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Balancing flash and ambient

MarkMark Member
edited August 2012 in wedding photography
The other day Neil posted an article on using high ISO and flash in combination


I came back to it today to re-read it and see that Mandy raised pretty much what I was going to ask but the answer left me still doubting my own thoughts and knowledge so here I am to ask and fill in a gap.

Walking around a reception taking photos of guest mixing and mingling I've almost got into a fixed set of settings - ISO 800 or 1600, f5.6 or thereabouts 1/200th and ETTL with a black foamie thing and ride the EV + or - until I'm happy then shoot with that in that area. Works pretty well.

But not always.

Sometimes I do see subject movement: in the photo shown in the article it might have been hands moving or lips if someone was laughing/talking.

If I try to increase ISO to get more ambient in or if I try to slow the shutter speed to 1/60th or the like for the same reason then I get more chance of subject movement.

Neils settings on that image he gave as 1/60 @ f3.2 @ 2500 ISO. I know the exact settings are not important...

In the reply to Mandy he said:
"Most importantly, the balance between flash & ambient, and specifically how much your ambient light is under-exposed. If there is a lot of subject movement, and your ambient light levels is close to the flash exposure, then you have a higher chance of blur."

Which I understand. If your ambient is high enough that an image will record then... well it will record something still after the flash has gone off... and if the subject has moved then it will record as motion blur.

Now 1/60 f/3.2 and 2500 ISO don't tell us anything aobut the balance.

In my own example I said ISO 800 and 1/200th. Usually the flash is doing nearly all of the lighting and ambient barely registers. But if I went to something much higher for ISO or slower for shutter speed the ambient is going to be less under-exposed, the flash will have to do less work to balance the exposure. And the ambient will have a chance to burn something in the image which could show up movement.

So my question is essentially to try to understand how to arrive at the settings to use so that the flash will record a crisp image with little or no subject movement and yet allow enough ambient that it doesn't just look like a flash-lit image.

What is the thought process? I would really like to get more ambient light into my shots but when I try there is a real risk of subject movement. I would dearly like to crack that issue. I would like to know, for example, how much far the ambient should be underexposed to minimise risk of movement showing.

Shooting in a pub at a music evening the other night I was shooting ISO 3200, 1/60th at f/1.8... no flash, ambient only. That's a light level only 1 and 2/3 stops below what Neil had, above. But I was shooting ok. If I had added flash... would I have had subject movement? I don't know.

Similarly, does it matter how far the bounce is having to travel? I've never thought about it before, but it strikes me that it might!

Sorry... long post... but I've been pondering this one for a while and was really pleased to see the blog post but it just made me ponder all the more. :)


  • depending on how much subject movement u have will be an important factor in deciding how many stops u will want to underexpose the ambient, so if u were shooting a Barmitzva and the guests were dancing the "Horah" (a fast dance) u will want to underexpose the ambient by at least 2 stops to ensure sharp images with little/no ghosting, basically take an "ambient only" shot to determine how underexposed ur ambient is and work from there.
  • MarkMark Member
    Which would explain why my "standard" settings work so well - I'm leaving most of the lighting to the flash.

    However, if the flash is doing the heavy lifting why bother with higher ISO at all?

    If I shoot in a room at ISO 800 f/5.6 and 1/200th and get an image the background will be fairly dark as the flash falls off. If I try to get the room mood lighting in by using 1/60th then I risk ghosting.

    If I shoot in the same room at ISO 3200 f/5.6 and 1/200th, once more I may get the mood lighting but risk ghosting.

    Maybe I am missing something here. Why go up the ISO levels to get more of a feel for the room but then risk ghosting?

    If I follow your advice, Naftoli, and keep ambient at least -2EV then what records will be pretty dark. The flash will still be doing most of the work. That seems at odds with the article.

    I thought the point of going up the ISO was to get more of a feel for the ambient. And that is what I want to do... without risking motion blur though.
  • good question! what u are not realizing though is that when u change ur iso from 200 to 400 not only does ur ambient record double the amount, but ur flash essentially becomes twice as powerful, so by using iso 1600 ur sensor is going to record much more of that light coming out of ur flash
  • MarkMark Member
    Ahah. Yes, of course.

    So essentially what you are saying is to still aim for -2EV to try to minimise motion but raise ISO to allow the flash to do more filling of the background. That's beginning to make sense. :) Thanks.
  • Very interesting debate. The same thing bothers me too from time to time, when I get motion blur from "proper" settings.

    Hmm, isn't raising ISO lowering flash power? Or am I wrong?
  • MarkMark Member
    sasko1 said: Hmm, isn't raising ISO lowering flash power? Or am I wrong?
    Raising ISO is lowering required flash power. That was what had me pondering at the start. ETTL will cut down the power used to get an even exposure. But what difference does it have for peripheral illumination?

    It did take me a lot of pondering but here is where I got to after Naftoli's post.

    Say I shoot fully manual, not ETTL.

    Let's start with a pretty dark room so that our exposure is coming from the flash.

    Assume we got an exposure of our subject as follows:
    ISO 200 and 1/200th and f/4 (ambient being somewhere far lower than that) but our flash was on manual and 1/2 power.

    Raising our ISO to 400 and keeping shutter and aperture the same we would need to half the flash so now 1/4 power.

    Raising to ISO 800 = 1/8 power, ISO 1600 = 1/16 power, ISO 3200 = 1/32 power.

    So we have turned the required power of our flash down an awful lot (four stops). Our subject is in theory receiving the same amount of light and therefore looks the same.

    How about our surrounding environment though? Well I said start with a pretty dark room - not a black one, just pretty dark. As we raise the ISO the ambient may start to be recorded so now our subject is in context rather than in a sea of black as the flash falls off away from them.

    But at higher ISO and ETTL if my flash was bouncing off a wall to fill the space then it may be doing so without nuking the subject provided we ride the ETTL and still illuminating the room to a degree.

    What I now understand is that provided the ambient isn't too high I should be able to get some decent results without risking subject movement.

    Hopefully the logic of that holds water! :)
  • Mark, even with bouncing the flash to reduce the effect of light fall-off, it still happens. More so if the subject is close to the light source. The other point that Neil was making in the article was about composing the shot to include something bright in the background (in this case the DJ's lights). The ambient in those 2 photos is under exposed to some extent and the bride and groom are lit mainly by flash. That's why there's (almost) no ghosting at 1/60th second.
  • MarkMark Member
    Allen, indeed. Got you.
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