Question on internal flash

[Tweek]

Got a question here, maybe a silly one, but I'd ask anyway

Does the internal flash of a camera compensate for the exposure by varying the intensity of the flash, or is it a fixed intensity? Yup I know the external flashes do their own metering (TTL, ETTL) and regulate the amount of flash, but does the "cheapo" internal flash do so as well?

Reason why I asked was I tried the following configurations on my C700UZ, indoors with auto-flash:

1) Aperture priority, F2.8, 1/40s, ISO100.
2) Aperture priority, F5.0, 1/40s, ISO100.
3) Aperture priority, F8.0, 1/40s, ISO100.

And for all three configurations I get pictures of the same exposure. And then I tried the following configurations, indoors with auto-flash:

1) Shutter Priority, F3.2, 1/40s, ISO100.
2) Shutter Priority, F3.2, 1/500s, ISO100.

And I get pictures of same exposure as well!

Since exposure settings between the configurations are different, the only reason I can think of that results in even exposure for all images, is that the flash intensity compensates.

And assuming that my guess is correct, I remember using my EOS 300 on P mode, with internal flash. It was at F4.0 and 1/90s (flash sync speed?). When I tried to increase the F number to get deeper DOF, the shutter speed started flashing at 1/90s. Why can't the internal flash of the EOS 300 compensate for the larger F-stop then?

Would appreciate if anyone can share any explanations and insights on this. Thanks!

 

[ckiang]

Originally posted by Tweek
Got a question here, maybe a silly one, but I'd ask anyway

Does the internal flash of a camera compensate for the exposure by varying the intensity of the flash, or is it a fixed intensity? Yup I know the external flashes do their own metering (TTL, ETTL) and regulate the amount of flash, but does the "cheapo" internal flash do so as well?

Reason why I asked was I tried the following configurations on my C700UZ, indoors with auto-flash:

1) Aperture priority, F2.8, 1/40s, ISO100.
2) Aperture priority, F5.0, 1/40s, ISO100.
3) Aperture priority, F8.0, 1/40s, ISO100.

And for all three configurations I get pictures of the same exposure. And then I tried the following configurations, indoors with auto-flash:

1) Shutter Priority, F3.2, 1/40s, ISO100.
2) Shutter Priority, F3.2, 1/500s, ISO100.

And I get pictures of same exposure as well!

Since exposure settings between the configurations are different, the only reason I can think of that results in even exposure for all images, is that the flash intensity compensates.

And assuming that my guess is correct, I remember using my EOS 300 on P mode, with internal flash. It was at F4.0 and 1/90s (flash sync speed?). When I tried to increase the F number to get deeper DOF, the shutter speed started flashing at 1/90s. Why can't the internal flash of the EOS 300 compensate for the larger F-stop then?

Would appreciate if anyone can share any explanations and insights on this. Thanks!

The internal flash usually has some sort of TTL or auto sensor which regulates its flash output by means of varying the duration of the flash. Longer duration = more exposure.

Flash exposure is only affected by the aperture and is totally unaffected by shutter speed. This is because flash duration is very, very short ( shorter than 1/1000). Which is why you don't get any difference when you vary your shutter speed while keeping the aperture constant.

In your other example, the camera will set the flash to the correct output for that particular aperture you are using. So exposure is the same as well.

I am not sure what the problem is for your EOS system. Canon flash system never fails to baffle me, they are rather, well, weird. However, it should warn you if your chosen aperture results in a shutter speed of greater than the flash sync speed.

Regards
CK

 

[Tweek]

Originally posted by ckiang


The internal flash usually has some sort of TTL or auto sensor which regulates its flash output by means of varying the duration of the flash. Longer duration = more exposure.

Flash exposure is only affected by the aperture and is totally unaffected by shutter speed. This is because flash duration is very, very short ( shorter than 1/1000). Which is why you don't get any difference when you vary your shutter speed while keeping the aperture constant.

In your other example, the camera will set the flash to the correct output for that particular aperture you are using. So exposure is the same as well.

I am not sure what the problem is for your EOS system. Canon flash system never fails to baffle me, they are rather, well, weird. However, it should warn you if your chosen aperture results in a shutter speed of greater than the flash sync speed.

Ahhh, I think I understand now. I always thought flash exposure is wholly determined by how strong the flash is, didn't know the duration contributes too. Thanks ckiang for your explanation!

Oh I still have one question: in that case, what's the definition of flash sync speed?

 

[roygoh]

Originally posted by Tweek


Ahhh, I think I understand now. I always thought flash exposure is wholly determined by how strong the flash is, didn't know the duration contributes too. Thanks ckiang for your explanation!

Oh I still have one question: in that case, what's the definition of flash sync speed?

Taken from "www.howstuffworks.com":

"Most SLR cameras use a focal plane shutter. This mechanism is very simple -- it basically consists of two "curtains" between the lens and the film. Before you take a picture, the first curtain is closed, so the film won't be exposed to light. When you take the picture, this curtain slides open. After a certain amount of time, the second curtain slides in from the other side, to stop the exposure. "

Then here's my explanation of the flash sync:

Since the curtains taken a finite amount of time to run across the film plane, at higer shutter speeds, the second curtain will have to be released before the first curtain has travelled entirely across the film plane. In this case, there will not be an instance when the entire film plane is exposed.

The flash is normally fired when the first curtain has travelled to the other side of the film plane. If the shutter speed is set too high, then at this moment the second curtain has already been released, thus covering part of the film. So when the flash fires, that part of the film covered by the second curtain will not be exposed. This is why when you set the shutter speed above the flash sync speed and take a flash shot, you will have part of you picture under-exposed.

Depending on the camera mechanism, the curtain travel speed is different. The faster the curtain can travel, the higher the shutter speed you can use with flash without part of the film being covered by the second curtain. The maximum shutter speed you can use on a camera is then called its flash sync speed. You can use any shutter speed lower than that to get a full-frame exposure by the flash.

Earlier SLR cameras have cloth curtains travelling horizontally across the film plane. The standard flash sycn speed is normally 1/60 s. I belief Nikon is the first to come up with the alluminum alloy leaf shutter mechanism, and the curtains travel vertically across the film plane. That is allowed higher flash sync speeds of 1/250s on cameras like the FE2, F801 and most of their latest models.

Hope this is not too confusing to read.

 

[ckiang]

For (consumer) digital cameras, I believe you can sync at any speed you want, if the cameras allow. I've tried up to 1/500s on my Coolpix 950 (the water drop in my avatar is taken at 1/500s with flash).

Regards
CK

 

[Tweek]

Originally posted by roygoh


Taken from "www.howstuffworks.com":

"Most SLR cameras use a focal plane shutter. This mechanism is very simple -- it basically consists of two "curtains" between the lens and the film. Before you take a picture, the first curtain is closed, so the film won't be exposed to light. When you take the picture, this curtain slides open. After a certain amount of time, the second curtain slides in from the other side, to stop the exposure. "

Then here's my explanation of the flash sync:

Since the curtains taken a finite amount of time to run across the film plane, at higer shutter speeds, the second curtain will have to be released before the first curtain has travelled entirely across the film plane. In this case, there will not be an instance when the entire film plane is exposed.

The flash is normally fired when the first curtain has travelled to the other side of the film plane. If the shutter speed is set too high, then at this moment the second curtain has already been released, thus covering part of the film. So when the flash fires, that part of the film covered by the second curtain will not be exposed. This is why when you set the shutter speed above the flash sync speed and take a flash shot, you will have part of you picture under-exposed.

Depending on the camera mechanism, the curtain travel speed is different. The faster the curtain can travel, the higher the shutter speed you can use with flash without part of the film being covered by the second curtain. The maximum shutter speed you can use on a camera is then called its flash sync speed. You can use any shutter speed lower than that to get a full-frame exposure by the flash.

Earlier SLR cameras have cloth curtains travelling horizontally across the film plane. The standard flash sycn speed is normally 1/60 s. I belief Nikon is the first to come up with the alluminum alloy leaf shutter mechanism, and the curtains travel vertically across the film plane. That is allowed higher flash sync speeds of 1/250s on cameras like the FE2, F801 and most of their latest models.

Hope this is not too confusing to read.

Very interesting, nope it's not too confusing. But then, in that case, flash sync speed will not be relevant in digicams that use electronic shutter?

 

[roygoh]

Originally posted by Tweek


Very interesting, nope it's not too confusing. But then, in that case, flash sync speed will not be relevant in digicams that use electronic shutter?

That's right. Flash sync is specific to focal plane shutters.

I believe the twin-lens reflex (hope I've got the name correct) cameras like the Seagull brand type has the shutter combined with the aperture mechanism. These cameras can sync at any shutter speed the camera is capable of providing. The aperture is normally closed. When the shutter is released, the aperture opens to the set value, and remains open for the duration of the shutter speed setting. In this case the film is always fully exposed regardless of the shutter speed setting. However, such mechanisms are not capable of very high shutter speeds. If I remember correctly, the max shutter speed on the Seagull is 1/800 sec. Anyone please correct me if I am wrong.

Thanks!

Roy