different times for same aperture

[skronwith]

I am photography and indoor shot with the flash. In aperture priority mode, I set 5.6 and the camera gives me a time of 1/3 of a second. If I swith to P mode, and shoot the exact same scene, the camera automatically sets the aperture to 5.6 but chooses 1/3 of a second. Why such a huge difference given that the apertures have been set to the same amount? Thanks

[rfreschner]

I set 5.6 and the camera gives me a time of 1/3 of a second

the camera automatically sets the aperture to 5.6 but chooses 1/3 of a second

Am I missing something? These are the same values, so where is the difference that you are speaking of?

[tim]

What happening is the flash sync speed comes into play. I don't remember enough about it to tell you, but have a read about sync speeds in this thread (http://photography-on-the.net/forum/showthread.php?t=46599), it should tell you everything you need to know. I'll read it too, I should know this stuff.

[skronwith]

I'm sorry, the P mode sets the timing to 1/30 th of a second, not the 1/3 as in the aperture mode.

[tim]

I think P mode automatically changes the ISO too. I don't think flash sync does come into play after all.

[pcasciola]

In Av mode, the camera meters for available light, and the flash is only used for fill. That's why you are seeing the big difference.

[tim]

So P mode knows about the flash and factors it into the exposure time it'll use? I've never used P mode...

[rfreschner]

So P mode knows about the flash and factors it into the exposure time it'll use? I've never used P mode...

I've never used P mode either, but according to the manual, yes it sets the shutter speed and aperature for your flash exposure. It says that the shutter speed will be set from 1/60 to 1/250 sec.

[Mikelite80]

FYI...In P mode you control the iso.

[pcasciola]

So P mode knows about the flash and factors it into the exposure time it'll use? I've never used P mode...Yes. It's a little odd. I never use P mode either, but it took me a little while to figure out the difference between Av/Tv and M even, which also behave completely differently.

Have a look here: (Link should bring you right to #10 of the top 10)
http://photonotes.org/articles/eos-flash/#faq10

[PacAce]

So why is everybody assuming that a flash was in use? :confused: (BTW, if the flash were being used, the slowest shutter speed would be 1/60 in P mode, not 1/30.)

At any rate, without the flash in use, the Av and the P modes should basically give the same shutter speeds for the same lens aperture. The only thing I can think of that will change that (short of there being something really wrong with the camera) is that the two shots were not really the "same" even though they were shot in the same room. For example, if the areas under the focusing point are different in reflectivity or maybe the room lighting was with one of those florecent(?) lights that always seem to fluctuate in intensity although it's not really visible to the naked eye, then that could account for the difference in shutter speeds. Without the EXIF and the actual images, we'd only be speculating at the cause of the discrepancy between the two shots skronwith is talking about. :)

[DavidEB]

I think this is documented in the camera manual, at least for the rebel and 20D.

In Av mode, the camera sets the shutter speed to correctly expose the background in ambient light(regardless of the flash on main subject). Therefore, indoors you get very long exposure times. If you're not using a tripod this can be very annoying. The 20D has a custom function to prevent this behavoir.

In P mode the camera does the same thing but limits the shutter to prevent excessively slow shutter speeds, hence it caps out at 1/60, or maybe 1/30 on whatever kind of camera you have.

[PacAce]

I think this is documented in the camera manual, at least for the rebel and 20D.

In Av mode, the camera sets the shutter speed to correctly expose the background in ambient light(regardless of the flash on main subject). Therefore, indoors you get very long exposure times. If you're not using a tripod this can be very annoying. The 20D has a custom function to prevent this behavoir.

In P mode the camera does the same thing but limits the shutter to prevent excessively slow shutter speeds, hence it caps out at 1/60, or maybe 1/30 on whatever kind of camera you have.
Do we know for a fact that skronwith was using a flash? And the cap for flash in P mode is 1/60, not 1/30 for all the Canon cameras that I'm aware of.

[tim]

So why is everybody assuming that a flash was in use?

I am photography and indoor shot with the flash.

It's in the first post.

[hmhm]

With life expectancy on the rise, as photographers we now have a glimmer of hope of someday eventually achieving a thorough understanding of the Canon flash system.

This helps:
http://photonotes.org/articles/eos-flash/

The short answer is that when you enable the flash, there are now 3 elements to exposure that the camera selects: aperture, shutter speed, and "how much flash". The camera only tells you its choices for aperture and shutter speed.
-harry

[PacAce]

It's in the first post.
Maybe you're right, although I didn't read it that way. But to be honest with you, I didn't know exactly what that first sentence was trying to say. However, if he was shooting with a flash, then how is that in P mode, the shutter was at 1/30? It should have been at 1/60. I guess that's what threw me off.

[Titus213]

With life expectancy on the rise, as photographers we now have a glimmer of hope of someday eventually achieving a thorough understanding of the Canon flash system....
-harry

You assume that with age comes wisdom? Sometimes old age arrives all by itself....:lol: