Lens hood length - avoiding vignetting

[Dan Fromm]

Hey, QG, why the vertex? Why not the entrance pupil?

 

[Q.G.]

Hey, QG, why the vertex? Why not the entrance pupil?

Why not indeed?

 

[orto]

..
Unless you mount a lens hood on the very lens vertex itself, the distance between the vertex and the mount where you can mount the thing must be taken into account, subtracted rom the hood length.
And that distance is different for different lenses. Is a feature of the barrel.

I know that some people put a lot of trust in theoretical considerations.
But unless you want to remain stuck in a theoretical world, it pays to be practical too.

I have now a good contact with George. And the more I know him, the more I like him - a very interesting person, indeed.
His lens shades are calculated for a given length from the lens vertex. The installation of the calculated lens is then done on the barrel according to the barrel and not the other way around. He said, putting the hood on a lens must be done so that the calculated distance is correct - that is no problem, as the hood mount must anyway be built for the given lens.
He told me stories about "clients" he helped - none of them, he said, had no problem with putting the hood on the lens so that the opening would have the correct distance from the lens vertex.
I myself cannot imagine that it would be impossible - after all, all barrels can take some mount, what's the big deal. He calculates the dimension, they build the lens mount.
What I start to see is that you just don't want to see that it is possible calculate a lens hood with a technical calculation... Why that? Jealousy, perhaps?

 

[orto]

Why not indeed?

Why not? Is it perhaps easier to measure the length distance from the pupil than it is from the lens vertex?? You, who thought the barrel shape was the thing to measure it from - you want to measure it now from the entrance pupil?? And your barrel?
How about your own calculations? Common, you are obviously so practical, it will be just a piece of cake,no? From the entrance pupil, of course!
Why not indeed?!

 

[Q.G.]

Why not? Is it perhaps easier to measure the length distance from the pupil than it is from the lens vertex??

No. Just as easy.
So why not indeed.

You, who thought the barrel shape was the thing to measure it from - you want to measure it now from the entrance pupil?? And your barrel?

See that it indeed is the way it was presented?


The length of the actual hood must be adjusted to the distance between lens (vertex/entrance pupil/principal plane/whatever) and the place the hood mount is in.
No use ignoring that bit, or the hood will be too long.

And (pardon me) it is rather silly to make fun of that, since one could easily think it shows that you do not understand what you write yourself, i.e. "the hood mount must anyway be built for the given lens".


So why the silly 'overprotective' stance?
I'm sure George understands that his method needs no 'protection' against the additional comment that the hood length does not depend on calculations alone.

 

[orto]

No. Just as easy.
So why not indeed.

See that it indeed is the way it was presented?


The length of the actual hood must be adjusted to the distance between lens (vertex/entrance pupil/principal plane/whatever) and the place the hood mount is in.
No use ignoring that bit, or the hood will be too long.

And (pardon me) it is rather silly to make fun of that, since one could easily think it shows that you do not understand what you write yourself, i.e. "the hood mount must anyway be built for the given lens".


So why the silly 'overprotective' stance?
I'm sure George understands that his method needs no 'protection' against the additional comment that the hood length does not depend on calculations alone.

Just as easy? Then why to change the way of measuring? For your (nonexistent) glory?
You yourself have presented your way of measuring - are you unhappy about it, now? It's still there, in your posts. (you can always delete them, if you thing you are not presented as you like)
Wow! Imagine - the lens hood George calculates and makes for himself are built in such a way that their construction allows to keep the calculated distance. Wonder how? Ask him.
Why don't you come with your own calculations instead of looking down at someone else work? Or you just have time for jealous remarks and don't know how to calculate it yourself? It seems so.

 

[orto]

FOR THE FUTURE REFERENCE IN THIS THREAD: George wrote to me, that anyone interested in his calculated lens hood dimensions can contact me (I'll be in contact with him) and he will gladly continue to indicate the values for the interested. You can PM me about your needs with the technical specifications of the format, lens, etc. The hood you will need to make yourself (George will help you with some construction details if you want but he doesn't sell the hoods).

 

[Dan Fromm]

Why not? Is it perhaps easier to measure the length distance from the pupil than it is from the lens vertex?? You, who thought the barrel shape was the thing to measure it from - you want to measure it now from the entrance pupil?? And your barrel?
How about your own calculations? Common, you are obviously so practical, it will be just a piece of cake,no? From the entrance pupil, of course!
Why not indeed?!

Goldfart, I asked the question because the object of the "design a hood exercise" is to vignette the cone of rays entering the lens as much as possible without losing image in the corners of the film. Doing this requires knowing where the cone's apex is. It can't be the front of the lens' front element.

 

[orto]

Dan,
if you read the posts George wrote in this thread, you can see that his hoods are calculated in such a way that any diminishing of its opening is already vignetting the film and the opposite makes the hood less efficient. Thus the practical result shows the hood is calculated correctly. It could not be so if the dimension were calculated incorrectly. I don't think you came with some new element that would proof it wrong. Somewhere he said - the proof is in the pudding. And the pudding was eaten by many members, who got his calculated dimensions...
Of course, he didn't write how he calculates them. I don't know it either. I think, knowing the theory and having made the calculation program I would keep the "secret" private too. As I said in my first post here, I admire that he managed to do a thing that you can't find anywhere else.

 

[Dan Fromm]

goldfart, you sound very much like George.

If the hoods he designs function as claims than the story that the calculations behind them use the position of the lens front vertex is a red herring.

Using the front vertex as a reference point for mounting is another matter entirely.

 

[orto]

...
Using the front vertex as a reference point for mounting is another matter entirely.

That's what I was saying and what I meant, nothing else. It's somewhere in his posts.
I had several exchanges with him about the hoods and the comments.

 

[Q.G.]

That's what I was saying and what I meant, nothing else.

Were you???

But however that may be, i wonder: why must we go through you to get in contact with George?
He is a member, and we can send personal messages to him, right?

 

[orto]

Were you???

But however that may be, i wonder: why must we go through you to get in contact with George?
He is a member, and we can send personal messages to him, right?

Just read the post n.75 - and wonder even more

 

[Dan Fromm]

QG, just between you and me I believe that George is a faker and that goldfart lives under a bridge and subsists on goats. I'd be happy to be proven wrong on both counts, but at this point all I expect is more smoke from the fire used to char the day's meal.

 

[dfoo]

I think, knowing the theory and having made the calculation program I would keep the "secret" private too.

The problem with a secret is that it remains so... if George gets hit by a bus what then?

 

[orto]

The problem with a secret is that it remains so... if George gets hit by a bus what then?

I didn't speak about it with him. Who knows?

 

[Q.G.]

Dan,

Though i (literally) don't know about George, and though i perhaps would have chosen different words, i agree.

 

[jd callow]

FYI orto, GoldFart and George are all the same user with different ID's on APUG and have been merged into Orto. Whether the APUG George and the lens shade George are one in the same is something I can't answer. Having multiple IDs are generally not allowed.

 

[makan]

George calculated all of my lens shades, all working perfectly. I'm horrified his posts got corrupted.

 

[Uhner]

...

 

[whojammyflip]

Lens hood according to f-stop?

What an enjoyable thread! I've done some back of the envelope drawings and concur that Helen's elegant simple arithmetic agrees with my drawings.

Simply explained for anyone wondering how to derive it, take half the FOV (call it theta), and HD=2.(HL.tan(theta)+LD/2), where HD is hood dia, HL is hood length and LD is lens dia. [theta is actually equal to arctan(filmDiag/2/focalLength), so I think Helen's calculation is perfect, the trig functions cancel out]

If you want a more efficient design, imagine fitting the rectangular hood inside the circumference of the round hood. For a 2:3 35mm format, the hood will have sides of 0.55xRHD:0.83xRHD, which again comes from simple Pythag stuff.

The rectangular hood will cut flare by nearly half as the area of the rectangle is 0.59 of the round hood.

For a further improvement, I'd be really interested to hear how to adapt this calculation for stopped down lenses!