Are Zooms more prone to focus problems than Primes?

[robertdrake]

Just wondering?

[Jon]

What are you calling "focus problems"? Front/back focus? Non-uniform focussing plane?

[robertdrake]

Front or back focusing issues. I don't seem to see much regarding these problems with the prime lenses.

[Jon]

That's because there are more people with zooms than with primes. If you do a search on those terms here, primes are also mentioned in connection with these problems, especially where the problem seems to be in the camera, not the lens.

[daveh]

A few possibilities: Primes are usually faster and faster lenses are easier to focus whether automatically or manually. The percentage of professional/highly-experienced photographers is probably higher among prime lens users than zoom users. More experienced users might be less likely to blame problems on equipment, and when it really is the equipment, are more likely to just get it fixed than to talk about it on a forum. Primes are simpler, so - all other things being equal - may have fewer quality control issues. And/or there may simply be more zooms in the world and/or amoung web forum users.

[dhbailey]

Another issue with zoom lenses and perceived focus issues is that most people are using zooms to shoot pictures from a greater distance. That means there are more items between the camera and the main target. I've found that my 75-300 IS lens appeared to be focusing correctly when I use the 9 AF points. But when I switch to just the center AF point I get exactly what I thought I was shooting at to be in focus.

With the 20D, it will focus on a little twig that just happens to be in one of the AF points, even while I think I'm looking at an object 100 feet further away.

And remember the AF points are actually a little bit larger than represented in the viewfinder, at least from what I've read about Canon cameras.

[KennyG]

A few possibilities: Primes are usually faster and faster lenses are easier to focus whether automatically or manually.

Not exactly true. Primes generally have wider apertures than an equivelant length zoom and therefore a shallower DOF. Lack of understanding of working with a shallow DOF often leads to complaints of poor or front focus.

Is one better than the other for focus, probably not as long as both are within specification and there is nothing wrong with the camera's AF system.

[robertwgross]

Zoom lenses are physically more complex inside than a single focal length lens. That complexity means there is more that can break. They don't break very often, but still...

I had one Canon zoom lens that broke. It would neither zoom nor focus right. Canon fixed it. Hard to tell how some other lens would have done.

---Bob Gross---

[daveh]

Not exactly true. Primes generally have wider apertures than an equivelant length zoom and therefore a shallower DOF. Lack of understanding of working with a shallow DOF often leads to complaints of poor or front focus.


I'm not sure what you're saying isn't exactly true so let's try it this way: wider apertures do make focusing easier. It's true of course that wider apertures also have less DOF but in the same light the prime and zoom will be shot at the same aperture leaving only the advantage of easier focusing for the prime since it could be focused wide-open.

[neo_xeno]

Not exactly true. Primes generally have wider apertures than an equivelant length zoom and therefore a shallower DOF. Lack of understanding of working with a shallow DOF often leads to complaints of poor or front focus.

I'm not sure what you're saying isn't exactly true so let's try it this way: wider apertures do make focusing easier. It's true of course that wider apertures also have less DOF but in the same light the prime and zoom will be shot at the same aperture leaving only the advantage of easier focusing for the prime since it could be focused wide-open.

Kenny is comparing a prime with a wider aperture then a zoom with the equivalant focal length. not comparing a prime and a zoom with the same aperture and focal length. so what he is saying is true.

[daveh]

Kenny is comparing a prime with a wider aperture then a zoom with the equivalant focal length. not comparing a prime and a zoom with the same aperture and focal length. so what he is saying is true.


Well neither was I. I wasn't comparing lenses of the same maximum aperture - I was saying that under the same conditions the lenses would be shot at the same aperture meaning DOF would be the same, but you would have the advantage of focusing the prime wide open. At lower light, you would have less DOF on the prime and the zoom would simply be unusable - or the user would switch to a potentially too-low shutter speed - and possibly interpret that as a poor focus (oops that's a new suggestion too isn't it?). These are fine possibilities as well, but they're just different possibilities than the one I mentioned. I believe they are all true possibilities.

[CyberDyneSystems]

In my humble opinion.. a wider aperture = harder to focus.

Shallower depth of feild = less leeeway.. focus needs to be more precise thus it is more difficult to achieve focus...

On the other hand.. wider aperture DOES mean faster and usually more accurate AUTOfocus...

Is this what you mean when you say wider aperture = "easier" focus? Do you mean faster more accurate Auto focus?

[daveh]

Yes - faster and more accurate was what I meant by "easier" ;) Ahhh is that the confusion? That makes sense.

Yes DOF is less, but under the same light you'll shoot at the same aperture and have the same DOF so the bigger MAXIMUM aperture doesn't make it any harder to obtain the focus and DOF under equivalent conditions (ie focus wide-open then shoot stopped down)

It's also faster and more accurate for manual focus. Most of the manual focusing aids don't work very well above F4 or so. (Some older SLRs didn't stop the lens down and release it during the shot - you had to open it up, focus, stop it down, meter, shoot. Open it up...)

[robertdrake]

Okay, well the whole reason I brought it up was I don't recall ever seening a post where someone complained that their prime lens was "backfocusing" or "front focusing". I've seem plenty for zooms, espc. the 70-200L f4. Because of this, I've pretty much decided against that lens in favour of a 100 micro 2.8, 200 f2.8 and 24 f2.8. Guess my next question should be, will I be collecting lots of dust? :lol:

[drisley]

I would say that atleast 90% of those complaints are due to user error (I was among those at one time).
The rest may be a problem with the camera or the lens.
However, if there is a problem with the lens, Canon will fix this under warranty without any problems.

[robertdrake]

Well, the 70-200L f4 looks pretty iffy to me. Seem to be a awful lot with backfocusing problems with this one, and lots who have no issues and think its the greating thing going. Very tempting and very worrying at the same time, since I tend to have rather bad luck buying stuff.

[Jon]

Okay, well the whole reason I brought it up was I don't recall ever seening a post where someone complained that their prime lens was "backfocusing" or "front focusing". I've seem plenty for zooms, espc. the 70-200L f4. Because of this, I've pretty much decided against that lens in favour of a 100 micro 2.8, 200 f2.8 and 24 f2.8. Guess my next question should be, will I be collecting lots of dust? :lol:

When you first posted your question, I ran a search here for front-focus and back-focus; one of the first threads that came up mentioned, among other lenses where the user was seeing the problem, the 50 f/1.8. Actually, I think most of the front- and back-focussing posts on here that have resolved to actual problems, not user error, were with the body.

[RDKirk]

It's also faster and more accurate for manual focus. Most of the manual focusing aids don't work very well above F4 or so.

Eh? Which ones didn't? I used everything from virtual cross hairs to microprisms with fast lenses--faster lenses were ALWAYS easier to focus with manual focusing aids than slower lenses. It was with slower lenses that microprism spots and split image halves might darken to uselessness.

(Some older SLRs didn't stop the lens down and release it during the shot - you had to open it up, focus, stop it down, meter, shoot. Open it up...)

That's 'way old, in terms of the SLR bodies. By the 60s they were automated (although some like the Alpa had a funky external automation). Some cheap telephoto lenses (like the old Spiratone 400mm) were full manual.

[RDKirk]

Well, the 70-200L f4 looks pretty iffy to me. Seem to be a awful lot with backfocusing problems with this one, and lots who have no issues and think its the greating thing going. Very tempting and very worrying at the same time, since I tend to have rather bad luck buying stuff.

I'd take the majority of the "front focusing" and "back focusing" issues with a grain of salt. Most of those problems stem from a misunderstanding of the design tolerances of the AF system.

If you've read Chuck Westfall's recent pamphet (as well as the FAQ on the Canon USA website), part of the problem is that they're looking too darned close. People are expecting to see perfection with their noses buried in the equivalence of a 30x40 print--the system was designed to look good on a 5x7 print from 20 inches away.

[ScottE]

Many consumer grade zooms are not built to the same manufacturing standards as most prime lenses. This is done in order to be cost competitive in the market place. If manufacturing standards are lower, you are going to have less reliable performance in function such as focusing.

With lenses built to the same standard, you will generally find that prime lenses have larger maximum apertures. For an example a 100 mm prime might have a maximum aperture of f/2.8 while a 28-105 zoom might only have an aperture of f/5.6 at 100 mm. Even if you are shooting at f/8 you will focus at maximum aperture. If you are focusing manually, the image will be 4 times as bright in the f/2.8 lens as in the f/5.6 and there will be much less depth of field. This means it is easier to select a plane of focus that is as close as possible to the distance of the subject.

I believe that autofocus depends on the angle of incidence of light striking the sensor from both sides of the lens. (Contrast will be greatest when light ariving from a point on the subject through all points on the lens arive at the same point on the sensor.) This angle is limited by the aperture which will be twice as wide in the f/2.8 lens as it is in the f/5.6. This also will allow autofocus to be more accurate with the f/2.8 lens.

[daveh]

Eh? Which ones didn't? I used everything from virtual cross hairs to microprisms with fast lenses--faster lenses were ALWAYS easier to focus with manual focusing aids than slower lenses. It was with slower lenses that microprism spots and split image halves might darken to uselessness.

Ground glass. ;) It always "works" though it gets "harder" to use stopped down - but that's just a user issue. It still works. Really though I said "most" because I said "f4 or so" and the exact cut-off varied by various designs and with split images you could argue where the cutoff was because the sweet spot for where your eye had to be to avoid the blackout kept getting smaller.

I'm glad that someone finally agrees with me that fast and accurate focusing = easier focusing.

[Jesper]

In my humble opinion.. a wider aperture = harder to focus.

Shallower depth of feild = less leeeway.. focus needs to be more precise thus it is more difficult to achieve focus...

On the other hand.. wider aperture DOES mean faster and usually more accurate AUTOfocus...

Is this what you mean when you say wider aperture = "easier" focus? Do you mean faster more accurate Auto focus?

Because of the larger aperture, more light enters the lens, which makes it easier to focus, either manually or automatically. Also, because the depth of field is so shallow, it's easier to focus more precisely - if you're slightly off, you'll notice it more than with a smaller aperture.

[RDKirk]

I'm glad that someone finally agrees with me that fast and accurate focusing = easier focusing.

I think people are refering to the fact that DoF is shallower with a faster lens, so it's more critical to be spot-on with focus. OTOH, it's much easier to see when one is spot-on with focus when using a faster lens.

However--if one is using a faster lens AND stopping down, then one has the advantage of easier focusing with no disadvantages.