Determining Lens Hood Dimensions

[nation_of_pomation]

I have kind of an odd question: how do I determine the appropriate opening diameter and barrel height for a lens? On an SLR, I figure that I could just look through the viewfinder and see whether or not it was in the way, but on a rangefinder, this isn't an option. So, I was wondering if there is a formula for determining this. I'm guessing it'll have something to do with the focal length and the filter ring. Any help would be greatly appreciated, and if it helps with an creating example, the lens I'm currently trying to find an appropriate generic hood for is an 85mm lens with maximum aperture f2 with a 49mm filter diameter.

 

[Particular]

Open the back of your camera, and hold the shutter open on B. You can check if a hood vignettes by looking through the lens from behind. Make sure that the hood isn't blocking the view right into the corners.
Obviously won't work as well with screwmount leicas.

 

[ferider]

Hi Adrian,

Does the following make sense ?

For a rectangular hood:

x = (l + 27.8mm - f) * 35mm / f

For the diameter of a round hood, you need to replace 35mm with the image diagonal.

Roland.

 

[Ronald M]

Place some matt acetate where the film goes. Now look at the corners from an angle like the X shown above. The acetate acts as a ground glass. Cover with a coat to keep out stray light.

Move an object in so you can see it.

The same thing is done with a view camera to check lens coverage. The full aperture needs to be seen.

 

[ray*j*gun]

Thanks Roland nice diagram and formula.

 

[nation_of_pomation]

For the diameter of a round hood, you need to replace 35mm with the image diagonal.

Roland.

It all makes perfect sense, except what do you mean by image diagonal? The diameter of the widest opening of the hood?

 

[ferider]

It all makes perfect sense, except what do you mean by image diagonal? The diameter of the widest opening of the hood?

If you make a rectangular hood, what will limit its width is the with of the film - 35mm (that's where in comes from in the above formula).

If you make a round hood, it's size is limited by the picture diagonal, around 43mm.

Roland.

PS: thanks, Ray.

 

[nation_of_pomation]

I understand. So, the formula for a circular hood would be:
x = hood diameter
l = length from hood to camera
f = length from rear element to film

x = (l + 27.8mm - f) * 43.3mm / f

Correct?

 

[ferider]

I understand. So, the formula for a circular hood would be:
x = hood diameter
l = length from hood to camera
f = length from rear element to film

x = (l + 27.8mm - f) * 43.3mm / f

Correct?

All correct, except f being the focal length.

And I am assuming you are using a Leica. If you don't, you need to substitute 27.8mm with whatever registration distance your camera has, here is a list: http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html

Best,

Roland.

 

[nation_of_pomation]

Okay, thank you! I wish I had a Leica, mine are LTM cameras, so 28.8mm should be about right. Thank you so much for your help!

 

[nation_of_pomation]

So, the lens hood I was trying to use is 40mm from the front of the camera including the 40mm to 40.5mm step up ring (since it's a Canon), the registration distance is 28.8mm, the focal length is 28mm (nominally).

x = (40+28.8-28)(43.3/28)
x = ~63
So, the diameter of the end of the hood would have to be 63mm at that distance, and this one is only 55mm, so I guess that explains it.

Alternately, I had another hood I was saving for a 35mm lens, but for the sake of practice:

x = (38+28.8-28)(43.3/28)
x = ~60

Again, the hood is much too small as this takes a 46mm cap that came with it. You know, I've hated doing math my entire life, but this is actually bearable! Photography is a beautiful thing.

 

[Haigh]

Thanks for asking this question. I have often wondered myself. I am most impressed with the wonderful diagram posted in answer to your question.

 

[nation_of_pomation]

I second that thought, Roland's diagram is great!