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03-09-2009, 10:59 AM   #1
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f-stop and metering

Does the focal length of a lens effect the metering if the f-stop is held constant?

If I put my camera on spot metering and used a 200mm lens @ f/4 should I get the same exposure time if I then used a 28mm lens @f/4? And, do zoom lens behave any differently?

I quickly tried this experiment but the ambient light kept changing.

03-09-2009, 11:23 AM   #2
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How does "probably so" work for an answer?

The reason is because the 28mm lens is going to cover a field of view 7x larger than the 200mm lens. And even with spot metering that means the area being metered is going to be 7x larger than on the 200mm. So unless you're shooting a uniformly lit, textured and colored wall then the metering will likely differ between lenses. Depending on the scene it may be either a negligible or dramatic difference.
03-09-2009, 01:54 PM   #3
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Right - the scene itself changes in most real world situations. But in theory, if you shot a plain wall with even lighting, you'd get the exact same exposure at any given f-stop independ of focal length. The aperture is given as a ratio (f/4, or 1:4) precisely to remove the focal length from the equation.

Of course, there still might be slight variations from lens to lens just because some might absorb more light than others, plus the aperture mechanisms might be slightly inaccurate.
03-09-2009, 02:43 PM   #4
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QuoteOriginally posted by wiseman Quote
And, do zoom lens behave any differently?
Not in principle, but there are many zoom lenses which change their minimum aperture ratio through the zoom range. If you keep the aperture ratio constant while zooming, the exposure will stay constant as well (provided that the scene illumination doesn't change with the field of view, hence the suggestion of the "white wall" as the ideal subject for a corresponding test).

03-09-2009, 05:55 PM   #5
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Thanks to all for such helpful information!
03-09-2009, 07:13 PM   #6
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Remember, an f-stop is an f-stop.

If you're on an aperture of f5.6 at 18mm, being on f5.6 at 500mm means you'll be able to theoretically gather the same amount of light through the lens.

The aperture of a lens is calculated by dividing the focal length of a lens by the diameter of the aperture (or the objective lens if you're shooting wide open).

f-number = focal length/aperture opening diameter.

So, let's say you've got an 100mm f4 lens, that means that the front - the objective - lens is 25mm in diameter (100/4). If you stop down that lens to, say, f11, then the diameter of the iris with be 9mm (100/11).

Or if you have a Pentax f2.4 DA 70mm Limited, then you know the diameter of the objective lens is 29mm.

In other words, if you want, say, a 200mm lens that'll be able to gather as much light as that 70mm Limited (ie, have the same maximum aperture), then you know that you'll need an objective lens that's 200/2.4, or 83mm in diameter.

But, of course, as Marc and venturi et al pointed out, in practical terms you may get more or less light, as wider lenses are able to capture more sources of light, if there're any there.
03-10-2009, 08:00 AM   #7
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As others noted, in theory it should be the same assuming the same subject is being metered. In practice though exposure can vary somewhat, even on constant aperture zooms, and some lenses will over- or under-expose at certain apertures. In particular, lenses are prone to falloff at wide apertures. I would bet that often you'll probably get slightly different exposures between an f/2.8 lens stopped down to f/4 than you would from a lens where f/4 is the maximum aperture, though the center of the frame may be fairly close.

Consider that handheld light meters return shutter speed & aperture without caring necessarily that you're shooting a 28mm vs an 85mm lens. You're supposed to be able to just shoot with those inputs without regard to which lens you're using...but it's up to you to learn your equipment for the small differences.
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