[Rondec]

I think the problem comes in if (like with the DA *55) you have a hood designed for APS-C and shoot the lens on full frame. You do get some vignetting with the hood, but typically a hood just blocks light that is actually out of the field of view from causing flaring.

[Sandy Hancock]

Unless you're shooting slide film for analogue projection, who cares about 1/3 of a stop?

[danielwritesback]

On some camera hands book that tell me a hood will decrease light value about 0.3 aperture, somebody says there is no effect at all. Which one is real?

Lens hood = no change
Lens hood + 81A filter = 1/3rd stop

If the camera handbook had been translated between languages, or compressed, then the 0.3 aperture surely came from a discussion of filters.

Here is the likely confusion:
You don't need a filter when using a lens hood, but...
You need a lens hood when using an uncoated filter.


I suspect that the handbook wasn't clear about the equipment and scenarios:
The exposure correction (for filter) is automatic/included with the in-camera meter of your slr/dslr/mirrorless-digital camera; however...
The exposure correction (for filter) is manual with the external/pocket meter for your antique TLR/View/Rangefinder film camera.
Example of pocket meter:

Using a device like this is the only time you need to worry about the exposure compensation (for a filter). This applies to an antique camera, or even a new camera if its light meter were either inaccurate or inoperable. Then, a filter is in front of the lens, but not in front of the external light meter. That's when you compensate for that specific difference, manually.


Or, in QA form:
What is the lens hood for? Because of the filter.
What is the exposure compensation for? A filter.
Does a lens hood need exposure compensation? No.
Does a lens hood need a filter? No.
Does a filter need a lens hood? Yes.


P.S.
If an explanation from me did better than the handbook, then...
A more reliable handbook is probably available.