Manual flash (Full,1/4,1/16) and shutter speed on 7i

When I am in ADI Flash Control and at aperture priority the shutter speed is ALWAYS 1/45 s regardless of the selected aperture. But when I am at Full Power, 1/4 Power or 1/16 Power the shutter speed varies depending on the selected aperture.

Consequently, I am getting a well exposed picture but blurred due to the slow shutter speed.

When, on the other hand, I set shutter priority to 1/45 the aperture turns red over the entire range of 2.8 to 8. This repeats itself at Full Power, 1/4 Power or 1/16 Power flash settings.

How then can I get a sharp picture when shooting with a flash setting other than ADI?

Thanks for your comments.
Josh

It's all about how much ambient light you want in your picture to balance against the flash.

Regardless of flash power you use, if you set the shutter speed to a 2000th you will get the same exposure. Depending on the distance of the subject the flash may or may not have enough power to reach the subject, depending on the power setting you choose, and also depending on the aperture you've chosen.

What's puzzling me is why you are using full power, 1/4 or 1/16 power?

What are you trying to achieve?

Jawed

ADI mode regulates the flash output based on exposure feedback obtained during the pre-flash and camera data such as focal length and focus distance. Since the exposure feedback circuit is varying the flash output/duration ADI is just not compatible with using the flash with the varous manual settings like you are trying to do.

If you want to use Full Power, 1/4 Power or 1/16 Power to manually contol the flash output set the Flash Mode to "Fill Flash" and Flash Control to "Manual" via the recording mode menu.

Chuck Gardner

Josh Halberthal said:

When I am in ADI Flash Control and at aperture priority the shutter
speed is ALWAYS 1/45 s regardless of the selected aperture. But
when I am at Full Power, 1/4 Power or 1/16 Power the shutter speed
varies depending on the selected aperture.

Consequently, I am getting a well exposed picture but blurred due
to the slow shutter speed.

When, on the other hand, I set shutter priority to 1/45 the
aperture turns red over the entire range of 2.8 to 8. This repeats
itself at Full Power, 1/4 Power or 1/16 Power flash settings.

How then can I get a sharp picture when shooting with a flash
setting other than ADI?

Thanks for your comments.
Josh

Click to expand...

Jawed,

I thought, probably in error, that when I am very close to an object, say 0.5 m, then by using 1/16 of the flash power I avoid burning the picture.

Josh

Jawed said:

It's all about how much ambient light you want in your picture to
balance against the flash.

Regardless of flash power you use, if you set the shutter speed to
a 2000th you will get the same exposure. Depending on the distance
of the subject the flash may or may not have enough power to reach
the subject, depending on the power setting you choose, and also
depending on the aperture you've chosen.

What's puzzling me is why you are using full power, 1/4 or 1/16 power?

What are you trying to achieve?

Jawed

Click to expand...

Yes, if you're very close then you can use less power. But you also need to understand that when you are using manual flash power, the camera is not going to adjust aperture and shutter speed for you in any meaningful fashion. It no longer has a "goal" for flash exposure.

It will probably adjust shutter speed for the focal length, using the approximation of 1 divided by the focal length (e.g. if you zoom to 100mm, the shutter speed will go to 1/90th) as the minimum shutter speed it will choose automatically. And it will probably adjust the aperture to get the best out of the ambient lighting available.

So, for manual flash exposure, you are entirely responsible for setting the aperture to suit the power of the flash. Remember shutter speed will not affect the amount of flash power in the picture.

So if you want to avoid blur set a high shutter speed, e.g. 1/180th.

If you want to minimise the amount of ambient light (e.g. indoor fluorescent tubes) then set the shutter speed to 1/2000th.

You use the aperture to fine-tune the flash exposure. Remember this also affects ambient light.

You may find it simpler to keep the camera in P mode. The flash power can be throttled-back (if needed) by adjusting the flash exposure compensation, e.g. -1 or -2. The camera will use a smaller aperture for underexposure, as well as using less flash power. In this situation, the camera will choose a shutter speed that matches the 1 divided by focal length rule, or faster if the ambient light is very bright.

So, I suggest you first experiemnt with flash exposure compensation in P, A and S modes (you can use both the normal exposure compensation and the flash exposure compensation settings - get a feel for how these two different methods affect the amount of ambient light in the picture). Once you've understood those, then go into M mode and do the same (play about with both aperture and shutter speed to see how it affects the balance between ambient and flash light). Compare the effects of the different settings you try both indoors under normal house or office lighting, and outdoors, e.g. at sunset or under the shade of a tree.

Once you've got the hang of that, using the manual flash power settings will seem more straight forward.

Another thing to bear in mind is that for every 1/4 of power, you get half the distance from the flash. So 1/16th power gives you one-quarter the distance that full power would give you. At f2.8, ISO 200, full power, the flash works to 3.8m. So at ISO 100 you will get 2.7m (divide distance by 1.4 per stop). At 1/16th power you will get 0.68cm distance (divide 2.7m by 4). So, for 0.5m focus distance you need to use f4 (one stop darker, divide 0.68m by 1.41 to get 0.47m).

So the correct exposure for 0.5m is ISO100, 1/16th power, f4. You can use any shutter speed you like.

Alternatively, if you look on page 92 of your manual, you will find a table for Guide Number.

In the ISO 100 column, the guide number is 8m at full power, 4m at 1/4 power, and 2m at 1/16 power. To use this table you divide the guide number by the focus distance to get the aperture you should use.

So in this case you divide 2m by 0.5m to get an aperture of 4. Voila, ISO 100, f4, 1/16th flash power. The same answer as before.

My D7 camera doesn't have manual power adjustment, so I can't test all this for you.

What I do know is that, in general, you can leave the camera in P mode and the camera will adjust flash power, aperture and shutter speed quite nicely. In general you shouldn't need to use manual flash power settings to avoid burnt out pictures. But of course there are always tricky subjects!

Best of luck.

Jawed

Josh Halberthal said:

Jawed,

I thought, probably in error, that when I am very close to an
object, say 0.5 m, then by using 1/16 of the flash power I avoid
burning the picture.

Josh

Click to expand...

Jawed,

I want to thank you for the very thorough explanation, I have learned a lot.

Thanks again,
Josh

Jawed said:

It will probably adjust shutter speed for the focal length, using
the approximation of 1 divided by the focal length (e.g. if you
zoom to 100mm, the shutter speed will go to 1/90th) as the minimum
shutter speed it will choose automatically. And it will probably
adjust the aperture to get the best out of the ambient lighting
available.

So, for manual flash exposure, you are entirely responsible for
setting the aperture to suit the power of the flash. Remember
shutter speed will not affect the amount of flash power in the
picture.

So if you want to avoid blur set a high shutter speed, e.g. 1/180th.

If you want to minimise the amount of ambient light (e.g. indoor
fluorescent tubes) then set the shutter speed to 1/2000th.

You use the aperture to fine-tune the flash exposure. Remember
this also affects ambient light.

You may find it simpler to keep the camera in P mode. The flash
power can be throttled-back (if needed) by adjusting the flash
exposure compensation, e.g. -1 or -2. The camera will use a
smaller aperture for underexposure, as well as using less flash
power. In this situation, the camera will choose a shutter speed
that matches the 1 divided by focal length rule, or faster if the
ambient light is very bright.

So, I suggest you first experiemnt with flash exposure compensation
in P, A and S modes (you can use both the normal exposure
compensation and the flash exposure compensation settings - get a
feel for how these two different methods affect the amount of
ambient light in the picture). Once you've understood those, then
go into M mode and do the same (play about with both aperture and
shutter speed to see how it affects the balance between ambient and
flash light). Compare the effects of the different settings you
try both indoors under normal house or office lighting, and
outdoors, e.g. at sunset or under the shade of a tree.

Once you've got the hang of that, using the manual flash power
settings will seem more straight forward.

Another thing to bear in mind is that for every 1/4 of power, you
get half the distance from the flash. So 1/16th power gives you
one-quarter the distance that full power would give you. At f2.8,
ISO 200, full power, the flash works to 3.8m. So at ISO 100 you
will get 2.7m (divide distance by 1.4 per stop). At 1/16th power
you will get 0.68cm distance (divide 2.7m by 4). So, for 0.5m
focus distance you need to use f4 (one stop darker, divide 0.68m by
1.41 to get 0.47m).

So the correct exposure for 0.5m is ISO100, 1/16th power, f4. You
can use any shutter speed you like.

Alternatively, if you look on page 92 of your manual, you will find
a table for Guide Number.

In the ISO 100 column, the guide number is 8m at full power, 4m at
1/4 power, and 2m at 1/16 power. To use this table you divide the
guide number by the focus distance to get the aperture you should
use.

So in this case you divide 2m by 0.5m to get an aperture of 4.
Voila, ISO 100, f4, 1/16th flash power. The same answer as before.

My D7 camera doesn't have manual power adjustment, so I can't test
all this for you.

What I do know is that, in general, you can leave the camera in P
mode and the camera will adjust flash power, aperture and shutter
speed quite nicely. In general you shouldn't need to use manual
flash power settings to avoid burnt out pictures. But of course
there are always tricky subjects!

Best of luck.

Jawed

Josh Halberthal said:

Jawed,

I thought, probably in error, that when I am very close to an
object, say 0.5 m, then by using 1/16 of the flash power I avoid
burning the picture.

Josh

Click to expand...

Click to expand...

wow Jawed.. that's a lot to digest and test!! i've learnt a lot from your post.. thanks!

one question - most of the times, i'm using P mode in flash work.. but i must say.. shooting people indoors with it.. jusst not to my satisfaction.. the picture is not totally lit.. underexpose.. i always thought the in-built flash is not powerful enough.. ( i'm eyeing sigma DG Super now.. ).. but now i think i don't know enuf

I wish I've a picture to upload for your comment, i'll take one later.. in the meantime.. what did i do wrong?

I'm afraid I can't say.

When doing indoor flash photographs of people (which, to be honest, is rare) I tend to stick with P mode and I often dial-in some under-exposure.

So, I'm afraid my experience is different from yours.

Jawed

sherkhan_my said:

wow Jawed.. that's a lot to digest and test!! i've learnt a lot
from your post.. thanks!

one question - most of the times, i'm using P mode in flash work..
but i must say.. shooting people indoors with it.. jusst not to my
satisfaction.. the picture is not totally lit.. underexpose.. i
always thought the in-built flash is not powerful enough.. ( i'm
eyeing sigma DG Super now.. ).. but now i think i don't know enuf

I wish I've a picture to upload for your comment, i'll take one
later.. in the meantime.. what did i do wrong?

Click to expand...