None constant aperture zoom lenses

[benjiboy]

I was just wondering if your using for example a f3.5-f4.5 none constant aperture zoom at what point does it change aperture during the zoom travel ? because this could be very significant if it is used with a hand held light meter or none TTL strobe, what I'm asking is, is the transition gradual throughout the zoom range ?.

 

[markbarendt]

I was just wondering if your using for example a f3.5-f4.5 none constant aperture zoom at what point does it change aperture during the zoom travel ? because this could be very significant if it is used with a hand held light meter or none TTL strobe, what I'm asking is, is the transition gradual throughout the zoom range ?.

It is gradual on the lenses I have.

On lenses of this type I tend to avoid using the wider/variable apertures. With the example you gave I'd treat it as a 4.5 and use A or M to keep it out of the variable range.

 

[Q.G.]

It's gradual on all variable aperture zooms.

Basically (though not exactly), the focal length changes while the pupil size remains the same. And given that the f-number is f(ocal length)/pupil size, any change in focal length, no matter how minute, will change the f-number.

And that provides the clue to how to use such a lens with non-TTL metering: first calculate the pupil size by doing the sum focal length/f-number at that focal length (always given for both extremes, so doable), and jot that down somewhere handy.
Then after changing focal length do the sum backwards again: f-number = focal length set/number calculated in first step.

What you also could do is do the sums and calculate at what focal length setting you would need to add 1/3 or 1/5 stop, and mark those points on the focal length scale of the lens.

Mind you: while close enough, it's not necessarily correct, nor exact. It depends on what happens inside that lens, and that varies with design.
But it works.

And another mind you: this changes with focus setting too. The closer you get, the more light you lose.
And that can be a significant amount not just when doing macro. Especially not when using wildly asymmeric lenses, which zooms are.

 

[benjiboy]

Thanks guys , I suspected that was the case, I just have one variable aperture zoom a 35-70 my others are constant aperture.