Differences in modes on 40D

topcatproduction

topcatproduction

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Hi, sorry if this seems a really stupid question but I have had a blank on it in my head! I normally shoot in Aperture priority, and last night (and day) whilst shooting in the fairly dim house I was getting a lot of hunting and very poor results, using my 70-300 is usm, built in flash (forgot speedlite). However if I turned it to auto, it would use the flash to focus and would do so quickly and accurately. It would also use a much lower ISO than I was using and still be brighter, with a much quicker shutter speed!

So I suppose the questions are:
Why on AV mode does it not use the flash to burst to focus, instead of hunting and not focussing?

How does it come up with a much nicer brighter picture on auto than AV with same aperture (4-5.6), lower ISO (400 instead of 8-1600), same custom settings, and quicker shutter (1/50-1/60 instead of 0.5)!

I haven't messed around with tonnes of settings and normally know what I am doing but this makes no sense to me! :thinking:

Uploaded a couple to see the difference, EXIF data should still be there.

AV mode:
IMG_9676.jpg


Auto mode:
IMG_9677.jpg
 
Think you probably have the focus assist disabled in the menu, but it always fires in auto mode no matter what if required
 
In Av mode the camera assumes that the ambient lighting is important. It determines the exposure from the ambient light, and then adds a bit of fill-in flash. In your photo it's chosen a shutter speed of 1/2 for the ambient light, which has caused camera shake. And the ambient light is illuminating the subject quite adequately, so the fill-in flash hasn't added much.

In P mode the camera assumes that the flash is the dominant source of light. It ignores the ambient light and always sets a shutter speed of at least 1/60 to avoid camera shake.
 
Last edited:
StewartR said:
In Av mode the camera assumes that the ambient lighting is important. It determines the exposure from the ambient light, and then adds a bit of fill-in flash. In your photo it's chosen a shutter speed of 1/2 for the ambient light, which has caused camera shake. And the ambient light is illuminating the subject quite adequately, so the fill-in flash hasn't added much.

In P mode the camera assumes that the flash is the dominant source of light. It ignores the ambient light and always sets a shutter speed of at least 1/60 to avoid camera shake.
Click to expand...
That makes sense- so the flash is not as bright when it assumes ambient light is more important? I'm still suprised that the first pic is not overexposed considering everything apart from a dimmer flash. Never used P mode I'm afraid, and have forgotten what I read about it- is it something I should look into using more?

dentedshed said:
Try this:

Menu > C. Fn I:Exposure > option 7 (Flash sync. speed in Av mode) > 1/250sec (default is auto)
Click to expand...
Do you mean check that the flash will fire under 1/250 sec?

tijuana taxi said:
Think you probably have the focus assist disabled in the menu, but it always fires in auto mode no matter what if required
Click to expand...

I thought that but just tried it in similar conditions and it used the flash to focus so not sure why it didnt before?!
 
Sorry I should've been clearer. If you follow my setting above, the camera will take a pic at 1/250 when the flash is up or an external flash attached. Basically you are overiding the cameras ability to choose shutter speed.
 
topcatproduction said:
whilst shooting in the fairly dim house I was getting a lot of hunting and very poor results, using my 70-300 is usm, built in flash (forgot speedlite). However if I turned it to auto, it would use the flash to focus and would do so quickly and accurately.
Click to expand...

If you are using AI Servo in AV mode you won't get focus assist from the flash.

topcatproduction said:
Or does it mean that when the flash is up the slowest shutter it will use is 1/250?
Click to expand...

Correct.
 
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