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# Thread: Lens hood length - avoiding vignetting

1. The quick and crude way to check lens hood vignetting on a camera with ground glass or a focusing screen is to illuminate the screen and check its corners through the lens and hood. This might not be perfectly accurate with SLRs with less than 100% viewing, but works good enough for practical use.

2. Here's a simple way of calculating the diagonal of a rectangular or square hood, or the diameter of a round hood. It assumes that the front element is only just big enough to avoid vignetting when the lens is wide open (this may not be true, and the assumption may lead to hoods that are slightly larger than they need to be).

f = lens focal length
S = film diagonal
L = diameter of front lens element
H = diameter or diagonal of lens hood
d = distance lens hood protrudes in front of front element, measured from the periphery of a convex front surface, not the vertex.

All measurements should be consistent, eg all in millimetres. It gives hoods that are slightly larger than George's. The formula is based on similar triangles, with the fundamental one being the one formed by the film diagonal and the focal length.

Strictly speaking it only applies for a round hood, but the correct calculation for a rectangular or square hood would require the aperture, and it would be more complicated. The error should not be great, though the error does reduce the size of the hood to slightly below what it would be with a rigorous calculation.

H = S.d/f + L

Best,
Helen

3. Sure -your calculation gives a lens hood with bigger than necessary opening so it increases the size of the hood, not reduces as you say. That leads to reduced efficiency - the common defect of all the shades on market. Also, calculating a lens hood according to the aperture in use has no practical meaning - you don't want to change your lens hood when you change your aperture. In itself calculating a square or rectangular lens shade (and only such a shade can be really effective in all situations) doesn't require aperture input more than a calculation of a round lens hood.

5. One more detail. In my calculations I use a way how to express the efficiency of the designed lens shade. I express it in the percentage of the eliminated flare causing light, given certain general way of illumination. In such a way I know where to stop prolonging the lens shade, as after a certain point you get (in a non linear way) too little (efficiency increase) for too much (shade's dimension). Even if in the real situation the efficiency could be less than calculated in the very majority of cases it is much, much more (given the way I pressume the lighting).

6. Helen,
sorry to say, but people must be warned ;-)
As you say, your calculation is just for a round hood. This means that in some cases they are of very small efficiency. In many cases, like for longer focal lengths, you can make a lens hood with a rectangular opening that has one side (the vertical if it is used on a panoramic format) even smaller than the front lens diameter! In the case of your calculation this dimension would be increased with several cm - making the shade insufficient almost like those rubber shades sold in shops (I call them "rubbish" shades, for that's what they are). Sorry to say, but someone who doesn't know could think that calculating with you equation he gets a good lens shade while obteining a nothing close to it. Only shades that copy the film format shape can be efficient - provided they are well calculated or tried. Com'on, you know better than that - give them the true stuff ;-)

7. George,

You didn't read what I said: "Here's a simple way of calculating the diagonal of a rectangular or square hood..." The results are similar to yours, and, using that simple formula (I can't call it 'my formula') one dimension could be smaller than the diameter of the front element. I agree that lens shades should copy the film format for maximum effect.

Where did I suggest that the lens hood should be sized for the aperture in use? The relevance of the maximum aperture for rectangular hoods is to try to get as close to the extremities of the ray bundles as possible without any obstruction of them. You run into difficulties for a rectangular hood because the envelope of the ray bundles is not a simple projection of the film format - it's generally barrel-shaped to some degree. This can be seen in the design of the entry baffles for some lenses. Of course you need to know a lot more than just the aperture to design the ultimate baffle or matte.

I only offered the simple formula in the absence of anything better - you had turned down my request to explain your method - and I outlined the reasoning and assumptions behind the formula I gave. In reality I wouldn't use a purely theoretical method to determine the geometry of a lens hood - I'd use it as a starting point, then use a method similar to the one Jim describes to fine tune it.

I, for one, would be very grateful to you if you were to explain your method.

Best,
Helen

8. Don't worry, I read it all. If the poor guy gets just the value H of the diagonal you still have to give him a way how to calculate the correct rectangular dimensions as there are more different rectangulars with the same diagonal - only one of them corresponds relatively to the film format.
You yourself say that the correct calculation for a rect. or sqr hood would require the aperture. Sorry, the barrel shaped projection is just of a purely academic importance when you think of the practical issues involved in the lens shade use. You have to have some space for not absolutely precisely placed hood, not absolutely precise distance from the lens, etc. You don't make the lens shade with a micrometer either. And because the given lens shade has anyway just a limited efficiency you cannot strive for any "ultimate" efficiency or a hood shape.
I use the calculations exactly to avoid a plenty of Jim's "fine tuning" - not at all easy when you have to move all for sides and keep the centre of them on the axis. Actually, when I put the lens shade on the lens and try then to shorten one or the other side it shows vignetting immediately - such is the precision of the calculation.
The reason I don't want to go to lengthy explanations of the calculation is mainly practical. I wrote it as a computer program 15 years ago and I didn't care about it from then. It's in the old Qbasic (remember?) and I never cared to rewrite it again - on my XP I use the Qbasic taken from the history. Nobody will pay me the time for explaning the whole theory. Whoever wants, can ask me the dimensions. Cheers.

9. As an added bonus here's a real story. I once calculated a lens shade for a guy with his Alpa camera with a wide angle, 47mm if I recall it well. He made it(very nicely, machined from a block of some hard plastic, I was impressed) only to write to me later that when he started to use this lens hood he noticed that he didn't need a centre filter for the lens anymore. And he sent me pictures taken with and without the lens shade and the filter. I was surprised, and sincerely, I don't know the reason for that effect on the film. Helen, any theory?

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