My 17-40 has two apertures?!

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trencheel303

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f number is related to the diameter of entrance pupil (the size of the iris opening seen from the front) and the focal length. This is why fast teles have bulbous front elements to magnify the iris and collect more light. In a zoom with a fixed f-number, changing the focal length changes the entrance pupil, so it needs to change iris to keep the f-number constant. I don't think your lens has two apertures, more likelythe same one changing.
 
Brachytron said:
f number is related to the diameter of entrance pupil (the size of the iris opening seen from the front) and the focal length. This is why fast teles have bulbous front elements to magnify the iris and collect more light. In a zoom with a fixed f-number, changing the focal length changes the entrance pupil, so it needs to change iris to keep the f-number constant. I don't think your lens has two apertures, more likelythe same one changing.
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Yes, just one aperture changing.

Most zooms do it, as the natural inclination for a zoom is for the f/number to drop as focal length is decreased. Variable aperture zooms take advantage of this fact, to give a lower f/number at the wider end. You can see the diaphragm changing as you zoom, to maintain a constant f/number

With zooms which have a constant f/number from the start, like the 17-40L, this characteristic is 'designed out' optically which makes the lens a bit smaller and lighter, and I guess improves performance slightly. But sometimes the optical correction isn't perfect and so needs a little bit of mechanical help from the diaphragm.

In theory, if this wasn't corrected then you would have a variable maximum aperture lens but the difference is so marginal as to be of no practical use, whereas in some situations it is a slight convenience to know that f/4 is f/4 at all times, and not f/3.862475 now and then.
 
The 17-40L (and 16-35L) actually has a secondary diaphragm to help control flare when the angle gets wider....I suspect that this is what you're seeing. Strictly not an aperture in way we normally think of one (ie, electronic and stops down to adjust the f number) but it is variable and mechanically coupled to the zoom ring.

Bob
 
Canon Bob said:
The 17-40L (and 16-35L) actually has a secondary diaphragm to help control flare when the angle gets wider....I suspect that this is what you're seeing. Strictly not an aperture in way we normally think of one (ie, electronic and stops down to adjust the f number) but it is variable and mechanically coupled to the zoom ring.

Bob
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This explains it exactly; I can guarantee they are seperate apertures because I can stop it down by holding the DOF preview, and zoom, and observe the two 'apertures' changing irrespective of each other.

Thanks everyone for the comments - I guess I was on the right track with my thinking in paragraph three of the OP.
 
Canon Bob said:
The 17-40L (and 16-35L) actually has a secondary diaphragm to help control flare when the angle gets wider....I suspect that this is what you're seeing. Strictly not an aperture in way we normally think of one (ie, electronic and stops down to adjust the f number) but it is variable and mechanically coupled to the zoom ring.

Bob
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That's interesting Bob. The bit I can see moving with the zoom in my 17-40L looks exactly like the regular diaphragm. But it is extremely hard to see identify exactly.

What you are describing sounds like a secondary variable baffle. You can see something like that marked on Canon lens construction diagrams, but there is no further information that I can find. Have you got any more info on that? A link maybe? Thanks!
 
HoppyUK said:
.... Have you got any more info on that? A link maybe? Thanks!
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Richard,

It gets a brief mention on some off the Canon national sites for this lens and the 16-35 (not all sites though) http://www.canon.lt/products/about.asp?id=1039 for example (see the last few lines on the page)

I have read a more detail appraisal of its intentions and design philosophy somewhere but it's proving elusive at the moment....I'll dig deeper in the morning but it may be in one of my books and not from an online source.
My recollection was that the basis for discussion was the 16-35MkII and it explained how the addition of the secondary diaphragm cut down the flare that the MkI (and older 17-35L) suffered from

Bob
 
hold on a second I'll upload a clip I made.
 
Canon Bob said:
That video shows it nice and clearly :thumbs:

Bob
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Yes, cheers guys. I can now see it clearly in my own lens with a bright light on it. My 24-105L is exactly the same, and other several other zooms by all accounts.

I found this reference in Canon Lens Work III talking about lens construction and control of internal reflections "...and fixed and movable diaphragms (in zoom lenses) which double as flare cutting devices." Can't find anything further.

So what is the other part of their 'double' function? This extra diaphragm must also affect the f/number :thinking:

Either way, Trencher's lens appears to be entirely normal :thumbs:
 
Edit: crossed post with Bacon Chops above!

GRECIAN said:
If you have to ask the question, does that not tell you something.
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Sounds entirely normal to me! :D Sign of a healthy enquiring mind :lol:

Not that I've been squinting down the barrel with a torch of course, but I have some further unhelpful observations, based on looking through Canon Lens Work III. There is a PDF version of it here http://www.canon-europe.com/Support..._educational_tools/en/ef_lens_work_iii_en.asp Tons of great info for you there Trenchers, or anyone else with Lens OCD :)

If this additional aperture is just a light baffle, then you would expect to see it in all wide zooms, but it is not. If it was for maintenance of a constant f/number, then you would expect to see it in all constant f/number zooms, but it is not. If it was an L lens feature, it would not appear in non-L lenses, but it does. It also appears in a couple of Canon primes - macros and the 24L 1.4 :thinking:

So on a casual glance through that stuff and a bit of unsuccessful googling, my theory doesn't really stand up, although I can see that it might yet hold water in some cases (related to internal focusing). But on what I've seen so far, it's probably best to assume that I'm talking rubbish on that one. Sorry about that ;)

What is certainly true is Bob's post about it being a variable light baffle, and I think we could safely leave it at that if it wasn't for Canon's statment above about this diaphragm having a 'double' purpose. Over to you Shirlock Trencheel :)
 
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i'm all out of suggestions... more bacon needed I think.

Ha - Hoppy has just started growing roots.
 
Mmm...bacon...I could murder a buttie right now... Germany is crap for trying to get proper bacon...

Apart from your new lens' baffling behaviour (geddit?) how is it?
Happy?
 
Sounds like it's there to cut out light bouncing around inside the lens to me.
 
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