Iris shifting at DVinfo.net

Kevin Sullivan · July 18th, 2006, 05:17 AM

Does anyone know how to stop the iris from shifting while zooming?

The manual says that this is just a fact of life. Any ideas?

Thanks,

Kevin Sullivan

Boyd Ostroff · July 18th, 2006, 07:39 AM

Welcome to DVinfo Kevin. Yes, it's a fact of life and there's nothing that can be done. All camcorders that have fixed lenses behave the same way. However you aren't correctly stating the problem. The iris isn't "shifting," it's staying the same. The f-stop is changing due to the optical properties of the zoom lens, which lets less light through at the telephoto end of the zoom.

There is a way to keep this from happening, but it probably isn't the solution you're looking for. I don't have my Z1 here, but I think the wide open f-stop is 2.8 at full zoom. If you set the iris at f2.8 you can zoom in and out all you want and the f-stop won't change. But it you set it at f1.2 then it will work its way up to f2.8 as you zoom in. This has to do with the design of the zoom lens and you just have to live with it....

Kevin Sullivan · July 18th, 2006, 11:46 PM

Thanks for the detailed response. Been designing a shot for a project and was concept testing and could not figure why the iris was changing. Will test again today with your info.

Greg Boston · July 18th, 2006, 11:54 PM

Quote:

Originally Posted by Boyd Ostroff

Welcome to DVinfo Kevin. Yes, it's a fact of life and there's nothing that can be done. All camcorders that have fixed lenses behave the same way. However you aren't correctly stating the problem. The iris isn't "shifting," it's staying the same. The f-stop is changing due to the optical properties of the zoom lens, which lets less light through at the telephoto end of the zoom.

There is a way to keep this from happening, but it probably isn't the solution you're looking for. I don't have my Z1 here, but I think the wide open f-stop is 2.8 at full zoom. If you set the iris at f2.8 you can zoom in and out all you want and the f-stop won't change. But it you set it at f1.2 then it will work its way up to f2.8 as you zoom in. This has to do with the design of the zoom lens and you just have to live with it....

Interestingly enough Boyd, some lenses don't exhibit this behavior. The auto lenses on the Canon XL's do this, but the 16X manual zoom maintains f1.2 all the way in. Same with the Fujinon lens on my new camera. It leads me to wonder what the design criteria has to be to allow this.

-gb-

Boyd Ostroff · July 19th, 2006, 08:18 AM

Heh, you didn't read my fine print :-) "All camcorders that have fixed lenses behave the same way." I know that there are interchangeable manual lenses for the XL series, the JVC HD100 and larger pro cameras which don't exhibit this behavior.

I don't know much about lens design, but I always assumed the trade-off was both the cost and the size, since the lenses you just mentioned are large and expensive...

Mark Utley · July 19th, 2006, 12:32 PM

Just to avoid confusion, the Z1's aperture goes up to F1.6, not F1.2.

Kevin Sullivan · July 20th, 2006, 04:41 AM

Thanks for the info everyone.

After testing, the lense definitely stays put at 2.8

A little more light and problem solved.

Thanks once again,

Kevin Sullivan
Lead Dog Productions

Michael Donne · July 21st, 2006, 01:31 PM

from still photgraphy and lenses my understanding of the criteria to achieve a fixed max aperture through a zoom range is basically lots of glass...to get enough light in at the longer zooms and thus achieve a good max aperture. look at a sports shooters 400mm f2.8 lens...its 7" diameter at the front element! Try making a wide that zooms to such lengths and you get one big heavy expensive lens. Its much easier to make a light, compact zoom and trade off the aperture. (aperture when expressed as for example f2.8 isnt an actual physical measurement of a hole size....such as 2.8cm...its a proportional measurement)

Lawrence Spurgeon · July 21st, 2006, 10:53 PM

Greg,
Price and quality.

A constant apature lens is certainly possible as you note, but the larger tha apature the larger the glass diameter relative to the distance to the sensor. When you zoom the lens, the distance between tha apature and sensor changes, and so does the f-stop value. If you have a huge diameter lens, you can keep a constant apature.

A few lenses of f/1.0 have even been created - they are huge, and cost thousands of dollars to build, have a depth of focus of inches wide open, but allow for filming with matchlight.

Betsy Moore · July 25th, 2006, 01:19 PM

Quote:

Originally Posted by Lawrence Spurgeon

Greg,
Price and quality.

A constant apature lens is certainly possible as you note, but the larger tha apature the larger the glass diameter relative to the distance to the sensor. When you zoom the lens, the distance between tha apature and sensor changes, and so does the f-stop value. If you have a huge diameter lens, you can keep a constant apature.

A few lenses of f/1.0 have even been created - they are huge, and cost thousands of dollars to build, have a depth of focus of inches wide open, but allow for filming with matchlight.

Most famously used in Barry London. Kubrick had to round up one of those lenses from NASA and mutilate a classic WWII combat camera, as it was the only thing that would work well with it.

Sort of on the same topic, is there some automatic override of manua iris that shuts the iris down when there is a bright light source in the background? If I pan my camera throughout a scene and it passes a window, a lamp etc., even though my iris is on manual, the camera will get dark. Is this to protect the chips or have I just accidentally tapped some obscure settings button?

Boyd Ostroff · July 25th, 2006, 03:25 PM

Quote:

Originally Posted by Betsy Moore

If I pan my camera throughout a scene and it passes a window, a lamp etc., even though my iris is on manual, the camera will get dark. Is this to protect the chips or have I just accidentally tapped some obscure settings button?

You know, it's interesting that you mention this; I've seen the same thing happen on my Z1 with everything in manual. It only seems to happen when there's a huge difference in light levels. I wonder what is going on there? I'm not sure it's the iris that's changing... it might actually be reducing the gain? Maybe someone else can shed some light (sorry ;-) on this phenomenon....

Betsy Moore · July 25th, 2006, 05:37 PM

It is sort of strange, isn't it Boyd? Sometimes I feel like I'm right back to my JVC HD1:)

July 18th, 2006, 05:17 AM	#1
Kevin Sullivan New Boot Join Date: Sep 2005 Location: Baku, Azerbaijan and London Posts: 14	Iris shifting Does anyone know how to stop the iris from shifting while zooming? The manual says that this is just a fact of life. Any ideas? Thanks, Kevin Sullivan

July 18th, 2006, 11:46 PM	#3
Kevin Sullivan New Boot Join Date: Sep 2005 Location: Baku, Azerbaijan and London Posts: 14	Thanks for the clarity. Thanks for the detailed response. Been designing a shot for a project and was concept testing and could not figure why the iris was changing. Will test again today with your info.

July 19th, 2006, 12:32 PM	#6
Mark Utley Major Player Join Date: Mar 2004 Location: Toronto, ON, Canada Posts: 479	Just to avoid confusion, the Z1's aperture goes up to F1.6, not F1.2. __________________ Mark Utley

July 20th, 2006, 04:41 AM	#7
Kevin Sullivan New Boot Join Date: Sep 2005 Location: Baku, Azerbaijan and London Posts: 14	After testing... Thanks for the info everyone. After testing, the lense definitely stays put at 2.8 A little more light and problem solved. Thanks once again, Kevin Sullivan Lead Dog Productions

July 18th, 2006, 07:39 AM	#2
Boyd Ostroff Wrangler Join Date: Dec 2002 Location: Mays Landing, NJ Posts: 11,802	Welcome to DVinfo Kevin. Yes, it's a fact of life and there's nothing that can be done. All camcorders that have fixed lenses behave the same way. However you aren't correctly stating the problem. The iris isn't "shifting," it's staying the same. The f-stop is changing due to the optical properties of the zoom lens, which lets less light through at the telephoto end of the zoom. There is a way to keep this from happening, but it probably isn't the solution you're looking for. I don't have my Z1 here, but I think the wide open f-stop is 2.8 at full zoom. If you set the iris at f2.8 you can zoom in and out all you want and the f-stop won't change. But it you set it at f1.2 then it will work its way up to f2.8 as you zoom in. This has to do with the design of the zoom lens and you just have to live with it....

July 19th, 2006, 08:18 AM	#5
Boyd Ostroff Wrangler Join Date: Dec 2002 Location: Mays Landing, NJ Posts: 11,802	Heh, you didn't read my fine print :-) "All camcorders that have fixed lenses* behave the same way."* I know that there are interchangeable manual lenses for the XL series, the JVC HD100 and larger pro cameras which don't exhibit this behavior. I don't know much about lens design, but I always assumed the trade-off was both the cost and the size, since the lenses you just mentioned are large and expensive...

July 21st, 2006, 01:31 PM	#8
Michael Donne Regular Crew Join Date: Feb 2005 Location: brighon uk Posts: 55	from still photgraphy and lenses my understanding of the criteria to achieve a fixed max aperture through a zoom range is basically lots of glass...to get enough light in at the longer zooms and thus achieve a good max aperture. look at a sports shooters 400mm f2.8 lens...its 7" diameter at the front element! Try making a wide that zooms to such lengths and you get one big heavy expensive lens. Its much easier to make a light, compact zoom and trade off the aperture. (aperture when expressed as for example f2.8 isnt an actual physical measurement of a hole size....such as 2.8cm...its a proportional measurement)

July 21st, 2006, 10:53 PM	#9
Lawrence Spurgeon Regular Crew Join Date: May 2006 Location: Honolulu, HI Posts: 31	Greg, Price and quality. A constant apature lens is certainly possible as you note, but the larger tha apature the larger the glass diameter relative to the distance to the sensor. When you zoom the lens, the distance between tha apature and sensor changes, and so does the f-stop value. If you have a huge diameter lens, you can keep a constant apature. A few lenses of f/1.0 have even been created - they are huge, and cost thousands of dollars to build, have a depth of focus of inches wide open, but allow for filming with matchlight.

July 25th, 2006, 05:37 PM	#12
Betsy Moore Major Player Join Date: Mar 2003 Location: Los Angeles Posts: 444	It is sort of strange, isn't it Boyd? Sometimes I feel like I'm right back to my JVC HD1:)