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F-stops and flash exposure

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Greg Matty

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While relaxing in my hotel room in Copan, Honduras, a couple of weeks ago, I decided to take some pictures inside the room to kill some time.

I had the Sigma 20mm 1.8 lens on my 10D with the 420EX flash attached. I would swear the exposure was not that different from f-11 to f-4 while using the same shutter speed. I know this doesn't make sense which is why it is so puzzling. I have since erased the images so I can't post examples.

I can't for the life of me see how such different apertures, with the same shutter speed, could yield such similar photos. Is this a flash photo phenomenon? It could be I wasn't paying close enough attention to my camera and indeed the shutter speed was changing, but I don't think that was the case. And for flash photography, shutter speed shouldn't matter all that much. The shades were drawn in the room and it was pretty dark. Without the flash, picture taking would have been almost impossible. Granted I had been hiking the Mayan city all day and maybe this was a heat stroke induced hallucination.

Greg
 
since it was dark there was very little ambient light contributing to the pic, even if it was somewhat bright in the room, but your shutter speed was fast, again the ambient light would have little contribution to the exposure, therefore the flash was the main light source

well for a given subject (ie your room) the flash compensates for the aperature. in other words the bigger your aperature, the flash senses this, and then the less light it outputs.

feivel
 
That is three stops difference, which might be possible depending on the conditions (distance, lighting, color, etc); but the answer lies in whether it was right or not. If the exposure was two or three stops over or under, you would notice the difference. If the pictures looked about right, then there was no problem.

Duke
Greg Matty said:
While relaxing in my hotel room in Copan, Honduras, a couple of
weeks ago, I decided to take some pictures inside the room to kill
some time.

I had the Sigma 20mm 1.8 lens on my 10D with the 420EX flash
attached. I would swear the exposure was not that different from
f-11 to f-4 while using the same shutter speed. I know this
doesn't make sense which is why it is so puzzling. I have since
erased the images so I can't post examples.

I can't for the life of me see how such different apertures, with
the same shutter speed, could yield such similar photos. Is this a
flash photo phenomenon? It could be I wasn't paying close enough
attention to my camera and indeed the shutter speed was changing,
but I don't think that was the case. And for flash photography,
shutter speed shouldn't matter all that much. The shades were
drawn in the room and it was pretty dark. Without the flash,
picture taking would have been almost impossible. Granted I had
been hiking the Mayan city all day and maybe this was a heat stroke
induced hallucination.

Greg
Click to expand...
 
Feivel probably got it.

I always assumed the flash was firing the same amount of light regardless of aperture. Of course that is not the case so it makes sense I got a pretty good exposure regardless of aperture. I don't know why I didn't think about this. I guess I am used the the sun which won't vary it's output to my satisfaction.

Although there was some differenece between the exposures, any of them by themselves would have looked more or less correct. Only when comparing them back to back could you see the slight difference.

Too much Honduran heat or I might have thought about that myself.

Greg
 
But I've personally been very frustrated with flash photography on numerous Canon cameras. Finally got around to reading every bit of information I could get my hands on about E-TTL. Discovered that P gives priority to illuminating the foreground, while Tv and Av give priority to the background. Because I normally shoot Av without flash, I never bothered to change it, and my flash pictures were a disaster. The subjects were always dark. And that was with a 550EX only inches away. Go figure! But I switched to P when shooting flash and wow, I at least get a properly illuminated subject.

You might try different settings, including M, to see if that helps. Good luck.
 
tdhg566 said:
But I've personally been very frustrated with flash photography on
numerous Canon cameras. Finally got around to reading every bit of
information I could get my hands on about E-TTL. Discovered that P
gives priority to illuminating the foreground, while Tv and Av give
priority to the background. Because I normally shoot Av without
flash, I never bothered to change it, and my flash pictures were a
disaster. The subjects were always dark. And that was with a 550EX
only inches away. Go figure! But I switched to P when shooting
flash and wow, I at least get a properly illuminated subject.

You might try different settings, including M, to see if that
helps. Good luck.
Click to expand...
Av, Tv and M modes are fill flash modes, while P mode is a full flash mode. In Av, or Tv you set an aperture or shutter speed to get correct exposure for ambient light and the flash will then output correct amount of light for subject. In M mode which I use a lot, I just choose a a fast enough shutter (say 1/125) and appropriate aperture for depth of field required and shoot, especially for macro work, where a tripod can be a hassle. In this case you are close enough to subject that underexposed backgrounds aren't an issue. P mode will never choose a shutter speed more than 1/60 as it exposes for subject regardless of how dark background will be.
 
feivel said:
since it was dark there was very little ambient light contributing
to the pic, even if it was somewhat bright in the room, but your
shutter speed was fast, again the ambient light would have little
contribution to the exposure, therefore the flash was the main
light source
well for a given subject (ie your room) the flash compensates for
the aperature. in other words the bigger your aperature, the flash
senses this, and then the less light it outputs.

feivel
Click to expand...
Yes. It doesn't matter what aperture he was choosing, the flash automatically compensates it's out put to give correct exposure of subject. Bigger aperture less flash and vice versa.
 
Best explination I have read yet...
And yes the flash will compensate for the aperture setting..

When shooting on the move in a dark room, I succeessfully shoot at 1/30 sec at wide apertures in P mode to save the batteries on the flash. The batteries last longer on a long shoot because the flash does not have to work as hard. And as Mr. Majestyk wrote, this is normally when more interested in the subject matter more than the surrounding area.

But with a slow enough SS with a wide aperture, it does a faily good job with the surrounding area as well..
Vaughn
Mr Majestyk said:
Av, Tv and M modes are fill flash modes, while P mode is a full
flash mode. In Av, or Tv you set an aperture or shutter speed to
get correct exposure for ambient light and the flash will then
output correct amount of light for subject. In M mode which I use a
lot, I just choose a a fast enough shutter (say 1/125) and
appropriate aperture for depth of field required and shoot,
especially for macro work, where a tripod can be a hassle. In this
case you are close enough to subject that underexposed backgrounds
aren't an issue. P mode will never choose a shutter speed more than
1/60 as it exposes for subject regardless of how dark background
will be.
Click to expand...
--
Vaughn T. Winfree
Friends Don't Let Friends Shoot Film :)

pBase supporter http://www.pBase.com/vaughn
 
Auto-flashes work that way. The output of the flash varies to provide the correct exposure for the f-stop you select. There are limits however depending on the flash unit.
Greg Matty said:
While relaxing in my hotel room in Copan, Honduras, a couple of
weeks ago, I decided to take some pictures inside the room to kill
some time.

I had the Sigma 20mm 1.8 lens on my 10D with the 420EX flash
attached. I would swear the exposure was not that different from
f-11 to f-4 while using the same shutter speed. I know this
doesn't make sense which is why it is so puzzling. I have since
erased the images so I can't post examples.

I can't for the life of me see how such different apertures, with
the same shutter speed, could yield such similar photos. Is this a
flash photo phenomenon? It could be I wasn't paying close enough
attention to my camera and indeed the shutter speed was changing,
but I don't think that was the case. And for flash photography,
shutter speed shouldn't matter all that much. The shades were
drawn in the room and it was pretty dark. Without the flash,
picture taking would have been almost impossible. Granted I had
been hiking the Mayan city all day and maybe this was a heat stroke
induced hallucination.

Greg
Click to expand...
 
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