Aperture of the iris and focal length

[lukemabat]

HV20 automatically closes the iris when the focal grows. It sounds strange that the iris closes exactly when F-stop grows (F1.8 - 6.1mm, F2.8 61mm): it is more like a firmware "bug"...
This behavior brings to 2 problems:

1. The global light, at the maximum focal length, goes down by 1.25 stops
2. You have more depth of field (usually you'd like to have a restricted depth of field)

But nothing is lost.... you can force the total iris aperture (even at the maximum focal length) by locking the exposure when you are at the minimum focal length (wide-angle), and then you could zoom. In this way you can get circular bokeh even at the maximum focal length.

Here you can find some exaples (explanation in italian and english): http://lukemabat.interfree.it/hv20/fiori.html

Examples stills: in the first the iris is totally open, in the second is close like F2.8 at 6.1mm.

F1.8:


F2.8:


I hope this can help to have images more "cine" like.

Ciao,
Luca

[Rich]

Most consumer zoom lens will have a variable relative aperture at different focal lengths. It is expensive and difficult to maintain a constant aperture at both ends of the focal range and maintain other lens qualities. I don't recall any camcorder zoom lens with the same aperture or f stop at both ends of the focal range.



Rich

[lukemabat]

Most consumer zoom lens will have a variable relative aperture at different focal lengths. It is expensive and difficult to maintain a constant aperture at both ends of the focal range and maintain other lens qualities. I don't recall any camcorder zoom lens with the same aperture or f stop at both ends of the focal range.

Please, keep attention: absolute iris aperture and f-stop are not the same. HV20 phisically closes the iris when the focal length grows. Obviously the F-stop (entrance pupil divided by the focal length) grows with the focal length. For example, at the minimum focal length, when the iris is totally opened, you have F/1.8; when it is totally closed, you have F/8 (this is not the truth, because the iris could close until F/22, but only when not in Av). So, at the maximum focal length, the F-stop should be between F/2.8 and F/12 (or F/35 with automatic exposure), but HV20 simply closes the iris following the F-stop measured at 6.1mm.
Canon had justified the forced iris closure with image quality, but I didn't see any difference in quality between the 2 frames: see the flower in focus, or try by yourself if you have an HV20.

However, this is a trick in order to have more control on the cam...

Ciao,
Luca