gauging light lost...

[JustinT]

just curious as to how to gauge the amount of light loss due to the 35mm adapter...

I did a test where i set iris to auto and it went to 2.0 (which is wide open) when i removed the adapter and refocused.. it went to 5.6 i think... does that mean i have lost 3 stops?

[Barry_Green]

Roughly, yes.

The question is -- how many were due to the adapter, and how many were due to your lens? If you had a 35mm camera lens with a maximum iris of f/2.4, then you'd be throwing away 1.5 stops as compared to a lens with a maximum iris of f/1.4.

So some light loss will be due to the adapter, and some will be due to the light transmission of the lens you have on in front of the adapter. At 3 stops, you're looking at needing 8 times as much light to get comparable brightness in the scene.

[Bobaristheguy]

Light loss is primarilly due to the gg not the lens. You must remember that a 35mm lens was designed to have a focus plane of 24mm x36mm. Your 1/3" ccd censor is much smaller. This means that in order to reach an f-stop of 1.8 at roughly 8mm or so you need a much smaller iris to achieve it than say a 35mm lens at f 2.8 at 28mm. This is because f-stop= focal length/by the diameter of the aperature. This means light moves relative to focal length. That is one of the reasons why dv camcorders have such low light capabilities compared to film cameras.

[JustinT]

dang, i guess i am losing a lot of light then :( thought it was actually pretty good.. might look into purchasing a premade GG ...

the lens i have is a canon FL 1.2 so that is like WIDE OPEN.. yet i still have to stop the dvx down to 2.0 (filming inside with natural daylight).

I assume a better GG would (under the same conditions) allow me to perhaps run the DVX at 3 or the SLR lens at 2.8 and still achieve the same look (ignoring the elarged DOF due to the closing SLR)?

[Dennis Wood]

I'm seeing 1 to 1 1.5 f/stop loss with my adapter with an f/1.4 lens fully open. This is typical.

To compare, light a scene and frame it identically with and without the adapter. 3 stops sounds like a lot.