Why are f2.8 lenses so sought after?

jeff_becker · June 18, 2009

OK. This may be a dumb question. I understand that f2.8 lenses are faster, more expensive and highly sought after. For example my Tamron 2.8 28-75 costs a lot more than my Nikon 18-55 f3.5 kit lens. My question though is the "sweet spot" for most lenses appears to be between f8 and f11 - or in that range.

If so, why is it so important and nice to have a f2.8 lens if your best photos are shot at f8? Is it simply to provide you better shooting ability in lowlight situations where the faster speed is needed?

Thank you.

Jeff

john tonai · June 18, 2009

I never pick an f/stop because it is in the "sweet spot". I choose it because it gives me the depth of field that I desire for that specific shot or because, like you said, I need to shoot in a low light condition that requires the fastest possible shutter speed.

So, to answer this:

<h1>Why are f2.8 lenses so sought after?</h1>

</blockquote>

for me, it is because they don't make 2.0 or 1.4 zoom lenses since I like an extremely shallow depth of field. I can always stop down if I want more dof.

Matt Laur · June 18, 2009

Yes, better ability in low light. And a much brighter viewfinder, too. It also drags in more light for the autofocus system to chew on, which improves performance. And when you can open up the lens to f/2.8, you are able to much more deliberately choose what's in focus, and what's not. This allows you to get a nice shot of someone's face in a social setting, while having a a busy, distracting background nicely blurred and out of focus (see "shallow depth of field"). Also: most f/2.8 zoom lenses are constant aperture lenses. Unlike your 18-55, which will get slower (a more narrow aperture) as you go to a longer focal length, most pro zooms will retain the aperture you choose no matter what focal length you're using. This means that the exposure will behave itself in a predictable way, even if you re-frame it using the zoom. Very important when doing certain sorts of strobe lighting, or when being very deliberate while working with available light and trying to control DoF. Because such lenses are usually meant for more demanding photographers, they also tend to be better built, use better materials, and hold up better. Of course, they're larger and heavier, too, compared to slower and variable-aperture lenses in the same focal length range. As for the sweet-spot issue: A lens that is at f/5.6 when wide open at the long end is only barely stopped down when it's at f/8. So it's just starting to approach the benefits of stopping down (for sharpness) and it's already quite slow. A good f/2.8 lens will hit that sort of tack sharpness while still gathering several times more light. Mind you, my Nikon 70-200/2.8 is great even wide open... but man, you sure pay for that (in cash, size, and mass).

phule · June 18, 2009

If all of your photos are going to be shot at f/8-f/11 then I would argue it is not important at all to have a f/2.8 lens.

But I have yet to find or hear of anyone shooting with only 2 apertures for every single photo. Seems rather limiting.

I would also point out (this is my opinion at least) that any apparent increase in image quality between wide-open and, say, f/8 (independent of depth of field discussions) is rather minimal except in cases of very large prints.

Matt makes very good points regarding the brightness of the image in the viewfinder and auto-focus (these are things I often forget about myself).

alec_myers · June 18, 2009

f/2.8 lenses not only supply more light to the AF system (assuming you have a camera with such a thing) but they also have a wider baseline so the AF is more accurate.

nathan_meador · June 18, 2009

Plus the wider range of lighting that the lens allows you to be able to shoot in. If you go to your kids (if you have any) basketball games for example, having the kit lens is almost pointless since it isn't 'fast' enough, as you've mentioned. I sometimes use mine for portaits when i want the background nice and blurred out by using the shallow DOF the lens offers.

peter_wang6 · June 18, 2009

I can think of a few reasons for coveting larger maximum aperture lenses:

<ol>

<li>Faster lenses mean better low-light performance, often as much as 1 stop, and sometimes 1.5 - 2 stops. That can mean the difference between handholding and having to use a tripod.</li>

<li>Better performance at wider apertures. Most lenses improve image quality as they are stopped down, up to the diffraction-limited aperture. A lens that starts soft at f/2.8 might become acceptably sharp at f/4, and excellent at f/5.6, whereas a lens that starts at f/4 may be less sharp until it hits f/8.</li>

<li>Shallower DOF. Subject isolation from the background becomes easier, and allows for greater creative possibilities.</li>

<li>Enables teleconverter use. Lenses that start at f/4 or f/5.6 lose a stop with a 1.4x TC and 2 stops with a 2x TC. Many cameras can't autofocus when the effective f-number is f/8 or smaller.</li>

</ol>

That's what I can think of. I'm sure there are others; but some disadvantages include:

<ol>

<li>Heavier weight. Big apertures require larger glass.</li>

<li>Costly, due to more challenging control of aberrations, and the larger glass.</li>

<li>Requires larger diameter filters.</li>

<li>Harder to conceal, due to size.</li>

</ol>

andylynn · June 18, 2009

"Sweet spot" can be misleading. If I need to stop down a lens for sharpness, my useful range of apertures on an f/2.8 zoom might start at f/4. The kit lens can't do f/4 at all above something like 24mm, and to stop it down for better sharpness might mean going to f/5.6 or f/8.

However, modern lenses usually will give you excellent results wide open and there will be no need to use f/8 unless you want expanded depth of field. So an f/2.8 lens has many applications that a kit zoom doesn't do well, like shooting indoor events, musicians onstage, guests at tables at wedding receptions, etc. Also, as others said, because the wide open aperture is larger, and you're composing the shot and focusing with the aperture wide open, you see a brighter / easier to use in low light image in the finder, and the AF system has more information to work with and will be faster and more accurate.

The other factor that you don't get just from the numbers is professional optical and build quality. Because f/2.8 zooms are marketed to pros, rich amateurs and other people who spend a lot for their lenses, they are usually built from higher quality components: larger pieces of glass that are made to tighter tolerances and give you a sharper image at the edges and corners, more metal in the construction than plastic, larger zoom and focus rings that feel more solid, etc.

ed farmer - mount laurel, · June 18, 2009

First, the "sweet spot" for most lenses is simply two to three stops down from the maximum aperture. f2.8 is really a compromise between cost and speed. Believe me when I tell you that if I could afford an 80-200f1.8, I would trade my f2.8 lens for it in a minute! However, the cost to produce something this would be astronomical. Look at how the prices rise for prime lenses as speed increases. You can pick up a Nikon 50f1.8 for $125. The f1.4 version goes for something over $300. The f1.2 is more yet and Canon 50f1 sold for about $2500. The same would happen for zooms going past f2.8. Other posters have also been correct in saying that the f2.8 lenses tend to get a better (pro) build quality, but I think that is because of the factors that I cite here. Lastly, is that a faster lens can take pictures in situations where others can't. With non-static subjects, even a VR or IS lens can't compete with a fast lens.

hjoseph7 · June 18, 2009

If I had the choice all my lenses would be f2.8 and below. This comes from the days of film where raising the ISO in low light situations would cause problems. I still have this hang-up.

Alan Marcus · June 18, 2009

The lens acts much like a funnel that gathers light. The larger the lens’s diameter the more light it can gather. The ability to gather huge a quantity of light is much sought after. Consider the great astronomical telescopes are hundreds of inches in diameter. In a camera, a large diameter lens affords the opportunity to take pictures in feeble light conditions. Additionally a large lens diameter allows super fast shutter speeds not available with smaller diameter lenses thus the more ability to arrest movement in sports photography and anywhere there is fast action.

Now a large lens diameter is only half the story. The lens’s focal length plays a big part also. We love long focal length lenses because they act like a telescopes thus they slash the apparent distance camera-to-subject. We want the bird or the elephant or the outfielder to record close-up and with detail. To accomplish we mount a long lens.

When we mount a long lens, the amount of light that is allowed to enter the camera is diminished. Each time we double the focal length, say from 50mm to 100mm or 100mm to 200mm the laws of optics causes us to lose four fold (4x) of the amount of light reaching the film or chip. The only countermeasures are elevated ISO, slower shutter speed, or build the lens so it has a bigger diameter.

The f/number you are taking about results by dividing the working aperture into the focal length. If a lens has a 200mm focal length, to achieve f/2.8 the working diameter (aperture) must be 10.23mm. If the lens is a 500mm telephoto the working diameter must be 178.6mm. It is difficult to make good lenses with large diameters because the curve of the glass at the edges must be very steep. It takes lots of time and meticulousness grinding of the glass to get this curve (called a figure). And f/2.8 is not all that wonderful. The next larger aperture is f/2 and the next larger is f/1.4. It would be wonderful to have a 500mm telephoto with a maximum aperture of f/1.4. Sorry to report that few of us if any can afford such a lash-up. The f/2.8 is a compromise price vs. ability to gather light.

William Michael · June 18, 2009

This is a few down and has relevance:

<a href="00TgOw">http://www.photo.net/beginner-photography-questions-forum/00TgOw</a>

***

On the general concept of a "sweet spot", and considering we seem to be talking only about zoom lenses - consider zooms to have two "sweet spots":

>one in regard to the sweetest Aperture Range (like for an F2.8 zoom that might be F5.6 to F11 just as an example).

> the other in regard to the sweetest Focal Length Range: like on a DSLR Kit zoom (18 to 55) it might that be 22mm to 50mm is the range where better IQ is attained, at any specific aperture.

The wider the zoom's compass, the more likey the poorer IQ at the extremes of the FL.

WW

jcoffin · June 18, 2009

To amplify on Alec's point (since I initially misunderstood what he was saying): a faster lens always transmits more light, so it's easier for the standard sensor to do its job with a faster lens.

Some cameras (mostly the more "advanced" models that they expect to be used with fast lenses) include a special sensor (or more than one) that only works with lenses that are f/2.8 or faster. This sensor has the equivalent of a "longer baseline", which improves focus accuracy.

In theory, a sensor optimized for an f/3.5 lens could be built, but in reality, most cameras only include two types: f/5.6 (or so) and f/2.8. Only a lens that's f/2.8 or faster can use the more accurate f/2.8 sensor.

fullmetalphotograper · June 18, 2009

One my favorite reasons for shooting f/2.8 is Bokeh. Which allows me to use a shallow depth of field to blow out backgrounds.

Another reason is low light shooting.

Exposure: 1/5, Aperture: f/2.8, ISO Speed: 6400

I shoot a lot of sports so it allows me freeze action and blur backgrounds.

Exposure: 1/500, Aperture: f/2.8, ISO Speed: 6400

Another reason for f/2.8 is it makes for a brighter viewfinder which makes it easier to focus and compose a photo.

With most modern lenses you do not have less of a sweet spot issue. If I have lenses that a major drop in sharpness when I stop down or open up that lens goes to the shop or is scrapped. One of the thing to remember depth of field (range of focus) is not the same as sharpness. What depth of field is about area in front of and behind the point of focus that is in focus.

craig_shearman1 · June 18, 2009

As a point of reference, 2.8 is fast for a zoom, but it's not a fast lens for those of us brought up in the pre-zoom days. Up until maybe 10 years ago, an SLR was typically sold with a 50mm lens rather than a "kit" zoom. The 50 was never slower than 2.0 and was frequently a 1.7 or 1.8. If you could afford it, you got a 1.4 (one stop faster than 2.0 and two stops faster than 2.8). A 50mm 5.6 (which is the f stop at 50mm on some variable aperture kit zooms) was beyond just laughable, it simply didn't exist. I'm not sure you could find even a 2.8 50. My 55mm MicroNikkor was slower because it was a specialty closeup lens, but even that was 3.5

William Michael · June 18, 2009

I like that "point of reference", as a beginner we've gained zoom facility, but at what cost?

"A 50mm 5.6 (which is the f stop at 50mm on some variable aperture kit zooms) was beyond just laughable, it simply didn't exist." . . . most of the kits lenses are also around F/5.6 at FL = 35mm - which is of keener relevance and just as "laughable"

(re FoV in a DSLR / APS-C to the “old” SLR / 135 format).

WW

akajohndoe · June 18, 2009

A fair number of the digital point and shoots (they like to call them DSLRs, LOL) have only two f-stops, usually f8 and f11. Even those with an Av mode.

In the days when prime lenses were the norm, zoom lenses were often atrocious. Nowadays, if you are willing to pay for it, you can get a very nice zoom. In fact, most of the lesser zooms (such as the ubiquitous 28-135/3.5-5.6 and variants) are much better than the zooms of yesterday.

The constant aperture and the ability to open up to f2.8 are the salient points in response to the original query, though.

kari v · June 19, 2009

why is it so important and nice to have a f2.8 lens if your best photos are shot at f8?

</blockquote>

"Sweet spot" and sharpness are highly overrated. It's always nice to hear that my 50/1.4 is unusably soft wide open and that I should stop down at least to f2.8 - 4 for good results, interestingly I've never trashed a shot because of this "fact". Sure, it's nice to know that when stopped down to f4-5.6 it just about hits sensor/film limits but I only shoot at those apertures when it fits the scene.

Sigma 18-50/2.8 and Tamron 17-50/2.8 are similar. You hear that they're bit soft wide open or something but in real world both perform very nicely and offer good bang for the buck at $400. And of course when stopped down to f4 they're very good indeed and still faster than the kit lens.

Also, there's one point that doesn't become evident until you try yourself. Expensive fast glass tends to be sharp and well built, that's nice, but there's more, that little something in the images. It's hard to quantify but for example basic 50/1.4 at f1.4-2 produces images that look nothing like the kit lens does and I'm not just talking about the reduced DoF. Sometimes that extra bit of *character* is actually due to optical *flaws* that mostly go away when you stop down a stop or two. But who cares, I don't shoot test charts, I want interesting images. I'm going to be a heretic here and say that even Sigma 18-50/2.8 has loads of more character than the kit lenses.

BTW: This is a topic that always ends in a big argument. ;)

From raw speed perspective f2.8 is not really fast. It's more like adequate for most things and keeps zooms in humane price and weigth class. f2 zoom would be insanely large. (Olympus has two but they use smaller and differently shaped sensor and even then the zooms are $2000+ and rather large and heavy.)

thomas_sullivan · June 19, 2009

available darkness...

..meaning...with digital SLRs now having ISO of 25600 a lens with f/2.8 or faster, pretty much allows photography in any available light situs you can be in.

ellery · June 19, 2009

not sure if this was already said, but note that for certain lower quality lenses the aperature can increase as you zoom in - ie at 18mm you might get 3.5 and at 50mm you might get 5.5 - so just look for that when you get to looking at the lenses - better lenses maintain the aperature that you set throught focal length.

bob_osullivan · June 20, 2009

"the "sweet spot" for most lenses appears to be between f8 and f11"

This statement probably refers to sharpness. And it's actually one or two stops down from wide open not F8-F11. So on an F2.8 lens it would be F4 or F5.6.

"why is it so important and nice to have a f2.8 lens if your best photos are shot at f8"

There's much more to a photo being "Best" than just sharpness. A sharp photo that is underexposed is worthless.

For the advantages re-read Peter Wang's post. He pretty much nailed it.

daniel_sach · June 21, 2009

Something else to consider, is that an F/2.8 lens is built better than say a f/5 lens. Even is ou stop it down to a lower F/stop it will give a better picture than the lower F/stop lens. An F/2.8 lens stopped to f/8 would be better than an F/5 lens stopped to f/8. I found out a good lens at f/5 is better than a bad lens at f/3. The f/2.8 lenses also usually use a high quality glass.

Just my 2 cents

Daniel

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