EOS 70d Live View question

[cadams]

Hi

I have just purchased an EOS 70D.  My question relates to using Live View under the "fully automatic" setting.  I understand that touching the live view screen causes the camera to focus on the point of touch, and this is exactly what happens on my camera.  However, I have also noticed that the exposure changes as well.  For example, if I touch a dark object the aperture or time will change to allow in more light and vice versa if I touch a light object. 

I cannot see any reference to this in the operating manual.  It is a very good function but I would be interested to know where this functionality is described in the manual if anyone knows.

Thanks

[amfoto1 · Accepted solution]

The 70D has a 63 zone iFCL metering system. In other words, the image area you see through the viewfinder is divided up into 63 different areas that are being measured individually, with the gathered info interpreted as best the system can and then used to arrive at what the camera "thinks" is a correct exposure.

When in "Fully Automatic" it likely uses what's called the Evaluative metering method. This method puts more emphasis on the area immediately around the active AF point(s). It's sort of like Spot Metering linked to the active AF point, but Evaluative metering also considers the rest of the image area in its calculations.

You also likely have Exposure Simulation activated (in fact it might not be possible to deactivate it while using a fully auto mode).

If you were instead using one of the "creative" (as Canon calls them) auto exposure modes, you would have more flexibility in setting the camera up. For example, you could choose a different metering method (Center Weighted would not react the same way as Evaluative, simply looks at the whole image area, with a bit more emphasis on the central portion of the image, but not linked to any particular AF point), or you could dial in some plus or minus adjustment using Exposure Compensation. Av, Tv and P are the "creative" auto exposure modes. Av stands for "Aperture priority", and is where you choose the aperture and the camera's automation selects an appropriate shutter speed to match. Tv stands for "Shutter priority", where you choose a shutter speed and the camera sets the aperture automatically. P is "Program" mode, where the camera sets both aperture and priority. (Note: in all three modes, you would have selected and ISO, first.)

You also could turn off Exposure Simulation, if you wished. Then the image seen on the camera's LCD monitor will not vary in the same way.

However, I'm not really suggesting you not use Evaluative metering or Exposure Simulation. They are both valuable tools available for your use. I'd suggest you learn to use them to your advantage. Also become familiar with the camera's three other metering methods (Center Weighted, Partial and Spot).

Evaluative metering actually does a pretty good job in a wide variety of situations. I'd suggest learning how to use it well.

First, try to think of the world in shades of gray, rather than colors. Like all reflective metering systems, it's designed to produce a "neutral gray" and is influenced by subject tonality. For example, if you photograph a black bear in a coal mine, the camera will try to overexpose it. Or, if you photograph a white polar bear in a snow storm, the camera will try to underexpose it. In both cases, the metering system is always trying to make the subject come out "neutral gray". It can't  think... you have to do the thinking and recognize situations where it's going to be skewed, override it and make appropriate adjustments.

Exposure Simulation can help warn you of these over and under exposure tendencies. You also can play back your images and inspect the histogram, to see if you have over or under-exposure issues, while you can still re-shoot the image if necessary.

Note: besides what the camera offers there are other types of metering that operate differently. For example, incidence metering measures the light falling onto the subject, rather than what's being reflected off of it (as reflective metering does). Used right, this can make for more accurate metering. However, there is simply no practical way to build an incidence meter into a camera and use it for auto exposure control. Incidence meters are mostly separate, handheld and many of them are also able to accurately measure flash and strobe lighting.

It sounds to me as if your camera is working just as it should. It's just a matter of you getting comfortable with it and learning to use it well.

Besides the manual that came with the camera, there are guide books available that can be very helpful and complement the manual. I'd suggest checking what guides are out for 70D. Whenever I start to work with a new camera model I always buy one or two of the guides, as a short cut to getting up to speed using the camera. From personal experience I can recommend guides by Charlotte Lowrie, Michael Guncheon and David Busch. If you search  for "70D" on Amazon in Books,  I'm sure you'll find guides by one or more of these authors, and likely some other very good ones (these are just authors I'm familiar with).   

If you are really new to photography, I'd also highly recommend getting a copy of "Understanding Exposure" by Bryan Peterson. It can be very helpful, too, and is more of an overview of how cameras work in general. I think it should be mandatory reading for all new photographers, or at least for anyone who wants to advance beyond the point-n-shoot skill level.

***********
Alan Myers

San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM 

[cadams]

Thanks Amfoto1 for the very detailed and informative reply.  I have just returned to the hobby after a forty year break from when it was my hobby during my teenage years.  The basic principles are still the same despite the technology changes.

The link of the AF with the metering seems like a good idea and I wish the manual had mentioned this.  The paper manual supplied with the camera surprisingly has no mention of the metering modes-- something Canon should add.

Chris

[amfoto1]

There is a brief description of each metering modes beginning on page 165 of the manual. However, "brief" is the key word and you should experiment with the modes to get a feel for how they work. All Canon cameras use similar metering methods. Some have more or less metering zones. Some don't have Spot metering or the size of the area Spot metered differs. But the basic idea has been the same for a lot of years and successive models now.

Cameras of 40 years ago that had built-in metering and forms of auto exposure generally relied upon something similar to Center Weighted metering on your 70D. A few cameras used a Spot metering method or offered it as an alternative.  

Since you have used cameras in the past and have some idea how they work, it will probably be much easier for you to get up to speed with the 70D. I'd still suggest a guide book as a helpful short cut, and "Understanding Exposure" as a great overview of the main features of a modern SLR. A lot has changed in the last 40 years and today's cameras are really loaded with useful features.  

Cheers!

***********
Alan Myers

San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM