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03-03-2011, 04:35 AM   #61
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QuoteOriginally posted by Lowell Goudge Quote
Dave

To put it in simple terms the
Petal is defined by the intersection of the lines that project out from the lens at any point along the angle of view of the lens for a particular sensor to the edges of the rectangular hood with the cylinder

They both offer the same exact protection since the edges follow the same plane of the rectangular permid.

Don't believe me just cut a hole in the bottom of the permid that fights tightly around a cylinder
Thanks Lowell, I understand and have understood that.

03-03-2011, 04:17 PM   #62
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I just knew this forum was seriously missing a thread on this topic. :-)
03-04-2011, 12:11 PM   #63
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QuoteOriginally posted by newarts Quote
. . . your proof of concept!
OK - Put up or shut up I guess.

I just submitted the step-by-step process as a DIY item - it should be up soon. I opted to go 'square' rather than mess about with the origami-fold trick for this one. There's no reason to go rectangular when you can achieve the same effect by trimming the square tube.

Which one of those hoods would you want to carry around in your kit bag?




H2
03-04-2011, 12:24 PM   #64
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pacerr

nice shot, BUT....

if the top half is optimum, then immediately below the edge, you are exposed to flare for off axis light, because the side does not cover sufficiently at the corner, and if the side is a correct length, then the top is in the field of view.

I think the longer top section needs to come down the sides a little, I would suggest 1/6 of the way in fact, considerint a 2 x 3 format.

just a simple observation because for me, the biggest issue is the sun is always in one corner of the frame

edit note, for me, it is simpler to do a tulip hood, because I have already modified the hood calc spreadsheet to calculate the tulip pattern, which I can then cut out of a sheet, and slide over an existing circular hood to extend it all the way to optimum distance,

03-04-2011, 12:38 PM   #65
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Just another thng to consider withy respect to the definition of "optimum" these days, and I only mention this because I did the measurement yesterday.

is your lens internal or external focusing?

you might add why, and what does this have to do with a lens hood, but...

Internal focusing is achieved in reality by modifying the focal length of the lens. To show you the impact of this focal length change (which many may assume is minimal, I tested my Internally focused Vivitar 85F1.4 and my Takumar 85F1.9 (which focuses by moving the entire group.

When focused at 1 meter, the width of the horizontal field of view of the vivitar is 585mm, compared to the Takumar which has a horizontal width of about 220mm.

what this means is that at minimum focus, my 85mm vivitar is actually about a 40mm lens!

Now, do I make my hood (rectangular, petal or cylindrical) for the entire focusing range, i.e. therefore giv up optimum and hav minimal, or make it adjustable?

this is a lot of work and discussion about less and less I feel
03-04-2011, 01:22 PM   #66
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QuoteOriginally posted by Lowell Goudge Quote
this is a lot of work and discussion about less and less I feel
Everything you say is probably true, Lowell, but I make no claim for optimum, just very low cost, adequate to it's intended purpose and likely to be in the bag when other large hoods wouldn't be.

Did ya notice I can twist that hood around the barrel to catch a stray light source and slide it back an' forth to adjust the length and vignetting? Even make it oversized or cut it up or bend a corner to compensate for the lack of a perfect tulip pattern while on site.

Oh, yeah, the metal Tamron hood weighs 3/4 pound where as the cardboard one comes in at 3/4 ounce (with the rubber band)!. An' the tabs are long enough it'll fit the SP 180 and SP 80-200 as well.

I'm a lifelong fan of pragmatic, field expedient solutions that I can afford an' I'd have to take out a mortgage if I had to replace the hood for that SP 300/2.8.

Besides, it's fun to watch the guys with $$$$'s-worth of gear go dumpster-divin' after the ones you throw away when they don't have anything with 'em in the field.

H2
03-04-2011, 01:39 PM   #67
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QuoteOriginally posted by pacerr Quote
Besides, it's fun to watch the guys with $$$$'s-worth of gear go dumpster-divin' after the ones you throw away when they don't have anything with 'em in the field.

H2
Need a photo of that one
03-04-2011, 02:01 PM   #68
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QuoteOriginally posted by Lowell Goudge Quote
Need a photo of that one
Actually happened to me many years ago at an aircraft mishap site. The low sun angle was not cooperating and shots needed to be made before sunset. The wind was too gusty to just hold a card in place and the terrain pretty much prevented using another body or two as a lens shade so I taped a tube made from a brown paper bag to the lens with electrician's tape.

I looked back as I left that site and saw two other guys rummaging in the truck bed where I'd thrown the makeshift hood. Shootin' film at the time I wasn't into snagging un-business-like pix for fun but I wish I had a sequence that told the whole story now. Ya had to've been there.

QuoteOriginally posted by Lowell Goudge Quote
. . . do I make my hood (rectangular, petal or cylindrical) for the entire focusing range, i.e. therefore giv up optimum and hav minimal, or make it adjustable?
I vote for square! Heck, I don' know. Just make it big enough to get the job done or wait for better conditions I guess.

QuoteOriginally posted by Lowell Goudge Quote
. . . immediately below the edge, you are exposed to flare for off axis light, . . . the biggest issue is the sun is always in one corner of the frame . . .
Yeah, but my cheap, pragmatic soul would require that I just cuss a bit an' take a coupl'a steps one way or t'other to avoid the issue -- that couldn't hurt 'cause I never get the composition right the first time anyway.

Or maybe just wait a few minutes; since the sun moves one degree every four minutes it'd clear that corner while I relaxed, soaked up a few rays an' chimped the first shots to discover I needed to adjust exposure too.

Now if it was an inconvenient street lamp there's always that Red Ryder BB-gun!

H2


Last edited by pacerr; 03-04-2011 at 02:31 PM.
03-04-2011, 02:09 PM   #69
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QuoteOriginally posted by pacerr Quote
I don' know. Just make it big enough to get the job done or wait for better conditions I guess.


H2
I think actually, making a cylindrical hood extension that slides over an existing (short) metal hood and then adjust as necessary for the optimum length seems the best idea.

On an IF lens I bet the hood is made for the close focus, otherwise it would cause vignetting, so a sliding second hood would be a great addition for these lenses.
03-04-2011, 03:09 PM   #70
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QuoteOriginally posted by Lowell Goudge Quote
so a sliding second hood would be a great addition for these lenses.
So one of those older lenses with sliding, rotating, integral hood would actually be a good idea then? And then I could make short, tulip-shaped "falsies" with a slip-fit to further accommodate the zoom/focus changes to install on the front of the hood?

Gonna have to break out all my old SMC-M teles an' dress 'em up like flowers.

H2
03-04-2011, 03:47 PM   #71
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QuoteOriginally posted by pacerr Quote
So one of those older lenses with sliding, rotating, integral hood would actually be a good idea then? And then I could make short, tulip-shaped "falsies" with a slip-fit to further accommodate the zoom/focus changes to install on the front of the hood?

Gonna have to break out all my old SMC-M teles an' dress 'em up like flowers.

H2
that's about the size of it, just so long as you don't exagerate the profile.
03-04-2011, 04:05 PM   #72
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QuoteOriginally posted by Lowell Goudge Quote
.....

Now, do I make my hood (rectangular, petal or cylindrical) for the entire focusing range, i.e. therefore giv up optimum and hav minimal, or make it adjustable?

this is a lot of work and discussion about less and less I feel
QuoteOriginally posted by pacerr Quote
OK - Put up or shut up I guess.

I just submitted the step-by-step process as a DIY item - it should be up soon. I opted to go 'square' rather than mess about with the origami-fold trick for this one.
I like pacerr's concept a lot. I agree with Lowell about curves on the box edges.

To add one more whack to this I submit the following estimate of how a hood decreases the unwanted light fraction. It is calculated for a circular image format and a conical hood, but I think it should be close to correct for a rectangular image format and rectangular hood or a rectangular image format and a conical or box hood with appropriate petals.


Notice that the hood's effectiveness depends on the Lens' minimum F-Number as well as on the hood's depth and angle.

Dave

PS The math involves integrating over what the lens sees originating from the scene and what light makes it to the lens from outside the scene (for a scene far away). The Cosine of the light's angle with respect to the optic axis is taken into account for both the source and the lens.

EDIT: I think that adding petals to the optimum circular format hood to block light from out-of-format parts decreases unwanted light by 40%; ie. by the ratio of the area of the circle to the 2:3 rectangle that fits inside it.

Last edited by newarts; 03-06-2011 at 02:14 PM.
03-04-2011, 04:22 PM   #73
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QuoteOriginally posted by Lowell Goudge Quote
that's about the size of it, just so long as you don't exagerate the profile.
Uhh . . . I thought that was the point!

H2
03-04-2011, 04:26 PM   #74
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QuoteOriginally posted by pacerr Quote
Uhh . . . I thought that was the point!

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It was ment as a joke about the "falsies" but forget it
03-04-2011, 04:41 PM   #75
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QuoteOriginally posted by Lowell Goudge Quote
It was ment as a joke about the "falsies" but forget it
I got it -- even gave ya credit for slippin' both those sly innuendos into one short sentence.

Hey, Robin. Ya want yer thread back now?

H2
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