With a tight shot on a person's face in manual mode I get a good exposure setting with the help of the zebra function, but when I zoom right in the iris closes down, even in the lock position. Why does this happen?

If I remember correctly some cameras don't have a wide f-stop range when you zoom all the way in. If you are full wide and have an f-stop of 1.6, when you zoom in it may change to 2.8, etc.

Don't ask me why, I just know that is what happens.

Cheers

It's the lens design. Same thing happens with even Canon L glass (or all non-fixed aperture lenses for that matter) if it doesn't have a constant aperture.

Nick,

The advice above is 100% correct.

At full telephoto the Z1 has a maximum iris setting of F2.8, but this changes to a wider F1.6 at wide angle. This is the case even if you are in manual mode, and is a limitation on the size of the objective lens - as James says a compromise used buy many lens manufacturers to keep the objective lens size and cost down.

The best way to set your exposure is to use manual mode - zoom in first with your zebra bars on (I use about 80 on caucasian faces) and when you have it correct, note the F stop in the viewfinder. Zoom back out and check whether the camera has reset the F stop to a wider setting - if so use the iris control to change back to the original correct exposure.

Hope this helps.

I found this thread tonight after discovering this with my fx1.
It's kind of dissapointing when you are trying to narrow the depth of field.

So here's a question (if anyone still wants to revive this old thread).

For interview shoots (talking heads) I set my camera guy up as far away from the subject as possible. That way he can use a lot of zoom and soften up the background while I sit closer for the interview.

Since you are loosing aperature size by zooming, it seems like that would offset some of the intended effect.

Has anyone discoverd where the threshold is on this?

Daniel

So you're saying that there might be a trade-off on DOF (instead of: zoom as far as possible, use largest f-stop possible, and if possible put some distance between the subject and the background). Why don't you try some tests at no-zoom (f1.6), then half way, and then something inbetween the best two results. At no-zoom, you may be to move the subject closer to the camera putting a lot more distance between the subject and background - maybe that'll do it?

Walter Graff's "The Headshot" DVD has some good info on this. It's basic DOF stuff (like my initial comments), but it's nice to see visually how he does it. I think Graff also mentions that 1/3" CCD have the most trouble providing a tight DOF. After seeing his video, I'm encouraged to focus more on lighting than DOF since that is much harder to create.
For the Video:
http://bluesky-web.com/dvd1headshot.htm
Here's a nice article by Graff:
http://www.film-and-video.com/oneroomexample.html

Also, this thread has some similar discussion but it's not obvious from the title:
http://www.dvinfo.net/conf/showthread.php?t=52846

Daniel,

Just for the record here is how the iris behaves as you zoom out.

First column is zoom % second is F stop.
0 - 1.6
10 - 1.7
50 - 1.8
65 - 2.0
75 - 2.2
84 - 2.4
92 - 2.6
99 - 2.8

Hope this is helpful. Sorry that the columns run together - won't let me seperate them - even in edit

Shawn, thanks for the quick response. That sounds like a good test, I'll wait a day and see if anyone has already done it and found the point where you reach the trade-off. I have a hunch (from that other thread) that 2.8 isn't going to be very different from 1.4, so it will be better to keep trying to use a long focal length.

Read the article, very helpful, thanks. I might pick up that DVD too.

Graeme thanks for the guide, that will be helpful.
Daniel

And if you want to get a narrower DOF, you can sometimes put on an ND filter and open the iris more.