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02-26-2011, 03:51 PM - 1 Like   #1
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optimum hood depth calculator?

Not sure if this is the correct sub-forum; the non-exclusive categories make my poor post-structuralist head hurt.

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.

Say I have a 135mm lens on APS-C and I want a circular metal hood for it. How long should it be?

(Strange thing but I have never seen this question asked... well, it was once years ago but the site and answer have been bit-terminated.)

02-26-2011, 04:43 PM   #2
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Not really a calculator but if you find out the field of view in degrees of a lens and then rest a protractor on the front element you can sort of visualise at what point visible vignetting would start then take at least a few centimeters off that for the maximum hood depth.

You can also look at the designed hoods for different lenses eg the 50-135 for a 50 on APS-C to get an idea of how deep you can go.
02-26-2011, 04:44 PM   #3
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It is pretty simple geometry.

One type lens hood is a cone with the same angular field of view as the lens so it cannot cause vignetting.

I think this basic design should work....the larger the ratio of height to focal length the better.


EDIT: there is no theoretical limit on how deep the hood can be, the deeper it is, the less light gets in from outside the field of view.

Last edited by newarts; 03-11-2011 at 07:09 AM.
02-26-2011, 05:55 PM   #4
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QuoteOriginally posted by newarts Quote
EDIT: there is no theoretical limit on how deep the hood can be, the deeper it is, the less light gets in from off the optic axis.
I would say that there is a limit since vignetting would occur at some point.

02-26-2011, 06:21 PM   #5
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As long as you kept the 'spread angle' of the hood larger than the field of view, there would be no limit to how deep the lens could be.

Practically speaking, I think the diameter of the front element / lens threads is the key dimension. The hood should be at least that deep for optimum results, I think. Then, it could be deeper as long as the field of view is kept in mind, to avoid vignetting.
02-26-2011, 08:11 PM   #6
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QuoteOriginally posted by yucatanPentax Quote
As long as you kept the 'spread angle' of the hood larger than the field of view, there would be no limit to how deep the lens could be.
Oh I see, you're talking about a conical hood and I'm talking about a cylindrical hood.
02-26-2011, 11:41 PM - 1 Like   #7
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QuoteOriginally posted by rparmar Quote
you're talking about a conical hood and I'm talking about a cylindrical hood.
Just ignore the sides of the "cone". It's only the diameter of the hood and the distance from the lens face that counts in a practical sense.

What makes MY head hurt is calculating stuff when I can accomplish the same goal through practical trial-and-error - mostly correcting the error!

Remove both ends of a soup can, round oat meal box, whatever suits the situation. Heck, even a square or rectangular box will suffice. Center it fairly well around the lens in question and play with it til you see vignetting. Take your dimensions from there. 'Duck' tape helps.

Also, remember that the solution for a one-time project doesn't have to be gold plated nor attach to the lens in a conventional fashion either. Take a tip from the cylindrical, built-in hoods on some lenses (which usually get bonus points in reviews, incidentally) and fabricate a make shift hood from rolled black paper and a rubber band. Adjust length as necessary. You can even trim the end like the flower hoods - the trim shape is dependent on the sensor proportions not the FL. The only important dimension is that the hood length be long enough to prevent unwanted light from reaching the front optics with no, or acceptable, vignetting.

H2

It's fun to watch the faces (sneers?) of guys with $,$$$-worth of equipment as they watch you use 'Scotch' tape and a small brown paper bag to fabricate a field-expedient hood. An' I'm still lookin' for an excuse to cut an 'X' in the bottom of a paper cup and slip it over a lens but it really doesn't make much sense unless you can make the inside of the cup black with something scrounged as well.
02-26-2011, 11:53 PM   #8
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QuoteOriginally posted by rparmar Quote
Oh I see, you're talking about a conical hood and I'm talking about a cylindrical hood.
oops.... my fault! In that case, I agree with pacerr!

02-27-2011, 04:06 AM   #9
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The conical hood design I showed will not vignette as it has the same angular field of view as the lens.

An oatmeal box will work well too; just mount it and cut it back until it doesn't vignette like pacerr said.

Square & hexagonal boxes will work too.

Last edited by newarts; 03-11-2011 at 07:12 AM.
02-27-2011, 04:38 AM   #10
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QuoteOriginally posted by newarts Quote
The conical hood design I showed will not vignette as it has the same angular field of view as the lens.
Exactly! And good idea to just try something with cardboard.
02-27-2011, 06:17 AM   #11
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Hoods are a Lens issue... moved to Lens Forum...
02-27-2011, 06:27 AM   #12
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Good question, Robin. I've thought about that, too--but not very hard ;~). Also good responses. The makeshift trial & error method appeals to me (which is why you should never drive over bridges designed by people like me.)
02-27-2011, 06:37 AM   #13
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QuoteOriginally posted by MRRiley Quote
Hoods are a Lens issue... moved to Lens Forum...
And also an accessories issue. But not a Pentax issue since it is not brand-specific.

Which is what I meant at the outset -- with the current organising scheme, there's no proper place for this thread.
02-27-2011, 06:47 AM   #14
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Do a search on the forum for an excel file I uploaded called hoodcalc it can be used to do what you want based even upon film and sensor size

Note in the dimensions you need the exact diameter of the front element ( which is different than the filter or accessory thread size and you need to know the set back of the front element from the end of the filter thread

Keeping these things inning it is pretty easy for circular hoods
02-27-2011, 10:27 AM   #15
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There are a couple of sites that have some lens hood templates you can print, as well as the ability to create custom hoods based on your own specs, however it looks like you need to be a 'subscriber' to get access to that custom functionality:

Custom lens hoods

and for APS-C:

Customized digital lens hoods
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