I am trying to stop using the auto mode. I think I sort of understand the following ISO rules:

  • Use an ISO of 100 or 200 when taking photographs outside in sunny conditions.
  • If the sky is overcast or it is evening time, then use an ISO within the range of 400 to 800.
  • Night time or in cases of low light you might need to set your digital camera ISO to 1600.

Should I follow the same rules when using a flash (direct/bounce mode)?


When you use flash outside in sunny conditions to fill shadows you should set it to a low value. When you use flash in low light and you want to illuminate just your model, it's better to use low iso values. If you want to illuminate the model and the scene you should use a higher iso value like 400 and up (the value depends on light and model of camera). For more information, have a look at: http://neilvn.com/tangents/flash-photography-tip-find-your-background/

  • do the same suggestions apply to bounce mode? – Max C Mar 10 '13 at 17:15
  • Essentially there is no difference between direct flash and bounce flash in this respect, although the flash has to work much harder if you bounce it because the light is spread out more so less of it falls on the subject. My approach to indoors flash, for the record, is to set ISO/shutter/aperture to whatever combination gives me the desired exposure for the background, and then add flash to give proper illumination to the foreground. This means that I don't actually pump the flash all that hard, as the overall exposure is in the ballpark to begin with. This gives a shorter flash recharge. – Staale S Mar 11 '13 at 14:19

I would say "no, do not use the same rules" - but that's because I wouldn't use your rules when not using flash. In most circumstances, I think you should start by selecting the shutter speed and/or aperture that you want, and then use the lowest ISO which allows you to expose the scene correctly. I can't think of many circumstances where I would choose the ISO before choosing other things.

  • 1
    "I can't think of many circumstances where I would choose the ISO before choosing other things." Except film photography! – James Nov 20 '14 at 0:45

flash is, imo, a lot trickier to set a rule around. it really is dependant on how you want the shot to turn out.

in low light, if you set to iso 100, 1/50 sec, flash on, the background scene will be dark/near black. there is a certain look some ppl like about the way that is shot but be wary of blowing out your subject unless you have a diffuser.

in low light, my preference with flash is to iso 100, 1/15, f1.8. this opens the shutter longer allowing the flash to freeze my subject while allowing the slow shutter to show up in a nicely lit ambient mood. as you move to iso 200++, this allows your shutter to be set faster without motion blur if your subject moves while still pulling up the ambiance. But that is entirely up to you as the slower shutter opens up some veryy creative motion blur with flash photography.

flash photography isnt as easy to "can" in respect to settings so while my rule may work for me, you may find it is inappropriate to your stylr or taste.


You can go pretty high with ISO, depending on light conditions. So I wouldn't rely on a specific ISO formula.

I've taken mine up to 5000 to get sharp, focused images with a fast shutter speed and no flash. Like the below - barrel racing, indoor arena, no flash allowed. And those horses were moving.

I usually start at 400 ISO. Then I decide which aperture and shutter speed I'll need, depending on the subject, ambient light and whether I can use flash.

Then if the exposure is too dark, I start raising the ISO until I get decent image exposure. More noise, but better than no images. enter image description here


Should I follow the same rules when using a flash (direct/bounce mode)?

It depends on how you plan to use the flash, and more specifically, how you plan to balance the flash against the ambient.

If you are planning on balancing the flash against the ambient so that the flash acts as fill (i.e., most of the light for the exposure will come from the ambient, and the flash is only used a little bit to "fill in" the shadows), then using similar exposure settings to what you'd use for ambient is a good rule.

However. If you plan on using the flash as your main source of illumination, then your ISO setting plays a role both in where you set your ambient exposure, as well as in trading off against how much light your flash can output.

Shooting at iso 100 or f/8 indoors can be asking for more light output from a hotshoe flash than it can generate. So you may have to increase either setting to a comfortable medium just to get an adjustable range of light from your speedlight for the image. But using a lower setting can help you underexpose or kill the ambient light in the image the way you want.

My starting point with flash is to typically set mid-range values: ISO 400, f/4, and my camera's sync speed. And then adjust from there, depending on what I want for my lighting ratios, keeping in mind my flash is only powered by AAs and can only output so much light.

See: Strobist's "Lighting 102" assignment on Balance.


The first thing I do when using flash is to look at the flash guide number to subject distance. It is assumed that you are using flash because the ambient light does not provide a satisfactory exposure. You therefore need to make sure your flash, either as a fill in or the main light, provides adequate illumination since it should reach your subject and should not blow out the subject.

With flash, only the aperture matters.

After this, then determine the ISO that will not introduce noise and you are good to go.

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