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FORUMS Photography Talk by Genre Bird Talk 
Thread started 12 Apr 2014 (Saturday) 14:57
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Struggling with Exposure in TV and AV Mode

 
jhayesvw
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Apr 14, 2014 12:10 |  #16

Amazing info and explanations in this thread. Thanks guys.



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Duane ­ N
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Apr 14, 2014 18:36 as a reply to  @ jhayesvw's post |  #17

After reading many of the excellent replies to this question it's no wonder some people are confused about how manual mode works. I've been shooting in manual mode for the last 4 years and have no idea what some of you guys/gals are talking about....lol.


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Duane ­ N
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Apr 14, 2014 18:40 |  #18

sandpiper wrote in post #16829178 (external link)
You need to assess your meter reading and the tones, set the exposure values you think will be correct and (unless the light changes) LEAVE IT ALONE. The meter needle may be moving around the scale quite merrily showing some shots as "overexposed" and others as "underexposed" but that will just be the effect of the background. If the light on the subject hasn't changed, your exposure doesn't change.

This is exactly why I don't even look at the needle in the viewfinder...to me it's just a dancing line that's constantly moving as I pan around or move to various parts/colors of the subject I'm photographing.

A very "understandable" way of explaining it. ;)


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hollis_f
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Apr 15, 2014 06:51 |  #19

frugivore wrote in post #16832140 (external link)
Nobody outside Canon really knows how the autoexposure program works, but I believe it works like this:

1. Take a reading of the parts of the scene you ask the camera to meter (e.g. spot vs center vs evaluative)

There's quite a difference between the way those first two works from the way evaluative works. The first two do just average the part of the scene you point the central focus point (for most EOS cameras) at. The only difference between spot and centre is the size of the sampled area.

Evaluative is quite different. Firstly, it uses the specific focus point that you (or the camera) select. It assumes that whatever is in focus under that point is the subject that you want to be properly exposed. Then it checks all the other focus points, looking for something at the same distance as your subject. If it finds something then that's included as part of the subject that needs to be correctly exposed.

It then divides the image into many portions and takes a meter reading for each section. It then looks through a database that contains similar meter readings from many, many sample images, trying to find something similar.

That's why Evaluative metering works very well in most situations. It can work out just what your subject is and what the background is - take the exposure information for subject and background - compare that info to a database of common images - find one that fits - and set what it thinks should be the correct exposure.

You can see the two methods in the image below. The spot metered image has measured the brightness of the white box, decided on the exposure needed to make it come out as 15% grey, and turned the background black. A user who knows how Spot metering works would have dialed in some exposure compensation, lengthening the exposure time. For the Evaluative shot, the camera has determined that the box is the subject (because that's what I focused on). It's database probably doesn't contain a 'grey subject on a black background' entry; but it does seem to contain a 'white subject on a grey background' entry. So it's chosen exposure values for that type of image - and got it spot (pun intended) on.

IMAGE: http://www.frankhollis.com/temp/Eval%20vs%20Spot.jpg

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frugivore
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Apr 15, 2014 16:40 |  #20

Frank, that's an interesting explanation. I'm not sure I understand how or why it checks other focus points and the corresponding metering zones. I have tested evaluative in the past to understand it but was never entirely successful.

After reading your post, I googled this a topic and found a detailed answer from Chuck Westfall:

http://www.prophotohom​e.com …etering-3.html#post469805 (external link)

It talks about the levels of priority of segments. This sounds a lot like center weighed, but connected to the AF point and with auto EC applied. Interesting!




  
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hollis_f
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Apr 15, 2014 16:54 |  #21

frugivore wrote in post #16836356 (external link)
After reading your post, I googled this a topic and found a detailed answer from Chuck Westfall:

I'm pretty sure that the process has evolved since Chuck wrote that.


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jefzor
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Apr 15, 2014 16:55 |  #22

Sandpiper sums it up nicely. I find myself using M more and more because of the inconsistent auto exposure when birds fly past different backgrounds. I usually take a metering off grass. Of course, on a day with small clouds passing the sun all the time, M can be a pain to use. Auto exposure is still the way to go if you need to take a quick shot without fiddling with the settings first.


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dino211
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Apr 17, 2014 13:19 |  #23

For changing light conditions and random birds/locations, I set my 7D to AV, high speed,
+/- one/two thirds and fire in bursts of three shots. This usually gets the shot when you don't
have time to analyse the scene.


Dean
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Struggling with Exposure in TV and AV Mode
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