Free Shipping on ALL Products
camera reviewsbest cameraslens reviewsphotography tipscamera forumvideo toursphotography bookssupport me
It is currently Mon Sep 01, 2014 9:08 am

All times are UTC




Post new topic Reply to topic  [ 6 posts ] 
Author Message
 Post subject: Flash and TV / AV Modes
PostPosted: Fri Sep 19, 2008 7:33 pm 
Query on flash on my eos 40D (built in).

I have noticed that when I go to full auto or P mode and flick up the flash, I get different shutter / aperture than if I go to AV or TV mode and this is confusing me.

Eg just tried a sample photo (indoor at night) and Auto gives me 1/60 at F4. Now, when I switch to TV mode and select 1/60, the cam selects F1.8. If I close tha flash the cam then flashes at F1.8 indicating not enough light.

Sooooo.... looks to me like in TV/AV mode the flash provides some light, but not as much as full auto.

Bit confused by this, any comments appreciated :)


Top
  
 
 Post subject:
PostPosted: Fri Sep 19, 2008 9:21 pm 
In the AUTO mode, the camera can tinker with the ISO to keep a relatively fast exposure. I believe this is the case here: In 'semi-manual' modes such as Av and Tv, the ISO setting is under your control - and if you haven't bumped it up beforehand, it's presumably lower. Therefore, you can only get that 1/60 shutter speed at f/1.8 and ISO200 in Av or Tv mode, while the AUTO mode can basically crank the ISO up to 3200 to balance things out, and you get the same exposure albeit with a stopped-down diaphragm.

In the default E-TTL mode, flash power is determined by a number of factors, including the metered light level, distance to subject, focal length, desired aperture (in Av mode), desired shutter speed (in Tv mode) and of course, ISO setting. So when you are in AUTO mode and the camera has cranked up the ISO value, the flash does not need to fire just as powerfully, and it may not fire at all.

All in all, I believe you left the ISO at a relatively low value in Av or TV mode (e.g. 200), which prompted the camera to choose a larger aperture or a slower shutter speed, and eventually fire the flash. In the AUTO mode, the camera took over things, and cranked the ISO up to compensate for the light level, and the same exposure is achieved without firing the flash.


Top
  
 
 Post subject: Further tests :)
PostPosted: Sun Sep 21, 2008 9:53 am 
Thanks for that - after further testing the ISO was indeed being altered in full auto, but in P mode it definitely isn't as tried this morning.

So, P mode, ISO 200, 1/60 F4, switch to tv ISO 200, 1/60 and F1.8 ....

In fact I can crank right up to ISO1600 and still get 1/60 F4 in P, it's only when I go to ISO3200 the cam closes down the aperture a little...

There must be a sensible reason for this, if I understood it maybe I could make use of this behaviour :)
Thanks all


Top
  
 
 Post subject:
PostPosted: Sun Sep 21, 2008 10:51 am 
hairyfrog wrote:
So, P mode, ISO 200, 1/60 F4, switch to tv ISO 200, 1/60 and F1.8 ....


OK, now that sounds weird. Because the ISO is under your control in both modes, the exposure narrows down to aperture and shutter speed, at which point the two exposures above will be different.

In P mode, the camera meters the scene and decides on the exposure all by itself, so unless you applied exposure compensation, 1/60 at f/1.8 is the 'almost' correct exposure. In Tv mode, shutter speed is under your control, but the aperture compensates for that. Therefore, the camera should have selected f/1.8 or a nearby value, not stop down to f/4, that's more than two stops slower.

So, check the exposure compensation value in both settings, also check metering (i.e. if you are using spot metering milimeter changes to the composition can cause this much difference). Under the same ISO, metering, exposure compensation settings, the same composition should yield the same exposure no matter what mode you use.


Top
  
 
 Post subject: more stuff
PostPosted: Mon Sep 22, 2008 6:13 pm 
Quote:
Under the same ISO, metering, exposure compensation settings, the same composition should yield the same exposure no matter what mode you use


The same exposure yes, but only the same aperture and shutter speed if the flash is giving out the same amout of light each time surely?

Couldn't the flash power be what's being varied (the experiment with changing ISO up to 1600 would indicate this no?)
Cheers


Top
  
 
 Post subject:
PostPosted: Tue Sep 23, 2008 10:22 am 
Sure, but you pop up the flash manually in those modes, and I assumed you didn't. My bad.


Top
  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 6 posts ] 

All times are UTC


Who is online

Users browsing this forum: Yahoo [Bot] and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to:  

All words, images, videos and layout, copyright 2005-2012 Gordon Laing. May not be used without permission.
/ How we test / Best Cameras / Advertising / Camera reviews / Supporting Camera Labs

Webdesign by Alphabase IT
Powered by phpBB® Forum Software © phpBB Group