Will using a lens at max aperture ("wide open") result in poor images?

I have the Nikkor 35mm f/1.8. I think that, at least for me, the bigger issue with using the lens at max aperture in low light is that it's hard to get it to focus on the right thing. With that shallow of a DOF, if your subject is turned slightly then only one eye will be in focus, which is the idea, but if they move before you get the shot taken you might get the nose instead. Not such an issue in good light as the camera will focus much faster. That being said, though, I have plenty of great shots of my kids taken at f/1.8 in low light (you can see examples at my blog).

In general, most lenses don't work at their peak at maximums or minimums. That includes both aperture and zoom, and probably other things as well. Still, if that's the DOF you are looking for, then go for it.

The performance of a lens at maximum aperture really boils down to two things: lens element material and lens element and group construction. A "lens" as we commonly refer to in the DSLR world is actually a collection of individual glass lens elements, often grouped into related sets. There are a wide variety of type of individual lens elements that aim to control optical aberrations and refract light appropriately so an image can be focused clearly onto the image plane. These lens elements include your standard convex, concave, and partial convex/concave lenses that normally refract light, apochromatic, achromatic doublet, and diffractive lenses designed to correct chromatic aberrations, aspheric lenses designed to correct spherical aberrations, high density glass or glass alternatives used to refract light with less aberration, ultra low dispersion glass to reduce dispersion (and therefor chromatic aberration), etc.

A cheaper lens will use cheaper lens elements, and have fewer correcting groups designed to eliminate optical aberrations such as chromatic aberration, spherical aberration, distortion, and a few others. For cheaper quality f/1.4 lenses, used wide open, you are most likely to encounter chromatic aberration, which usually presents as color fringing. You may also encounter focus shift problems as you stop down the aperture in a cheaper lens, due to spherical aberration. In some cases, spherical aberration is a desirable attribute, as it creates extremely soft bokeh (background blur circles). Such a trait is particularly desirable in portrait lenses. Simpler lenses with fewer elements have the potential to offer superior sharpness, as the more lens elements, the more compromises you will ultimately have to make. This superior sharpness is usually only realized when the lens is stopped down beyond f/2.8, however, due to the lack of correction for optical aberrations at wider apertures.

A more expensive lens will usually use higher quality glass, and include elements with ultra high density/ultra low dispersion elements, apochromatic and/or apsherical lens elements, and possibly lens elements made of flint or flourite. There will often be more corrective groups in a higher quality lens to maximize control over how light refracts, disperses, and focuses. Lens elements in a high quality lens will also sport high quality multicoating to eliminate lens flaring and ghosting, which results in a sharper image than with lens elements that are not multicoated. (Cheaper lenses will usually include multicoated front and back lens elements, but may not include multicoated lenses for all elements.) A top of the line fast prime, while it will still exhibit some optical aberrations wide open, will usually be FAR superior to a cheap lens. The background bokeh of a quality lens is also usually much more desirable as the design of the lens diaphram is usually more advanced as well, with more blades, curved blades to avoid polygonal out-of-focus circles, etc.

When it comes to lens quality, you ultimately get what you pay for. If you need top of the line, wide-open performance, you're going to need a top of the line lens that is designed to deliver top of the line wide-open performance. This would be the case if you were doing portrait or wedding photography for a living, and needed that optimal bokeh at f/1.4 (or f/1.2 even) for superb facial portraits. If you need a fast lens, but will mostly use it for night photography or in situations where capturing supreme focus with ideal background blur is not key factor, then an f/1.4 that has moderate bokeh some chromatic aberration issues wide open will likely not be an issue.

These days, most f/1.8 lenses offer decent quality wide open, and pretty fantastic quality when stopped down past f/2.8 (and up the the diffraction limit of the camera, which usually falls around f/8 or so.) There are a variety of f/1.4 lenses from several brands, ranging from pretty cheap to really high quality. A middle-ground f/1.4 lens can be had for about $500 or so that will deliver very acceptable quality wide open, and better stopped down a bit (most lenses improve in sharpness when the aperture is closed down by a stop or two.) If you want the best quality as wide as you can get it, most big name brands like Nikon and Canon, as well as Zeiss, make very expensive, very wide lenses (as wide as f/1.2) that offer unsurpassed quality from their widest aperture through f/2.8, and possibly beyond. You'll have to pay for this quality, though, as these lenses usually roll in for around $2k.

An interesting consequence of achieving maximum wide-open quality, however, is often less than maximal quality through the "middle" aperture range, from f/2.8 through the diffraction limit. Compare a cheap $90 nifty fifty f/1.8 at f/4 to a $2000 f/1.2 at f/4, and the cheap lens will probably edge out the expensive lens in the area of sharpness. The additional and non-standard lens elements (like apochromatic, aspherical, etc.) can correct optical aberrations wide open, but usually at the cost of some sharpness. The higher quality lens, despite having slightly lower sharpness, will still usually provide superior color, contrast, and clarity.

It is normal for lenses to become sharper as they are stopped down, as everyone already said so.

What varies is the amount of softness at the wide apertures and what someone considers unusable. I do own certain lenses which I think this would apply but not all, some are almost as sharp wide-open as at their sharpest.

When 35mm F1.8G is reasonably sharp wide-open and only improves noticeably by F2.8, still the difference is far from huge. This is a dedicated cropped-sensor (DX) lens, so that is the only story you'll get.

For an FX lens, there are always two stories because DX cameras like the D7000 use the best portion of the lens and so some of them like the 50mm F1.4G are terrible wide-open on a full-frame but reasonable on a DX body.

First point: it's pretty safe to say that even a "really poor" lens for a typical DSLR has higher resolution than (for example) most of the lenses used by Adams, Weston, etc. (though, of course, they were using a much larger "sensor", so the total amount of information captured was still quite large). Even if we stick to small-format photography, some stunningly great pictures have been taken with lenses that would barely even qualify as "poor" today.

Second point: for portraits (in particular) "soft focus" lenses are quite popular (as are various "filters" and tricks to reduce sharpness). In short, maximum sharpness isn't necessarily a particularly desirable quality for this kind of picture.

Third: I'd say at least 95% of the people who say a particular lens (or camera) is poor are basically making excuses for the fact that the pictures they take pretty much suck. In most cases the problem is about six inches behind the camera, not in the lens.

From a technical viewpoint, it's true that nearly all lenses improve in quality when stopped down a bit (optimal performance is typically around 2 stops down from wide open). In all honesty, there's usually a bigger improvement in coma and astigmatism than in sharpness, but there's usually an improvement in sharpness as well.

Just for completeness, I'll mention one other option: at one time, Nikon made the 58mm f/1.2 Nocturnal Nikkor. This was optimized for use with the aperture wide open, and (by reputation -- I've never used one personally) had excellent sharpness and contrast, even at maximum aperture. This is a mostly theoretical option though: first of all, it hasn't been made in quite a while, so you can only buy them used (for around $3,000 the last I heard). Second, as 50mm lenses go, they're almost ridiculously big and heavy (and, as mentioned above, expensive).

There is, however, a somewhat more reasonable alternative that bears mentioning: the Sigma 50 f/1.4 EX DG HSM is better wide open than most of the "first party" manufacturer's lenses (Canon, Nikon, Sony, etc.) Like the Nocturnal Nikkor, however, this is big, heavy, and relatively expensive -- though certainly to a lesser degree (in all respects). It also has the advantages of being autofocus, current, and easily available. Whether you would really benefit from spending extra money for this is open to question -- a lot of people use Nikon's f/1.4 lenses to produce very nice pictures (both wide open and stopped down), and I have no doubt at all that if you have sufficient skill, you can do the same. OTOH, if you really want optimal performance from a "normal" lens wide open, the Sigma probably provides the best optical performance you can get right now.

Edit: I suppose since I mentioned the Nocturnal Nikkor, I should give equal time, and correct that last statement a bit: I meant something like "the best you can get for your Nikon". If somebody's truly fanatical about optimal performance and maximum aperture, there's also the Leica 50 f/0.95 Noctilux. For most people, this is impractical in too many ways to even think hard about it, but it's at least as practical a choice as the Nocturnal Nikkor (at least it's still in current production).

Bottom Line: the lenses you're looking at are certainly usable, and capable of taking great pictures, wide open. When you do have enough light, the sharpness will generally improve by stopping them down somewhat, but (as with most lenses) the real limiting factor on picture quality will usually be how the lens is used, not the lens itself.

The short answer:

Don't spend more on a f/1.4 lens if you only intend to shoot it stopped down.

Tests indicate that the Nikon 50 f/1.8 is sharper at f/1.8 than both 50 f/1.4 offerings are stopped down to f/1.8. This is the same situation with Canon so I assume it's to do with the design of the 50mm lens.

• http://www.dpreview.com/lensreviews/nikon_50_1p4g_n15/page3.asp

• http://www.dpreview.com/lensreviews/widget/Fullscreen.ashx?reviews=30&fullscreen=true&av=1.667&fl=50&vis=VisualiserSharpnessMTF&stack=horizontal&&config=LensReviewConfiguration.xml%3F2

• http://www.dpreview.com/lensreviews/nikon_50_1p4_n15/page4.asp

In summary get the 50 f/1.8, yes relatively soft wide open but it's also very fast and there are times when you need it.

It is true that the image at maximum aperture is somewhat softer.
How much so depends very much on the lens in question.
The only way to answer the question properly is with careful measurement.

As an example, the graph below shows the variation in edge blur (in pixels, near the center of the image) against aperture for a Sigma 50mm f2.8 macro lens.

This is a moderately priced high quality lens that delivers an edge blur of less than 1.5 pixel between F4.0 and F11.0, an excellent result.
At F3.2 and 2.8 the image is a little softer with an edge blur of 1.8 and 2.0 pixels, respectively, which is not half bad. But this is for the center of the image, near the borders it falls off more sharply.

The measure of sharpness I use is edge blur, using the slant edge technique and the QuickMTF program. Edge blur is used instead of MTF or lines/mm because it has immediate and intuitively obvious meaning.