Lens hood question

Funnily enough there has been quite a bit of hood talk recently, just while I'm having a hood dilemma of my own.

Does anyone know of an online hood size calculator? I'm trying to figure out what would be the best screw on hood for a 50mm lens (I was planning to take into account that it is on an APS-C camera, so I was looking for a longer than a 'normal' 50mm hood).

Hood sizes are a mystery to me. Take for example the giant hood on the Pentax 50-135 - that hood has to work when the lens is at 50mm right? So why aren't hoods for 50mm lenses that size as well?

Another example: the (original) round hood on my 85mm Takumar is tiny (and presumably not much use) - it's only about 2cm long! Whereas the petal hood on my Sigma 70-200, which works when the lens is at 70mm, is enormous, at least 7cm long. Why didn't the Takumar ship with a longer hood? Should I get a longer hood for it? Will it do anything?

Does the front element turn? besides that I don't think you have flower screw-in hoods to begin with only circular hoods.
I think you can just take the 1.5X crop rule into account, so you should be looking for a hood for a 75mm lens. Don't hang me on it though.

This article provides a pretty good explanation http://toothwalker.org/optics/lenshood.html

Regards - Steve

Thanks for the replies. Stefan but as I already said I already have a hood for an 85mm lens, and it is puny. Steve I had already read that one. Interesting but doesn't really answer any of my questions.

Tim, my understanding is that there is benefit (up to a point) to be gained by using the longest hood possible that does not cause vignetting. The benefit I believe is more pronounced on digital than with film.
As to why 50mm lens hoods are not longer, a hood the length of the one supplied with the 50-135 would probably cause vignetting at the wider apertures usually associated with a 50mm lens. That aside there is a a law of diminishing returns and it would make such a lens quite unwieldly.

I agree with Stefan, in that the crop factor needs to be taken into account , when considering a replacement hood. So in the case of your 85mm Takumar I would be looking for something originally designed for a 135mm lens. I purchased one of the expandable types described in the article for use on a 35-105 lens and it does work pretty well.

By way of illustration many DA lenses will show less vignetting on a full frame SLR with the lens hood removed.

Regards - Steve

85mm shouldn't work to begin with, that will vignette most likely. Also as you said lens hoods have different sizes, you've long and short 85mm lens hoods.
The longer the lenshood the more effective they will be.

The hood of the 50-135 is probably so big because it should also be effective at 135mm and also the lens itself is quite long so if you reverse the hood it isn't longer then the lens. That's why the hood of the DA55 is shorter because the lens is shorter.

Thanks both of you for your input!

Essentially I am trying to find out the longest hood that will not vignette on a 50mm lens for use on APS-C. Does the aperture make a big difference or is it minimal? I'm looking for the longest hood possible for my 50/1.2 and my 50/1.7, as well as a new hood for my 85/1.8 to replace the tiny original one.

Stefan regarding the 50-135: the 135mm end surely doesn't have any relevance as the hood must work at 50mm right? So it is a 50mm lens hood. Shouldn't we be using hoods that size for our 50mm lenses?

Yes the longer the better but when you reverse mount the hood on the lens for storage you don't want the hood to be longer than the lens.
Also the larger the aperture the larger the opening of the hood must be, it's the same that the elements in the lens or larger as well.
It's explained on the toothwalker page.

Indeed you're right about the 50-135 hood ^^

You often see telescopic hoods, you might want to use one of those to get the most effective hood. Simply slide back till it doesn't vignette.
Hama has some for example link