I posted the following as a response to a question in the CANON CORRAL where the importance of the issue might be missed by many non-Canon users and so, if it is permissable, I am posting it here as well. The initial query had concerned the difference between round and scalloped lens hoods - with additional comment on the shock absorbing qualities of lens hoods in the event of a mishap.
- The lens hood used as bumper bar is a long-standing ancillary benefit - in fact the rigid lens hood is considered by many to be a better option in this case than the collapsible rubber type because it will prevent the front element or filter smooching damaging surfaces with greater certainty.
But back to the real purpose of lens shades and why some have 'petals' or are scalloped:
The primary purpose of the lenshood is to reduce the entry of non-image-forming light into the optical system. There is an inherent level of flare in all optical systems which is increased by the addition of this non-image-forming light. Coating and then multi-coating of lenses reduces flare somewhat but still the most effective way of containing the ill-effects is to block the culprit before it enters. Enter the lens hood!
Just how much more effective the scalloped lens hoods are than regular round or rectangular lens hoods is a matter of the precision of alignment with the edges of the image with which they are engineered and manufactured.
I primarily use a view camera - 'What place have you here then? I hear you ask ... Well the principles of many aspects of photography apply universally across the board and are sometimes better addressed in other kit. Thing is that with the view camera the lenshood is often another bellows mounted in front of the taking lens which can be adjusted to best shield the optics from extraneous light. In fact, for my Linhof there is a device which can fit in front of this lens hood bellows with four independently adjustable board masks. These are aligned with the very edges of the image by inspection at the shooting aperture through cut-off corners of the ground glass in an effort to eradicate ALL but the actual image-forming light from entering the system.
The 'petal' lens hood is an attempt to address such a situation with the SLR or DSLR but without the same level of sophistication. For them to make one that would be 100% effective at f16 it would vignette at f4.
Another oft-used device is what is known a a FLAG. This is a separate device- usually a piece of flat black meterial like a card - which is placed in such a way as to prevent harsh and intense light falling on both the front element of the lens and THE LENS HOOD. You see despite the ribbing and matte finish, the internal surface of the lens hood can actually bounce strong light back into the lens, albeit at reduced level, but strong enough to degrade image quality. Flags can also be employed in studio set-up close to lamps to ensure that they do not shine directly into the lens also. The best check is to look at the front element of the lens with it in its shooting orientation and 'flag' any sources that can be seen directly reflecting the front element.
The optical system and its internal reflections is not the only source of image degrading due to flare. BELLOWS FLARE or internal body flare is also a major issue. This is light forming part of the image circle of the lens which is not contained within the image area rectangle but falling on the walls of the mirror box or bellows or whatever. Where this is of particular relevance to DSLRs with APS sized chips is that full-frame 35mm lenses are filling the camera with an image circle of light far greater than is necessary - a great point in favour of using DX lenses on DSLRs.
One last comment on flare: It is also greatly increased by dust and marks on the lens elements. Keep your lens clean and minimise the air-to-glass interfaces by using filters ONLY when essential.
Cheers,
