Aperture Question

[mike_downs]

<p>I have a 18-55 3.5/5.6 lens. Should I be able to shot at 3.5 in all camera modes. Its likes to shot at 4/5.6 must of the time. I went to manual mode and could not get it below 4. I would think in manual I could select any aperture.Both the camera and lens are new and I just must be missing something. That most likely will be knowledge. If its not user error I need to re-package and send back for a exchange. Thanks for any help!</p>

[andylynn]

<p>You can only use f/3.5 when zoomed out all the way. That's common with consumer grade zoom lenses.</p>

[songtsen]

Using the Canon EF-S 18-55mm f3.5-5.6 as an example, f3.5 is only available at focal lengths between 18mm and 24mm. At 24mm, the maximum aperture is f4; at 35mm, f4.5; and at 55mm, the lens doesn't open wider than f5.6.

[lex_jenkins]

<p>That's how a variable aperture zoom works. The effective maximum and minimum apertures change as the focal length is changed from the wide end to the long end. You'll also find the minimum aperture will change, from the nominal f/22 or f/32 to an even smaller aperture, altho' sometimes this shows only in the EXIF data.</p>

<p>Effective maximum aperture also changes with some non-zoom lenses, particularly macro lenses. The effective maximum aperture decreases with most (not all) macro lenses at minimum focus/maximum magnification due to the long extension of the barrel. This also occurs with some non-macro lenses but the effect is usually negligible, only around -1/3 EV loss.</p>

[tommyinca]

<p>If you want to shoot at low aperture, you might want to check out an EF50/1.8 or a EF35/2, those 2 are great fast lens at bargain basement price in the Canon line up. </p>

[Alan Marcus]

<p>The camera lens acts much like a funnel in that it gathers light. In other words, the working diameter of the lens is a key component, determining how much light will transverse the camera lens. We are talking about the light energy that will arrive at the film or imagining chip plane (location) to produce the exposure. In concert with the working diameter of lens is the working focal length. The focal length is a distance measured from about the center of the lens barrel to the film/imagining chip plane. As you zoom you are altering the focal length. As you zoom in you are increasing the focal length. Another way to say this, the working focal length of the lens is increasing. Now focal length determines magnification. As you zoom in you are magnifying the subject. This is magnification like looking through binoculars. When you zoom out the subject size get smaller as does the focal length. Now the focal length (the amount of zoom) controls magnification. If you double the zoom, say from 50mm to 100mm, the subject size (magnification) doubles (2x). While the magnification is increasing, the amount of light reaching the film/imagining chip is diminishing. This change is large, for each 2x increase in magnification the light reduction will be 4x (2 f/stops). Now the f/number is derived by dividing focal length by lens diameter. What does this mean to me and my camera? As you zoom in, the light energy received by the camera drops. To compensate the lens must be opened wider. There is a linkage that opens and closes the camera’s aperture in concert with the zoom. This is an attempt to keep the f/number constant throughout the zoom. Regretfully there is a limit as to how much additional diameter the lens can bestow over the entire span of the zoom. This is a price consideration in lens design. Your lens is performing normally for a zoom in its price range.</p>

[aldrich]

<p>The simple way to look at any lens in the future, Mike, is basically as such:</p>

<p>If you see two apertures on the lens, be it 3.5 - 5.6, 4 - 5.6, or whatever, it is as our colleagues described above so well. The larger aperture coincides with the widest angle on the lens, while the smaller aperture coincides with the telephoto end.</p>

<p>If you see only one aperture, i.e. 70-200mm f/2.8, you can safely assume that all focal lengths of that zoom lens can be used at f/2.8. This is usually preferred, and usually more expensive too.</p>

<p>--Ryan</p>

[dan_south]

<p>Mike, think of the aperture as a circular hole. A circle can be measured by its diameter. That diameter is expressed as a fraction. The numerator is your focal length at the time you take the shot. The denominator is the f-stop number that you select. The f-stop number can be only as small as the maximum (really minimum) f-stop number that's printed on the lens.</p>

<p>At 18 mm, the maximum diameter of your lens' aperture is 18mm / 3.5 = 5mm.</p>

<p>At 55 mm, the maximum diameter of your lens' aperture is 55mm / 5.6 = 10mm.</p>

<p>If your lens could maintain an f-stop of 3.5 throughout its range, the maximum diameter of the aperture at 55 mm would be:</p>

<p>55mm / 3.5 = 16 mm</p>

<p>Your lens is too small to open up its aperture to a 16 mm circle. That's why you can't shoot at 55 mm @ f/3.5. You're limited to 55 mm @f/5.6, a smaller opening (10mm vs. 16mm). So called "fast" lenses are physically larger (and heavier). This enables their apertures to open up to a larger circle when a wide aperture is desired.</p>