Program Mode vs Shutter Priority?

[Jesse17]

My photography club is putting on a free camera/photography workshop for beginners just learning how to use their DSLR. I was just working on my presentation about the exposure triangle and the M A S P modes when it occurred to me I don't really know what Program mode does exactly. I tried looking it up and it seems like it's just auto mode, but it adjust the aperture by letting you adjust the shutter and doing the opposite number of stops on the aperture. That makes sense to me, but I can't figure out how that is any different than using shutter priority in the first place.

Is it just that the camera pics a more artistic aperture/shutter combination for it's starting place than Auto would have? Or is it something like the camera takes more responsibility at guessing if it's a night shot or bright snowy shot, that shouldn't be exposed to a middle gray like auto would try to do?

I'm just not understanding what is different about using P instead of S/Tv. Can someone explain it to me? Thanks!

 

[ac12]

A vs. P also depends on the camera and brand.
On the Nikon (D70 and D7200) here are a couple differences that I can think of. I don't know the particulars for other brands or models.

AF; A = closest subject, P = you get to choose what to focus on

This was the killer item for me.
Example, take a picture of a group at a dinner party. A will focus on the closest subject/object, which could be the dinner table, not the guests on the other side of the table. IOW, the camera will focus on anything between you and the real subject. So for me, worse than useless, because the WRONG thing will be in focus.

There are other AF differences, but the "closest subject" killed it for A mode, and made any other comparison irrelevant.
​

Flash; A = automatic, pop-up is used, P = you get to choose to use the pop-up or not.

Since I use an external flash, I do NOT want the pop-up flash to pop-up.​

In P vs. S.

In P, the camera will make the first selection of aperture and shutter speed, and either or both can change as the lighting changes.

In S, YOU specify the shutter speed, and it is LOCKED down.​

 

[TCampbell]

It helps to understand how "Automatic" mode comes up with its exposure decision before understand how Program mode is different.

In both Automatic and Program mode, there's something called the "program line". Here's a Canon article that touches on it. In that article there are a couple of diagrams to look at.

Shooting modes - Canon Professional Network

In the diagrams, you'll see a red line that starts low and is horizontal to a point, then takes a diagonal turn up, then hits a plateau where it goes horizontal again. That's the "program line". Along the bottom you'll see the shutter speeds (aka Tv or Time value). Along the right you'll see the aperture values (aka Av). But along the top you'll see the Ev's (Exposure values... and these are represented by diagonal black stripes). Every exposure combination along those black lines represents the ability to take an "equivalent" exposure).

Look at the bottom graph (the more interesting of the two). This is the "program line" of an f/3.5-4.5 lens. Notice the program line starts at the lower left at the f/3.5 value... the wide-open aperture available on that particular lens. So when lighting is poor... it'll use the widest possible aperture (lowest f-stop). It remains there until the shutter speed gets to somewhere around 1/50 sec... and then suddenly the camera is willing to start stopping down. It will stop down more and more as more light is available. But at this diagonal section of the program line, it's basically splitting all extra light by giving half the light to increases in f-stop values (smaller apertures) and half the light to increases in shutter speed.

It continues to do this until it reaches f/22... (possibly the minimum aperture for that lens) and from there on it will just increase the shutter speed only.

If you were to use that particular lens in full Auto mode, that's how the camera would pick the exposure.

ALSO... that's the exposure it will naturally pick if you use Program mode.

But up to this point, we've ignored those diagonal black stripes created by the "Ev" values.

It turns out in Program mode you can do something called "Program Shift". If you look at those diagonals, all the exposure combinations that fall along those lines are "equivalent". Look at the lower diagram (the one for the EF 24-85mm f/3.5-4.5 lens). I'll call your attention to the point where the Program Line passes through f/8 at 1/250th second. Notice the diagonal Ev line is indicating Ev 14.

At this point I should take a side-bar to mention what Ev's are. Ev was a simple way to map all equivalent exposures based on a table and a very simple formula that says "Ev = Av + Tv". But each "Av" and "Tv" value was mapped to a simple integer.

The baseline for the Ev table was that Av of f/1.0 is mapped to 0 and each "full" stop increases the Av value by 1. So f/1.0 is Av = 0. f/1.4 is Av = 1. f/2.0 is Av = 2. f/2.8 is Av=3. f/4 is Av=4. f/5.6 is Av=5 and so on.

The same is done for the Time values (Tv, aka shutter speeds) except it starts at 1 full second. 1 sec is Tv=0. 1/2 sec is Tv=1. 1/4 sec is Tv=2... and so on.

Why do this? Because if your light meter says you have "Ev = 14" worth of light, you can use ANY combination of Av & Tv values as long as their integer values add up to 14 ... and you'll get a decent exposure.

If you made a table of Av & Tv values... the Ev values would all fall on diagonals (those are all "equivalent" exposures.)

Back when I did weddings using the Hasselblad camera, the lens had both the shutter speed ring and the aperture ring ON the lens itself (the shutter was in the lens on this camera... it used a leaf shutter in the lens instead of a rolling "curtain" shutter back at the focal plane). And of course it also had the aperture ring in the lens. Since the two rings were adjacent, you could lock them together at an "Ev" value. So when you adjusted those two "locked" rings, it would shift both the aperture and shutter speed together to maintain the same Ev. We really could use a light meter to take the Ev reading, lock the rings on the lens at the prescribed Ev value... and adjust as needed. This was old-school "program shift" before cameras went digital.

So I'll end the side-bar and get back to the normal explanation.​

This means that for any light meter reading, there are many possible combinations of aperture and shutter speed that will collect the same equivalent amount of light.

And this is where Program mode comes in....

In full Auto mode, you are locked out of most changes. But in Program mode, the camera will offer you an initial exposure... but you can perform "Program Shift" to slide to different Av/Tv combinations that collect the SAME amount of light but using different settings.

If you look through the viewfinder... while you meter the shot, you'll see the proposed exposure based on following the "Program Line" (this line is different for every lens... based on the possible aperture values available on that lens as well as based on the focal length of the lens (or the focal length you've selected if it's a zoom lens.) But since you can see the proposed exposure through the viewfinder... if you think the shutter speed should shift higher... or the aperture should shift to provide greater depth of field ... you can just shift the exposure and the camera will move BOTH the shutter & aperture together to the next "equivalent" exposure.

In full-manual mode, you'd have to adjust each of those ... but in full manual you could move to exposures that are not "equivalent" (to increase or decrease the exposure). In Program mode you could do that too... but you'd have to use the Exposure Compensation (EC) control to make that change.

This makes Program mode start at just like full Auto mode... except unlike Auto mode where you are locked out of most changes, in Program mode you are not locked out... you get the camera's suggested exposure but you can shift or compensate as you want.

BTW, I tend to tell people that the "program line" tries to find a "safe" exposure. It is worried that if it just kept everything at max aperture, you might not like the depth of field. But if it went for max depth of field (e.g. f/22) then you'd never be able to hold the camera steady enough based on the length of the shot. So it's trying to strike a balance... to get an exposure that will work based on the amount of light it has (of course if there's not much light to work with... you an push it into a corner where it simply has no choice but to offer an exposure that's probably not realistic (since the amount of light isn't much to work with.)

One more side-bar... notice that ISO isn't part of this? Keep in mind that on a digital camera, ISO isn't truly part of the exposure at all. The imaging sensor has some given level of sensitivity based on it's design. You can dial in aperture and shutter speed settings... and that will control how the image is exposed on the sensor. Then the shutter closes and the image is complete. Notice there was no ISO involved?

AFTER the image is exposed... the camera will apply some "gain" or "amplification" to the image... THAT's where ISO comes in. But this happens only after the image was captured. It controls how much gain is to be applied. It's a post-processing step performed in the camera.

You'll get a less-noisy image if the non-ISO part of your "exposure" didn't require applying any amplification.

​

 

[ac12]

Thanks Tim,
That is a neat explanation of how the camera determines the aperture and shutter speed.

 

[Designer]

I was just working on my presentation about the exposure triangle ..

For the "advanced" presentation, you can show the club that ISO has no bearing on exposure, only the display.

 

[Derrel]

Jesse17,
The biggest difference is that in Program mode, the camera itself makes the first decision, based on light level, and focal length and ISO value, as to what the "best guess" is for the conditions. With longer lenses, the program's baseline will tend toward faster shutter speeds; with shorter lenses, there's less immediate need for fast speeds. "P-mode" can also be considered "user-shiftable programmed auto" mode, since one control wheel can be used to shift the pairing of f/stops and shutter speeds, in lock-step; P-mode is very rapid for making changes! One control wheel can shift the aperture and shutter speed simultaneously! Very handy!

Shutter-priority mode differs from P-mode in that the USER sets the initial shutter speed that is desired, and the camera matches that speed up with the appropriate f/stop value. This is very useful when a specific speed is desired, such as when panning, or when photographing aircraft or helicopters, or vehicles or athletes in motion, or for any subject when a specific, exact shutter speed is desired, or when there is a specific "floor" below which the speed must not go, such a say, 1/1000 second for baseball batters, or say 1/125 second for horse-racing panning shots, when you want a specific amount of motion blur, but not "more blur".

 

[keen.observer]

Manual Exposure forces you to set both Aperture, and Shutter Speed. Aperture Priority lets you set the Aperture, while the camera sets the Shutter Speed. Shutter Priority lets you set the Shutter Speed, while the camera sets the Aperture. Program lets the camera set both the Shutter Speed, and Aperture...according to a program you select. A Sports Program will use a fast Shutter Speed, and consider the Aperture secondary. A Portrait Program will use a wider Aperture, and consider Shutter Speed to be secondary. One program may be a Shutter Priority mode, while another Program is an Aperture Priority mode. The differences between Program and A, or S, is that A, and S, let YOU set the Aperture, or Shutter Speed, as you wish, and P-Program-lets the camera decide the settings.