[Wheatfield]

Did some testing today. First photo, no hood. Second photo with hood. The lens hood definitely makes my photos darker when in place. This is a Pentax hood that came with the lens. I shot these with the lens wide open which is not the sweet spot for any lens. I thought it might show the most dramatic results. I see little difference in IQ with or without the hood. Just the hood makes for darker exposures even though the aperture and shutter speeds were equal.

Your light has changed. That is the culprit in this instance.

[fstopfanatic]

No filters used on the lens. My thought is if the lighting had changed that much, why wouldn't the aperture or shutter speed have changed. To my eye the lighting was identical in both shots and evidently the camera thought the lighting was the same also.

[pakinjapan]

This test might be interesting for you
About Lens Hoods IMPORTANT! - PentaxForums.com

Yes, I think the light on your images are the same too. Image 2 should be a lot easier to deal with in post process for me. Hood seems to help increase mocro-contrast and it definitely not reduce IQ.

[BrianR]

the shadow on the fence is definitely different between the two photos

Exactly this. The light changed, look at the edge of the shadows. Hard edge on the shadow means a small light source (e.g. the sun), soft edge means a large light source (e.g. the sun behind some clouds). Shadows don't change like that unless the light did. This will be much more noticeable as the distance between an object and its shadow is larger (the distance of the roof to the fence vs the distance of the roof to the siding closest to us).

Why didn't the camera adjust exposure to compensate? Who knows what auto-picture mode has planned, maybe it was trying to avoid diving too deep into the highlights in the sky in both cases? Maybe it was on the cusp of changing in opposite direction for each photo? Maybe it was feeling lazy?

Repeat the test and make sure the light doesn't change in-between. It's a fact that whenever I want to test two different setups and have the time to do it, it will be a day with fast moving spotty cloud cover. If you're doing it outdoors, have a look at any clouds near the sun, wait for a large enough opening. I'd also suggest full on manual mode to rule out any auto-corrupting from the camera's little brain.

[Bophoto]

The photo without hood is lighter and the subject has a higher contrast due to stronger sunlight.
But with and without hood, the camera's light meter seems to respond a bit different, try 1/3 more exposure compensation with the hood.

ups, there was a page 2.
To compare the contrast of images, they should be as bright.