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I use a Sony NEX-5R, and I bought the Sony 18-105 F4 constant-aperture lens.

I'm aware that the constant-aperture F4 gathers more light than, say a F3.5 - 5.6, and it gives you more control over the depth of field. I'm also aware that constant-aperture lenses are heavier, bigger and more expensive.

What are the other tradeoffs? Would I get a lens of better quality for the same price if I don't insist on a constant aperture?

Do constant-aperture lenses make a practical difference for low-light shooting, in aperture or shutter speed priority mode? If the aperture changes as you zoom in with a variable-aperture lens, the camera can always compensate by varying the shutter speed and / or ISO, so does it make a noticeable difference in practice, for low-light shooting?

And, taking a step up from F4 constant-aperture zooms, how do F2.8 constant-aperture zoom lenses compare? Are they generally as sharp as prime lenses? In other words, if I don't need a wider aperture than F2.8, would an F2.8 constant-aperture zoom lens substitute for multiple prime lenses in its focal length?

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    Your f/4.0 lens doesn't necessarily gather more light than a f/3.5-5.6 lens. Exactly the opposite is true at some zoom settings. – RoG Nov 29 '16 at 7:48
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Do constant-aperture lenses make a practical difference for low-light shooting, in aperture or sheet speed priority mode? If the aperture changes as you zoom in with a variable-aperture lens, the camera can always compensate by varying the shutter speed and / or ISO, so does it make a noticeable difference in practice, for low-light shooting?

Many times when shooting action in limited light you are using manual exposure mode with the widest aperture and the slowest shutter speed you can without getting camera blur or blur from the motion of your subject(s). By using a constant aperture zoom lens you don't need to change the ISO to compensate for the change in aperture as you zoom. Many cameras allow the use of Auto ISO in manual shooting mode but some don't. Even with Auto ISO, the way some cameras handle "partial stop" ISO settings (i.e. ISO 125, 160, 250, 320, etc.) make them less than ideal compared to "full stop" ISO settings (i.e. 100, 200, 400, etc.). In such cases the preferred way of using Auto ISO is with "full stop only" ISO values enabled, which means as a variable aperture lens moves through maximum apertures in 1/3 stop increments the exposure "bounces" up and down. For what I shoot on a regular basis there is a world of difference between a constant aperture zoom and a variable aperture one.

Would I get a lens of better quality for the same price if I don't insist on a constant aperture?

In general constant aperture zooms tend to have better optical quality than variable aperture zooms. It is not so much an inherent quality of a constant aperture design as it is an indicator of what the market demands out of higher priced lenses. Usually, but certainly not always, the tradeoff isn't better optical quality in exchange for a variable aperture design. Rather it is variable aperture in exchange for a cheaper price.

And, taking a step up from F4 constant-aperture zooms, how do F2.8 constant-aperture zoom lenses compare? Are they generally as sharp as prime lenses? In other words, if I don't need a wider aperture than F2.8, would an F2.8 constant-aperture zoom lens substitute for multiple prime lenses in its focal length?

That all depends on the particular lens. The Canon EF 70-200mm f/2.8 L IS II is one such lens that can answer (a qualified¹) yes to your last question. A few other zooms are very close or equal to their prime counterparts at the same aperture settings. But there are other zooms whose image quality falls far short of their prime or narrower aperture counterparts. The Canon EF 16-35mm f/2.8 L II is one such lens in this category. The EF 16-35mm f/4 L is better at pretty much every common combination of aperture and focal length than the first two versions of the 16-35 f/2.8 and is also quite a bit more affordable. The newly released EF 16-35mm f/2.8 L III, however, is significantly better than all of Canon's previous 16-35mm zoom lenses. It's also quite a bit more expensive than the others.

¹ As Roger Cicala has pointed out more than once in his blog series at lensrentals.com, the lens-to-lens variation between all zoom lenses, even the most expensive ones, is much higher than between even mid-grade prime lenses. So a "good" copy of a very good zoom such as the EF 70-200mm f/2.8 L IS II or EF 24-70mm f/2.8 L can be pretty close with regard to measurable I.Q. to prime lenses in its focal length range. But the less measurable characteristics of any two lens designs will vary from one to the next. How the out of focus areas are rendered, for example. Even two very different prime lenses, such as the EF 50mm f/1.2 L and the EF 50mm f/1.4 differ more on things that don't show up on a flat test chart than they do with regard to absolute resolution, geometric distortion, astigmatism, coma, etc.

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  • "Some cameras allow the use of Auto ISO in manual shooting mode but most don't" -- is this still true today? Even my cheap 2000D supports manual + auto-ISO. Also, my EOS RP does support manual shutter+aperture with automatic ISO. – juhist Dec 14 '19 at 11:21
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    @juhist It was pretty much the case in 2014 when it was written. Even cameras such as the Canon EOS 5D Mark II and EOS 1D Mark III did not have usable Auto ISO. By 2014 Canon's upper tier (1D X, 7D, and 5D Mark III) had it and it was beginning to filter down to the lower tiers. Other brands were similar. – Michael C Dec 14 '19 at 15:55
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Something the other answers don't touch on: fast f2.8 constant-aperture zoom lenses are typically the manufacturer's best, which is more than just a larger aperture. Special elements and coatings on elements are used to make these lenses better (and more expensive) than variable-aperture lenses. I'm really only familiar with the Nikon line-up, where that means more elements with nano crystal coatings ("N"), more extra-low dispersion elements ("ED"), and more aspherical elements, as well as better weatherproofing.

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It's an old question ( EG http://www.dpreview.com/forums/thread/2714173 ).

The short answer is that it is most advantageous for Video, because when you zoom in everything doesn't get darker.

For single frame Photos you can adjust ISO and Shutter speed (the automatic Setting will do it for you) on a shot by shot basis and get all Photos similar; in a Video such variance would not be tolerated.

The design and manufacturing of varying aperture Lenses is less expensive than constant aperture; but that doesn't mean you couldn't use a variable aperture Lens at a constant aperture, simply set your Aperature to the higher number (or greater) and the higher value is your minimum (and constant).

The lower the F number, the ability to zoom and constant aperture for Zoom Lenses are all factors that can increase the cost.

A higher F number and a variable aperture Zoom Lens will be the least expensive.

Some constant aperture Zooms are less expensive than some Primes, and vice versa; quality being the main factor for expense in either of those cases.

If you will almost always take Videos either wide field of view (not zoomed in) or in bright sunlight then you can buy a non constant aperture Zoom and save a bit of money.

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  • Great answer. I'd have accepted it if I hadn't already accepted Michael's answer. – Vaddadi Kartick May 3 '16 at 4:43
  • Another important consideration for Video along with constant aperture is breathing, both 'constant focus' and 'constant position'. When you zoom in or out you don't want the focus to 'breathe' in and out, nor for the Lens to wobble. With still Photos that usually won't matter unless you need each image to correspond with the others (for focus or aperture stacking). Thanks for the props, always better than the 'driveby downvote'. – Rob May 3 '16 at 10:26
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    While you can adjust ISO and shutter speed to offset the changing aperture as you zoom, that's incredibly difficult to do when shooting fast moving sports. For that, a constant aperture zoom (especially indoors or under high school field lighting) is a near must. – FreeMan May 5 '16 at 20:19
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The advantage of a constant aperture zoom is that you can zoom in and out as much as you like without changing exposure - it's just one less thing to worry about, that's it.

A f/3.5-5.6 can gather more light at the wide end (where you can use f/3.5) then a constant aperture f/4 lens - but if you need to zoom in suddenly you have to change the ISO or shutter speed to compensate for you aperture changing.

And as for you question about how constant aperture zoom compare to primes, it's like everything else in lenses, the latest generation top of the line lenses are really amazing(but also really expensive), older or cheaper lenses vary in quality.

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    You're quite likely to be zooming into different light, especially with long zooms though, right? (In which case you (or the camera) will need to adjust ISO or shutter speed.) – mjs Feb 8 '15 at 13:20
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You should also consider that you will never find constant aperture zoom lenses of the superzoom type (like Tamrons recently released 16-300mm for DX sensors). Sigmas 1.8 (crop format)/2.0 (full frame)-constant aperture lenses have zoom factors of less than 2, typical 2.8 zooms are less than triple zooms (24-70, 70-200, 200-500), F/4-zoom glass has typically less than a fivefold zoom factor (28-135), sometimes only 2x. The reason behind this is, it gets more and more difficult (and expensive) to compensate the various kinds of optical aberrations within a wide zoom range and even more with a wide constant aperture, if not impossible.

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    It is not so much a question of balancing and compensating aberrations as it is one of physical size and design. A constant aperture lens must only zoom in front of the aperture stop. To a certain point moving the elements in front of the stop further and further away will increase the focal length of the lens. Bringing them closer to the stop decreases the focal length. If you want a 10x zoom ratio that means you must reach a short focal length, which may not be physically possible. – Brandon Dube May 3 '16 at 18:53
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Constant aperture lenses have only advantages (except lenses which are total crap). There is no decrease of light when you zoom out and this is the reason photographers pay sometimes huge money to get such lenses.

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  • Which constant aperture zoom do you recommend that can match the price, weight and size of a typical 50-200mm kit lens? – Philip Kendall Sep 21 '15 at 16:27
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    So constant aperture lenses are always better, except the ones that aren't! – MikeW Sep 21 '15 at 19:15

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