Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Canon EOS dSLR

Reply
 
Thread Tools Search this Thread
Old Nov 4, 2004, 1:21 AM   #1
Member
 
Join Date: Jul 2004
Posts: 38
Default

Hi there,

I was doing a little test shooting with my 20D (using in-built flash and a tripod and constant lighting) and came up with some unexpected results. Can someone help me understand the following:

Why does the camera set shutter speed in Av mode to a different level to when in Potrarait or Auto mode, when using the flash? Note all variables were the same except shutter speer(auto set), sharpness and contrast parameters. I only epxerience this when using the flash and can reproduce always (usually with more extreme results). If you look at the reduced sized pics you can see that the first picture is slightly more exposed.

Any ideas?


---------------------------------------
File Name
IMG_0009.jpg IMG_0010.jpg IMG_0011.jpg


Shooting Mode
Aperture-Priority Auto Portrait


Tv( Shutter Speed )
1/40 1/601/60


Av( Aperture Value )
4.54.54.5


Metering Mode
Evaluative Evaluative Evaluative

Exposure Compensation
0 0 0


ISO Speed
400400400


Flash Exposure Compensation
0 0 0


White Balance Mode
Auto Auto Auto


Parameters Settings
Contrast Standard Mid. High Mid. High
Sharpness Standard Mid. High Mid. High
Saturation Standard Mid. High Mid. High
Color tone 0 0 0




Attached Images
 
timmyB is offline   Reply With Quote
Sponsored Links
Old Nov 4, 2004, 5:56 AM   #2
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

IMO it's not Av, but your metering mode is the culprit: Evaluative.
This mode "evaluate" the scene based on several areas and certain round off in computations may result in a slight difference... especially if you let the camera chose its own AF point where the metering areas around it may shift.

Try to switch to Partial or Centerweighted metering and the results will be more consistent...


:idea: -> BTW you can also put the camera in manual: The camera will automatically switch to Centerweighted metering and you can fix both the shutter and aperture with flash.
NHL is offline   Reply With Quote
Old Nov 4, 2004, 6:43 AM   #3
Member
 
Join Date: Jul 2004
Posts: 38
Default

sounds like good advice NHL.

I will have a play with the metering modes and manual modeand see what happens.

cheers
timmyB is offline   Reply With Quote
Old Nov 4, 2004, 8:19 PM   #4
Member
 
Join Date: Jul 2004
Posts: 38
Default

I just did a few more tests and still am a little confused as to why the metering is different in 'Creative' modes (eg Av, Tv, M) than in 'Basic' modes (eg Portrait, Auto).

What I found was:

- When using a flash the camera was wanting to expose the shotssignificantly more in Creative modes than in Basic modes. These photos also looked more correctly exposed in my test shots.

- Yes the metering mode did hava a small effect but only small (i.e. not enough to account for the difference.)



Does anyone have any idea why camera will meterand recommendhigher exposure in Creative modes than in Basic modes?




timmyB is offline   Reply With Quote
Old Nov 5, 2004, 6:28 AM   #5
NHL
Senior Member
 
NHL's Avatar
 
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,547
Default

timmyB wrote:
Quote:
I just did a few more tests and still am a little confused as to why the metering is different in 'Creative' modes (eg Av, Tv, M) than in 'Basic' modes (eg Portrait, Auto).
-> Very simple explanation: In the 'Basic' mode, all exposures are done with 'Evaluative' metering only (ie the randomness of the segments selected couple with the AF); Whereas the 'Creative' mode let you pick fixed patterns like 'Partial' or 'Centerweight'

The 'Evaluative' metering has been archille's heel with the Canon's flash system: http://www.stevesforums.com/forums/v...42&forum_id=37

You just have to understand how it works that's all...
NHL is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 12:21 PM.