'Real' Point To "Program" (P) Mode

How do you adjust the exposure? Since I have only read the manual 3 times in the past 2 weeks, give me some help please

Go to a party, put the flash on put the camera in P and have a coctail.

That's all true. But Av and Tv mode work exactly the same way. Spinning the main dial has the same effect, also keeping the exposure "correct".

You obviously understand the effect of aperture on DOF and have a preference, depending on the photo you're taking. If you're going to use the "program shift" feature on every shot and override the defaults P mode chose for you, then you might find Av to be easier. Instead of worrying that P mode reset your 8.0 aperture back to 4.5 between shots, forcing you to spin the program shift dial again, you can be confident your aperture won't change in Av.

There are a few caveats, but basically I think P, Av, and Tv work the same way: by spinning the main dial you can get any combination of shutter and aperture you want, for the calculated auto exposure. The only difference is the initial settings for each shot, before you spin the dial. Av remembers the aperture you used last time. Tv remembers the shutter speed you used last time. P does something pretty random each time (well it's not quite random, but close enough for me). But once you start spinning that dial, it's hard to tell the difference between the modes.

P mode is useful when you have CFn 4-3 or 4-1 activated, and you need to pass the camera because you need to be in the shot. In these cases, just switching to P mode cancels the Cfn 4-1 and reverts back to normal “shutter release button focuses and meters. The camera picks the default exposure settings and tries to get a reasonable shutter speed and aperture that is pretty idiot proof. You don’t have to worry about the shooter holding his breath, or trying to freeze before releasing the shutter if you intentionally set up the shot for a slower than normal shutter speed. E.g. if you want nicer ambient light and your chosen settings mean shooting with a 1/30 or 1/15 shutter speed, passing it to a non-photog could mean trouble. P mode then offers a quick temporary point and shoot setting.

P mode also works well when using a ETTL flash. For those who don’t want to screw up an indoor event, P mode always works well.

The auto mode defaults to auto flash, and sometimes you just prefer available light.

The above is, unfortunately, mostly incorrect - at least in the case of a 20D. If it's true for other Canon models, I apologize, but I'd be surprised. I think you may be confusing P with the green-square mode. All custom functions are retained in P mode (English manual page 148, and confirmed on my 20D itself). Also, remember exposure compensation as also retained - another potential "gotcha" for the person you handed the camera to.

Green-square mode, on the other hand, is a great setting for handing the camera off to somebody. JPEG quality and red-eye reduction are basically the only two settings the camera pays attention to.

I agree here, though it depends on the way you want your picture to look. ETTL does a nice job of exposing the subject. P tends to keep a shutter speed of 1/60" or faster, so you are unlikely to get motion blur or camera shake at most focal lengths. The only issue for me is that at lower ISOs and most indoor situations, the background is going to tend to be very very dark. If you don't like the dark background look, you'd want to be shooting at a higher ISO or using a different shooting mode than P.

Very true - this is a good reason for using P instead of Green Square. In fact, just yesterday a nice man offered to take a picture of me with my family using my 20D. It was outdoors but a bit overcast. I dutifully switched the 20D into Green Square mode and explained that he just needed to point and push the shutter button. But gosh, was he surprised when the flash popped up and bopped him on the nose.

so in P mode, how does the flash differ from Av or Tv? is it just the same 'expose for the overall and fill flash', but with a minimum starting shutter speed? so less nasty surprises than Av mode when in poor light?

I've set mine up so that it always uses 1/200 as the shutter speed in Av mode, but thats sometimes way too fast, so if P mode provides a compromise that may be a way forwards

In P mode, the slowest shutter speed will be 1/60 which will give you a little more ambient lighting than 1/200 does. But then, you can also get a little more motion blur of your subjects if they are moving.

Now I am confused. I have a 350d with the kit lens. Sitting indoors with the flash popped up with ISO 400, I have the following:
In P mode the camera sets 5.6 and 1/60. I cannot alter the speed or aperture with the jog dial
In Av mode I can go from 4 sec exposure with aperture 36, to 1/10 sec with aperture 5,6
In Tv mode the range of settings is 8 sec with aperture 36, to 1/10s with aperture 5.6. OUtside of this range the aperture is flashing

According to the manual I should be able to use Tv mode to set the speed to say 1/200 and the camera will set the flash aperture to the correct value. Why is it then that at 1/200 the shows me the aperture as 5.6 but flashing to imply that it is incorrect?

If there is another thread explaining this, please point me there!

Thanks

TudorDoc

In Tv and Av modes, the camera will meter to expose for the ambient lighting. In the room you are at, you need 1/10 @ f/5.6 to get a good ambient light exposure. When you increase the shutter speed to 1/200 in Tv mode, you are decreasing the amount of light used for the exposure. To maintain the correct exposure then, the camera will need to increase the aperture opening but it's already at it's max of f/5.6. And that is the reason the aperture is flash. To warn you that you are going to be underexposing as far as ambient light is concerned. If you are going to be using the flash as your primary light source, you can pretty much ignore the flashing aperture and shutter speed in Tv and Av modes if you want.

Thanks - I am getting there I think!
Next question is regarding ISO. If the flash (420EX) adjusts the exposure to be correct, why is that when I shoot on Tv mode at ISO100 the image is darker than when I set ISO400. What I am trying to achieve is how to shoot with flash, at ISO400 at 1/200 to avoid camera shake/blur - and ensure the correct exposure

Thanks in advance
TudorDoc

Until I had decent lenses where I really wanted more control over DOF or a particular shutter speed I used P all the time and program shifted where necessary..