1) Because your metering mode changed between live-view and regular.
2) Because the angle of the shot changed a tiny fraction which tipped the balance in the meter - esp if it was on the cusp between one and the other
3) Because the meter system for the viewfinder is different to that of the live-view (one would assume) and as such, like the AF is slightly different, the meter reading is a little differently read by the camera.
4) Postprocessing settings differ between the two modes.
The only difference between both shots was flipping the live view lever between shutter releases. Using a Nikon D 5500. The angle of the shot admittedly change slightly on the lightbulb picture, but was pretty much identical on the kitchen utensil picture. Regardless of the image viewed, The captured image is always darker on live view.
I have to then assume that the internal meter is working a little differently. Remember when you're using the viewfinder part of the light is reflected up to the mirror prism whilst some is sent to the AF and metering sensors; with live-view this doesn't happen as the mirror is flipped out of the way and thus the camera uses a new set of metering sensors which likely changes how they read the light and thus how they'd meter the results.
That would be my best guess unless you've got some exposure compensation going on that you're not aware of.
I found this on page 104 of my D5100 manual:
Depending on the scene, exposure may differ from that which would be obtained when live
view is not used. Metering in live view is adjusted to suit the live view display, producing
photographs with exposure close to what is seen in the monitor. In P, S, A, and % modes,
exposure can be adjusted by ±5 EV in increments of 1/3 EV (070). Note that the effects of
values over +3 EV or under –3 EV can not be previewed in the monitor.
I also remember reading somewhere that the viewfinder should be covered when using live view because of light entering the system and changing the reading. (I'm not joking, even though this seems so odd. I also seem to remember having a little piece of plastic supplied with the camera for just that purpose, but I never bothered to use it.) Afterthought: I wonder if that item would be listed on "what's in the box"?
What otherprof mentioned above, that was my thought process as well-- that light entering the viewfinder eyepiece area is affecting the meter readings; this happens a LOT in certain situations where light in a significant percentage is coming in from a viewfinder eyepiece that does NOT have a person's eye or face squished up close to the eyepiece, and which is not shuttered or closed-off to ambient light entering the eyepiece. For example, at night in urban/suburban areas, even the low light level of streetlights can enter an unblocked, open eyepiece and that light entering the camera through the eyepiece can alter or affect the readings the meter is making of the viewfinder screen inside the camera. For timed exposures for example, it's a good idea to use the camera's eyepiece shutter blind (found on high-end cameras only) or to use the small, plastic slip-over eyepiece blind that many cameras come with.
I suppose a good way to cross check would be to repeat the type of tests the OP did above, but to add a third option: using an eyepiece blind, and then determining if it makes a difference or not.
Just put your lens cap on, open your shutter in bulb mode, then hold the viewfinder up to a light source for a few seconds. The resulting image will be anything but black, and may be completely white if held there long enough!
In the old days all Nikon's had a little switch next to the viewfinder to use an internal viewfinder blind. Now this is only found on higher end bodies such as the d700, d8x0 and higher.
When I first got my d7000 I went out and started doing longer exposure shots during the day on moving water. ALL my results were affected by the open viewfinder. After a bit I realized that switch wasn't there and I had to cover it myself.
One of a few reasons I want to move up to at least a d8x0 even though I prefer the d750.
Live view and viewfinder use different metering methods.
When you meter through the viewfinder the camera uses a dedicated metering module to measure the light split off from the viewfinder prism. When live view is engaged the mirror swings up and cuts off light going to the viewfinder so metering is done by measuring the light actually falling on the camera sensor. The viewfinder cover is only needed when using the viewfinder and it has no effectwhile in live view.
Yep. What you are seeing is the result of using 2 different light metering systems that can not be calibrated to each other.
In most DSLRs the light metering sensor is part of the viewfinder system in the top of the camera.
Light metering using that sensor can only take place when the reflex mirror is down and reflecting light up through the viewfinder mirrors/prism.
Live View requires the reflex mirror be up out of the light path to the image sensor.
The metering sensor in the viewfinder then has light through the lens blocked from reaching it because the reflex mirror is up against the focusing screen.
That's why the viewfinder goes black when you release the shutter to make a photo when not using Live View.
Consequently the image sensor itself has to be used to meter the light.
A small handful of film SLR cameras - Olympus OM-2, Nikon F3, Pentax LX, Minolta 9000 - have had the light metering sensor at the bottom of the reflex mirror box.
Light that passed through a semi-transparent portion of the reflex mirror was reflected by a secondary mirror, a special reflective coating on the front shutter curtain, or the surface of the film itself, or combinations thereof, down to the metering sensor in the bottom of the mirror box.
An advantage of this method of metering light is that the measuring result requires no adjustments when changing focusing screens or viewfinders. Also, some of the cameras using this configuration (f.e. the Minolta 9000) are virtually immune against measurement errors caused by light reaching the metering cells at larger angles, like when using a Shift/Tilt lens. The method is also very useful for real-time TTL flash metering.