Zoom closes Iris? Why?

[Graham Bernard]

Ok .. Something I really don't understand. I've got 2 XM2s. So I'll cross ref this with the other .. in the meantime.

Set up = Manual

f = 1.6

ND = ON

Focus = Manual

WB = Manual & set correctly

Shot: Back garden with deep shadows about 20 feet away. Otherwise nicely lit areas, grass bricks and stuff nice colours. Focus is set to furthest point and sharp. Now, as I zoom in to darker areas I can see the F value get bigger. Why? Why shouild it move - I'm in manual, what does I know I don't? And, why should it close the f value as I go into deeper shadow?
Shorry people, I must be missing something so obvious I really don't care if the answer is going to embarrass me - I'm beyond caring!

Anyways myu Matte Biox and the 2 filters are a real treasure .. love 'em.

Grazie

[Boyd Ostroff]

Actually it isn't closing the iris at all. Most zoom lenses have different f ratings at the wide and telephoto ends. If you look at the GL-2 specs you'll see that the lens is 4.2mm f1.6 at the wide end and 84mm f4.2 at the narrow end.http://consumer.usa.canon.com/ir/con...4&modelid=7512

So as you zoom in the f number varies between those values. That's a pretty big difference, and a trade off for the 20x zoom capability. I've never used a GL-2, but if behaves like other cameras then you should set your iris at f4.2 for starters and it won't change as you zoom. Since you're using the ND filter I assume you can switch it off and still be able to expose properly.

[Graham Bernard]

Thanks Boyd for the swift reply.

"Actually it isn't closing the iris at all. " But why do the numbers increase at the same time getting darker? I can understand it getting darker, I'm zooming into shadows, but why or rather WHAT is making the numbers "move" if I'm in Manual? Again what does it know I don't? And more to the point if I'm in Manual why don't the numbers stay put? What is going on, internally, that "allows" the numbers to increase when I'm in manual?

Thanks for your patience . . . I had the same problem in understanding that the Manual WB isn't actually manual, it is set against a prescribed notion of WB and adjusted to that. Maybe this is similar logic.

Grazie

[Boyd Ostroff]

Graham, you're just seeing the characteristics of the lens. It is slower at full telephoto - you can't zoom all the way in and still shoot at f1.6, it isn't physically possible. When the iris is wide open and you zoom in all the way the f rating for the lens is f4.2. If you zoom halfway in the rating will be somewhere between f1.6 and f4.2. So the numbers are changing to reflect the f rating for the lens at whatever focal length you're at.

Try what I suggested - start by setting the iris at f4.2 while zoomed out full wide. Now zoom in. The f number should remain constant. Instead of a zoom lens, imagine that you had three different fixed focal length lenses you could choose between. Now imagine that you opened the iris of each lens all the way and put them on your camera one at a time. The wide angle lens would open up to f1.6, the medium lens would open up to F2.8, and the telephoto would only open up to f4.2. The camera wouldn't be changing anything, these would just be the maximum opening availabe for each of those lenses due to their design.

[Ken Tanaka]

Boyd is quite correct, Grazie. The GL2's zoom lens is of a variable aperture design. Look at the lens specifications in the manual and you'll find it called out as "f/1.6-2.9", with 1.6 at the wide end and 2.9 at full tele. (That's still an awfully fast lens.)

To my knowledge the only lens in the Canon XLx/GLx range that has constant aperture is the 16x Manual Servo lens for the XL cameras.

[Graham Bernard]

Thanks lads . . and I take all you've said on board.

What I still want to know .. please be patient, this is really REALLY difficult for me tio get across - what is going on that allows the f numbers to rack up? What is going on that is telling the numbers to increase? Is it purley the internal functioning of the zoom pushing up the numbers?

Grazie

[Ken Tanaka]

It might actually be a more accurate question to ask what is not going on.

Maintaining a constant large aperture throughout a large zoom range becomes a very expensive proposition for optical design and manufacturing. Yes, certainly, it can be done. For example, Canon's famous EF 70-200mm f.2.8L photographic lens is perhaps one of the finest mid-range zoom lenses ever made. But it costs as much as a GL2.

So the GL2's designers realized that (a) providing a constant-aperture lens on this camera would make the camera far too expensive for its target market, and (b) GL2 owners for whom this has become a big issue have likely exceeded the sophistication level for the GL2 and are ready to move into bigger equipment.

[Robin Davies-Rollinson]

<<<-- Originally posted by Graham Bernard : What is going on that is telling the numbers to increase? Is it purley the internal functioning of the zoom pushing up the numbers -->>>


Graham,
I think you are spot-on there. Since the lens cannot operate wide open on a long focal length, there is some feedback from the iris system telling you the actual working stop at that particular lens setting.
I've had the same thing happening on far more expensive lenses, so it's nothing new!

Robin.

[Graham Bernard]

Ken thanks .. I understand the experience thing . . .but back to the numbers .. does the XM2 do the math and then "relate" this to the postion of the zoom ? And then disply the numbers?

Grazie

[Graham Bernard]

Robin! Thank you ..

That's exactly what I wanted to know. So, are the numbers are really "dumb" they must be woorking on assumoptions of where the zoom is adnd NOT taking into account the ambient lighting? - Go on boys hang on in there with me - I think Robin knows what I'm driving at ..

Grazie

[Ken Tanaka]

<<<-- Originally posted by Graham Bernard : Ken thanks .. I understand the experience thing . . .but back to the numbers .. does the XM2 do the math and then "relate" this to the postion of the zoom ? And then disply the numbers?
Grazie -->>>

Yes, that is what I suspect is generally happening with variable-aperture lenses.

But there is reason to doubt that this is always simply an arithmetic process. Since focus and depth of field are directly affected by aperture it may well be that the lens is really setting the maximum aperture to a new value and physically adjusting the iris in the event that it was previously wider. Otherwise there might be an impact on the lens' sharpness at it longest tele focal lengths.

Someone who knows much more about lens design and manufacturing than I will have to answer this question.

[Graham Bernard]

Ken - Thank you too.

I wasn't trying to count angels on pinheads here .. i as really seeing that which might be non-sensicle and that which might be a useful referenec for my shooting! See now?

Thank you all

Graham Bernard

ps Robin, I'm having load of fun with the Mate Box. Some guys are saying I should do stuff in post! Well .. not with what I just diod. I had a beautiful sky line with mixed lighting options and shadows and an angled 0.3 SOFT edge grad - great! I stumbled on a a chaps work when I get the link I'll forwardit to you . . er Jo Cornish? I think that was him . ..

[Brendan Marnell]

You've done it again, lads.
That discussion has revealed the mysteries and the relevance of focal length like nothing I've ever read before.

Questions
1.Is it true to say that focal length is an internal response (almost like a reflex action) by the camera to all the conditions and settings prevailing when taking a shot?
2.If so, is it true that there's nothing I can do to alter the focal length directly, without changing other settings or conditions?
3.If all that's true then may I assume that info. showing focal length is simply info., solely to enable me to seek to reproduce those conditions again if I wish or adjust them if I wish?
4. If I'm all wrong please tell me the purpose of focal length or what should I be reading?

Brendan Marnell

[Boyd Ostroff]

I think you have your terminology confused. Focal length is controlled by the zoom button on your camera (telephoto, wide, everthing in between). Maybe you meant f-stop or depth of field?

[Ken Tanaka]

Hello Brendan,
Actually no. Focal length is a property of the configuration of a lens' optical elements. On lens designs often referred to as prime lenses the the internal elements are locked into fixed positions. Consequently, the focal lengths of these lenses are fixed. (Ex: 24mm, 35mm, 50mm, et.al.)

In contrast, the internal optical elements of zoom lenses can be repositioned to widen or narrow the lens' field of view. Consequently, their focal lengths are variable. (Ex: 17mm-40mm, 24mm-70mm, et.al.)

So a lens' focal length is a principal optical property, not a consequence of exposure. That is, on the GL2's zoom lens the effective focal length of the lens changes as you zoom back and forth. The amount of light actually reaching the GL2's CCD's any given zoom setting (i.e. focal length) will vary; more with wider shots, less with tighter tele shots. Shutter speed and aperture are the compensating user controls to achieve proper exposure at any given focal length of the lens. (I side-stepped the sensitivity of the recording medium, or ISO rating, purposely since it's not immediately relevant to video cameras.)