Zoom lenses and quality & sharpness

[ausphotoy]

Hi all,

Im a little confused as to why you would not want a lens with the biggest focal range.

I have a zoom lens 18-135mm which I believed would be as sharp as say, a 18mm-55mm lens. I am having trouble with some landscape photography however... Someone had mentioned that lenses with large zooms, even if left at the shortest focal length, will not be great for sharpness.

Given that its not a bad lens, what difference is there between these two lens between the lengths of 18-55mm? Why should there be a difference in their performance? If they have the same amount of elements to help rid the picture of aberrations, what makes the 18-55mm more attractive for landscape?

Thanks in advance.

 

[robbins.photo]

The more focal length a lens covers the more compromises they have to make in it's design. All lenses will be at there "best" performance at a given focal length. So when your looking at a zoom that covers a relatively small variance in focal lengths it will often give you better results in that range than one that covers a much wider range of focal lengths. Now this is not set in stone of course, there are other factors that influence this including the quality of the optics and the overall build quality of the lens. But in general you will not get the same image quality you will from a "superzoom" that say covers 18-200 mm as you will from a zoom that covers a much shorter focal length, say 18-55.

 

[sk66]

+1.
But probably 90+% of the time, for anything printed or published to the web, the lens will not be "the problem" if you are not getting good/excellent results.

 

[ronlane]

The more focal length a lens covers the more compromises they have to make in it's design. All lenses will be at there "best" performance at a given focal length. So when your looking at a zoom that covers a relatively small variance in focal lengths it will often give you better results in that range than one that covers a much wider range of focal lengths. Now this is not set in stone of course, there are other factors that influence this including the quality of the optics and the overall build quality of the lens. But in general you will not get the same image quality you will from a "superzoom" that say covers 18-200 mm as you will from a zoom that covers a much shorter focal length, say 18-55.

I think robbins is close here. It is the compromises that have to be made but that is usually a result of trying to keep the cost down. The basic problem with a wide focal length lens is the limitations on the glass. You have to match up the glass to be able to get sharp focus in all ranges and focal lengths. This is difficult to do and I am sure takes time and more money, which drives the cost up. The compromise is to get an "all-in-one" lens for a decent price, you give up some sharpness.

 

[Solarflare]

Well, just like cameras, lenses are always a compromise. Every feature you demand comes at a cost for other variables of that lens.


Zooms are for example optically very expensive. That means they are much heavier than prime lenses, and/or have a much smaller maximum aperture. So if you get a huge zoom range on a lens, you will get a lot of optical compromises as well, resulting in a lower image quality, and only a small sensor size on top of it. Historically, the only reasons zooms even have been considered was because

(a) In the 1970s when Zooms finally started to get popular, film speeds got better and better and photographing with a lot less light than from prime lenses became a practicable option.

(b) Lens coating and quite soon multicoating was invented, reducing the reflection of light from the usual 4% you see from every surface with uncoated glas, such as windows at a house (these reflections are why we actually can see the window glass), down to values like 0.1% for the most excessive kinds of coatings, making much more complex lens designs with many lens elements, as they are required for zooms, an actual option.

Compare for example the 35mm f1.8 DX lens from Nikon with the new 18-35mm f1.8 zoom from Sigma. The 35mm f1.8 DX is a small and lightweight ~200g, 70 x 52mm lens (and great metal build, by the way), while the Sigma is a huge 800g, 78 x 121 mm sized block of a lens that dwarfs the DX cameras of Nikon. Yes the zoom has the same maximum aperture and optically its pretty great, but its only a 2.5 zoom (in the normal to near wide range, no less) and its already really huge and heavy.


The highest quality lenses with the least errors are thus prime lenses with a small maximum aperture that have been moved physically for focus (i.e. there is no inner focusing mechanism), like they are used for large format cameras, which typically start at f/5.6 for a "normal" lens.


Also, with practice one cant fail to notice that the ability to change focal length is often almost completely unimportant as a variable, and changing focal lengths in itself is a problem, because it also changes your perspective as well. Thus, in many situations, much more important than the ability to change focal length is often the ability to have large maximum apertures, for an ability to photograph in low light, or as a creative element for shallow depth of field.

Thus prime lenses remain very popular with more serious photographers.

 

[KmH]

As mentioned, as the zoom range increases so do the design restraints.
While the cost of allowing for the design constraints is certainly a factor, many of the restraints caused by the physics of optics can't be overcome with money.

Even prime (no zoom, fixed focal length) lenses are subject to design limitations imposed by the physics of optics.

Camera Lens Quality: MTF, Resolution & Contrast
Tutorials - Sharpness

 

[bratkinson]

Back when zoom lenses were in their 'beginning' stages in the 70s or so, they were noticeably inferior when compared with prime lenses having only a single focal length. Part of the problem was that to keep the retail price of the lens 'affordable', a lot of compromises had to be made. Maybe cheaper glass, fewer coatings, etc. But for the average amateur that I was in the 70s (and still am), the results were more than satisfactory to my eyes. As I was shooting almost exclusively slides, even when projected on a 10 foot x 8 foot screen, the clarity was still good enough. But, when compared to images shot with my primes - my FD 135 f2 was my favorite - the clarity/IQ of the primes was visibly more 'crisp', using the same Ektachrome with the same ASA in the same camera. Not earth shaking better...but better. Back then, some of the third party manufacturers entered the market with some real low-priced junk, too. Unfortunately, some still believe that all zooms are optically inferior to primes based on the 'real world' of '70s photography. Zooms have come a long, long way since those early days. So have the third party manufacturers.

My favorite lens back then was the Canon FD35-70mm f/2.8-3.5 S.S.C. and used it for perhaps 75% of all my photography (mostly trains). I was never disappointed with the images taken with that lens. By todays' standards, it was a rather 'tame' 2x zoom. Today, 10x and beyond are commonplace. Not surprisingly, my most used, all-around lens today is the 24-105 f4 L and gets used for maybe 60-70% of my shots.

So, are zoom lenses today still optically inferior to primes? That's still true, but to a much lesser degree. Everything has improved in their design and manufacture. Slide rule calculations are now done to what? 10 places? On a computer. Automated lens grinding and coating ensure uniformity across all copies of a lens, and so on. So, all lenses have improved in the last 40 years or so.

But just as the lenses have improved, so have testing abilities and those abilities have been taken out of the lens makers factories and made available to everyone. But now, 'we the people' on the web have come to expect images sharper than our own eyes perceive them, even with glasses or contacts. 'We the people' expect that super-high quality for a less-than $500 price, too. Guess what, folks. Ain't gonna happen any time soon. Same reason my $20,000 7 year-old car, even when new, won't go 200mph at Indianapolis, either.

So, to actually MAKE lenses that retail for less than $500, especially zooms, a few compromises have to be made. Maybe the images aren't at their sharpest in the corners when zoomed out...or zoomed in. Maybe the lens coating isn't as super-duper as the high priced models have. And, quite obviously, the super special 'rare glass' is only used in platinum-priced lenses, not the budget lenses.

And yes, I think every lens manufacturer has produced more than a couple of 'duds' that just don't focus as perfectly as well as the rest of the pack, have a little more pin cushioning or barrel distortion than they should, etc. Somebody tried to save a nickel in manufacturing and produced a bunch of duds. Happens everywhere, not just lenses.

But, perhaps the biggest reason of all why I chose zoom lenses over primes in my 35mm film days and now in my digital days is that I can most often get the shot I want, framed like I want, without the necessity of having to change lenses. Even with Canon EF lenses, it's still a minute or so to put the front lens cap on, remove the lens, put the rear cap on, remove the hood and flip it over, put the lens in my bag, take out the next lens, remove the rear cap, put it on the camera, remove the front cap, install the hood, then aim, frame, and shoot. Meanwhile, "the perfect moment" just passed by 45 seconds ago! At one point 2 years ago, I had 5 zooms and 3 primes in my bag. So, for my shooting 'style', I was able to test what worked best for me. I've since reduced that to 3 zooms and 1 prime...the 135 f2 L. While the 135 produces super sharp photos, so do my other 3 lenses.

But as I have come to realize in the past 2 years or so, the small advantage in sharpness, or IQ, of primes over zooms is nearly gone. And even more so, neither my 'audience' - 90% 'me', and the other 10%, people who wouldn't know the difference, realize there IS a tiny-smidge IQ difference between one lens or another. They're happy with cell-phone shots! For what it's worth, I was perfectly content with the IQ an EF-S 18-135 f3.5-5.6 IS on a 30D produced 4 years ago. I think our perception of what is 'good enough' has, as a group of web-savvy photographers, become too 'perfectionist' for our own good.

Just one more thing. To minimize the time lost changing lenses, many prime photographers have two cameras. In my film days, I had two, frequently mounted on a T-bar mounted to my tripod, or on a double-camera strap around my neck. I've now opted for convenience and less hassle over laboratory-perfect image quality able to read the writing on a dime from 500 yards.

 

[Gavjenks]

From my brief stint designing lenses for fun, the main issue I found was that if you have a prime lens, you can sort of "cheat" in a lot of ways. You can place certain types of elements in certain orders such that they just perrrrrrfectly cancel out various aberrations right at the moment they get to the focal plane, and you can do this fairly efficiently, without much glass.

For instance, you could have an element or two up front that add spherical aberration, but an element at the back that begins to remove it, and have them = 0 right at the sensor. Think of it like a spaceship thruster. You can fire the engines north a bit, and drift south gradually, then halfway to your destination, fire the thrusters south and end up hitting exactly.

With a zoom, many of the ways of "cheating" become instantly impossible. You can balance the contrasting forces at any one focal length, but not all, at least not as easily. Instead you have to do it more responsibly. In the above example, instead of canceling out spherical aberration once for the whole lens, you'd instead make sure that each individual lens group sends out net zero spherical aberration, so that no matter how they move around, it doesn't cause much of a problem. In the spaceship analogy, you have to make sure you travel in a straight line or your correct trajectory the entire way, equalizing your north south trajectory at every single moment, because you never know when mission control (the photographer) is going to suddenly tell you to go somewhere new (new focal length)

That adds a lot more glass (more corrective elements in every group) which weighs and costs a lot OR forces small apertures, more internal reflections = more vignetting, etc. etc.

 

[sk66]

Also, with practice one cant fail to notice that the ability to change focal length is often almost completely unimportant as a variable, and changing focal lengths in itself is a problem, because it also changes your perspective as well.

I agree with everything except this line.
IMO changing FL is of great importance because subject distance determines the perspective and FL determines the composition. They are separate functions but somewhat dependent on each other.
The idea that you can "zoom with your feet" is just wrong. You can "compose with your feet" but that will affect other things such as perspective and DOF. Just like zooming in from a fixed distance changes the composition but also affects the DOF (perspective remains the same).

 

[Derrel]

Hi all,

Im a little confused as to why you would not want a lens with the biggest focal range.

I have a zoom lens 18-135mm which I believed would be as sharp as say, a 18mm-55mm lens. I am having trouble with some landscape photography however... Someone had mentioned that lenses with large zooms, even if left at the shortest focal length, will not be great for sharpness.

Given that its not a bad lens, what difference is there between these two lens between the lengths of 18-55mm? Why should there be a difference in their performance? If they have the same amount of elements to help rid the picture of aberrations, what makes the 18-55mm more attractive for landscape?

Thanks in advance.

Zoom lens optical quality is determined to a great extent by three things. First is the ERA the lens was designed in. Second is the original retail price of the lens. Third is manufacturing and assembly quality and tolerances- also known as QA or Quality Assurance, AKA QC Quality Control.

MODERN zoom lenses have become very,very good. Many modern zoom lenses offer better image quality than 1960's prime lenses. ED and UD glass, nano-coating, and better computer design and ray-tracing software and supercomputers allow today's designers to compute and test designs within minutes. This is NOT the era of the 43-86mm Zoom~Nikkor.

The second factor, original retail price, is a pretty good guide to the quality one can expect. The $99 18-55mm kit zooms are not all that great. The $699 to $2499 zoom lenses are typically VERY good to excellent zoom lenses.

AN 18-55 is a low-cost lens, designed to be made of low-cost parts, and easily and cheaply built. It is a "throwaway lens" when it is damaged....don't fix it, junk it. You'll probably find that the expensive, higher-end consumer lenses like the 18-135 are better lenses AND much more-useful. The focal length range from 55 to 135 is a HUGE span of exceedingly useful lengths. I am NOT a fan of the 18-55 zoom: I've shot a pair of them....uggg....55mm is just NOT long enough for me.

 

[bratkinson]

AN 18-55 is a low-cost lens, designed to be made of low-cost parts, and easily and cheaply built. It is a "throwaway lens" when it is damaged....don't fix it, junk it. You'll probably find that the expensive, higher-end consumer lenses like the 18-135 are better lenses AND much more-useful. The focal length range from 55 to 135 is a HUGE span of exceedingly useful lengths. I am NOT a fan of the 18-55 zoom: I've shot a pair of them....uggg....55mm is just NOT long enough for me.

I'm in Derrels camp, too. 18-55 is too short of a focal length zoom range. Although the 35-70 was adequate for me in the 70s, I ended up breaking it after 4-5 years and replaced it with a Canon FD 35-105 f3.5. That extra 35mm zoom 'reach' was just what I needed, and minimized the number of times I'd carry 2 cameras. When I had a 30D, the EF-S 18-135 was a fantastic lens for me. But when I started doing low light, no flash photography, its limited aperture size (f5.6 when zoomed out) was just too slow.

 

[ausphotoy]

Hey guys thanks so much. There's so much information here, it takes a while to really get to the bottom. I do see what you mean. I think my next question would be, if all lenses (both zoom and non-zoom) have a preferred focal length, how do you determine what it is?

 

[Derrel]

Hey guys thanks so much. There's so much information here, it takes a while to really get to the bottom. I do see what you mean. I think my next question would be, if all lenses (both zoom and non-zoom) have a preferred focal length, how do you determine what it is?

Preferred focal length is determined by 1) the desired creative impact that the LENS can bring and 2) the working distance preference, or the imposed working distance.

1) a wide-angle lens shows a wide angle of view, and makes the foreground objects appear of good size, and makes the background objects appear small, and also more-distant. A wide-angle lens has the effect of increasing apparent distances.

3) A telephoto lens has a narrow angle of view, and it makes foreground objects appear large, and at the same time, it makes background objects LARGER, and it also appears to "compress" distances.

2) In between 1 and 3, is the "normal" lens. It gives a neutral appearance to spatial relationships, and produces neither a smallish nor a large-ish background, and also produces a normal-size main subject.

So, the creative choices are exaggerated foreground size and exaggerated impression of increased distance, or larger foreground object, with magnified size of background objects and a compressed apparent distance, or a "NORMAL" size and "normal" distance impression.

Pretty much, this is why the wide-angle to medium telephoto zooms are so,so handy as general walkabout lenses. And this is also why the superzooms, like the 18-200, 18-270, and 18-300mm "all-in-one" lenses are desired by people who want one lens that can do many things.

In Nikon, a lens like the 18-105, 18-135, or the new 18-140 VR is the typical newer, step-up from the 18-55 kit zoom. Nikon's 18-200mm and 18-300mm are significantly higher-priced zooms, and are some of the world's best "superzooms", while the 3rd party manufacturer's offerings are not quite in the same league.