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03-18-2010, 01:34 AM   #1
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Different AE values in live-view?

SooOo I've had my K-7 for a few weeks now, and, to put it mildly, I'll give the thing up when you pry it from my cold dead hands. Love it. This thing is the first SLR of any kind that I've really loved since the FM2n.

Getting to the point: I've noticed that I sometimes get different AE computations in live view mode vs. exposures computed with live view turned off. Nothing distressing - usually in the neighborhood of one-stop difference (sometimes the live-view computation is slower, sometimes the inverse is true), but it still got me thinking about why this might be the case. Did some searching here on the forums, but couldn't find anything about it. I should also mention that I haven't tested this thoroughly - just noticed it a half-dozen times in spot-ae, in both shooting modes.

My theory: the camera uses different methods for calculating exposure with the mirror up and down (light sensors at the prism vs. calculating exposure off of the CMOS data feed, similar to how contrast detect af is available in live view). If that assumption is true, then it follows that it would be impossible for both systems to meter every scene identically, which would explain the phenomenon.

Can anyone confirm or deny the validity of this theory? Ultimately this isn't hampering my shooting at all - just one of those curiosities that my brain's going to squirrel away on until I get to the bottom of it

Cheers!

p.s. just for fun (and totally unrelated), here are some of the shots that I've been producing with this baby... Nothing special, but this camera's definitely accelerating my learning curve.








Last edited by Timichango; 03-18-2010 at 01:41 AM. Reason: grammatical error
03-18-2010, 02:23 AM   #2
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Great start. I do like the strong blue tones arouind the buildings in the first shot contrasting with earth tones on the ground. Call me crazy, but its appealing to me.
03-18-2010, 03:10 AM   #3
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Well, that was a distressing find when I went from a Fuji S9500 to a K10 : Dslr metering is often poorer than Compacts or bridges, as they have to rely on low-res metering cells instead of analysing the actual pic and applying an highlight %.

So, I'm pretty sure the liveview metering is more often spot-on...
03-18-2010, 12:14 PM   #4
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One of the circumstances in which I noticed the discrepancy was taking a picture of black fabric against a white wall - initially, in live-view, the K-7 seemed to overexpose the shot, as the white wall was rather overexposed. The K-7 was set to spot-AE, and the frame was centered on the edge of the black fabric, adjacent to the white wall; closer inspection of the shot showed that the texture of the black fabric was properly exposed, which caused the wall to blow out.

Taking the camera out of live-view caused the scene to meter 1-stop faster, and average the exposure between the wall and fabric (this is still in spot ae). This resulted in less detail in the fabric, but no clipping on the wall. My suspicion here is that one of the two following scenarios is behind this:

a) the spot-AE area used in live-view is more constrained, and therefore only based the exposure on the fabric; the non-live-view spot-AE area is marginally wider, resulting in an exposure area which overlapped onto the white wall

b) the spot exposure area in standard mode (using the viewfinder) isn't a 'hard' spot area, and experiences some light diffusion, resulting in a more 'blended' exposure.

Not sure if any of this is accurate, or which is more desirable – an extremely rigid spot area, or something a tad broader. Probably the former, as I reckon the latter is what 'center weighted average' is for.

03-18-2010, 12:16 PM   #5
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QuoteOriginally posted by imtheguy Quote
Great start. I do like the strong blue tones arouind the buildings in the first shot contrasting with earth tones on the ground. Call me crazy, but its appealing to me.
Also, thanks for the comments, imtheguy!
03-18-2010, 02:37 PM   #6
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Doesn't the fact that your eye isn't up to the eyepiece have something to do with it, because light leaks in and corrupts the metering?

Try using LV but covering the viewfinder.
03-18-2010, 02:47 PM   #7
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Nah, I had the same instinct, but covering the viewfinder had no effect on the exposure in the circumstances that I was testing it under.

If the camera is, indeed, metering exposure off of the CMOS during live view, it's likely that the exposure cells near the prism (where they usually are, I believe), would be disabled. *edit*...I'll test this by nailing the viewfinder with a flashlight next time I'm shooting in live-view, and see if it has any effect on the exposure readings...

For that matter, I think there would be no way to read exposure using light cells at the conventional location near the prism during live-view, since all light entering the lens would be blocked from entering that area by the mirror, which is locked up during live-view. Hence why expect that the CMOS data is being used to determine exposure during live view.

I could be way off the money here though – I'm no camera technician, and this is all me making, at best, half-educated guesses

Last edited by Timichango; 03-18-2010 at 02:49 PM. Reason: added note to message
03-18-2010, 02:58 PM   #8
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Sorry not to make that clear in my previous post, but you are right about live view not using the metering cell located on the pentaprism.
It's based on the CMOS reading, using a meter/adjust loop until it reaches a satisfying level. You can see it easily by panning your liveview between a dark scene and a bright one, as you'll see it first overexposing, then darkening step by step, with accordingly increasing values for ISO/Aperture (or decreasing speed, it depends on what mode you're in).

You don't have these steps using the conventional meter cell, as it delivers a straight Ev metering ranging from 0 to 22 Ev (if I remember the figures correctly), without having to dance the "meter/adjust" number...

03-19-2010, 12:48 AM   #9
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This likely adds nothing useful to the thread but I wondered just how much light DOES come in from the optical viewfinder. Here is a shot from my P&S looking into the mouth of my A*400/2.8 with the K20D attached. Its enough light to recognize the trees in my backyard and houses across the river from me. Yes, those are the focus screen etchings also. A picture in reverse. More light than I had imagined. Thanks for humoring me on this tangent.

03-19-2010, 07:32 AM   #10
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On my K10D, I ran a quick, informal test, with the camera on a tripod, shooting in my backyard. The difference between having the eyepiece covered and uncovered was about two stops on a sunny day.

So, yes, a significant amount of light comes through the viewfinder. I wish that the K10D had a built-in viewfinder shutter, like my ES-II has. I don't always have my camera bag, where I keep the viewfinder cover, handy. It would be nice if I could just slide a lever and close the viewfinder. I usually just end up putting my thumb over the eyepiece.
03-19-2010, 09:03 AM   #11
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Yes, but the K10D doesn't have liveview. Without thinking it all the way through, my gut impression would be that whether the mirror is up or not would have a pretty significant effect on how much light entering the viewfinder could possibly affect results.

I personally don't bother with the viewfinder cover when using a tripod. I typically meter with my face up to the camera, then leave exposure locked there. Someone once posted an image where the light entering the viewfinder actually affected the image itself. That threw me for a loop, but I've never seen an issue with that myself.
03-20-2010, 11:46 AM   #12
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QuoteOriginally posted by Marc Sabatella Quote
Yes, but the K10D doesn't have liveview. Without thinking it all the way through, my gut impression would be that whether the mirror is up or not would have a pretty significant effect on how much light entering the viewfinder could possibly affect results.

I personally don't bother with the viewfinder cover when using a tripod. I typically meter with my face up to the camera, then leave exposure locked there. Someone once posted an image where the light entering the viewfinder actually affected the image itself. That threw me for a loop, but I've never seen an issue with that myself.

I realize that the K10D doesn't have live-view. The conversation had drifted slightly, and IMTHEGUY asked how much light really entered the viewfinder. From that respect, I don't think that the K10D and K20D are that much different.

Would light from the viewfinder affect exposure during live-view? I thought that, with LV, when the shutter is tripped, the mirror flips down, to allow normal metering, then flips up, as in a normal exposure. If that is true, it would seem that the image seen during LV would have no affect on the exposure. Also, if true, the light entering the VF could be affecting his exposure. As my little experiment showed, that can be significant.

I don't see how light entering through the viewfinder could directly affect the image, since the mirror is up during the exposure. It can, of course, affect the exposure, but you should not see light streaks or anything like that.
03-20-2010, 03:47 PM   #13
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Well, in liveview, the k7 does not lower the mirror to meter.
Metering is done on the CMOS feed itself, so it relies on an absolutely different method. Think about it as a 1280x720 metering sensor...
03-21-2010, 08:12 PM   #14
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I agree with the above - in liveview, the K-7 doesn't flip the mirror down to meter - metering takes place 'on-the-fly' during live view; the metering is performed while the image is being displayed on the screen in real-time.

Since the 'traditional' ttl metering sensors are at the prism (several people have asserted to me that this is an absolute fact), and the mirror is up during live-view, then the camera must, by deduction, be calculating exposure based on what the CMOS is seeing. Therefore, in this mode, it's unlikely that any light entering the viewfinder would affect an photo taken using live-view, since the sensor is effectively 'cut-off' from the viewfinder by the mirror being locked-up.

*edit* What might be adding to the confusion about live-view metering behaviour here is that the K-7 does drop the mirror to focus when using phase-detect focusing in live-view.

For non-live-view exposures in bright light, I follow the same method as Marc described above: I meter with my eye against the camera, and lock the AE to prevent any light that happens to scatter into the viewfinder from affecting the exposure.

imtheguy: Neat shot taken through the viewfinder! I've been playing around with doing something similar with an old broken Yaschica TTL camera for creative effect - there are whole flickr groups dedicated to this sort of thing!


Last edited by Timichango; 03-21-2010 at 08:15 PM. Reason: added note
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