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Hood lenght calculation?
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PostPosted: Tue Apr 08, 2014 7:11 am    Post subject: Hood lenght calculation? Reply with quote
What is the right lenght for 150 2.8 which has 82mm thread?

(Kaleinar 3B for p6)


PostPosted: Tue Apr 08, 2014 7:54 am    Post subject: Reply with quote
The maximum length of a cylindrical hood without vignetting depends on the sensor size. You can measure the length by fold a cardboard in front of your lens.


PostPosted: Tue Apr 08, 2014 8:13 am    Post subject: Reply with quote
If sd = sensor diagonal, fl=focal length of the lens, d= diameter of the cylindrical hood and l = length of the cylindrical hood
then
d/l >= sd/fl will give you the maximum hood length without vignetting as

l <= (fl x d)/sd


PostPosted: Tue Apr 08, 2014 8:35 am    Post subject: Reply with quote
I found a thread on pentaxforum which it worth to read.
http://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/134700-optimum-hood-depth-calculator.html


PostPosted: Tue Apr 08, 2014 8:57 am    Post subject: Reply with quote
FF it is, sorry.


PostPosted: Tue Apr 08, 2014 9:02 am    Post subject: Reply with quote
dan_ wrote:

l <= (fl x d)/sd


I'll try:
(150 x 90) / sqroot(35^2+24^2) =
13500 / 42,4 mm = 318 mm

ok, I'm not very good at math
Confused


PostPosted: Tue Apr 08, 2014 9:51 am    Post subject: Reply with quote
(150x82) /43,3 =~284mm

As you can see the hood diameter is about double then the sensor diagonal =>the maximum lens hood length is about double the focal length.
But the hood length should be measured from the diaphragm position (center of the lens) and not from the front lens, as the focal length is measured similar.
The formula that calvin has pointed to should be more accurate as it takes into consideration the f/number as well.


PostPosted: Tue Apr 08, 2014 10:17 am    Post subject: Reply with quote
Most generic screw-in hood is not longer than 3 inches. Wink


PostPosted: Tue Apr 08, 2014 10:50 am    Post subject: Reply with quote
calvin83 wrote:
Most generic screw-in hood is not longer than 3 inches. Wink

Of course, but I counted the maximum length of a hood. For safety the real length should be shorter.
It's simple geometry, as in this schema:



If my calculations are wrong please correct me.


PostPosted: Tue Apr 08, 2014 11:08 am    Post subject: Reply with quote
dan_ wrote:
But the hood length should be measured from the diaphragm position (center of the lens) and not from the front lens, as the focal length is measured similar.


Not the sensor? (edit: sorry didn't see the drawing for some reason)

284mm sounds a bit long?


PostPosted: Tue Apr 08, 2014 11:12 am    Post subject: Reply with quote
calvin83 wrote:
Most generic screw-in hood is not longer than 3 inches. Wink


this is what I'm thinking.

Click here to see on Ebay

so it could be fine for a 150mm lens?


PostPosted: Tue Apr 08, 2014 11:21 am    Post subject: Reply with quote
Sigma standard hood is pretty long for same specs I think?



PostPosted: Tue Apr 08, 2014 11:38 am    Post subject: Reply with quote
All the maths here are over my head.

I know somewhere where you can download templates for DIY lenshoods. Specific to the lens - perhaps not for you but others may find it useful

http://www.lenshoods.co.uk/


PostPosted: Tue Apr 08, 2014 12:29 pm    Post subject: Reply with quote
A most accurate calculation of the hood length should take into account the diameter of the front lens and the angle of view of the intended focal length for the intended format.

It should look like this :


a = (angle of view) / 2 and the angles of view for different focal lengths and formats can be found here:

http://rabinergroup.com/pdfs/LensangleofviewtableLong.pdf


PostPosted: Tue Apr 08, 2014 1:55 pm    Post subject: Reply with quote
so a = about 7, d = I guess 76mm, D = (guessing again) about 90mm.
That makes L = (90-76) / (2xtan(7)) = 14/0,25 = 56.
75 mm is then way too long?

If the hood's Diameter was 100mm the the correct lenght it would be 96mm.

well: 75 = (x-d) / 0,25; 75*0,25=x-d; x=d+(75*0,25); x=d+18,75
the hood is perfect if it is 76+18,75mm=94,75mm Diameter.
Confused Smile


PostPosted: Tue Apr 08, 2014 11:24 pm    Post subject: Reply with quote
One of Hasselblad's neatest accessories is there adjustable bellows lens hood. It is too big and bulky for 35mm/digital. I really wish someone would make a scaled down version for the smaller format

As for yours, the Hassy one will most likely fit yours with some modification.


PostPosted: Wed Apr 09, 2014 7:54 am    Post subject: Reply with quote
NikonAIS wrote:
One of Hasselblad's neatest accessories is there adjustable bellows lens hood. It is too big and bulky for 35mm/digital. I really wish someone would make a scaled down version for the smaller format

As for yours, the Hassy one will most likely fit yours with some modification.


That would look nice too, unfortunately they are more expensive than the lens itself.


PostPosted: Wed Apr 09, 2014 12:36 pm    Post subject: Reply with quote
kansalliskala wrote:
NikonAIS wrote:
One of Hasselblad's neatest accessories is there adjustable bellows lens hood. It is too big and bulky for 35mm/digital. I really wish someone would make a scaled down version for the smaller format

As for yours, the Hassy one will most likely fit yours with some modification.


That would look nice too, unfortunately they are more expensive than the lens itself.


There are a few on Fleabay that are running around $100 +/-


PostPosted: Wed Apr 09, 2014 3:23 pm    Post subject: Reply with quote
NikonAIS wrote:
kansalliskala wrote:


That would look nice too, unfortunately they are more expensive than the lens itself.


There are a few on Fleabay that are running around $100 +/-


Lens was 25 €. Smile Smile Smile


PostPosted: Wed Apr 09, 2014 3:45 pm    Post subject: Reply with quote
kansalliskala wrote:
NikonAIS wrote:
kansalliskala wrote:


That would look nice too, unfortunately they are more expensive than the lens itself.


There are a few on Fleabay that are running around $100 +/-


Lens was 25 €. Smile Smile Smile

Pretty cheap for Kaleinar 3B. Congrats!


PostPosted: Wed Apr 09, 2014 4:03 pm    Post subject: Reply with quote
calvin83 wrote:
Pretty cheap for Kaleinar 3B. Congrats!


Yes I know. Seller sold it "as is", aperture slightly sticky.
But it was actually very clean, just a little shake it works. Don't even think about opening it. Almost mint.


PostPosted: Wed Apr 09, 2014 4:31 pm    Post subject: Reply with quote
kansalliskala wrote:
NikonAIS wrote:
kansalliskala wrote:


That would look nice too, unfortunately they are more expensive than the lens itself.


There are a few on Fleabay that are running around $100 +/-


Lens was 25 €. Smile Smile Smile


I am sure it can be used on many other lenses too
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