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# Thread: Lens hood length - avoiding vignetting

1. Originally Posted by George
... 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.
...
Well, I thought that it was too simple to have to explain. All you do is multiply the relevant film dimension (the height or the width) by the ratio of the hood diagonal over the film diagonal. For example:

Hood height = Film height x H/S

I’d be interested to see the precision of your method in comparison to the crudity of the one I gave. Could you give me a range of hood sizes for the following lenses:

55 mm lens with 46 mm diameter front element;
80 mm lens with 44 mm diameter front element;
400 mm lens with 69 mm diameter front element.

The film dimensions are 94 mm x 120 mm.

Many thanks,
Helen
PS If a good lens hood reduced the difference in illumination between the centre and the edges of the frame, then it is likely that a significant portion of the difference was caused by flare being greater in the centre than at the edges. That's not unusual. I've never felt the need for a centre filter for a 50 mm lens on 6x9.

2. Here it goes (doubble checked) -
1) 55mm lens - for L (length of the shade) 60.8mm, A and B sides = 92.4 x 118mm
2) 80mm lens - L = 86.2mm, AxB = 89.6 x 114.4 mm
3) 400mm lens - L= 99.3mm, AxB = 56.9x72.7mm
Don't forget that I measure the length L from the lens vertex. Let me know how far it is from your model.

You explanation seems to be logical. It somehow goes against the common sense that most of the flare could be in the center. Simply because the smaller the lens diameter the better chances against the flare the lens has... Anyway, I believe you. George

3. Curious, I just checked it for at least the first of your examples. Hm, something is not good in your model - it gives very unnecessary big dimensions for the hood. If I didn't make a mistake, your model in the first case gave me 214mm hood diagonal against my 148.9 mm. That is too a big difference, you would be loosing too much of the efficiency. In fact if I were not so lazy I would find my original algorithms for my program but I have too much to do in this period of time. I even sent, some 12 years ago, the computer program with all the geometry to the Lee hoods firm and they answered me with a letter, telling me that my hoods are more efficient than theirs (not in production yet), for several reasons. (As a matter of fact my prototype of the first self supporting lens shade preceded theirs with 1 year, yet another story).

4. Your maths is correct: there is a huge difference in the two methods. Could you do one more to help me understand what is going wrong with the classic simple method?

Lens focal length 52 mm
Film sides 55 mm x 55 mm
Diameter of front element 50 mm

Thanks very much,
Helen

5. Here it is: L=70.7mm; AxB = 88.2 x 88.2mm. Good luck!

6. Thanks George,

I'm finding with these wider lenses and long lens hoods that your hood is well within the field of view. For the last case, the 52 mm lens, I measure a side (A and B in your notation) of about 115 mm at 71 mm from the front vertex as being just in the field of view, definitely not 88 mm. The 55 mm and 80 mm examples are also much too small. I haven't tried the 400 mm lens.

Any thoughts?
Best,
Helen

7. How are you finding it is in the field of view? Mathematically, on a drawing or practically? If on a drawing someting must be wrong with the drawing, sorry to say. I calculated and constructed under a 100 lens shades with the program and didn't have any encrouching lens shade at all. The program is bullet proof. I have lens shades for all my film formats (24x36, medium formats, panoramic formats, LF and special formats of my special homemade cameras) all my friends who wanted them have them too and I had never 1 simple case of a vignetting shade. All of them were right on the first use. Try it practically on you lenses and I am sure you will not get vignetting. Otherwise we don't speak the same language.
The shade cuts all the image circle that is not on the film format.

8. Let's check it the other way - what is your proposed dimension for 600mm focal length, 58mm diameter front element, lens hood long 92mm, for 56 x 82mm film format?

9. I correct the length of the shade in the last example on 95mm...

10. Helen,
I know where the devil lies - in details, of course. I made a mistake in the results I gave you for you examples. I mistankenly took horizontal values for vertical ones and it gave smaller dimension than necessary. So here it goes corrected:
1) 55mm lens: length of the shade 54mm, sides AxB = 129.1 x 101.1 mm.
2) 80mm lens; length 80mm, sides AxB = 129.1 x101.1 mm.
3) 400 mm lens; length 98.9m, sides AxB = 77.7 x 60.9
That is of course bigger than what I wrote before. Hope we get closer now in our methods? George

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