Constant iris?

[Tony Tibbetts]

Maybe this is off topic now. :) I was curious if the cameras iris could stay full open all the way into the telephoto range or does it stop down the more it zooms in?

[Stu Holmes]

Originally Posted by Tony Tibbetts

Maybe this is off topic now. :) I was curious if the cameras iris could stay full open all the way into the telephoto range or does it stop down the more it zooms in?

Well max.apertures are f1.6 at max.wideangle and f2.8 at max.telephoto, so its inevitable that it'll have to stop down from f1.6 to f2.8 as you zoom from one end to the other.

[Chris Hurd]

Originally Posted by Tony Tibbetts

Maybe this is off topic now.

Not any more... it is its own topic now (split out from another thread).

[Boyd Ostroff]

Originally Posted by Tony Tibbetts

Maybe this is off topic now. :) I was curious if the cameras iris could stay full open all the way into the telephoto range

In fact the iris does stay completely open when you zoom. However, due to the design of the lens optics the f-stop changes with the focal length. This is the same with all camcorders that have fixed lenses unfortunately. But it depends on the context. If the iris is set at f2.8 then it will remain constant throughout the zoom range. But, as Stu says, if you're shooting full wide at f 1.6 then the f stop will ramp down to 2.8 as you zoom in.

[Emmanuel Plakiotis]

Originally Posted by Boyd Ostroff

If the iris is set at f2.8 then it will remain constant throughout the zoom range. But, as Stu says, if you're shooting full wide at f 1.6 then the f stop will ramp down to 2.8 as you zoom in.

I believe you are mistaken. When you zoom in - due to the design of the lens - less light reaches the sensor than when the lens is in its widest focal length (for this particular lense is almost 2 f-stops). Hence in any given iris setting, the widest focal length will be 4 times brighter than the longest focal lenght.

If you have auto iris on, because the camcorder automatically compensates for the loss of light, you really don't see the difference except when the iris is full open. But if you shoot with fixed iris you will see a difference in ther brightness of the picture when you zoom in.

[Chris Hurd]

Boyd is not mistaken, however, if the camera is in an automatic program exposure mode, yes, it will compensate for the difference.

[Boyd Ostroff]

Originally Posted by Emmanuel Plakiotis

I believe you are mistaken.

I don't think so. I shoot a lot of performance where the light levels are low, so I'm pretty familiar with this phenomenon. For a wide shot of the stage I open the iris up all the way (f1.6 in your example), but then when I zoom in everything gets darker. However in a brighter scene when the iris is set to a smaller opening (f2.8 in your example), then I don't see any change in brightness as I zoom all the way in. This phenomenon is exactly the same on my VX-2000, PDX-10 and HVR-Z1.

BTW, it's interesting that on the VX-2000 when you open the iris all the way the viewfinder says OPEN instead of showing an actual f-stop. I think this is a nod to the phenomenon we're discussing. Interestingly, the newer Sony cameras don't do this; they display the actual f-stop and you can see it change as you zoom in when the iris is fully opened.

[Emmanuel Plakiotis]

When the lens is in the longest focal lens, lets less light to pass than when it is in the shortest focal length. It is a physical attribute of the lens and has nothing to do with the iris.
The iris (actually a resizable hole) that resides at the back of the lens, is just another way to control the light intensity by altering its size.
The f stop is just a mathematical indication of the amount of light that reaches the sensor so we can compare different lenses and/or iris settings in terms of brightness.

No matter where the iris is set, the telephoto end of the lens, will always yield a higher f stop than the wide end.

There are expensive zooms (both in photography and video) that held the same f stop rating throughout their focal range.
In older photographic zoom lenses where the iris was a manual ring, there were two dots on the f stop scale. The one represented the f stop of the telephoto and the other of the wide angle. So if you wanted a given f stop and you choose the telephoto end, you had to open the iris a bit more. Of course with the iris full open, there is no room for compensation and the telephoto lens would indicate a higher f stop on the scale.


Boyd, I can suspect two reasons why you don’t see the difference in brightness with your camcorders.
The zoom of the 2000 is smaller and therefore the difference in f stops between wide and telephoto ends is not that great (could be about 1/2 stop). In a bright lit scene and viewed from the LCD monitor or the viewfinder which are far from ideal monitoring solutions, the difference will be minimal and can go unnoticed, especially if you are not looking for it. When the light decreases the viewfinder/LCD screen have problems to accurate reproduce the scene and the difference is more profound. The other reason could be that there is automatic gain or automatic shutter, that doesn't turn off, even in manual f stop setting and compensate for the change in the iris.

[Betsy Moore]

speaking of fixed iris' but another of another kind, I wonder if this camera is going to have the irritating trait of the FX-1, where the iris closes automatically (even in manual mode) if it aims at the window or another bright object. Douglas Spotted-Eagle do you know?

[Douglas Spotted Eagle]

I don't have an FX1, so don't know.
But I do know the Z1 doesn't do this. Are you sure you're completely in manual mode? You can put the shutter in manual and iris still be in auto...
But that said, no...the V1 does not close the iris when in manual mode when pointed at direct sunlight.

[Tom Hardwick]

DSE is right, Betsy. And your FX1 is exactly the same as the Z1 in this regard. Thing is you're not in the fully manual mode if you experience this. Remember that to lock the exposure (and not just the iris diaphragm) you have to have the shutter speed, the gain and the aperture values all displayed in your v'finder.

Go back and check - I bet you didn't have the gain displayed. Better yet replay some of this footage that shows exposure changes with the display info turned on. See the gain value bumping up and down constantly?

As to ramping lenses (ie lenses that reduce their maximum aperture as you zoom) this is a fairly recent idea. It means lenses can still be described as f/1.6 (sounds good) whereas they're only this aperture for one of their very many focal lengths. IT means the lenses are a lot smaller and lighter and cheaper and have smaller filter sizes, too.

In Super-8 days many manufacturers (Canon, Nikon and Minolta etc) had cameras with 10 and 12x zooms that maintained f/1.4 throughout the zoom range. But they were big heavy zooms.

tom.

[Betsy Moore]

Thanks Tom and Donald... I'll do that thing--people have been complaining about this elsewhere on the site--I'll try to spread the word where I can...

L,

B