Tuesday, 15 July 2008

The PASM thing

Updated: 24th, July 2008

Most serious photographers really jaded with trying to explain the four main exposure modes as well as the "green" full AUTO mode and the plethora of SCENE modes in these cameras. So newbie camera owners get pale and listless descriptions of what these modes do. Often, the question is "should I use P or A or S" for this scene or that trip - is one better than another? Well, here goes:

Full AUTO mode - (often spelled AUTO and/or marked a special colour) - Use this mode when you can't or won't think about exposure and putting some human effort into choosing the right exposure. The camera will decide the exposure for you. Often, you can't adjust anything else, it's that automatic. Sometimes, you can set focussing to Manual still.

SCENE modes - these are like full AUTO but the camera maker has thought out particular scenarios - shooting action shots, candle lit pictures, documents, landscapes and so on. Whilst full AUTO is a good "middle" adjustment of various parameters, each SCENE mode has been optimised for what that mode describes. Thus if you shoot bright snow or seaside scenes, Full AUTO may make the photo too bland (i.e. more gray) whilst picking a "Snowman" SCENE mode will bias the photo to record white snow.

P is for balanced Program Mode. This is like full AUTO but you can adjust lightness, darkness (otherwise known as EV Compensation), Sharpening and so on. The camera will choose a shutter speed and aperture that is optimum - not too wide an aperture, so that it can give you balanced good sharpness and balanced deep Depth Of Field for the light you have, not too slow a shutter speed so that you can get hope to freeze some action and avoid handshake. Why use P instead of A or S? Because P varies two parameters instead of one parameter. This allows the camera to handle a wider range of light levels. For example, if you chose the a maximum aperture for A mode and the sun suddenly came out from behind the clouds, your camera would now have to react and hit fastest shutter speed to cope. If the fastest shutter speed was not enough, you've now got an overexposed photo. If it was on P, the camera would have the option of automatically reducing the aperture. Some DSLRs have Ps which is Program Shifting (You control the curve of EV vs (Shutter Speed x Aperture) combination. This allows the camera to recommend a balanced Aperture and Shutter Speed but if you feel that this combination is not what you like and you know that this combination is not the only one for the same light level, you could roll a thumbwheel and advise the camera to choose a faster shutter speed - the camera would accordingly change the aperture. So Ps is like using A or S with easy human supervision in case you want a different combination.

A is for Aperture Priority Mode. This is my favourite exposure mode - the human chooses what Aperture to set and the camera calculates the corresponding shutter speed based on the light meter reading. So I can choose a wide aperture to push my shutter speed as high as it can go for that lighting (for action shots) or I can narrow aperture so that I get maximum Depth of Field. However, if the light changes quickly or if it is a high Dynamic Range scene and pointing the camera to another place suddenly changes what it sees, the Aperture I have chosen may not be wide enough or narrow enough to cope - the calculated shutter speed may be at the maximum limit of what the camera can do.

S is for Shutter Speed Priority Mode. I set a Shutter Speed and the camera calculates the Aperture from the meter reading. This is useful if I have decided on a particular Shutter Speed, for example, a fast one, and despite lighting change, I want the camera to react by adjusting the Aperture. Or, if I purposely want a slow Shutter Speed so that I get a blur of flowing water or movement. I don't enjoy Shutter Speed Priority:

  1. If I choose an impossible Shutter Speed, then the light and the camera may not be able to react within the limit of Apertures that the lens has.
  2. If I choose too humble a Shutter Speed, whenever there is more light, I am not able to benefit by increasing the Shutter Speed because the camera is not in charge of Shutter Speed, it is in charge of Aperture size.

M is for Manual Exposure. Don't confuse that with MF which is Manual Focus. The two are independent of each other. If you choose M, you are not really "going cold turkey" - the digital camera's meter is still functioning and it will recommend or signify in some way, whether your chosen combination of Shutter Speed and Aperture is deviates from what the meter recommends. However, in Manual Exposure mode, you can certainly ignore the camera's recommendations and most importantly, the camera cannot take control of exposure - you are firmly in control. Use Manual if you are sure the meter reading is / will be wrong (shooting a small moon image against a dark night sky) or when you think the meter will jump around and fluctuate wildly by your waving the camera around an unevenly bright scene.

More importantly, PASM is not the end - it is the means to an end. Whether you used any of the PASM to get f/8 x 1/100th sec, somehow you and the camera got there.

Further reading:

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