Flash exposure is controlled by aperture … but only for manual flash
Quite often the short-hand descriptions of the fundamentals of flash photography become misleading ‘facts’. I frequently see the following statements repeated on the various photography forums and in questions that are emailed to me:
- flash is controlled by aperture
- shutter speed controls ambient light.
(This is sometimes twisted around to a completely misleading version:
ambient light is only controlled by the shutter speed.)
Even though these descriptions are half-right, they are also half-wrong and will obscure a proper understanding of how shutter speed, aperture and ISO interconnect for ambient light and flash photography.
But first, for the love of all that is good on this planet, if your camera has the ability to set 1/2 stop indents for shutter speed and aperture, please take it off that setting and change it so that your camera has 1/3rd stop indents for everything. This is essential.
This becomes important in situations where you already have correct exposure, but you want to change any of your settings in tandem. Then you can change either your shutter speed or aperture or ISO (or any combination of them), simply by counting clicks. Eg: If you want a full stop more shutter speed (ie 3 clicks), then you can count 2 clicks wider on your aperture, and 1 click more on your ISO, to still give you correct exposure for your new shutter speed.
TTL flash vs Manual flash
The topic of TTL flash vs manual flash has been covered here before. However, this time I want to concentrate specifically on those two statements mentioned earlier:
- flash is controlled by aperture
This depends on whether you are shooting manual flash or TTL flash. There are distinct differences.
With TTL flash, if you are within the range of what your flash can put out, then your choice of aperture and ISO in effect becomes transparent. (Please re-read that statement until the implications become clear.)
With manual flash however, aperture does have a direct influence on flash exposure …
but so does ISO. We should not leave ISO out of the equation with manual flash.
Shutter speed has no effect on flash exposure (below max sync speed),
BUT … with ambient light, any chosen shutter speed (at a chosen ISO), will dictate a specific aperture. Therefore your choice of shutter speed clearly does have an indirect influence on your flash exposure.
For example, the reason why we choose max flash sync speed when working in bright ambient light, is that at max sync speed we have the widest possible aperture (while remaining outside of High-Speed Flash settings), and therefore have the most range from our flash. Which is also the reason why it makes most sense to shoot in manual exposure mode on our cameras when doing this, even when using TTL flash.
So in the case of mixing flash with available light, the shutter speed does dictate the aperture, and therefore affects our flash exposure, or how hard our flash has to work.
Now, about the other comment:
- shutter speed controls ambient light.
This is true. But it neglects to mention the two other players here – aperture and ISO. This somehow slips by quite often in descriptions of balancing flash with available light, and it becomes another misleading short-hand description. And as explained earlier, choice of shutter speed actually often does have an important bearing on flash photography.
The theme of this post has been extended by a subsequent article, explaining it with some images:
when aperture does not control flash exposure. This might help to make it even more clear.
- When aperture does NOT control flash exposure
- Shutter speed controls background exposure?
- Balancing flash and ambient exposure (model: Anelisa)
- Manual flash vs. TTL flash (model: Aleona)
- Tutorial – balancing flash and ambient (model: Camille)
- Tutorials: Flash Photography Techniques
27 Comments, Add Your Own
Hey Neil, another insightful read!
It’s not a false statement for me to say that your site has been my greatest source of insight for flash photography and even certain aspects of photography (eg. exposure, metering, shooting in manual mode).
Your teaching has done wonders to my pictures and in the end, that’s probably what matters most!
2Iztok Grilc says
Neil – good article.
In the middle of summer, I also take some time and wrote simple article on my blog about flash. If someone want to read about simple flash using, welcome to
let me see if I get this right.
Constant flash-subject distance
Constant flash power
Constant ambient lighting on subject
Changes made keep subject within flash’s usable range for given settings
given a reference shot taken at mid iso, mid Aperture, mid Shutter speed …
Changing Shutter Speed alone will affect ambient exposure but not flash exposure. In essence Shutter Speed controls ambient light
changing ISO will affect BOTH flash and ambient lighting equally, in essence ISO *controls* both flash and ambient
Changing Aperture alone will affect BOTH flash and ambient equally, in essence Aperture *controls* both flash and ambient
Given the peculiarity of Shutter speed only affecting Ambient exposure, you can use that in order to negate the Ambient component of a change in either Aperture or ISO.
So given the reference shot I mentioned above…
Change the shutter speed alone faster or slower and you get a shot exposed no differently in terms of flash but darker or lighter in terms of ambient.
Change the Aperture or ISO and make a corresponding *negating* change in shutter speed (open lens increase shutter, stop down lens decrease shutter, +iso faster shutter, -iso slower shutter) and you get a shot exposed no differently in terms of ambient but darker or lighter in terms of flash.
I think I’m correct
and if so I think this little thought exercise was INVALUABLE to me as I was *stuck* on just the shutter for ambient and aperture + negating shutter for flash… I had NOT realized that ISO can also be used with a compensating shutter move to affect flash (and as a result, keeping a given DOF!).
And please do correct me if I’m wrong (though I will undoubtedly play a bit… but I think I’m right )
Thanks SO much for all the time you put into this site. It’s an incredible resource!
5Paul Hodgson says
Neil, as far as I’m concerned, use as many words as you like…you type and they will come!
I don’t understand why shutter speed does not affect the flash exposure.
If I shot @ 1/250s @ f/4 with flash settled to correct exposure my subject
if I shot @ 1/125 f/5.6 (so I increse 1 stop shutter speed and aperture for more dof)
I change the flash exposure ?
I reduce the aperture by one stop, so this mean half light.. in order to compensate it I increase the shutter speed by 1 stop. So, the flash light sould seem to be equal.. or not ?
I’m missing something…. sorry Neil…
I think I can answer this one :-)
ambient light is a contiunous source… the sun is on for the whole time of that 1/250 and it’s on for the whole 1/125, and the intensity of the sun doesn’t change (relatively speaking).
The result is that if you shoot at 1/125 the sensor is exposed to a constant amount of light for twice as long as if you shoot at 1/250… hence you get one stop brighter exposure.
Flash is an instantaneous (for lack of the technical term) source. For the purpose of this post we’ll assume that the flash burst lasts 1/4000th of a second. If you set your camera to 1/125 you get a flash burst that lasts 1/4000th of a second and then for the remainder of the exposure there is no flash. If you set the camera to 1/250 you get a flash burst that lasts 1/4000 of a second and then for the rest of the time there is no flash. Therefore your exposure will get the same exact amount of light from flash regardless of what shutter speed you select.
Hope this Helps
By the way… what you posted is true assuming MANUAL flash. If you set your flash to TTL it will most likely give you the same flash exposure in both cases by varying the intensity of the flash.
P.S. and that last bit is interesting in and of itself. I was doing some reading up on this last night and found out that most (if not all) flash (at least portable strobes anyway) accomplish their varying intensity (power) feature not by actually varying the intensity of the light… but by varying the DURATION of the flash. So for instance say that at 1/4 power your flash fires for 1/8000th of a second. Then at full power it might fire for 1/2000 of a second or thereabouts…. but the INTENSITY is for all intents and purposes, constant. I thought that was pretty interesting.
Pla .. It is like Allesandro explained. But to add a bit to it:
Shutter speed has no effect on flash exposure (while remaining below max sync speed.) This has to do with the way a shutter works with two curtains opening. The duration of how long the curtains are open, determines the shutter speed duration.
However, flash can be considered to be nearly instantaneous and hence needs the entire frame to be open / clear so that entire frame will receive proper exposure from the flash.
This means that as long as you keep to ANY shutter speeds such that the entire frame is open for that instantaneous burst of flash, then you will have your frame evenly exposed. These range of shutter speeds are ANY shutter speed below max sync speed. Above max sync speed, one of the shutter curtains will partially obscure your frame, and you won’t get even flash exposure.
Tutorial: High-speed flash sync
Maximum flash sync speed
Pla .. regarding the example settings you mention. It is as I said in the original posting here:
In the case of mixing flash with available light, the shutter speed does dictate the aperture, and therefore indirectly affects our flash exposure, or how hard our flash has to work.
Therefore in your example, the flash exposure is affected by the change in aperture .. which happened as a result of your change in shutter speed. But shutter speed in itself doesn’t affect flash exposure (for manual flash), or how hard your flash works (for TTL flash).
You have to get out there and try it for your self, on in experimenting can you really understand this concept..
Great article Neil..
What if i for example, used say F.8 aperture, and set my ISO up to say 1600 so the ambient was just underexposed about 2 stops. Shutter speed set to 200. Is my Aperture diminishing my flash output still?
Or is it necessary that my Aperture alway remain very low?
11.1Neil vN says
Compared to what? Before I can say whether your aperture is diminishing your flash output, we would need to know to what we are comparing it.
Also, what do you mean by an aperture that is “low”?
Wide aperture? Small aperture?
Ok, I’ll try to explain it better. If both examples have the ambient down by two stops. But one is using F.8, the other is using a wide open aperture. Is the flash output equal?
12.1Neil vN says
In TTL, the flash exposure will be the same – in other words, how the flash exposure looks.
But at f/8 the flash will have had to push out a lot more light to get to correct exposure, than for a wider aperture … if the ISO setting is the same for both scenarios.
Using manual flash. Each ISO is different, (regulating the exposures rather than the F-stops). So both exposures are the same (ambient) even though one is using F/8 and the other is using a wide open aperture.
So if both ambient exposures are the same, but with different F-stops, does it mean that the flash output is equal still?
13.1Neil vN says
For different apertures (and no corresponding change in ISO), the exposures will be different.
For different apertures (and corresponding changes in ISO), the exposures will be the same.
I am not following your direction with this line of questions though.
Yes, corresponding changes in ISO. Ok, the exposures will be the SAME. That’s what i was asking.
Since you use all three (Shutter speed, f-stop, and ISO) to juggle exposure, is it safe to say that you can use a very small F stop with manual flash as long as the ambient exposure isn’t too dark.
15.1Neil vN says
Pat … again, I am not quite following your question.
If you rephrase it to be more specific with the context (and whether you are referring to ambient and / or flash), I think you will find you can answer your own question.
This is one thing that really keeps me in question. I’ll try here. ISO and Aperture both control the ambient plus flash exposure. Ok. That being said, would I have the same flash output level using say F.22 as it would say F1.8 IF i raised my ISO to compensate the aperture? Or doesn’t that work?
17Neil vN says
Pat, similar to the other questions you have asked, you have to make very sure that you state your question within a specific framework. Very often, a general answer doesn’t cover everything, and can become misleading.
Since this article is about manual flash, I assume you are asking specifically about manual flash, and not TTL.
Now the next hurdle – what do you mean by “flash output level”?
The actual power level of the flash, or did you mean the flash exposure?
The flash’s actual output is controlled by YOU, as you dial in more or less power.
(With TTL, the camera does it.)
So in your question , the “flash output level” remains the same, regardless of your camera settings.
However, if you were referring to the flash exposure, then the camera settings do affect it.
I am going to assume you meant f/22 (and not f.22)
Yes, you could have the same flash exposure if you raised your ISO accordingly in tandem while you made your aperture smaller.
Neil, you’re last scentence explained it. Ok, I want to underexpose my ambient by using 1/8th second shutter speed-F/22 aperture-and ISO at 160. At this setting can i still have full flash exposure? I understand how using ISO and Aperture in tandem works to maintain proper flash exposure when changing settings, but in the case of these settings example, because of the slow shutter speed, i don’t need to have a quite high ISO necessarily in order to have full flash exposure, correct?
19Neil vN says
“At this setting can i still have full flash exposure?”
I don’t know.
You have given me ISO and aperture.
To solve the equation for the flash exposure, I would need the Distance.
Remember, flash exposure is dependent on 4 factors, as explained here:
– Tutorial on manual flash exposure.
Please step through that exercise with your flash and camera in your hand.
Also go through these articles to connect the dots if things aren’t quite gelling yet:
– manual flash vs TTL flash.
– Getting the most power out of your flash.
– Flash Guide Nr and the Sunny 16 rule.
Also, at the end of every article, there are links to other related articles.
Get your proper exposure for ambient, then if i wanted to stop down aperture by 6 stops for example, i would compensate by raising up my ISO (not the s.speed) (my flash output will stay (assuming same distance from subject) the very same as it was at a low aperture)
This is my conclusion. As long as I use ISO to compensate and not shutter speed, i can retain the same amount of flash exposure assuming it’s the identical distance in both cases.
I happen to learn by chance that shutter speed does not affect the flash exposure and proper ambient light can be achieved by adjusting the shutter speed. When I searched on Google to confirm my understanding, I came across your lucidly written article. This and a few other articles of yours not only clarified the subject but also instilled more curiosity in me to explore flash photography.
I recently started using my flash off-camera, triggering it wirelessly, in manual (M) mode. In the manual (M) mode, my flash (Samsung SEF-580A) gives an option to adjust both flash power and aperture value. I read the manual, and it says the aperture value should be the same as the one in the camera. However, in most of the manual flashes (non-TTL) tutorials that I’ve seen, adjustments are only made to flash power – not aperture. But with this flash, I have to set both aperture and flash power manually, in addition to flash zoom. This has become a little daunting as a beginner to flash photography.
It is unclear how the aperture and flash power on the flash are related to each other and the camera. What happens if I don’t match the aperture value on flash? Is there any manual mode that I am missing? I searched online to see if there are any other flashes that require adjustments to the aperture in manual (M) mode, and Nikon SB28 appears to have that kind of feature. I would greatly appreciate if you can shed some light on this topic. Thanks a lot!
22Neil vN says
Krish — with manual flash, the flash exposure is controlled by 4 things: PAID
Distance (& diffusion)
These are all interlocked, as explained here:
– Tutorial on manual flash exposure.
To specifically answer your one question — If you change your aperture after you have achieved correct exposure, you will either get over- or under-exposure, depending on how you changed your aperture.
For the rest — There are several tutorials on this site that explains it in more detail.
Definitely check this out: a video tutorial explaining High Speed Flash Sync, where I describe the relationship between those 4 factors: PAID.
I think things will start to fall into place if you watch that video, and also follow the related links in that article. Let me know if this helped.
Hi Neil, thank you so much for taking out your time to reply. The PAID concept definitely makes sense. I think once I go through your other referenced materials, I will have better understanding of this topic. Have a good rest of the week!