Canon 7D iris problems

[jpenna]

I explain it here:


Anyone know what's up?

 

[Josh66]

That's normal.

...I don't know how good I'm going to be able to explain it though. I can 'see' it, but I'm not really sure how to say it, if you know what I mean.

Wide open, it's stopped down more at the long end - but really the aperture is the same size. The f/# is only different because the focal length is longer.

At the tele end, it's maintaining a constant f/# - but the only way to do that is to change the actual size of the aperture.

f/8 @ 50mm will not have the same size opening as f/8 @ 135mm.

EDIT
For the same reason, it can probably stop down more at the long end. f/32 (or maybe f/22) is probably the smallest it goes at 18mm, but at 135mm it probably can go down to f/45. Not sure about that lens specifically, but some are like that.

 

[RyanLilly]

I figure in real world applications you never going to rack the zoom back and forth like an idiot anyway.

 

[Derrel]

That's normal.

...I don't know how good I'm going to be able to explain it though. I can 'see' it, but I'm not really sure how to say it, if you know what I mean.

Wide open, it's stopped down more at the long end - but really the aperture is the same size. The f/# is only different because the focal length is longer.

At the tele end, it's maintaining a constant f/# - but the only way to do that is to change the actual size of the aperture.

f/8 @ 50mm will not have the same size opening as f/8 @ 135mm.

EDIT
For the same reason, it can probably stop down more at the long end. f/32 (or maybe f/22) is probably the smallest it goes at 18mm, but at 135mm it probably can go down to f/45. Not sure about that lens specifically, but some are like that.

Not very well-said, but I think you're right on the money. And Ryan's observation is quite eloquent as well.

 

[Enough Already]

I figure in real world applications you never going to rack the zoom back and forth like an idiot anyway.

So you would not use the zoom in any fashion if you were using video mode??

 

[Stosh]

Josh is right on - doesn't sound like a problem at all.
First thing most people need is a better understanding of aperture and f-ratio. Aperture is a physical size determined by the iris. F-ratio is the ratio of focal length divided by aperture. So when people say my aperture is f/2.0 or whatever, that's technically wrong. Your f-ratio (or f-number) is 2.0, your aperture is half your focal length (so for a 50mm lens at f/2 your aperture is 25mm).

That being said, when you zoom, you change focal length. To maintain a constant aperture, the f-ratio must change. To maintain a constant f-ratio, the aperture must change. This is why you hear the iris changing size - it has to if you want it to maintain a constant f-ratio. Sounds like it's working exactly like it should.

 

[inTempus]

Anyone know what's up?

You can always start here.

 

[KmH]

Anyone know what's up?

You can always start here.

linky, no worky.

 

[ErectedGryphon]

Anyone know what's up?

You can always start here.

linky, no worky.

Linky worky, your computy no worky:mrgreen:

 

[inTempus]

You can always start here.

linky, no worky.

Linky worky, your computy no worky:mrgreen:

Hehe.

 

[RyanLilly]

I figure in real world applications you never going to rack the zoom back and forth like an idiot anyway.

So you would not use the zoom in any fashion if you were using video mode??

I would zoom, just not in THAT fashion.

 

[musicaleCA]

I figure in real world applications you never going to rack the zoom back and forth like an idiot anyway.

So you would not use the zoom in any fashion if you were using video mode??

I would zoom, just not in THAT fashion.

Uh...huh. Well I zoom like mad sometimes in sports, just to see what the heck is going on without moving my eye away from the viewfinder. Or sometimes the play moves so fast that I have to zoom out or in quickly. It happens.

I figure in real world applications you never going to rack the zoom back and forth like an idiot anyway.

So you would not use the zoom in any fashion if you were using video mode??

The world of modern film is one of primes, you know.

Anyway, this whole "problem" sounds like newbies to photography picking up 7D's hot off the shelf and not knowing how the camera works. (No offence to the op intended.)

 

[Derrel]

The world of modern film is one of zooms, as well as primes.

Arri-Zeiss Ultra Prime Lens Set
Primes

RED / Lenses
Zooms, and primes.

THALES ANGENIEUX cinema
Zooms.

 

[KmH]

You can always start here.

linky, no worky.

Linky worky, your computy no worky:mrgreen:

Doh! It would seem my PDF reader is AFU. :blushing:

 

[Tiberius47]

Here's what's happening....

The aperture value (the number after the F in f5.6 etc) is determined by two things: the actual diameter of the iris, and the focal length of the lens.

The lens you are using changes the aperture value when you zoom because the focal length is changing. The actual size of the iris is staying the same.

But when you set it to f18, the aperture value doesn't change as it did before. When you change the focal length this time, it would normally change the aperture value, but you've told the camera to remain at f18. So in order to compensate for the change in aperture value due to the zooming, the camera must change the size of the iris. That's what causes the sound you hear.