Bad exposure with 1/60 sync and flash

[cumi]

I have a Canon PowerShot A520 and shoot mostly in aperture-priority mode. When I use the (build-in) flash and aperture-priority mode, the photos are mostly overexposed. I think it's because, the shutter-speed is not changing when I change the aperture. It stays allways 1/60 sec, regardless how open is the aperture.

What am I doing wrong? Is this normal? How should I use the flash and how can I live with constant 1/60 sync time? Is maybe the P-mode better?

I recognised, that when I move a bit back and zoom to subject (also bigger F-Number), the photos are not so overexposed anymore.

Any experiences?

Thanks,

 

[Azuth]

If you can't adjust the power of the flash (perhaps a fill-in setting?) you're probably a little stuck. You need to set the aperture correctly to expose for the flash power. If you can't manually adjust the aperture then it's an issue.
Can you adjust the ISO setting?
Or perhaps some sort of exposure compensation?
Or try using something to difuse the flash?

 

[cumi]

If you can't adjust the power of the flash (perhaps a fill-in setting?) you're probably a little stuck.

I can adjust the flash power only in manual mode (3 stops).

You need to set the aperture correctly to expose for the flash power.

Why doesn't do this the camera for me? Without flash it does it!

Maybe, I should take the photos in shutter-priority mode and set 1/60 sec to have the correct aperture set automatically...?

If you can't manually adjust the aperture then it's an issue.

I can do it, but the problem is that the shutter speed stays the same. Normally, without flash, when I move 1 step lower with aperture, the shutter moves 1 step higher automatically. That's how the aperture-priority mode works. The problem comes with flash, when the shutter speed stays always 1/60, regardless if I move 1 or 10 steps lower or higher with aperture. The reciprocity is not working...

 

[Rob]

Generally speaking cameras have a constant fixed synch speed usually around the 1/60th to 1/500th range. The aperture affects the exposure, but with a built-in flash, the flash power should be stepped accordingly. It sounds odd that it doesn't work as-is.

Perhaps your camera does not permit flash synch in AP mode, but I would be surprised if this were the case.

Do you have another adjustment for the flash output? If you can alter flash power using EV or stops, then try stopping the flash back at wide apertures to avoid overexposure. Or, stick a rizla or something over it!

Rob

 

[cumi]

An example:

1.

Face overexposed.

File: IMG_0800.JPG
File size: 941KB
Image Serial Number: 107-0800
Camera Model: Canon PowerShot A520
Firmware: Firmware Version 1.00
Date/Time: 2005:10:21 18:57:46
Shutter speed: 1/60 sec
Aperture: 2,6
Exposure mode: Program
Flash: Built-in + Red eye reduction
Flash EF guide number: 3,28
Metering mode: Center-weighted average
Drive mode: Single frame shooting
ISO: Auto
Lens: 5,8 to 23,2mm
Focal length: 5,8mm
Subject distance: 0,41 m
AF mode: Single AF
Image size: 1704 x 2272
Rotation: none
Image quality: Fine
White balance: Auto
Picture Effect: Normal


2.

Moving a bit back and zooming using a F5 instead of F2.6 and it looks a bit better.

File: IMG_0801.JPG
File size: 895KB
Image Serial Number: 108-0801
Camera Model: Canon PowerShot A520
Firmware: Firmware Version 1.00
Date/Time: 2005:10:21 18:58:00
Shutter speed: 1/60 sec
Aperture: 5
Exposure mode: Program
Flash: Built-in + Red eye reduction
Flash EF guide number: 6,31
Metering mode: Center-weighted average
Drive mode: Single frame shooting
ISO: Auto
Lens: 5,8 to 23,2mm
Focal length: 18,7mm
Subject distance: 0,97 m
AF mode: Single AF
Image size: 1704 x 2272
Rotation: none
Image quality: Fine
White balance: Auto
Picture Effect: Normal

One more thing: what does the Flash EF guide number means? Why is it double on the second photo?

 

[Rob]

The flash guide number is the camera altering the output of the flash. Essentially, the number is the f stop multiplied by the flash to subject distance at 100 ISO. In this case the distance is measured in meters.

First Photo: GN 3.28 = f 2.6 * 0.41 m (ISO unknown) This is obviously a camera cock-up as a GN of 1 would be appropriate at this aperture and distance. Hence the picture is over exposed.

Second Photo: GN 6.37 = f 5 * 0.97m (ISO unknown) This is about right as the GN works out at about 4.85.

Remember that the ISO needs to be known to calculate exactly. It looks like your camera isn't very good at close range flash shots at a wide aperture. No real surprise there, just move back a bit and zoom to narrow the field of view and the aperture.

Rob

 

[Rob]

If you set the ISO to exactly 100 (if possible) then you should be able to confirm the maths easily. You can compensate if you know the ISO is say 120, but the formula gets a little more complicated.

Rob

 

[cumi]

First Photo: GN 3.28 = f 2.6 * 0.41 m (ISO unknown) This is obviously a camera cock-up as a GN of 1 would be appropriate at this aperture and distance. Hence the picture is over exposed.

...

It looks like your camera isn't very good at close range flash shots at a wide aperture. No real surprise there, just move back a bit and zoom to narrow the field of view and the aperture.

So I didn't do anything wrong.

The manual says the flash can be used from 0,45m. Maybe I was 4cms to close...

And actually the manual also says, that the flash sync is between 1/60 and 1/500, but I've never seen something different than 1/60...

 

[markc]

Cumi, I hope you don't mind. I'm not posting this as an edit. I just wanted to show you the difference between wide angle and telephoto. I got the size and exposure close to make an easier comparison.

I'm guessing the one on the right looks more like your son really does. Wide angles exagerate distances from the lens. Notice how his face looks like it's been inflated like a baloon? His nose looks bigger on his face and his ears smaller. When shooting portraits, it's usually best to use a slight tele.

 

[cumi]

Thanks markc! This was also a reason, why I took the second photo. You illustrated the difference perfectly, could be a nice school-example! :thumbup:

Any ideas for the reason of the overexposed face on the first one?

 

[markc]

If you were closer than what is recommended for the flash, the flash output may not be able to drop low enough. You can get pretty harsh light by being too close. And because of the odd way light works (inverse square law), you notice light drop-off much quicker. See how the bed goes so dark compared to him? If you are .41m from him, the light will be 1/4 strength at .82m. At .97m, the light doesn't dropt to 1/4 strength until 1.94m. The graph looks like this.

A flash's output drops remarkably just out of the flash. Sunlight has already traveled an amazing distance, so several meters will have no visible effect on it's dropoff.

.41m is only .04m from the minimum recommended .45m, but .04 is 10% of .45m. If my math is right, that's something like 22% brighter.