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How is a zoom aperture fixed?

Why does the aperture change as you zoom in?

  • I'm totally new to photography and lens. I notice on an typical Canon 18-55 lens, as I zoom in , the aperture closes. why? would it over expose if the aperture stays open? i just don't understand why you can't keep the aperture open when you zoom in, to allow the same amount of light in as you do at 18mm, zoomed out. Thanks.

  • Answer:

    The technical answer get's into a long winded discussion in geometry and physics but, the short answer boils down to cost, size and weight. There are "constant aperture" zoom lenses like the EF-s 17-55mm f/2.8 IS USM and EF 70-200m f/2.8L IS USM that maintain the same aperture from one end of their zoom range to the other but, you also pay premiums in size, weight, and cost for that convenience. The EF-s 17-55mm f/2.8 for example; costs almost seven times the price of your 18-55mm zoom, weighs three times as much, and is considerably larger overall. Similar story for the 70-200mm f/2.8L IS USM compared to the 70-300mm f/4-5.6 IS USM; the latter only costs about $550 where as the latter costs $2200. A lot of pros, and some amateurs will choose the constant aperture lenses specifically to address the problem you've mentioned with respect to changing exposure due to a change in focal length. They are willing and can afford the more expensive lenses to resolve this issue. The workaround would be to simply use the maximum aperture of the long enod of your lens as your maximum aperture. For example, if I know my 18-55mm lens can only shoot f/5.6 at 55mm, then I choose to never shoot wider than f/5.6 even at 18mm. Most of us don't particularly care for that solution so we shoot fast primes (f/2.0 or faster) or we shoot fast zooms (f/2.8).

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It's a variable aperture lens. Note how its description says 18-55mm f/3.5-5.6 This means that at 18mm it has a maximum aperture of f/3.5 and at 55mm a maximum aperture of f/5.6 The reason is that the aperture is measured as a fraction of the focal length. At 20mm focal length an aperture of f/4 means that the aperture opening is 20mm/4=5mm wide. At 50mm one would need a much bigger opening for the same aperture value: 50mm/4=12.5mm For such big aperture openings at long focal lenghts a lens needs to have a big diameter and big lenses. That's expensive...and hence such a thing isn't done in starter kit lenses. They use less big maximum apertures and in order to make the most out of it they allow a variable aperture, opening as wide as possible at the short focal lengths.

Jens

With most zoom lens the maximum aperture decreases (lowest available f number increases) as you zoom in. There are some zooms that don't, but they are expensive. If you have your 18-55mm lens set to 18mm f3.5 and zoom to 55mm, the camera knows that it has no f3.5 at 55mm and recognizes that you are at f5.6. If it is in an automatic or semi-automatic mode, the camera will adjust shutter speed accordingly. If it is in manual mode, it is up to you to do that, but the camera will adjust the meter reading. If you started at f8 and zoom in, your aperture setting will still be f8, because the camera knows that is available to you.

thankyoumaskedman

when you are zoomed all the way in and all the way out the opening of the appature changes

maj

The aperture is a ratio, not an absolute measurement of the opening diameter. It is a ratio of opening diameter to focal length. Therefore, consider a 100mm lens with an aperture of f/4; the ratio of 4 means the focal length is 4 times the aperture, so in this case, the opening would be 25mm as 100/4 = 25. Now zoom that lens to 200mm. Maintaining the same physical 25mm opening now changes the ratio to 8 (or f.8), as while the 25mm is constant, 200/25 = 8. Some pro lenses do have a constant aperture, so in the example above, the aperture would have to increase from 25mm to 50mm as you moved the lens focal length from 100 to 200mm to maintain a constant f/4.

AWBoater

The aperture is what yoiu get if you divide the focal length by the diameter of the hole in the middle. So, if you are set at 35mm focal length and the "iris" is 10mm across you get an aperture of f3.5. If you now zoom to 110mm and the iris is unchanged, still 10mm, then you get an aperture of f11. I don ;t don't know your lense but if the aperture at 18mm remained the same at 55mm then, if at 18mm it is as wide as it can be, the actual lense would have to physically increase in width for the aperture to remain constant.

BigAl

Because its a variable aperture zoom lens (these are usually the cheaper consumer grade lenses. The maximum aperture of the lens at 18mm is f3.5, at 55mm it's f5.6. This is cheaper to make than a constant aperture lens. For example, the 70-200mm f2.8 lens is capable of a massive f2.8 aperture at 70mm AND at 200mm - but it comes at a price - £1200!

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