There are probably two different answers here which in some cases may be
related.
The answer affecting small apertures is that the flash may simply not
have enough power to light the scene with a small opening such as f/16.
The flash fires but the camera doesn't sense enough light coming back
to declare a good exposure and doesn't set the good exposure indicator
to flickering. These shots will be found to be underexposed. You may,
however, be able to use f/16 if you're close enough to the subject. The
strength of the light from the flash diminishes very rapidly with distance.
The answer with larger apertures such as f/2.8 is not that you didn't
get enough light but that you got too much. If you have the complete
specifications for the 266d somewhere you may find that there is a
limitation on minimum allowable distance at various apertures and ISO
ratings. This has to do with the speed of the electronics in the flash.
If you're close enough to the subject and have a large enough
aperture/ISO combination the camera may sense proper exposure and tell
the flash to shut off but the flash may not be able to quench the flash
tube fast enough to stop the exposure when told to do so. These shots
will be found to be overexposed. Generally speaking, the more expensive
the flash unit, the shorter the distance allowable in TTL or auto mode.
The little Vivitar 550FD, for example, has a significantly greater
minimum shooting distance than does a T-32. I know the 266d has a
stated auto range of 2.3 to 33 ft. but I don't know what other
conditions are imposed within that range and can't find any detailed specs.
Finally, the combination effect. Even if you're at f/16, if the subject
is close enough the flash may still not be able to quench in time and
you'll still end up with overexposure.
To remove the random good/bad exposure element from your TTL flash
photography you need to check the specs for your 266d for
1) Minimum allowable distance
2) Guide number at various ISO settings.
The guide number is used for manual flash exposure calculations. When
using the guide number you can:
a) divide the guide number by the subject distance to get the required
aperture for correct exposure or,
b) divide the guide number by the aperture in use to determine the
maximum effective range of the flash.
The 266d has a zoom head and various guide numbers depending on the
position of the zoom head. At the 35mm position the ISO 100 guide
number is 66 feet. At the 135mm position the ISO 100 guide number is 92
feet. Using 66 for the 35mm position and setting the aperture to f/16
yields a maximum distance for good TTL exposure of 66/16 = approx. 4
feet. If set at the 50mm position the range will be a little longer at
5 feet because the guide number is 80. But something that must be kept
in mind is that the flash manufacturers (all of them) lie about the
guide numbers. Every flash I have ever measured is at least 1/2 stop
less bright than advertised and a full stop is not uncommon. To be on
the safe side you should assume that the true guide number may be as low
as the stated guide number divided by 1.4. That will allow for a 1 stop
error (or lie depending on how you see it).
But all of this calculation is meaningless if you use the bounce head.
If you use the bounce feature the distance to the subject is not the
straight line distance between you and subject but the distance the
light beam has to travel from flash to ceiling to subject. In addition,
the ceiling will absorb an unknown amount of the light making
calculations next to impossible.
Fortunately, there is a simple way around all this calculation mess that
will tell you if you're going to get a good exposure or not and without
using the camera and wasting film.
To check the flash exposure capability switch the flash from TTL mode to
normal auto mode. The 266d has two auto modes. One for f/2.8 and the
other for f/5.6. Choose the mode closest to the aperture you'd like to
use on the camera when you go back to TTL mode. Let's assume that you'd
like to use f/8 and ISO 100 on the camera. Set the flash for normal
auto mode at f/5.6. Since you're going to be using f/8 on the camera
you need to lie to the flash about the ISO rating. Since the camera
will be set one stop slower than the auto mode on the flash you need to
tell the flash that the ISO rating is only 50 or one stop slower than
the true ISO. If you wanted to use f/11 on the camera (two stops
difference) you'd have to set the flash ISO two stops down to ISO 25.
Then fire the flash in the same room at the same distance with the same
zoom and bounce setting you will use for the real shot and watch the
good exposure indicator on the back of the flash. If the good exposure
indicator on the flash lights up you've got a good chance of getting a
good exposure on the camera. Your subjects will go nuts if you do this
for every exposure but a few test shots in an unfamiliar room using only
the flash and auto mode will quickly tell you the minimum aperture
you'll be able to use for reliable exposure when you switch back to TTL
on the camera. I think you'll find that the power output from this
flash unit is small enough that you'll find the two auto modes of f/5.6
and f/2.8 have been chosen with a purpose. Without using higher speed
film you will likely be confined to larger apertures. You may also
discover that using the bounce feature will require higher speed film
(eg: ISO 400) as a practical matter.
Of course, the camera and the flash will both measure the exposure
somewhat differently. There is no guarantee that the camera will also
consider that there was enough light but they should be pretty close.
Plenty close enough to get a good print if you're shooting negative film
which has lots of exposure latitude.
Hope this helps,
Dr. Flash
Peter Bell wrote:
> I have a question regarding the flash indicator readings in the viewfinder
> of my OM-2s. I use an OM dedicated Sunpak 266d unit. I was under the
> impression that when I switch the flash to the TTL setting with my camera on
> auto, I am free to choose the aperture, and the camera will tell the flash
> when to shut off, thus giving me a correctly exposed image. I understand
> that when the correct exposure is acheived, the flash indicator LED in the
> viewfinder is supposed to flicker. After some experimentation, I've found
> that not all aperture settings will produce a flickering flash indicator.
> Indoors, its mostly f/11, f/8 and f/5.6 that will produce a flickering LED.
> With apertures above or below this range, the LED doesn't blink, it just
> lights up again once the flash has recycled. I've been using the 50mm f/1.8
> lens and 100 speed film.
>
> My experience has led me to conclude that when using a flash, my choice of
> aperture is limited, even on the TTL setting. Could someone confirm this and
> explain why I can't use any aperture I please? Thanks.
>
>
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