Aperture priority

Hello

I have a Pentax ME camera that picks the shutter speed automatically. How does the camera decide on the right shutter speed for the photo? how does the camera sense the amount of light in the photo?

Best to start here: http://www.cameramanuals.org/pentax_pdf/pentax_me.pdf

For some quick advice, chose an aperture that forces the shutter to 1/60th or faster for hand held

For the simplest through the lens(TTL) metering systems, there is a something called a photocell situated somewhere inside the camera. There are a couple of different types used and you can study up on them if you care to, but basically the relevant thing about them is that they have some measurable property that changes(usually resistance) depending on the amount of light hitting them.

The camera takes the signal from the photocell and uses it with information like the set film speed and the set aperture to calculate an appropriate shutter speed. It then fires the shutter at that speed.

That's effectively all there is to it. In the 70s and 80s when auto exposure became common, most companies favored aperture priority. Canon was the lone one out that MOSTLY used shutter priority on their cameras. Which one was favored depended somewhat on just how the lens mount/communication works on a particular mount.

The simplest system, used by the usual Polariod pack cameras, charges a capacitor through a phototransistor, with current proportional to light intensity. The time to charge to a specified voltage is then inversely proportional to intensity, as required. This also works if the light changes during the exposure.

Most TTL systems, especially those used on SLRs that I know, measure the light before the exposure, and lock the reading.

On most cameras, this is simpler than shutter priority, as shutter timing is an easy on/off.

The standard aperture priority setup will allow the user to choose the aperture by selecting it on the lens barrel, and the camera will choose the appropriate shutter speed to give the correct exposure. So the short answer is that the camera chooses the shutter speed that gives the correct exposure. As Ben has said, there is a light sensitive cell in there that measures the amount of light coming through the lens. The camera takes this and assumes that you want an average grey tone ( let’s assume you are using B&W film), because most normal pictures are comprised of various tones from pure white to jet black. If you “ mush up” all these tones you get an average grey, and so the camera takes the amount of light coming in and selects a shutter speed that will give an average grey on the film, for the aperture selected. It’s a reciprocal arrangement, so you will notice that for any given scene, if you reduce (make smaller) the aperture by 1 stop ( say from f4 to f5.6) then the camera needs to extend the shutter speed to double what it was to get the same exposure.

Let’s say you are shooting a landscape and you choose f11 for the aperture. The camera takes a light reading and comes up with 1/125th second for the correct shutter speed to give you an average grey tone on your negative. You might choose to use f16 instead ( to give you extended area of focus), so the camera will in that case change the shutter speed to 1/60th of a second to keep th ‘amount’ of light hitting the film the same as it was.

how does the camera sense the amount of light in the photo?

It has sensors reading the amount of light visible on the focusing screen. - They will get fooled when you put the camera on a tripod and a lot of light shines into it through the viewfinder. The ME Super's meassuring characteristic is a bit but not extremely center weighted. So yes, it can make more mistakes than a more modern matrix metering system. - I think the chapter on exposure compensation in the manual gives you a rough idea and from there on it takes experience or handheld metering.

Thanks for the reference to my website.

On your lens, there is a bar that stick out. The distance of that bar (left or right) moves as you change the aperture and tells the camera what F-stop that lens is set at by an arm inside the camera measuring where that arm is located. So you don't have to "stop down" to measure the lighting. Then using the camera's light meter and basic computer, figures out a shutter speed. Many other Pentax camera have auto and manual abilities, yours does not. So you can only pick the aperture (but it reflects the shutter speed). You need a faster shutter speed, open the aperture (but loose depth of field). You do have an OVER (bright sunlight with most films) and trying to shoot at F2 will most likely set off the OVER warning. Just stop the lens down a bit. You should not try to shoot hand held under 1/60 sec. If you have a 135mm or longer lens then no slower then 1/125th.