iris stops down on zoom

metimo

metimo

Active member
My boss uses the sony z1u, and we were trying to figure out the limitations of this camera.

Well, I've noticed that when I zoom in, the iris starts stopping down. I've seen this is my sony digital still camera as well, but I don't think the dvx does it.

Can anyone explain why this might have to happen for some cameras but not others?

thanks.
 
Easy: the maximum aperture of the lens is the ratio (lens opening)/focal length. Like 1/2.8 usually expressed as f2.8 (just the denominator is mentioned).

It is easy to make the wide angle fast; for something like 8mm focal length the opening needs to be only 4 mm to achieve this. Lets say you have a 20x zoom on your video camera (like Canon XL2). For 160mm telephoto end you would need 20x4mm=80mm opening. Even that $$$ Canon lens closes down at the zoom end, because if they would make the zoom hold the max aperture all the way, if would be BIG, heavy and cost a LOT. And amateurs would not buy it.

You can see the same thing in cheap SLR lenses, like 28-200mm f:3.5-6.3 or something. That 3.5 is the speed at wideangle, 6.3 at the zoom end.
 
hm, interesting. I thought it had something do with the focal length. I need to brush up on my photography 101

thanks
 
It has.

Aperture = (opening or iris) / (FOCAL LENGTH). Like I said in the above post...

When the zoom's focal length gets long enough, it is not (economically) possible to make the iris big enough. For this reason even good prosumer video cameras let the max aperture get smaller when zoomed to telephoto.
 
The DVX does do it, as do all the cameras in this price range and pretty much all low-cost zooms. The DVX and Z1 are 1.6 at the wide end, and 2.8 at full telephoto. The XLH1 is 1.6 at the wide and 3.5 at full tele; it has a much longer zoom so the iris is smaller the further out you go.
 
sorry, I meant, I thought it had something to do...


and Barry, thanks for the info. I think the dvx might be a little misleading when it says "open" instead of any value, which is why I was confused. all cleared up now!
 
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