Why does the iris close when i zoom in>?
- Thread starter Rhydon
- Start date
Steve Ogden
Well-known member
Re: Why does the iris close when i zoom in>?
Rhydon,
I presume it's a cost issue in the lens design. In zooms for still cameras, the wider a fixed aperture is, the more expensive the lens (compare a fixed f2.0 80-200 to a f3.5-5.6 80-200). So they make it affordable by having it stop down as the focal length gets longer. My favorite lens for my Nikon still camera is an 24-85 f2.8 - 4. F2.8 at 24mm, f4 at 85mm.
I sure don't know for sure thats what's goin' on with the iris, but I'd bet it is.
Rhydon,
I presume it's a cost issue in the lens design. In zooms for still cameras, the wider a fixed aperture is, the more expensive the lens (compare a fixed f2.0 80-200 to a f3.5-5.6 80-200). So they make it affordable by having it stop down as the focal length gets longer. My favorite lens for my Nikon still camera is an 24-85 f2.8 - 4. F2.8 at 24mm, f4 at 85mm.
I sure don't know for sure thats what's goin' on with the iris, but I'd bet it is.
J.R. Hudson
Spirit Animal
Re: Why does the iris close when i zoom in>?
Assuming you have it on AUTO IRIS (But why?):
Lets say you have a WIDE shot of a building with the bright sky as it's backdrop and then you ZOOM in and the sky is no longer visible, just the building front.
The IRIS would adjust itself to compensate for the lack of light in the FRAME.
That's my take. ANYONE?
Assuming you have it on AUTO IRIS (But why?):
Lets say you have a WIDE shot of a building with the bright sky as it's backdrop and then you ZOOM in and the sky is no longer visible, just the building front.
The IRIS would adjust itself to compensate for the lack of light in the FRAME.
That's my take. ANYONE?
Steve Ogden
Well-known member
Re: Why does the iris close when i zoom in>?
Pardon my last post (wish my mouth had an 'undo' button) - variable f-stop sounded like a good theory until I read the spec, saw it's a f1.6 lens, and got to thinking how silly it would be to have that on this camera's lens. Oh well. . .
Pardon my last post (wish my mouth had an 'undo' button) - variable f-stop sounded like a good theory until I read the spec, saw it's a f1.6 lens, and got to thinking how silly it would be to have that on this camera's lens. Oh well. . .
J.R. Hudson
Spirit Animal
Re: Why does the iris close when i zoom in>?
Even in manual? :-/
Jarred? Barry?
Even in manual? :-/
Jarred? Barry?
Steve Ogden
Well-known member
Re: Why does the iris close when i zoom in>?
Rhydon, I doubt that I'm correct, - the camera's specs shoot my theory down. (variable f stop was not mentioned, f 1.6 was.)
But hey, I'm gonna climb out of this hole before I dig in any deeper & leave it for someone who knows. If I wasn't on the road, I'd test mine to see if it does it too.
How much does it close down between 85-99?
Rhydon, I doubt that I'm correct, - the camera's specs shoot my theory down. (variable f stop was not mentioned, f 1.6 was.)
But hey, I'm gonna climb out of this hole before I dig in any deeper & leave it for someone who knows. If I wasn't on the road, I'd test mine to see if it does it too.
How much does it close down between 85-99?
Rhydon
Well-known member
Re: Why does the iris close when i zoom in>?
thats the funny thing. The F-stop meter stays the same (no changes, at full open all the time) yet the image gets noticably darker (im shooting a white wall in lowish-light conditions.) Wheres the man himself, Jarred, im sure he knows why the camera does this.
thats the funny thing. The F-stop meter stays the same (no changes, at full open all the time) yet the image gets noticably darker (im shooting a white wall in lowish-light conditions.) Wheres the man himself, Jarred, im sure he knows why the camera does this.
J.R. Hudson
Spirit Animal
Re: Why does the iris close when i zoom in>?
Wait a minute.
How can the iris change and the f-stop stay the same?
I think you're trippin!
Wait a minute.
How can the iris change and the f-stop stay the same?
I think you're trippin!
Z
Zoomforce
Guest
Re: Why does the iris close when i zoom in>?
You need to understand how autoexposure works to understand why shooting a wall changes things. Autoexposure expects in any given image that there will be light areas, dark areas and mid areas.. which if all averaged out equals a middle grey. So if you are pointing at a completely white wall.. autoexposure is gonna try and expose it to make it grey. Same if you shoot a completely black wall.. you would get the same grey image (of course if the F-range wasnt limited) because its looking for a balanced ratio.
That is why you have exposure controls in the menus, so if you know you are shooting a white wall you can set the offest correctly.
As to the zoom shift.. as sort of mentioned, this is done automatically. The DVX100 uses a retrofocus lens, which needs exposure compensation at the end of the lens because the lens gets slower at longer focal lengths.
You need to understand how autoexposure works to understand why shooting a wall changes things. Autoexposure expects in any given image that there will be light areas, dark areas and mid areas.. which if all averaged out equals a middle grey. So if you are pointing at a completely white wall.. autoexposure is gonna try and expose it to make it grey. Same if you shoot a completely black wall.. you would get the same grey image (of course if the F-range wasnt limited) because its looking for a balanced ratio.
That is why you have exposure controls in the menus, so if you know you are shooting a white wall you can set the offest correctly.
As to the zoom shift.. as sort of mentioned, this is done automatically. The DVX100 uses a retrofocus lens, which needs exposure compensation at the end of the lens because the lens gets slower at longer focal lengths.
Barry_Green
Moderator
Re: Why does the iris close when i zoom in>?
sfogden was right in the first place. It's f/1.6 at the wide end, and f/2.8 at telephoto. If you watch it, you can see the iris gauge step down as you zoom in, gradually it goes from OPEN to 1.7 to 2.0 to 2.4 to 2.8. Zoom back out and it'll open up again.
It's the way the lens is designed. It's not constant-aperture (and offhand, I can't think of any lenses in this price class that are constant-aperture). The fastest it'll get, at full telephoto, is f/2.8.
That's one reason why, when people are looking at making their homemade mini35's, I wanted to see the results of various diopter experiments to see if you could fill the frame at the wide end of the zoom scale, because when you zoom all the way in, you give up about 1.5 stops of light!
sfogden was right in the first place. It's f/1.6 at the wide end, and f/2.8 at telephoto. If you watch it, you can see the iris gauge step down as you zoom in, gradually it goes from OPEN to 1.7 to 2.0 to 2.4 to 2.8. Zoom back out and it'll open up again.
It's the way the lens is designed. It's not constant-aperture (and offhand, I can't think of any lenses in this price class that are constant-aperture). The fastest it'll get, at full telephoto, is f/2.8.
That's one reason why, when people are looking at making their homemade mini35's, I wanted to see the results of various diopter experiments to see if you could fill the frame at the wide end of the zoom scale, because when you zoom all the way in, you give up about 1.5 stops of light!
Z
Zoomforce
Guest
Re: Why does the iris close when i zoom in>?
yes.. how ironic.
That effect is called exposure compensation. To ride Barry, it is also why when you use something like the Pro35 mini35 adapter, you need to use really fast lenses (read expensive) as the adapter looses between 1 stop and 2 stops of light all by itself.. which, as a bonus, helps with the shallow DOF.
I went and saw a film out projected in a theater of the pro35 adapter today.. looked really good except when used with really wide lens. The filmout wasn't that usefull, as if anyone saw the dvd, they shot in a huge white room with really soft light.. not really a normal shooting arangement.
yes.. how ironic.
That effect is called exposure compensation. To ride Barry, it is also why when you use something like the Pro35 mini35 adapter, you need to use really fast lenses (read expensive) as the adapter looses between 1 stop and 2 stops of light all by itself.. which, as a bonus, helps with the shallow DOF.
I went and saw a film out projected in a theater of the pro35 adapter today.. looked really good except when used with really wide lens. The filmout wasn't that usefull, as if anyone saw the dvd, they shot in a huge white room with really soft light.. not really a normal shooting arangement.
Barry_Green
Moderator
Re: Why does the iris close when i zoom in>?
When we used the mini35 on the DVX, we also shot comparison shots using just the anamorphic adapter. We had to stop down the anamorphic version by about 4 stops to match the exposure of the mini35 version. That takes into account the 1.5 stops you lose from going telephoto, plus the stop or so the adapter costs you, plus the slow 35mm lens we were using (f/1.8 or f/2.4, if I recall correctly). 4 stops! As in, we needed 16x as much light for the mini35 version as we did for the anamorphic version!
When we used the mini35 on the DVX, we also shot comparison shots using just the anamorphic adapter. We had to stop down the anamorphic version by about 4 stops to match the exposure of the mini35 version. That takes into account the 1.5 stops you lose from going telephoto, plus the stop or so the adapter costs you, plus the slow 35mm lens we were using (f/1.8 or f/2.4, if I recall correctly). 4 stops! As in, we needed 16x as much light for the mini35 version as we did for the anamorphic version!
Z
Zoomforce
Guest
Re: Why does the iris close when i zoom in>?
yes.. one downfall for sure. The pro 35 is even worse at loss, but the "ND" effect can be adjusted to some degree.
Bottom line for the adapter is, if your shooting exteriors at night or low light.. good luck... specially if you are using slow zeiss lenses.
yes.. one downfall for sure. The pro 35 is even worse at loss, but the "ND" effect can be adjusted to some degree.
Bottom line for the adapter is, if your shooting exteriors at night or low light.. good luck... specially if you are using slow zeiss lenses.
J.R. Hudson
Spirit Animal
Re: Why does the iris close when i zoom in>?
My head starts hurting when I read these kinds of posts. Reminds me of sitting in Algebra in 10th grade.
See. I like Jarred's answer:
That effect is called exposure compensation.
That's all I need to know.
My head starts hurting when I read these kinds of posts. Reminds me of sitting in Algebra in 10th grade.
See. I like Jarred's answer:
That effect is called exposure compensation.
That's all I need to know.
Steve Ogden
Well-known member
Re: Why does the iris close when i zoom in>?
Hi everyone - first of all, let me apologize for the wishy-washy nature of my posts in this thread. My background is still photography and I'm a total newbie when it comes to moving pictures of any kind and video in particular. I'm here to learn - and like so many others, have found the posts here extremely valuable while starting to climb the steep learning curve. At the same time, having learned still photography shooting Kodachrome 25, a 5 stop exposure latitude and many lighting issues are things I am aquainted with.
Barry - is it customary in the video world to call a lens f1.6 when it's really a f1.6 to 2.8? I was really surprised to see what I thought was a fixed f1.6 Leica lens on an already well done camera - and for $3400. Wow. Glad to realize that's what it is, so I can compensate exposure when using manual iris and long zoom in. All my interview shooting so far has been manual iris without much zooming - and I hadn't considered the implications of this style lens (never did much zooming during my exposures before.)
Hi everyone - first of all, let me apologize for the wishy-washy nature of my posts in this thread. My background is still photography and I'm a total newbie when it comes to moving pictures of any kind and video in particular. I'm here to learn - and like so many others, have found the posts here extremely valuable while starting to climb the steep learning curve. At the same time, having learned still photography shooting Kodachrome 25, a 5 stop exposure latitude and many lighting issues are things I am aquainted with.
Barry - is it customary in the video world to call a lens f1.6 when it's really a f1.6 to 2.8? I was really surprised to see what I thought was a fixed f1.6 Leica lens on an already well done camera - and for $3400. Wow. Glad to realize that's what it is, so I can compensate exposure when using manual iris and long zoom in. All my interview shooting so far has been manual iris without much zooming - and I hadn't considered the implications of this style lens (never did much zooming during my exposures before.)
Barry_Green
Moderator
Re: Why does the iris close when i zoom in>?
" is it customary in the video world to call a lens f1.6 when it's really a f1.6 to 2.8?"
Well, not really, I'm used to seeing the variable aperture stated. But it's also exceptionally uncommon (at least I think it is) to find a fixed-aperture lens at all in this price category. So I wasn't surprised to find it change aperture.
Of course, you could always just go into manual exposure and lock it at f/2.8 -- that way it would maintain a constant 2.8 throughout the zoom range...
" is it customary in the video world to call a lens f1.6 when it's really a f1.6 to 2.8?"
Well, not really, I'm used to seeing the variable aperture stated. But it's also exceptionally uncommon (at least I think it is) to find a fixed-aperture lens at all in this price category. So I wasn't surprised to find it change aperture.
Of course, you could always just go into manual exposure and lock it at f/2.8 -- that way it would maintain a constant 2.8 throughout the zoom range...
Steve Ogden
Well-known member
Re: Why does the iris close when i zoom in>?
Completely reasonable at that price. f2.8 it is, except when staying wide (love that 1.6 dof)
Thanks
Completely reasonable at that price. f2.8 it is, except when staying wide (love that 1.6 dof)
Thanks
scottchapin
Well-known member
Re: Why does the iris close when i zoom in>?
[quote author=John Hudson link=board=Events;num=1075421927;start=0#2 date=01/29/04 at 19:09:27]
Assuming you have it on AUTO IRIS (But why?):
Lets say you have a WIDE shot of a building with the bright sky as it's backdrop and then you ZOOM in and the sky is no longer visible, just the building front.
The IRIS would adjust itself to compensate for the lack of light in the FRAME.
That's my take. ANYONE?[/quote]
In that event, the iris should open to compensate for the lack of light.
[quote author=John Hudson link=board=Events;num=1075421927;start=0#2 date=01/29/04 at 19:09:27]
Assuming you have it on AUTO IRIS (But why?):
Lets say you have a WIDE shot of a building with the bright sky as it's backdrop and then you ZOOM in and the sky is no longer visible, just the building front.
The IRIS would adjust itself to compensate for the lack of light in the FRAME.
That's my take. ANYONE?[/quote]
In that event, the iris should open to compensate for the lack of light.