A stupid question

[jakub_ozimek]

OK, here comes one of my stupid questions, hope I don't get banned or anything:

The way I see it, being a complete layman, in a given scene lit by a given

available light and shot at a given ISO speed there are a few possible

time/apperture settings that will result in correct exposure [that is, in my

layman language, the little arrow showing zero. [Not to mention that when you

change ISO to 'Auto' the possibilities expand, don't they?] How does the camera

decide which one of these to choose when I'm in the P mode? Is there a way [i'm

using Canon 40d] to discard a setting chosen by the P-mode and make it come up

with a different one? [i know there are still A and T modes if I'm after a

specific time or apperture setting, but can't I not be and still get a few

options with P?]

[alan_ginman]

Not a stupid question at all. I think I know what you mean. I love the freedom that the P mode gives me and I use it quite a lot. On my camera (5D) it tends to go for somewhat shorter shutter speeds and wider apertures than I would have chosen. I can see why it's doing it, as it's generally better to do that than to have too slow shutter speeds.

 

Sorry, I don't think there is an answer to your question, but I thought you might like to know that you are not alone.

 

Regards

 

Alan

[sattler123]

It is whatever Canon thought was right when they programmed the camera and that's where the problem comes in. Why would you let the camera make such important decisions for you? OK, if you were using a P&S MAYBE I could understand. But both of you are using very sophisticated cameras (5D and 40D) and you use them mostly in "P" mode? I honestly think that you are waisting the capabilities of your equipment. At the very least you shoud learn and use Aperture Priority and/or shutter priority.

[Matt Laur]

Most modern cameras have a database of "scenes" to which they compare what they're seeing as they meter. When you put the camera on cruise control, it's looking at focal length, the appearance of hot-spots (like the sun) in the shot, etc. There are many clues that the software in the camera can follow to make a guess about whether to err on the side of a wider aperture or a slower shutter speed. If a camera provides for "sports" mode or "portrait" mode, that's just a way of helping it decide how to interpret the light its dealing with, and to prioritize for you.

 

I happen to be a Nikon guy. As someone who also writes software, I'm always intrigued by what my camera body decides to do when left up to its own devices. I always leave my D200 in "P" mode when it's just sitting in the bag... the better to simply grab and shoot in an unexpected situation... and it's what my wife calls "wife mode." In programmed auto mode, it seems to tend towards a moderately wide aperture (say, f/4 - 5.6, even when it could open up to 1.4 or 2.8) and reasonably quick shutter speed. I don't usually shoot higher than ISO320 unless I must, and I don't let the camera make that call, generally. But if I have ANY time to consider the shoot, I find that I'm in aperture priority mode most of the time - it's almost always the approach that gives me the control I'm after.

[jakub_ozimek]

Told You all it was a stupid question :) No, I don't usually shoot P mode, but A mode, that's what my camera is set to 'when in the bag' - it's just a question out of pure curiosity. As for the 'database of scenes' - my 40d has it and, as far as I know, 5d doesn't. But these modes are mutations of the 'full auto' mode: you only handle your lens [if it's a zoom lens, that is] and the shutter button - everything is decided for you, you cannot even shoot RAW, or decide whether you want flash or not. In the P mode you can decide if you want flash, what quality of picture to save plus you can override the exposure with a dial on the back of the camera

[zml]

What the camera determines is EV (exposure value) and sets the "best" shutter/appreture pair to match it at a given ISOE (ISO equivalent) setting. The exact values for shutter and appreture are based on the camera "program bias", i.e. the guesses taken by the designers what is "best" shutter/appreture at a given EV (i.e. for a given quantity of light.) This can actually be a very simple firmware code (as it was in many pre-computerized cameras) or a fairly complex program. In new(er) cameras, the "program mode" checks also for the lens type, max. f/stop, etc.

 

 

Since each EV can be expressed in many shutter/apreture combinations(say, 1/125 & f/5.6 = 1/250 & f/4 = 1/30 & f/11, etc.) program can be easily "shifted" by the user (i.e. instead of 1/125 & f/5.6 selected by the camera, the user can select any appreture/shutter combinations that result in the same EV ) using the "shift button.

 

Some camera manuals were/are actually including the program table together with the algorithm for built-in "bias." The concept of EV is older than dirt, BTW, and not only light meters but many a camera (Hasselblad, Rollei, ec.) could be set this way.

[matt_needham]

"As for the 'database of scenes' - my 40d has it and, as far as I know, 5d doesn't. But these modes are mutations of the 'full auto' mode:"

 

He's not talking about the icon or scene modes (portrait, landscape, action, etc...). He's talking about data in the camera. The camera manufacturers have been describing it as "scenes" programmed into the cameras since the 1990's. I sold cameras back then; I remember that the Nikon N-70 was programmed with hundreds or thousands of "scenes" that when coupled with the matrix metering (or whatever Nikon calls it) insured perfect exposure every time, or so the sales literature read.

 

Somewhere, probably on the Canon website, you should be able to find information on how the camera chooses it's settings when in the fully auto modes. Or just do some testing of your own. For a given amount of light and a particular ISO there are at least as many potential exposure combinations as whichever you have less of: aperture settings or shutter settings. I have always assumed that in the full auto modes the camera was taking focal length into account to determine a minimum safe hand held speed.

[jason_hall4]

Read page 85 of the manual. P mode is shiftable with the control dial. True of all Canon SLR bodies.

 

Jason

[jeremy_podolski]

If you have the time (i.e. you are not photographing an elusive target), maybe the best solution to your problem is to meter in P mode and see what it tells you is a proper exposure. Then quick switch over to M, knowing you can adjust proprtionately. For example, if the camera gives you 1/200 at f/4 and you want more depth of field, you can switch to M and close down your aperture by a stop while decreasing your shutter speed by a stop. You probably know all this, but I don't thikn there's a way to tell your camera to "think" a certain way in P mode.

[jason_hall4]

From the manual...

[michael.gregory]

Jakub,

 

while looking through the view finder ata scene half press the shutter button so that the speed and aperture reedings are displayed at the bottom ofthe viewfinder window. then move your right index finger back from the button towards the body of the camera until you feel that little scroll wheel. Twiddle that a bit and watch your exposure values shift. the exposure will remain the same, ( meaning for every one stop increase in aperture there will be a corresponding one stop decrease in shutter speed and vice versa). The camera will try to hit the middle of the available range of settings based on lots of things including which lens is mounted. But once the middle value has been decided for you, its just that easy to override it.

[jason_hall4]

Jeremy, that would completely defeat the purpose of M mode. Also P mode is shiftable to "make" it give you the aperture that you want. The M mode also keeps the meter active so you can adjust Shutter and Apature to your hearts content and the meter will tell you where the exposure is going to land. No need to go back and forth between modes. :o)

 

Jason

[jason_hall4]

Or is it that you are looking for Exposure Compensation while in P mode. That is a completely different subject. That is were you are trying to change the actual exposure of the image and not just select a particular Shutter or Aperture.

 

Jason

[stevepamp]

Or... spend a little time learning about exposure and do it yourself. I never trust the computer in the camera with any decisions. Only you (as opposed to some programmer over at Canon) know what you want the photo to look like.

[jakub_ozimek]

Jason and Michael, thank You very much, this is precisely what I was asking about! The funny thing is I did read the entire manual - while I was waiting for the parcel with the camera I downloaded the pdf and read it, just to ease the pain of waiting - and I must have not noticed or forgotten this piece of information. Once again, thanks!

 

And Matt, I didn't [obviously :)] know about the 'scenes' inside the camera. BTW, the 'perfect exposure every time' part is clearly taken from an ad brochure :)

 

All in all, it did turn out to be not so stupid of a question after all since some of the replies it got implied not everyone is aware of the 'Program Shift' feature

[jason_hall4]

As they say, the only stupid question is the one you did not ask.

 

BTW, its an easy thing to miss as it is not part of the regular instruction but rather a note at the end of the section.

 

Jason :o)

[mendel_leisk]

I'm not sure if anyone has mentioned this yet:

 

In P mode, while metering is active, I believe rolling the small top wheel will shift the combination of aperture/shutter speed, maintaining "proper" exposure It might be the power switch has to be in "hockey stick" postion (not sure, rarely if ever use this).

 

The big wheel at the back is for exposure compensation, either negative or postive, a different adjustment. You *do* need power switch in "hockey stick" position for this to work.

[sarah_fox]

In case nobody else gave you this answer:

 

Look in the back of the manual. There is a table that tells you the aperture and speed selected for each EV in the program mode. That will tell you exactly how the camera decides.

[luisarguelles]

Use the camera in manual mode for a while. That will teach you a lot about

photography.