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03-01-2011, 02:48 PM   #31
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QuoteOriginally posted by sewebster Quote
If you have a hood that is a cylinder of the same total length of the tulip then it will vignette. If you have a hood that is only the length to the cutouts of the tulip then you won't get as much light shielding. The tulip shape is "longest" where the angle of view is smallest and "shortest" where the angle of view is largest.
I gather the cut-outs in the tulip correspond to the corners of the sensor where vignetting is likely to appear when a round hood is used--right?

03-01-2011, 02:57 PM   #32
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QuoteOriginally posted by dadipentak Quote
I gather the cut-outs in the tulip correspond to the corners of the sensor where vignetting is likely to appear when a round hood is used--right?
That's the idea.
03-01-2011, 03:58 PM   #33
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Can you use a tulip hood when shooting other flowers?
03-01-2011, 03:58 PM   #34
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Kidding!

03-01-2011, 04:17 PM   #35
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QuoteOriginally posted by dadipentak Quote
I gather the cut-outs in the tulip correspond to the corners of the sensor where vignetting is likely to appear when a round hood is used--right?
Hold a tulip hood to your lens rotated 90-degrees out of normal alignment and observe the effect.

H2
03-01-2011, 04:48 PM   #36
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QuoteOriginally posted by sewebster Quote
If you have a hood that is a cylinder of the same total length of the tulip then it will vignette. If you have a hood that is only the length to the cutouts of the tulip then you won't get as much light shielding. The tulip shape is "longest" where the angle of view is smallest and "shortest" where the angle of view is largest.
I understand that but it amounts to only a little savings of material for the same total protrusion in front of the camera.

The best shape is probably a rectangular pyramid with cross sectional shape similar to sensor and an angle that matches the field of view of the lens. I recall seeing hoods with that shape.
03-01-2011, 05:36 PM   #37
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QuoteOriginally posted by newarts Quote
I understand that but it amounts to only a little savings of material for the same total protrusion in front of the camera.
Material savings compared to what? If they didn't make the tulip cutouts there would be vignetting if the same total length were retained.

QuoteQuote:
The best shape is probably a rectangular pyramid with cross sectional shape similar to sensor and an angle that matches the field of view of the lens. I recall seeing hoods with that shape.
Yup.
03-01-2011, 05:37 PM   #38
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QuoteOriginally posted by newarts Quote
The best shape is probably a rectangular pyramid with cross sectional shape similar to sensor and an angle that matches the field of view of the lens. I recall seeing hoods with that shape.
I associate that shape with movie cameras.

03-02-2011, 04:39 AM   #39
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QuoteOriginally posted by sewebster Quote
Material savings compared to what? If they didn't make the tulip cutouts there would be vignetting if the same total length were retained.
I think a better way to look at it is the tulip shape cuts out light not used due to the rectangular frame.

The material savings I refer to is compared to a simple cone as high as the tulip tips at an angle that avoids vignetting.
03-02-2011, 04:41 AM   #40
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QuoteOriginally posted by newarts Quote
I understand that but it amounts to only a little savings of material for the same total protrusion in front of the camera.

The best shape is probably a rectangular pyramid with cross sectional shape similar to sensor and an angle that matches the field of view of the lens. I recall seeing hoods with that shape.
If you come across a picture of one could you post it here. My mind is having a hard time trying to picture it for some reason.

Thanks
03-02-2011, 04:43 AM   #41
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Here is a link on hoods: Lens hood - Wikipedia, the free encyclopedia

Here is a link for those DIY: How to make a hood for your lens

It helped make things a little clearer for my aging brain..

Last edited by glee46; 03-02-2011 at 04:44 AM. Reason: Misspelled words
03-02-2011, 05:47 AM   #42
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Optimal Rectangular Pyramid Hood

QuoteOriginally posted by rparmar Quote
...
How can one calculate the optimum hood depth range for a given focal length? That is, the minimum length that will do any good or the maximum length that will work with no vignetting. Of course this will depend on sensor size as well....
Here's a design for a rectangular prism shaped hood. It is optimum in the sense that there is no other shape that lets in less light from outside the field of view for the same hood height. It also uses almost the minimum possible amount of material in its construction.


The fraction of light from outside the field of view is proportional to (1+Focal.Length/Hood.height)^2, so there is no optimum height (the higher it is, the less extra light fraction enters.)

Dave

EDIT: I'm not recommending that anyone build one of these (although it'd be easy to build and I'll post a cutting template if anyone asks); rather, I'm addressing the optimum hood question. It is likely that a conical hood just a bit deeper than the optimum rectangular hood can do as well (hmmm...I'll think about this...)

RE-EDIT: I've changed the drawing to remove an error involving masking of the lens.

Last edited by newarts; 03-02-2011 at 06:48 PM.
03-02-2011, 05:55 AM   #43
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QuoteOriginally posted by glee46 Quote
Here is a link on hoods: Lens hood - Wikipedia, the free encyclopedia

Here is a link for those DIY: How to make a hood for your lens

It helped make things a little clearer for my aging brain..
Good DIY link. The proper hood height (depth) can be estimated from the relationship posted earlier:

Height = Focal.length (Top.diameter/Filter.diameter-1)

This presumes that the filter ring is just on the verge of vignetting, hence is safe but may not be optimum.

Dave

Last edited by newarts; 03-02-2011 at 06:39 AM.
03-02-2011, 08:12 AM   #44
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QuoteOriginally posted by rparmar Quote
Can you use a tulip hood when shooting other flowers?
only if you promise to wear rose colored glasses
03-02-2011, 08:16 AM   #45
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the big issue with respect to hoods, regardless of rectangular or petal hoods is the ability to gaurantee regestration to the sensor, to avoid unwanted vignetting at the corners.

also note that rectangular hoods, while better than circular hoods are still not optimum.

as someone stated earlier the shape of a petal hood is defined by the field of view at that specific point along the sensor . if you are shooting very close to the sun, the difference could still be quite noticebale.
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