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Let's say that I have a 24-70 F3.5-4.8 variable aperture zoom lens. If I were to stop down that lens to 4.8, would it then be equivalent to a 24-70 F4.8 constant aperture zoom lens from a user perspective? That is, would it in this configuration be possible to tell the two lenses apart, just by looking at the output they produce? For example, would the output look different when zooming in and out with them?

(To frame the question in a somewhat less abstract way: If I planned to shoot with my lens stopped down to F4.8 anyway, would I benefit from the lens being a constant aperture lens?)

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Typically, the constant aperture zoom would be sharper. The vast majority of lenses get sharper when stopped down, this includes constant aperture zooms and prime lenses too. So when you stop down an F/2.8 lens to F/4.5, let's say, you get would get an image which is very sharp. The variable aperture lens though would be wide-open at some focal-length and so would exhibit more softness & vignetting. How much is impossible to say but you would most likely be able see the difference.

Images from both lenses would have the same properties though in terms of angle-of-view and depth-of-field. So they would look the similar from the viewfinder. The constant aperture zoom though would let more light in though so would show a brighter view. Additionally, most cameras focus faster since autofocus is always performed wide-open, even you selected a smaller aperture for the exposure.

  • "The constant aperture zoom though would let more light in though so would show a brighter view" - Even if (successfully) set to the same f-ratio as the variable aperture lens? :S – Speldosa Jul 9 '16 at 5:29
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    @Speldosa yes, the aperture is only closed for actual recording of image/video; while you're framing and focusing, aperture is kept open. The "they would look similar from the viewfinder" mostly holds because depth of field is very hard to tell from a modern viewfinder. – Imre Jul 9 '16 at 6:22
  • I don't think the "sharper stopped down" part is going to be true in practice here, though. The constant aperture zooms are effectively cutting off their worst apertures at the wider focal lengths, and because they tend to be more expensive lenses, are likely to be better even wide open. – mattdm Jul 9 '16 at 7:04
  • I do agree with the point about the viewfinder and autofocus, though. – mattdm Jul 9 '16 at 7:04
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    Based on plenty of testing, I can say that even constant aperture lenses do sharpen when stopped down. Good ones only need one stop or so to reach maximum sharpness, but some take around two. – Itai Jul 9 '16 at 14:38
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All other things being equal, yes — mostly. In terms of exposure and depth of field, they would be as close to functionally equivalent as things get in the real world.

The shape of the aperture blades will have some impact, so in some cases you could probably tell by looking closely, even if that were the only difference. But in the real world, those lenses probably differ in other more significant ways.

For example, it is usually the case that constant aperture zoom are higher-end, so you might have reasons to buy one other than this particular feature. It's likely to have better environmental sealing, and will probably better correct for various distortions and artifacts.

In fact, in the real world, I expect that even though stopping down a bit usually improves technical image quality, the constant-max-aperture zoom wide open at the wide end will beat the variable lens one stop down. On the other hand, the variable aperture zoom will likely be lighter and cheaper, which can both be important considerations.

Also, as Itai points out, cameras use the widest aperture for composition and focus, so the viewfinder may be a bit brighter and autofocus a bit faster with the lens with the wider aperture (so possible advantage to the variable max aperture lens in this case).

You probably will find How do constant aperture zoom lenses work? to answer a lot of your question,as well.

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In theory, it makes no difference how the lens attained focal ratio 4.8 (f/4.8). However, every lens is constructed via a series of compromises. In other words, the perfect lens, able to yield a faithful image has yet to be made. As to the constant zooms design: The problem is; as the focal length is changed, the size of images of objects is changed. Long lenses magnify thus the image they produce is enlarged. An enlarged image means that the image forming light is caused to play on a larger area. This larger image is cropped by baffles inside the camera so we only image the central portion. Nevertheless, this enlarged image is more feeble than an image yield from a shorter lens. In other words, the shorter lens yields a bright wide-angle view and the longer lens yields a far dimmer magnified view. The change in brightness with focal length change is quite substantial. Each time the focal length doubles, the image size increases by twice (2X). The image brightens of this enlarged image is now 4X dimmer. Thus the problem is, how do we cope with this changing image brightness throughout the zoom?

A 2X change in focal length must be compensated for by opening up or closing down the lens aperture (iris diameter) to allow 4X more or less light to play on the image chip (or film). It would be neat to this with a mechanical linkage. If true, the zoom mechanism would open up or close down the aperture to achieve constant image bigness. Such a design proves to be too complicated and too costly. Another way to achieve constant image brightness is to use a design whereby the lens array in front of the iris diaphragm is a strong magnifier. This design has the front lens array change its distance to the iris during the zoom. The result is, when viewed from the front, the iris diameter appears more enlarged or less enlarged depending on the zoom setting. In other words, the front lens array changes the apparent size of the iris. This change in size allows more or less light to transverse the lens.

Maintaining the image brightness throughout the zoom requires a super compromise in lens design. To get it right (never happened) the design is complex and thus costly to make. Constant zoom lens must be good to work but they remain compromised. A fixed focus (prime) requires less compromises thus they yield a more faithful image. A non-constant zoom can be good. They may outperform a constant zoom. The problem is always cost to make. Will the photographic community pay the price?

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