problem. It is NOT to be confused with "optimum for our situation".


Even with high ISO, we still have choices about the orange. We can choose to underexpose the ambient a bit, a stop or more, so we see it less well (it is our choice how we see it).

If we set -1 EV Exposure Compensation for that ambient purpose, this reduces the flash too, so it is necessary then to set +1 EV Flash Compensation to restore it.

Or we could use camera Manual mode, and set ambient exposure as we wish. The TTL flash is still automatic in any camera mode, so it still does its thing the same as before.

I'm going to read your replies a few times and try and learn what you've said as you obviously know your stuff, and its fascinating. Having said that though, one of my weaknesses is knowing what to think when trying to capture an image, as most of photos are of my 4 yo child and she is always bloody moving, and so i find myself wanting to take snaps quickly and without thinking. Sometimes i just want to pick up the camera turn it on and snap.

Am i safe in saying that if i want to reduce the flash power indoors to stop it brightening my subject too much that i can do so, but should make up for the light loss by increasing ISO to compensate. In increasing ISO the permanent orange incandescent lights would become the main lights and so its important to use an orange filter on the flash to match, if i want to be able to correct white balance accurately?

I'm going to try the flash outdoors too from now on then. That should be interesting.

paul_b said:

I'm going to read your replies a few times and try and learn what you've said as you obviously know your stuff, and its fascinating. Having said that though, one of my weaknesses is knowing what to think when trying to capture an image, as most of photos are of my 4 yo child and she is always bloody moving, and so i find myself wanting to take snaps quickly and without thinking. Sometimes i just want to pick up the camera turn it on and snap.

Am i safe in saying that if i want to reduce the flash power indoors to stop it brightening my subject too much that i can do so, but should make up for the light loss by increasing ISO to compensate. In increasing ISO the permanent orange incandescent lights would become the main lights and so its important to use an orange filter on the flash to match, if i want to be able to correct white balance accurately?

I'm going to try the flash outdoors too from now on then. That should be interesting.

Click to expand...

In those cases if you have flash point it up at the ceiling and you 'should' get a good shot, just remember if you change from landscape to portrait shot on the fly you need to change flash direction lol.

paul_b said:

I'm going to read your replies a few times and try and learn what you've said as you obviously know your stuff, and its fascinating. Having said that though, one of my weaknesses is knowing what to think when trying to capture an image, as most of photos are of my 4 yo child and she is always bloody moving, and so i find myself wanting to take snaps quickly and without thinking. Sometimes i just want to pick up the camera turn it on and snap.

Am i safe in saying that if i want to reduce the flash power indoors to stop it brightening my subject too much that i can do so, but should make up for the light loss by increasing ISO to compensate. In increasing ISO the permanent orange incandescent lights would become the main lights and so its important to use an orange filter on the flash to match, if i want to be able to correct white balance accurately?

I'm going to try the flash outdoors too from now on then. That should be interesting.

Click to expand...

Using Flash Compensation to adjust the TTL flash level is a good thing of course, and (in the same situation, same room), you should be able to get the first shot right (like you want it), and then the rest (in same general situation, same room ceiling and height, etc) should also be reasonably proper too. In the same general situation, you should be able to shoot as quickly as you wish, after you get the first one like you want it.

That said, general metering rules always apply. If you aim your meter at a black subject/background, or at a white subject/background, neither will be correct, or rather, correct is that both come out middle gray. True of ambient or flash, just how reflected meters work.

No, not quite correct about ISO. ISO will not adjust to flash level (flash level will adjust to ISO). But ISO only adjusts to ambient level. Which can change as you move around a room, and aim lens at a window or a table lamp, etc. So this changing ISO and changing ambient will shift the flash around (in order to follow ambient, not necessarily a bad thing), but flash compensation or metering will not affect ISO. But I am saying "in same general situation" which rules that shifting out. Saying, it will be still be a lot better than if you did not properly adjust the flash compensation. We can also do substantial correction on Raw files later (if we avoid overexposure burning out detail).

Yes, the idea of using fill flash with incandescent main light is to use a version of a CTO filter (Color Temperature Orange) on the flash, so you can use Incandescent white balance. There are different shades of incandescent bulbs, and also different shades of CTO (half CTO, 1/4 CTO, etc).
3/4 CTO is a popular choice, and it will knock your flash power down about a stop (to about half).

See Four Flash Photography Basics - Color filters on flash

I use the swatch book mentioned there, one each of a jillon choices, $2.50.
Or the kit mentioned there, a few each of a few popular choices, $8
But you will want to see a few choices. Once you decide your choice, a Rosco 20x24 inch sheet is $6.50 (B&H).
I use masking tape to attach them to the flash. I don't use them for matching ambient light, but instead use them to add color to water drop splashes.


I think very few non-professional people are willing to bother with this, and it is not my best specialiity either.
But, it certainly is the conventional wisdom.
A more popular conventional wisdom says "Do not mix light sources".

For tips, search Google for CTO filter

Thank you all for giving me your advice. I've learnt so much in such a short time today. I've been practising with my camera and it's making sense now.

I'm going to read this topic here many times in the next few days