Live view underexposure in Canon XSi

[Carlos Lindado]

Hello, I've just registered to the forums, I think this is the most comprehensive Canon-related site on the net!


Currently, I have a Canon XSi with EF-S 18-55mm f/3.5-5.6 IS and EF 50mm f/1.8 Mk I lenses, and I've noticed that when I use Live view while in Av or Tv mode, the camera underexposes the image by 2/3-1 stop. This doesn't occur all the time, but in certain lighting situations (low light, light in front of camera). Why do you think this is happening? I guess is CMOS sensor metering vs dedicated AE-meter metering, but I'm not really sure. The point is, sometimes I'm shooting on a tripod with Live view and I have to re-take the picture because I didn't account for Live view underexposing the image.


Thanks for reading!

[Scott]

Not to sure Carlos but welcome to the forum. I know one of these smart guys / galls will know what is going on. Have you got your LCD brightness set right.

[Sheiky]

Hey Carlos, welcome!


Funny that you stated this "problem". I never use liveview(at least not in AV/TV or auto mode) so I haven't experienced it yet. But I've tried it myself just yet with a Sigma 50mm 1.4 on a 5D2 and I too see the darker exposed shots when using live-view.


I tried 5 subjects inside, (only one set had the same exposure), so constant light and there's definitely some degree of difference. From 2/3 to even 2 stops. The photos don't look "badly exposed" or something. But they are a little off...most noticeable when viewing the histogram.


Interesting point and at this moment I don't know what causes it. I'll do some more testing later on...


Jan

[mikehillman89]

I've noticed this problem as well Carlos! I'm not entirely sure what it is as well. To be quite honest I just adapted myself to exposing a bit up, but in any case I've rarely used Live View... As well, (for 7D owners) I've noticed that the camera way underexposes the video in live view as well so beware!

[Carlos Lindado]

Thanks for your replies! I didn't think this was an issue on other Canon bodies too (I've used a friend's 7D, but not in live view mode). My best guess is that the CMOS sensor isn't as accurate as the dedicated AE meter, and it tries to calculate the "best" exposure its own way.


Regarding live view, I like to use it for tripod shots (awkward angles, manual focusing, more accurate framing) so I think it's a useful tool in specific situations.

[alex]

Doesn't LiveView set the exposure based on where you have the small rectangle in LiveView moved to? I use LiveView on my XSi anytime I have it on a tripod and I want to set focus to something that is not dead center in the screen. I've noticed that the exposure on the screen will vary when I move the rectangle around to place it over where I want to zoom in to set focus.


So if it's sampling from a smaller zone for metering (like Spot metering, kind of), there is the possibility of it getting a different exposure than if you didn't use LiveView and had your camera set to Evaluative or Partial Metering.


I could be completely incorrect. [:$]

[Carlos Lindado]

Hey Alex, you're completely right! I just tested moving the focusing rectangle and yes, the exposure changes drastically, however, the manual of the camera says "During Live View shooting, the metering mode will be set to evaluative metering regardless of the current metering mode setting".


Intrigued by that, I tested changing the focus point while in "normal mode" (no live view) and, as in live view mode, the exposure changed drastically, so I think Canon's evaluative metering is strongly biased toward the focus point. Nevertheless, live view calculated exposure continues to be different from the one calculated without live view.

[Sheiky]

Well that clears some things at least [] It's not changing anything for me personally, but I like it when a "problem" gets solved [Y] Good stuff detective Alex!

[alex]

Glad to help, but that still doesn't clear up why LiveView, which supposedly uses Evaluative Metering, would get different metering results than normal view Evaluative Metering.


Anyone have any ideas about that one?

[neuroanatomist]

Glad to help, but that still doesn't clear up why LiveView, which supposedly uses Evaluative Metering, would get different metering results than normal view Evaluative Metering.

I suspect Carlos was on the right track:



<div>

I guess is CMOS sensor metering vs dedicated AE-meter metering

Both are 'evaluative' in the sense that the camera is using (undisclosed) algorithms to compare the luminance of subject and the surrounding area in determining autoexposure. But there are several differences:

• 'Standard' AE and Live View AE use different sensors (dedicated AE sensor vs. imaging sensor)
• The areal coverage of the sensors is different (standard AE uses 35 zones on the XSi, Live View uses the entire frame, including the edges that extend beyond the coverage of the dedicated AE sensor)
• AF points are different - evaluative metering determines the subject based on which AF point is selected; for standard AE that's restricted to the 9 AF points, whereas for Live View the AF point can be anywhere in the frame.
• The algorithms used to determine exposure likely differ between the two modes (and of course, the inputs to those algorithms also differ).

All of this will add up to situational differences in the way exposure is determined.


I tried a little experiment with my 7D, comparing exposures in standard shooting vs. Live View. I shot some trees this morning, backlit by the near-dawn sky. Changing from standard to Live View shooting, I could see the underexposure that has been mentioned - the Live View shot was about 1/2 stop darker (visually and based on the histogram). Conversely, last night I had shot some indoor test shots in the living room. A torchire lamp in the corner of the room was a bright spot at the edge of the frame. For that composition, the Live View was about 2/3 of a stop overexposed compared to the standard VF shot. So, it seems not to be a consistent, systematic underexposure with Live View, but rather simply a difference in the way exposures are calculated, which is impacted by composition and lighting.


Just as a side note that I uncovered while reading up on these issues - evaluative metering is linked to one and only one AF point. So, if you're shooting with automatic AF point selection and several AF points 'light up', that simply means that all of those points are within the depth of focus (determined by the AF system). However, only one of them will be used as the basis for the evaluative metering and unfortunately, there's no way to know which of the 'lit' AF points the camera is using as the basis for the metering evaluation.


I suppose all of this just reinforces the need to check the histogram as you shoot!
</div>