Bruce's Photo Blog
Bruce Foreman | October 2013

Auto or manual exposure...which should I use?

by Bruce Foreman

Most of today's cameras have up to five auto exposure modes plus “manual”. Photo instructors and advanced hobbyists will tell you to use manual to really learn photography and photographic exposure. But to someone less advanced, trying to follow this advice leads to frustration and unnecessary “stumbling”. So what is the best way to go?

The truth is that many advanced hobbyists and more than a few professionals will use the automated modes. Once these modes are understood the reasons for choosing one or another make the choice easier. So here's a quick rundown:

Green box (often referred to as “idiot” mode): This is fully automatic, the camera makes all choices for you and all you do is press the button. If there is adequate light you get a good exposure or the flash fires.

Auto: Fully automatic exposure but the flash does not pop up on it's own.

Program(med) Auto: The camera uses automatic exposure based on a built in table of programmed parameters. If lighting levels are low the program will favor the wider lens apertures and slower shutter speeds. As the lighting levels increase the program will use medium apertures and the shutter speed will increase until the lighting gets stronger (like bright daylight and sunlight where the program will tend to set smaller apertures and faster shutter speeds.

Going in reverse as the lighting weakens the program will select lower shutter speeds until the shutter speed reaches the reciprocal of the lens focal length. Systems where there is an electrical link between the body and lens facilitate this and supposing a focal length of 100mm, the shutter speed that is a reciprocal value (1/100th) is the slowest that one should attempt to handhold the camera for that shot.

So there's a lot of “communicating” going on between the camera and lens and that helps Program mode to work well. When I'm heading out the door I often select Program Auto so that if a picture situation presents itself I can raise the camera and try to get it with minimum “fuss”. After getting one or two “insurance” shots I'll often switch to Manual mode and try to “refine” it.

Programmed Auto mode can also be “fine tuned” on many cameras. Depending on what make and model you have, there may be either a “Program Shift” or simply “Exposure Compensation” you can use to “bias” or “shift” exposure to lighter or darker. If your camera has a provision for “Live View” to display exposure changes with the image going lighter or darker as changes are made, you can couple this with “Exposure Compensation” to give you a degree of manual control over Programmed Auto.

I just tried this out on my Panasonic Lumix GH3, setting “Constant Prevue” to “ON” I started adjusting Exposure Compensation up and down and sure enough saw the changes on the LCD and in the EVF. So what this does for you is it allows you to “refine” the “look” visually while Programmed Auto handles the “exposure chores”.

Aperture Priority: You select the aperture for Depth Of Field effect desired (large aperture for shallow zone of focus, small aperture for deep zone of focus, or medium aperture for somewhere in the middle. The camera exposure system selects ISO (if you have ISO on AUTO) and shutter speed. Or you can also directly set the ISO you desire (according to lighting situation) and that only leaves the shutter speed for the exposure system to pick for you.

I also frequently use this mode when I want to be able to react quickly.

Shutter Priority: This mode comes in handy when rendition of motion is a primary consideration in your pictures. Preselecting a faster shutter speed to freeze motion (1/250th or faster) or a slower shutter speed to allow a moving subject to blur some leaves the exposure system to select ISO and aperture to get you correct exposure. As in the previous mode you can also directly select the ISO value you prefer and the system will only have aperture to select for you.

Manual Mode: Here you select both aperture and shutter speed, some cameras will allow you to leave ISO on AUTO and others will make you also select ISO manually. This is the mode many will suggest “real photographers” use, but I don't subscribe to that. I do use this mode a lot, especially when taking my time to make the picture “right”

Manual exposure often left those trying to learn photography “gasping for air” or totally lost but fortunately today's cameras have some real help. Once you have grasped the concept of larger aperture for “shallow zone of focus” or “selective focus”, and smaller aperture for “deep zone of focus” this makes a good starting point. ISO should be chosen for lighting conditions to start with, and adjusting shutter speeds until the exposure meter readout indicates correct exposure completes the process.

If your camera has “live view” this often makes the LCD image darken with underexposure or get lighter with overexposure. When you have learned to properly interpret what you see on the LCD this becomes an important tool that makes manual exposure much more fun. So you have the meter readout plus the appearance of the image on the LCD to help you out.

One HUGE word of caution here: Bright daylight and direct sunlight really interfere with being able to effectively see the LCD image so I suggest a very useful tool to toss in the bag would be some kind of loupe that excludes extraneous light. The one I recommend is the Hoodloupe 3.0 from Hoodman USA ($79.95 at B&H). This is a hard rubber housing that blocks out extraneous light and has a diopter adjust eyepiece. It provides no magnification but gives a clean 1:1 view. Canon DSLR users (Txi series, 60D, 7D) may benefit from the 3X magnifying eyepiece with eyecup (HoodMag 3.0 at $39.95 at B&H) tends to have finer definition than most LCD's have had. This Hoodloupe comes with a neck lanyard and I often let it dangle on that until I need to “see” something, hold it in place over the LCD then let it go to dangle on the lanyard. Canon DSLRs have a sharper LCD image than some other brands, otherwise the 3X eyepiece seems to just magnify pixels.

A third “helper” with manual exposure is the histogram as long as we realize it is NOT an exposure meter. It is merely a graphical display that shows where the image tones fall over about a 5 F stop range. There are some good articles on the web that explain how to interpret the histogram, just do a search for “histogram” and “reading histogram”.

Where the camera allows previewing the exposure effect in manual mode on the LCD it gives me a much better “feel” for how the picture or motion picture clip is going to turn out and it should let you come to “grips” much better with your images.

So back to the original question: Auto exposure or manual?

Try this, set Program at first (or Aperture Priority) and take a shot. Take note of the ISO, aperture, and shutter. Then switch to manual mode and dial those in. If your camera has “live view” activate that and look at your image on the LCD (use something like the Hoodloupe 3.0 if outdoors). Adjust the aperture up or down and notice any change in the image, do the same with the shutter speed. Then bring those back to where the meter readout indicates proper exposure.

The LCD will lighten or darken with exposure changes ONLY IN MANUAL mode. In any of the AUTO modes the exposure system will compensate and show you only a properly exposed image.

By the way this will work in video mode also on the Canon DSLR and Panasonic Lumix GH series cameras.

So the auto modes are not all as “automatic” as we've been led to believe. Each mode has a purpose and when we understand what they do and use them intelligently they no longer are a “crutch”.