Camera Question about semi auto exposure

[Vinny]

I was in aperture priority and had the same f stop, same ISO, same focal length and same lighting conditions and my camera, a D90 used 3 different shutter speeds ... underexposing, correct and overexposing a few times. I had center metering chosen.

Can anyone explain why this would happen, almost sounds like I hit the bracketing button (I know my fingers did hit it once ... I still say too many buttons ) but I don't think it happened more than once. Any suggestions?

 

[myfotoguy]

That does sound like you had bracketing on. Was your camera at the exact same position (same thing in the center of the frame each time)?

Possibly that info is in EXIF if you have the right exif reader. I checked NX2 and ACDSee but didn't see anything, but maybe it doesn't show up unless you had used it. Hmmm now I'm interested- I may have to test and see.

 

[Vinny]

Yes, it was pretty much the same. I guess I could have had bracketing on but when looking at them with Nikon's view Nx it says 0 ev ... I guess I could have had pressed the bracket button and activated the bracketing feature but I know when I found that feature activated I put the controls to 0 to turn it off. I just find it a little weird that it wasn't taking consistent photos, of course it could be me sucking after not taking photos for too many years ... LOL!!

Here's three of the photos, but there's more that are over and underexposed.

 

[KmH]

Yep, looks just like auto bracket is turned on.

 

[Vinny]

Should have bought a cheaper camera, or one with less freakin buttons!! :lmao:

Yep, looks just like auto bracket is turned on.

 

[emh]

Hm... looks like auto bracketing. Even smells like auto bracketing -- the earliest _DSC0047.jpg (second one posted) is the correct exposure, _DSC0048.jpg is the -ve (first one posted), and _DSC0049.jpg is the +ve (third one posted) which matches the auto bracket order...

But EXIF data doesn't show any exposure compensation and no flash. Exposure Bracket Value is also 0. So it's not auto bracketing.

The real culprit may be the AF point selected. The 3 photos are using 3 different AF points:

_DSC0047.jpg: Center, Bottom
_DSC0048.jpg: Center, Top
_DSC0049.jpg: Lower-left, Lower-right
​

Since you were using spot metering, the AF point difference could quite easily be what led to the different exposures. You can probably confirm or debunk this theory by correlating the AF points to the full pictures (I can't really tell for sure since you only posted crops).

 

[myfotoguy]

This is interesting.

Your shutter speeds are:
DSC0047 = 1/40
DSC0048 = 1/125
DSC0049 = 1/400

That's 1 2/3 apart. The D90 can be set to 1/3, 1/2, 2/3, 1 or 2 EV for bracketing, unless I'm missing something, this shouldn't be bracketing, does that sound right?

Are your other images three in a row of normal, under, over? What does your menu have set for bracketing order, is it MTR>under>over, or is it Under>MTR>over?

I noticed exif said you were in spot metering. This looks like a situation where the shutter speed could swing that wide of range depending on where your meter was reading. You were in auto-focus area, so if you moved the camera slightly the camera could have been focusing on different areas and spot metering on different areas. Just a thought. Not sure what other ideas other folks might have. Would D lighting come into play?

EDIT TO ADD: While posting this, emh also posted similar findings.

 

[Derrel]

I would also cast my vote with those who think that spot metering is what caused the exposure fluctuations.

 

[Vinny]

My auto bracketing is set for the MTR, over, under. I could see the shutter speeds in the Nikon Nx software but the software did say ev was 0, my assumption which could be wrong is the camera electronically adjusted the image (which I thought was the ev) and didn't do it physically (didn't change the ISO, shutter speed or aperture). Some of the other images do follow a difference in light pattern - dark, correct, overexposed, others are a series of underexposed, others are overexposed ... few came just right.

The auto-focus points that the Nx software says the camera chose:

DSC0047 - mouth area and the stand that's in front of him
DSC0048 - eyes and around the knot in his tie
DSC0049 - by the front shoulder and the curtain right above where the drumstick and drum intersect

Since I'm new to auto focus and different metering types a camera has - is there a correlation between where the camera focuses and it's light meter? I thought the 2 were independent. I chose center weighted since I wasn't sure how "zoomed" I was going to be but knew my son was going to be in the center.

Question about spot metering then - the first photo is really underexposed is it possible that the meter was reading off of the light reflection of the drums even though it wasn't a focus point? Everything else is really underexposed.

I appreciate everyone's help!

 

[emh]

According to EXIF data, you were using spot metering not center weighted. Spot metering is based on a small area around the selected AF point. So that probably does explain the variations in exposure.

When I use spot metering, I generally find it useful to manually select the AF point (see pg 56 of the D90 manual).

 

[Vinny]

Thanks for that!

I was trying to get a center weighted reading (reason my son is in the middle) and I know that at one point the indicator was in the center. Too many buttons.

I guess it's time to read the book a 3rd time ... I miss my FE!!!

 

[myfotoguy]

Spot is related to the location of the focus point, and as was mentioned, will work best in single point AF. If you move that single point to the center, it won't be center weighted, but meter off that spot that happens to be in the center.

Center weighted will always meter the center of the frame, and put more importance on the center of the frame, regardless of where the focus point is. As a side Note: This is also typically where in TTL flash mode the flash is metering.

So if you have a subject off center, and there is a room behind them, say a shelf 20 ft beyond them. It is likely you will blow out the subject, because the flash meter is metering from the center of the frame (read up on FV lock and for overcoming this). If your subject is closer than whatever is in the center, than they will easily get overexposed from the flash. In TTL-BL you may end up with less chance of blowing out the subject, but indoors this often creates underexposed image, and TTL-BL is it's own little technical trail of explanation.

Sorry about the side trail about flash. But since I was explaining center weighted it reminded me about where the flash meters and thought I would mention it. That was something I wish I had known early on.

 

[Vinny]

Yeah, I screwed up royally on these photos. I had the spot in the center in the beginning and I thought it was being center weighted vs spot. The way I accidently put the camera into bracket mode I probably was changing the spot area as well!