How do cameras expose automatically?

I am wondering about how cameras choose the exposure automatically. So you put in 400 speed film. Does the camera take as a base point the lowest shutter speed and then choose the highest F stop against it or... there must be a bias towards higher F-stops, no? Rather than a bias towards high shutter speeds.

I am just thinking about street photography. For me, I always want the highest F-stop possible so everything will be in focus. I don't care much if it is 8, 11, 16 as long as it is in that range and not down at 2, for example. To overcome this I would normally shoot aperture priority and set it to 16 and if the conditions were too dark then open it until was OK (always of course maintaining the shutter speed stays above camera shake level) Since I am using the aperture this way one could argue I don't need any manual control over the camera at all other than ISO. 400 would reduces the changes of the camera shooting wide open.

Just curious how cameras think...

Depends on the camera; some allow choosing aperture priority or shutter priority which fixes that parameter. Obviously at some point, the opposite parameter being varied by the automatic function can run out of range. Other systems additionally offer a "program" mode that walks some sort of decision sequence trying to use reasonably small apertures until that forces shutter speeds low enough to be problematic for hand holding. (And arguably, they don't "think" all that much.) I would imagine the exact algorithm for the "P" mode varies quite a bit between various makes and models. With interchangeable lens cameras, dealing with different maximum apertures is another complexity that has to be factored in also.

As Dave stated each camera is programmed. There is not standard program for all cameras. The camera manual will tell you how the program works.

For years photographers recorded their exposurers for the images they made. Camera manufacturers used this information to program their camera chips.
In the case of Aperture or Shutter proirity each the aperture and shutter speed dials have a resistor ring and depending on position the voltage changes between the center lead (corresponds to the setting) and the end points.
The meter produces a voltage porponital to the light reaching it and correct exposure will be a given voltage. If in Aperture proirity then the camera locks out the shutter speed dial and controls the curtain release via the perdetermined program, in Shutter proirity the aperture is locked out and the camera controls the aperture. The voltage being produced by the control selected by the photographer set the bias that the other control functions by.

There are even simpler systems, that still function quite well.
The Olympus Trip 35 uses a trap needle approach, and as long as light levels are reasonably high, it can work well. And no batteries required ! (the camera relies on a built in selenium cell).

A simple automated film camera will know, for example, that you've set your film as ISO 400, your chosen aperture is f/11, and then based on the light level its meter sees will try to select the correct shutter speed.

If the speed is too low, for example under 1/30, the camera may give you a warning or it may even lock the shutter.

As mentioned above, each camera is different and has varying levels of sophistication.

Lower end Minolta and Pentax SLRs have a set of icons, a face, a person running, flowers, mountains. The use selects the icon that matches what he/she is shooting, the action Icon and camera gives a nod to fast shutter speed, landscape best aperture. The camera knows the set ISO and what lens is being used. For street photography I have dug out my old Pentax PZ 30, entry level, 50mm lens, Tmax set to 800, set camera to action, I tend to control for faster shutter speed to avoid shutter shake.

everything being in focus can be achieved easily with a wide angle lens (24mm or 19mm) - amazing depth of field, especially if you move the infinity marker towards the f-stop marking on the lens for "hyper-focal focusing". Even at relatively fast f-stops - give it a go! You may need to get closer to your subject!!! Also watch out for exaggerated angles as distortion gives very different viewpoints the closer you get to something.

The first and probably simplest system is found in the Yashica Electro 35 series. When you press the shutter release a capacitor is charged by a photo cell. The brighter the ambient light the more current that is stored in the capacitor. While there is still current in the capacitor the shutter is keep open. The capacitor is connected a resistor that drains the it at a known rate. When thee stored current reaches a certain value the shutter closes. If the photocell is located behind the lens as in later models then readings can be taken in stopped down mode. Setting the film speed sets the value of the resister. A faster film speed setting selects a lower resistance so the capacitor drains faster. A -very reliable circuit with few parts. More modern cameras essentially use the same method although they may have more bells and whistles.

Gerald C Koch said:

... the Yashica Electro 35 series. When you press the shutter release a capacitor is charged by a photo cell. The brighter the ambient light the more current that is stored in the capacitor. While there is still current in the capacitor the shutter is keep open.

Click to expand...

Aside from the fact that a capacitor stores charge and not current, there is a problem with the description above. According to that scheme, more light would imply more charge which would keep the shutter open longer? More light should close the shutter sooner.

... A faster film speed setting selects a lower resistance so the capacitor drains faster. A -very reliable circuit with few parts. ...

Click to expand...

Yes, that is elegant.

JamieB said:

I am wondering about how cameras choose the exposure automatically. So you put in 400 speed film. Does the camera take as a base point the lowest shutter speed and then choose the highest F stop against it or... there must be a bias towards higher F-stops, no? Rather than a bias towards high shutter speeds.

I am just thinking about street photography. For me, I always want the highest F-stop possible so everything will be in focus. I don't care much if it is 8, 11, 16 as long as it is in that range and not down at 2, for example. To overcome this I would normally shoot aperture priority and set it to 16 and if the conditions were too dark then open it until was OK (always of course maintaining the shutter speed stays above camera shake level) Since I am using the aperture this way one could argue I don't need any manual control over the camera at all other than ISO. 400 would reduces the changes of the camera shooting wide open.

Just curious how cameras think...

Click to expand...

for you, aperture priority might be the thing to get,which is common and sensible.

JamieB said:

... I would normally shoot aperture priority ...
Just curious how cameras think...

Click to expand...

Depends on the engineer/designer.

I have a DSLR (who shall remain nameless) that has both an Auto mode and an "intelligent" mode (or some such nomenclature). I shoot aperture priority the vast majority of the time, but the Auto setting will give me equivalent exposures. The fancier intelligent setting puts exposures all of the map. I have no idea what that setting is thinking ...

As for film cameras, I've used all kinds, but my latest and favorite 35mm is a Minolta Maxxum 7. On any automatic setting, it give me consistent and surprisingly accurate exposures. It's a pleasure to just point and shoot!

JamieB said:

For me, I always want the highest F-stop possible so everything will be in focus. I don't care much if it is 8, 11, 16 as long as it is in that range and not down at 2, for example.

Click to expand...

There are practical and technical difficulties with this approach. Ireland isn't the sunniest of places so to freeze motion and obtain sufficient depth of field, you need to aware of the compromises. Street photography requires a shutter speed of 1/250 upwards and a minimum aperture of f8 - before anyone shouts up there are many exceptions but the above holds true for "classic" street photography. To get front-to-back DoF at f8 means a wide lens, probably a 28mm. It will also mean pushing 400 ASA film much of the time by at least one stop, probably two. Image quality drops at apertures smaller than f8, whether that reduction is a problem is in the eye of the beholder.

Personally speaking I'm happier to set a camera manually for the street, but some people make auto exposure work for them. You can shoot SP with a 50mm lens and 100 ASA film (Cartier-Bresson did), but the reason many people don't is it's hard to square the exposure circle. Grain, blur, depth, quality - something's gotta give.

Paul Howell said:

Lower end Minolta and Pentax SLRs have a set of icons, a face, a person running, flowers, mountains. The use selects the icon that matches what he/she is shooting, the action Icon and camera gives a nod to fast shutter speed, landscape best aperture. The camera knows the set ISO and what lens is being used. For street photography I have dug out my old Pentax PZ 30, entry level, 50mm lens, Tmax set to 800, set camera to action, I tend to control for faster shutter speed to avoid shutter shake.

Click to expand...

The Nikon N75 has both a program mode and icons with a face, a person running, flowers, mountains. That is why I posted earlier that different cameras have different programs and different program modes.

At the beginning autoexposure was pure electromechanic. The digit of the meter was caught at releasing by a staggered wedge and its position controlled exposure.
That went as far as programmed autoexposure.

Agfa was leading at this field:
1956: first mass produced autoexposure camera, Agfa Automatic 66 (time priority)

1960: first programmed autoexposure camera, Agfa Optima

For street photography manual photography is the way to go.
You probably use B&W decently "fast" film.
You only have - typically - two situations: in favour of light, and backlit. Those two situtations are some 2 or 3 stops apart.

You keep in mind which is your current aperture and shutter speed. You know for which exposure it is.

For instance, you have a 1/125 f/8 for a subject in the shade with ISO 400.
If you see a subject that needs "attention", and the subject is in shade (because it is in shade or because it is backlit) you keep that exposure. You probably have your camera pre-focused to some intermediate distance which gives you "acceptable" sharpness within your likely subject focus distance. You raise your camera and take the picture without focusing and without calculating exposure, because in street photography "readiness is all", and in street photography you know your subject is not going to be very far, nor very near.

If your subject is in the sun, you have to change exposure. Let's say you adopt 1/250 f/11, you adjust your exposure without looking at the camera, and before bringing the camera in front of your eyes. One click, one click. That's why "clicks" are provided, after all. (or, maybe, two clicks if you only close aperture, or if you only change the shutter speed). Your hands must know what is the exposure setting of the camera.
Then you take the picture, again, without focusing, without calculating exposure. Possibly without bringing the camera to your face.

You have your exposure and focus values pre-set as mush as possible, and you adjust them as fast and discreetely as possible.

Automatic exposure won't work with street photography because it will force you to deal with backlighting situation, or situations where the subject is in sun while the rest of the picture is in the shade. A common occurrence in urban streets, where you have shadows (projected by buildings, cars, vans, billboards etc.) crossing the picture. Also automatic exposure is likely to fail (overexposing for the presence of the sky in the picture) if you take a picture from below and upward, such as a picture from your waist line of the face of a near subject.

As an alternative, you use B&W, you expose always for the shadows, you don't touch exposure at all and you only concentrate on catching the moment.

Keep it as simple as possible. B&W films have a lot of latitude and will give you a printable negative unless you don't make a grossly wrong exposure. Don't focus on "focus" and "exposure", keep you attention entirely on the subject. Always know which is the current setting.

Theo Sulphate said:

Aside from the fact that a capacitor stores charge and not current, there is a problem with the description above. According to that scheme, more light would imply more charge which would keep the shutter open longer? More light should close the shutter sooner.



Yes, that is elegant.

Click to expand...

You are assuming the circuit holds the shutter open. If the shutter is tripped and held open by some sort of latching mechanism, then the capacitor could build up enough charge to eventually fire a solenoid that releases the shutter and allows it to close. That would make the capacitor's charge proportional to the exposure time.

As Jim points out it is more likely that the shutter is held open by a timer such as a count up or count down timer.

jim10219 said:

You are assuming the circuit holds the shutter open. If the shutter is tripped and held open by some sort of latching mechanism ...

Click to expand...

OK - that makes sense.

JamieB said:

I am wondering about how cameras choose the exposure automatically. So you put in 400 speed film. Does the camera take as a base point the lowest shutter speed and then choose the highest F stop against it or... there must be a bias towards higher F-stops, no? Rather than a bias towards high shutter speeds.

I am just thinking about street photography. For me, I always want the highest F-stop possible so everything will be in focus. I don't care much if it is 8, 11, 16 as long as it is in that range and not down at 2, for example. To overcome this I would normally shoot aperture priority and set it to 16 and if the conditions were too dark then open it until was OK (always of course maintaining the shutter speed stays above camera shake level) Since I am using the aperture this way one could argue I don't need any manual control over the camera at all other than ISO. 400 would reduces the changes of the camera shooting wide open.

Just curious how cameras think...

Click to expand...

it's all magic!

If you are using an automatic compact the most common way is centre weighted meter with the common theme reported across brands being that it will change shutter speed before changing aperture. This can mean the camera could end up giving you 1/8th s @f5.6 rather than the more practical 1/30 @ f2.8. Not all auto compacts will do this but just because yr lens is fast enough to shoot handheld in lowlight it does not mean camera will give you those settings!

Pick yourself up a cheap automatic. Just make sure you can either set iso manually or it has +-2 stop exposure compensation and after a couple of rolls you will understand when to under or overexpose from what camera is saying. Once you can do that you have a very powerful street photo tool