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As an example, say you have two lenses:

300mm f/2.8 being used at f/8
300mm f/4 being used at f/8

My understanding is that by definition the amount of light coming through the aperture means that you will still have to take both shots at the same shutter speed (for the same exposure).

That said, why then pay for a lens that can work down to f/2.8 if, for example, you're going to use it regularly at f/8? (eg. at f/2.8 sometimes the depth of field can simply be too shallow - in the above example, take bird photography).

Some have told me the f/2.8 lens requires more glass to achieve f/2.8 and therefore it lets in more light and can shoot at a faster shutter speed at the same aperture - but I really don't think that makes sense.

The only other possibility that makes sense is that typically lens manufacturers will produce a better build quality on a lens that can achieve f/2.8, so, for example, you might get sharper images because of better build quality, but that has nothing to do with the lens' ability to stop to f/2.8.

  • Coming back to this 1 year on, for the benefit of those new to understanding aperture, it helps to realise aperture is a fraction of focal length. Eg. f/2.8 is really "focal length divided by 2.8" = 1/2.8 of the focal length. Hence for a long focal length like 300mm to support a large fraction of this for aperture (eg. f/2.8 = 300/2.8 = approx 107mm) it needs a very wide diameter lens (in order for aperture to open to 107mm). Hence "more glass" in my question really means larger diameter glass, which is why there is more light entering the camera during auto focusing. – youcantryreachingme Aug 25 '15 at 9:46
  • To summarise the selected answer, faster lenses (with wider aperture) should produce a better result with autofocusing and metering before taking the shot because these functions are carried out with the lens wide open. Additionally, aberrations at the edge of an image can be blocked by shutter blades on a stopped-down shot because the faster lens is wider, thus allowing the blades to better block the aberrations. However, a wider lens actually accentuates the aberrations and these lenses generally have corrections built in. Hope I got that right! – youcantryreachingme Aug 16 '18 at 5:35
7

Some have told me the f/2.8 lens requires more glass to achieve f/2.8 and therefore it lets in more light and can shoot at a faster shutter speed at the same aperture - but I really don't think that makes sense.

Basically incorrect. The entrance pupil for 300mm at f/8 will be 37.5mm in diameter, regardless of the diameter of the lens' front element. So you won't gain any shutter speed advantage by using the f/2.8 lens at f/8 as compared to using the f/4 lens. When you're stopped down to f/8, the light striking the parts of the front elements of either the 150mm diameter f/2.8 or 75mm diameter f/4 lens beyond the 37.5mm diameter circle in the center of the element is not being allowed to pass through the aperture diaphragm.

You will get an advantage with regards to the amount of light used for focusing and composition as most cameras meter and focus with the lens wide open and only stop the lens down to the specified aperture setting a split-second before the shutter opens. This will very likely mean faster speed and more precision in terms of autofocus performance. Exactly how much difference will depend on the capabilities of the camera's AF system.

The only other possibility that makes sense is that typically lens manufacturers will produce a better build quality on a lens that can achieve f/2.8, so, for example, you might get sharper images because of better build quality, but that has nothing to do with the lens' ability to stop to f/2.8.

That's pretty much it in terms of image quality. There is an additional IQ consideration, though, that does have to do with the lens' ability to open up to f/2.8.

The typically larger front element of the f/2.8 lens mean that when stopped down to f/8 the aberrations introduced on the edges of the larger objective are blocked by the aperture blades. Since these aberrations increase in severity with an increase in the diameter of the front element when shot at maximum aperture most telephoto f/2.8 lenses have more extensive aberration correction than narrower aperture lenses of the same focal length. It is theoretically possible to make an uncorrected f/2.8 telephoto lens that would have poor image quality and would be outperformed by a good f/4 lens, but the market doesn't demand such a lens. Almost all constant aperture f/2.8 telephoto zoom lenses and f/2.8 telephoto prime lenses are of very good optical quality because the market demands it.

In general, with long telephoto lenses the larger aperture models will slightly outperform their smaller aperture counterparts when both are from the same manufacturer and the same generation of lens design. But there are no real "consumer grade" first party lenses in the prime supertelephoto category with which to compare them. They're all very well corrected. (There are much cheaper third party fixed narrower aperture mirror lenses with lower optical quality and very cheap fixed narrow aperture refractive lenses with abysmal optical performance because there are few, if any aberration corrections in the design of the lens.)

With wider angle lenses it is usually the case that optical quality compromises must be made to allow wider apertures at such wide angles of view if field curvature is to be corrected to a flatter field of focus. It is much easier to correct a narrower aperture wide angle lens for field curvature than to correct a wider aperture wide angle lens for field curvature.

Depending on their intended usage, some wide aperture prime lenses in the 50-85mm focal length range do not correct for field curvature. When tested at a single focus distance optimized for the center using a flat test chart as a target these lenses appear to be very soft on the edges and in the corners. But often they are very sharp on the edges and in the corners, just not at the same focus distance as the center of the field of view because the shape of the sharpest focus at any point in the frame is part of a sphere.

Lenses corrected for field curvature to give them a flatter field of focus (often called plane of focus though that is somewhat of a misnomer for all but a pinhole camera - no lens is perfectly corrected for field curvature) will do much better across the entire field when tested using a flat target at a single focus distance. Such well corrected lenses tend to produce bokeh that is harsher or 'busier' than lenses less corrected for field curvature. Thus, a lens like the EF 85mm f/1.2L II which is not well corrected for field curvature has a distinctively smooth bokeh on the edges of the frame, but is totally unsuitable for flat field work, such as document/art reproduction.

  • Case in point: Canon 400mm f/2.8 ($10k) and 400mm f/5.6 ($1.4k). – user13451 Oct 25 '14 at 23:53
  • There's no distinguishable difference at f/8 between the EF 400mm f/2.8 L IS and the EF 400mm f/5.6 L. the-digital-picture.com/Reviews/… – Michael C Oct 25 '14 at 23:58
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    The difference comes more when comparing lenses such as the EF 70-200mm f/2.8 II IS ($2.3K) and the EF 70-300mm f/4-5.6 ($650). the-digital-picture.com/Reviews/… – Michael C Oct 26 '14 at 0:02
  • I was trying to point to a highly corrected that opened up to f/2.8 to something more affordable that was wide open at f/5.6. It is quite likely that for the lens test, both lenses outperform the sensor. Canon's MTF charts suggest that the f/2.8 has better contrast across the board than the f/5.6 does, even when both are stopped down to f/8. – user13451 Oct 26 '14 at 0:05
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    And in the context of the original question, there is no distinguishable difference when mounted on any available Canon body. – Michael C Oct 26 '14 at 0:26
19

It's widely believed that a lens with a faster aperture will perform better stopped down compared to a slower lens wide open. Whilst this is often the case it's not a hard and fast rule.

With DSLRs the lens is wide open until you take the shot (at which point the iris quickly closes). Having a faster lens therefore means you get a brighter viewfinder image. It also means the AF system can use a wider baseline which in many bodies activates a set of more accurate AF sensors.

This is why Canon & Nikon make f/2.8 lenses all the way up to 400mm, in addition to providing more light for sports photography, they make autofocus more accurate as well. There is important number with regards to AF and that's f/5.6. This is the limit of most camera's AF systems and so you won't find very many AF lenses slower than f/5.6 at all.

Having a faster lens to begin with also means you can use teleconverters and still have a large enough aperture for AF.

  • 1
    "It's widely believed that a lens with a faster aperture will perform better stopped down compared to a slower lens wide open. Whilst this is often the case it's not a hard and fast rule." Do you have any evidence (i.e. lens tests) to back that up? All the comparisons that I can remember have shown that the smaller aperture lens performs better. – JenSCDC Nov 30 '14 at 13:54
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    I don't have time to dig out lens test and try and equate them between brands but there are plenty of 50mm f/1.4 lenses that aren't amazing at f/1.8 or f/2 and would easily be beaten by something like the Sony/Zeiss FE 55 f/1.8 or the Leica AA 50 f/2 wide open. Likewise many ultrafast lenses f/1 or f/0.95 aren't great one stop down compared to the best f/1.4 lenses. – Matt Grum Dec 3 '14 at 9:49
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    In the context of long telephoto lenses as mentioned in the OP it is more often the case that the more expensive lenses with wider apertures perform better optically even when stopped down. In the case of shorter lenses with wider fields of view, the issue of field curvature is more pronounced and forces designers to balance the design between a flat field plane of focus vs. center sharpness with uncorrected or less corrected field curvature. That's one reason for the multitude of various 50mm designs from each of many different makers. – Michael C Nov 26 '16 at 2:51
3

Mr.Grum offered some very good reasons why faster lenses exist - because of fast and reliable focusing, and of easier framing and following due to brighter viewfinder.

To add a little to what he said, focusing with teleconverters is much better too when the lens is fast. Apart from focusing, optics in slower lenses often is not designed to be used with a TC, results with TCs are marginal.

One of very important characters of a lens (and telephoto lenses are very special in this regards because by their nature if there is a background it is nearly always out of focus) is the artistic quality past DoF. Slower lenses are not usually as good at that as faster. Not a big issue with birds in flight, but usually it is not the only shooting situation. Another issue is how the specular highlights are rendered (bokeh).

Stopping fast telephoto lenses to f/8 is something to be avoided, btw. It's performance usually start to degrade. Those lenses are often designed to be used near wide open, stopped down 1 or 2 stops.

Big question is why f/8. What is the size of the subject and shooting distance? How you calculate your DoF?

  • Typically, for birding you're at or near the minimum focusing distance for the lens (and might even be using a short extension tube to get within the normal minimum). The idea that lenses can or should only be used wide-open or near wide-open is utter nonsense. – user32334 Oct 26 '14 at 8:31
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    That's right, extension tubes are very useful. However my 300mm is focusing as near as 2.3m. Hardly the case I'm going to shoot a bird this close. 10m to 30m is more typical. If you are going to look at DoF tables you will see the the DoF is comparable or larger than a typical bird in this case even at f/4. With tubes, yes, one may need to stop down more to get DoF, but focusing with tubes needs all the aperture opening to facilitate lens AF. As to "utter nonsense", well, those lens are indeed designed to be used wide or near wide open to allow fast shutter speed for less shake. – Iliah Borg Oct 26 '14 at 11:58
  • I read that the viewfinder doesn't get any brighter for faster lenses, as they are optimized for a modest f/stop and inspection shows that light from the edges of the (faster) lens doesn't reach the screen even if opened farther. I think it was a Clarkvision.com, but I can't find the citation now. I did see mentions of the same effect in other places, though. – JDługosz Oct 27 '14 at 1:24
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    If you have a depth of field preview button, easy to check. My viewfinders easily change brightness between f/2 and f/2.8 (200/2 lens mounted). – Iliah Borg Oct 27 '14 at 1:38
  • You stop seeing an improvement in viewfinder brightness at about f/2.5 (depending on the particular focusing screen used), so there is a benefit to using an f/2.8 rather than f/4 lens. – Matt Grum Oct 27 '14 at 10:39
3

I'm reminded of one of Feynmann's anecdotes about one of his first engineering jobs. He was told to pick gear tooth count from the middle of the catalog in the desired diameter. On either end they are pushing limits; if they could do that last one perfectly, they'd offer another one after that! The extremes are less than optimum.

I think it's the same with glass. Although it's possible that a lens could be as perfectly made as a larger one with the edges cut off just to make it smaller/lighter, I think that's not the common trend.

A larger maximum f-stop is often seen as a general figure of merit. Except for novelty lenses, you expect the larger diameter one to be generally better in other ways too.

OTOH, that doesn't always turn out to be the case, “At f/4 all these lenses are equally super sharp.”

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That said, why then pay for a lens that can work down to f/2.8 if, for example, you're going to use it regularly at f/8? (eg. at f/2.8 sometimes the depth of field can simply be too shallow - in the above example, take bird photography).

If you use them at f/8, they will both allow the same amount of light, but shooting at f/8 with tele lenses is not typical - at least not for me. Fast apertures allow fast shutter speeds in sports photography, shooting wildlife at dawn and dusk etc. The shallow depth of field is often beneficial, because it isolates the subject from distracting background. You can always stop down fast lens, if you want the depth of field... The fast tele lenses are large, heavy and expensive, but allow you to bring home more keepers.

Besides amount of light, these lenses are usually made better than their slower and cheaper counterparts. The f/2.8 teles have faster AF, better IS, better build and better image quality.

Some have told me the f/2.8 lens requires more glass to achieve f/2.8 and therefore it lets in more light and can shoot at a faster shutter speed at the same aperture - but I really don't think that makes sense.

They require larger diameter of the lenses. They also need better correction of optical aberrations, which is often achieved by adding more glass. So yes, they usually have more glass than slower lenses.

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