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08-13-2018, 09:56 PM   #1
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Is this formula accurate for hood length?

Formula max round hood length =((n15/2)/(sin(radians((180-90-((180-m15)/2)))))*(sin(radians((180-90-m15)/2)))) n15=diameter m15=max diag aov

I found this online...

08-13-2018, 11:03 PM - 3 Likes   #2
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I've got an easier way for us non-math majors...

Put the hood on the lens. Put the lens on the camera. Set lens to the smallest aperture. Shoot a bright sky. If it doesn't vignette your hood is a keeper.
08-13-2018, 11:22 PM - 2 Likes   #3
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That method makes it hard when you are buying online and wish to have a good chance of being correct in your selections.
08-13-2018, 11:24 PM - 1 Like   #4
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I am old fashioned:
Take a sheet of dark paper/cardboard and roll it around the lens. Move it in Liveview F16 until it vignettes. Measure.

08-13-2018, 11:36 PM - 2 Likes   #5
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QuoteOriginally posted by UncleVanya Quote
Formula max round hood length =((n15/2)/(sin(radians((180-90-((180-m15)/2)))))*(sin(radians((180-90-m15)/2)))) n15=diameter m15=max diag aov
I have no idea, but I guess you could "reverse engineer" a formula from this Excel sheet:
https://www.pentaxforums.com/forums/attachments/pentax-slr-lens-discussion/4...d-hoodcalc-zip

Posted by @Lowell Goudge; several years ago. Found the link in this discussion: optimum hood depth calculator? - Page 2 - PentaxForums.com
08-14-2018, 02:12 AM   #6
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I might be well off (I'm dyslexic and don't much like math), but...

Does(?) that formula turn out to be something like;

Take the field of view for the lens, then look at the diameter of the hood, and the length of the hood, then draw a triangle - Compare triangle to field of view.
08-14-2018, 02:55 AM - 6 Likes   #7
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I was told that there would be no math...

08-14-2018, 07:20 AM - 1 Like   #8
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QuoteOriginally posted by pepperberry farm Quote
I was told that there would be no math...
Lol. I'm afraid that was inaccurate. You need to solve this to win entry into the raffle for the last k1. (Just kidding!)
08-14-2018, 08:34 AM - 2 Likes   #9
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That original formula isn't right. The link provided by @savoche to that spreadsheet is better although the numbers in it need to be changed to work with the Pentax SR system.


Note: checking for hood vignetting by stopping down can determine if a hood is bad but provides no guarantees that it is good. A hood can pass the stop-down test but still cause vignetting under two conditions:

First, a hood could pass the stop-down test but also create up to 1-stop of vignetting when the aperture is wide-open.

Second, a hood could pass the stop-down test but create severe vignetting when the IBIS system has the sensor shifted. The required unvignetted image circle is larger than the 43.3 mm circle of a 36x24 frame. It's a 47.4mm circle to handle the ±1.5 mm of 2-axis shift on top of the 36x24 frame.
08-14-2018, 10:51 AM - 1 Like   #10
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QuoteOriginally posted by photoptimist Quote
That original formula isn't right. The link provided by @savoche to that spreadsheet is better although the numbers in it need to be changed to work with the Pentax SR system.


Note: checking for hood vignetting by stopping down can determine if a hood is bad but provides no guarantees that it is good. A hood can pass the stop-down test but still cause vignetting under two conditions:

First, a hood could pass the stop-down test but also create up to 1-stop of vignetting when the aperture is wide-open.

Second, a hood could pass the stop-down test but create severe vignetting when the IBIS system has the sensor shifted. The required unvignetted image circle is larger than the 43.3 mm circle of a 36x24 frame. It's a 47.4mm circle to handle the ±1.5 mm of 2-axis shift on top of the 36x24 frame.
while i agree with the second point, the only way to determine the first is to have a before and after test with and without the hood. note that many lenses have light fall off, , not vignetting exactly of up to 2-3 stops in terms of exposure value between middle and edge when wide open and it is almost impossible to detect without making a measurement.

but common sense suggests if you use the sheet, then simply take the maximum length and take it back a few mm. also since the hoods, unless you are making it yourself , are in fixed sizes, you simply pick the next one in sequence that is shorter than the spreadsheet predicts is optimum
08-14-2018, 02:25 PM   #11
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QuoteOriginally posted by Lowell Goudge Quote
while i agree with the second point, the only way to determine the first is to have a before and after test with and without the hood. note that many lenses have light fall off, , not vignetting exactly of up to 2-3 stops in terms of exposure value between middle and edge when wide open and it is almost impossible to detect without making a measurement.

but common sense suggests if you use the sheet, then simply take the maximum length and take it back a few mm. also since the hoods, unless you are making it yourself , are in fixed sizes, you simply pick the next one in sequence that is shorter than the spreadsheet predicts is optimum
Great points. Yes, the only to really tell is in taking a pair of images with and without the hood, at the widest aperture, widest zoom, and composition adjustment all the way in a corner. The difference between those two images will reveal any hood-related vignetting.

The spreadsheet (with frame size numbers adjusted for SR range of motion) should give a very accurate hood size. I like that the spreadsheet uses the outer diameter of the front element which is a decent proxy for the aperture effects.
08-14-2018, 04:06 PM - 1 Like   #12
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QuoteOriginally posted by photoptimist Quote
the only to really tell is in taking a pair of images with and without the hood, at the widest aperture, widest zoom
This is my experience as well. Tests done at F16 as others have recommended are only valid at F16. That test is not reliable for wider apertures.
08-14-2018, 05:10 PM   #13
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QuoteOriginally posted by UncleVanya Quote
That method makes it hard when you are buying online and wish to have a good chance of being correct in your selections.
.
Works for me because I buy only two types of lens hoods.

1. The original OEM hood. It's usually quite expensive (Zeiss metal hoods can exceed $200 new). Obviously, they will be a perfect fit.

2. $3.99 made-in-China generic hoods. If I guess wrong and they don't fit I just throw them in my "lens hood bag" and buy another hood slightly larger or smaller. Currently, I've got about 30 of these extra Chinese lens hoods in my generic hoods bag. At this point, I generally can accommodate just about any lens with hoods from my extra lens hoods bag.

Last edited by Fenwoodian; 08-14-2018 at 07:30 PM.
08-14-2018, 05:19 PM - 1 Like   #14
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QuoteOriginally posted by Fenwoodian Quote
.
Works for me because I buy only two types of lens hoods.

1. The original OEM hood. It's usually quite expensive (Zeiss metal hoods can exceed $200 new). Obviously, they will be a perfect fit.

2. $3.99 made-in-China generic hoods. If I guess wrong and they don't fit I just throw them in my "lens hood bag" and buy another hood slightly larger or smaller. Currently, I've got about 30 of these extra Chinese lens hoods in my generic hoods bag. At this point, I generally can accommodate just about any lens with hoods from my hoods bag.
Another 20 generic hoods and you could have bought another Zeiss one 🙂

And they probably don’t include a microfiber cloth like many of my cheap ones have...

-Eric
08-14-2018, 07:17 PM   #15
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I've been known to use the collapsible rubber hoods.
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