orrie smith Loc: Kansas

I agree, a lot of photographers will want the finest focus that is possible, although, and maybe unfortunately, their are a lot of people that will except what comes from their cameras without the hassle of perfection. focus tuning, in my opinion, becomes more important if you are doing portraits, wildlife, macro, and such. maybe not so important for landscape, as there usually are no central focus points noticeable, but rather the entire scene is the subject.

SharpShooter Loc: NorCal

TD, of course I'm the same person!! :lol:

No, actually, I PM'd fishwhistler and asked him if he would say that!
And bless his little heart, he did!!! :lol: :lol:
SS

CO

It is important to find out where the lens has its best performance. I just got the Tamron 45mm f/1.8. I checked the image resolution chart for that lens on LensTip.com. That chart shows that it's sharpest at f/4 to f5.6. I took a lot of test photos to verify that. I can see from the test shots that it really is the sharpest in the f/4 to f/5.6 range.

JBruce Loc: Northern MN

I've noticed something interesting with in-camera micro-focusing. I have a T3i and a 60D that do not have the feature, and they both seem to focus nearly perfectly with my best L lenses. My 70D and 5d MIII do have micro-focusing and both of them have required MF on most of the same lenses. I wonder if Canon is just a little less diligent, or more careless, and lets the purchaser do the work that should have been done at the factory? You think, and has anyone else noticed this?

John_F Loc: Minneapolis, MN

I tried that search on YouTube and all the options had to do with focusing, particularily auto-focusing. Would that not imply setting the lens rear principal plane to sensor surface distance such that the subject is in focus? In which case, that is not 'tuning the lens itself.'

BBurns Loc: South Bay, California

Sorry Charlie.......

amfoto1 Loc: San Jose, Calif. USA

It's actually Micro Focus Adjustment...

Your camera came out of one factory. Your lens was made at another. They both were tuned to "factory" standards in many ways (shutter speeds, aperture size, light meter, auto focus and more), but each has some small tolerances... a "range" that's considered "good enough" for a particular camera or lens. (More advanced and expensive items might be held to a higher standard, with tighter tolerances.)

When it comes to focus, though, it can happen that a camera might be ever-so-slightly skewed one way while the lens you put on it is ever-so-slightly skewed the opposite way.... So the problem might compounded to the point where AF isn't as accurate as it could be. That particular camera and lens combo "front focuses" or "back focuses" a little.

This is further compounded by larger lens apertures, longer focal lengths, and/or very close focusing and high magnification (such as with a macro lens)... all of which render particularly shallow depth of field so that even a slightly mis-focused image can be problematic.

Now the next thing that happens today is that everyone is looking at their digital images on their computer screens... and they can all-too-easily enlarge them to utterly ridiculous sizes, only 18" from the tip of their nose, and get hyper-critical about things like focus accuracy. There's almost an obsession with this now, I'd argue way beyond what's really needed to produce wonderful prints in many standard sizes or great looking digital images to share online at typical Internet sizes and resolutions.

So a few years ago the camera manufacturers were seeing their DSLRs and lenses being sent in for warranty service in huge numbers... flooding them with relatively minor adjustment work (a lot of it rather silly).

A solution... Give users a means of making much of the minor adjustments themselves. As a result, many of today's more advanced DSLR models, targeted at more serious users (i.e., the folks most likely to have become hyper-critical) have a means of fine tuning the autofocus performance of individual lenses on that particular camera. On Canon, for example, none of the Rebel-series models have this feature, but all the current higher-end models do (70D/80D, 7D-series, 6D, 5DIII, 5DS/5DS-R, 1DX, 1DX II). I think you'll find similar with other camera manufacturers. Sigma also provides means of adjusting some of their more recent lenses via a "USB dock".

Again on Canon, the Micro Focus Adjustment feature has been upgraded. On older models it could accommodate a single adjustment on up to 20 different lens models. That meant that any adjustment you dialed in for an "EF 50/1.4" lens would be applied to any and all EF 50/1.4 lenses used on that camera. It also meant you had to compromise on a single adjustment with a zoom, that might actually need different settings at the extremes of it's range of focal lengths. Newer Canon models (all those mentioned above) now can accommodate up to 40 lenses, recognized by individual lens serial number, and offers two adjustments on zoom lenses.

Now, with many lenses MFA more likely isn't needed. For example, an 18-135mm "kit" lens with f3.5-5.6 aperture just naturally has greater depth of field that "hides" minor focus errors. Stopping any lens down from it's maximum aperture also will make for greater depth of field and more tolerance of minor focus errors. Focus accuracy is much more critical with a high performance 400/2.8 telephoto, 85/1.2 portrait lens or a macro lens focused very close near it's maximum 1:1 magnification. All these make for very shallow and unforgiving depth of field.

Also, again using Canon examples, the type of focus drive can make a difference. Their cheapest lenses use a micro motor focus drive system. While fine for general use and many purposes, those just aren't as accurate or consistent focusing as some of the more advanced focus drive systems. For example, the EF 50/1.8 Mark II lens was nearly impossible to MFA, because it was just too inconsistent focusing (it's now been superseded by the EF 50/1.8 STM, which is better). Canon's STM (stepper motor) and especially USM (ultrasonic motor) focus drive systems that are used on their "better" lenses give more faster and more accurate AF.

But, there also are many other things that can effect image sharpness and are often mistaken for focus error. First is using a larger aperture than necessary for the purpose, as already mentioned. Most lenses are just not their sharpest wide open. Many improve a little or a lot one or two stops down from wide open.

Camera shake blur of various sorts is also often mistaken for focus problems. Using slow a shutter speed for the situation is just asking for trouble. This might be from trying to handhold at too slow a speed... or can be caused by wind or even internal camera vibrations from the movement of the mirror and shutter at certain, slower shutter speeds.
There's also subject movement blur, again caused by using too slow a shutter to freeze the action.

Another common problem is unnecessarily using a filter on a lens. Especially cheaper filters can cause a lot of loss of image quality. But some lenses simply don't "play well" with filters... the original Canon 100-400mm L is well known to have problems with even high quality filters. Folks slap a filter on their lenses thinking that thin piece of glass is somehow going to "protect" it... when in fact they are ruining the quality of their images. A lens hood actually provides better protection... certainly can't hurt and actually might help with AF performance by keeping oblique light off the lens.

Another consideration with zoom lenses is that some are varifocal designs, which do not maintain focus when the focal length is changed. This type of design is used to keep costs, size and complexity down... and becoming more common since AF can automatically correct for it, when it's used right. But, in some AF modes (such as Canon One Shot or Nikon AF-S or similar from other manufacturers), it's very important to know if your zoom is a varifocal. And, if it is, to always remember to re-focus after zooming the lens.

And, all too common... is user error. As AF systems have become more and more complex and customizable, it's also very easy to set things up wrong and cause focus errors. Often when I'm shooting sporting events I hear people using the wrong focus mode. I hear their camera's "beeping". With most cameras, that's focus confirmation that only works in the focus mode for stationary subjects... which means their camera is locking focus, but the subject is moving away from that point before the shutter trips. I guarantee that the majority of their shots are out of focus... and that most of the time they blame the gear, when in fact the problem is that they are setting the camera up incorrectly. They should be using a focus mode for moving subjects (which never locks, continuously updates itself to track the movement). AFAIK, that doesn't (actually, can't) give focus confirmation with any camera.

How to tell if you have a problem that needs adjustment? Well there are tests you can do... but they have to be done very carefully, eliminating a lot of the pitfalls mentioned above. Also, first check if your camera (or lens, if it's a Sigma) even has means of making the adjustment. If not, maybe you are better off not knowing!

But, if your camera doesn't have MFA feature and focus is seriously off, the lens can be calibrated by a professional technician. And, lenses will commonly lose calibration gradually over time, simply from wear and tear on their mechanisms.

A professional uses specialized devices to test lenses. You can do similar simply by taking test shots. You need to set up the camera and lens on a very stable tripod and, ideally, use some sort of target such as this: http://www.bhphotovideo.com/bnh/controller/home?O=&sku=970599&gclid=CIH0lKfku8sCFVKDfgodBY0DQg&is=REG&ap=y&m=Y&Q=&A=details. There are DIY homemade targets, too, if you don't want to buy one. Heck, even a newspaper taped to a wall, weathered wood fence or brick wall can tell you a lot (just be sure to carefully set up so that it's as parallel as possible with your camera's sensor).

In the manuals for their models with MFA, Canon outlines a relatively simple method using a flat target and switching between the viewfinder-based phase detection AF and Live View contrast-detection AF (the latter is always most accurate.... though it's much slower). If you see a difference between the two, you know that the phase detection AF needs adjustment with that particular lens and can dial some in.

There also is software that can be used in conjunction with targets to fairly automatically fine tune focus... Reikan FoCal is one example (http://www.reikan.co.uk/focalweb/). DXO Mark is a more sophisticated lens evaluation software that can be used for testing, too.

Hope this helps answer your questions. I have cited a lot of Canon examples only because those are what I'm most familiar with. I imagine most DSLR manufacturers today offer similar on at least some of their cameras.

fishwhistler Loc: Kittanning, Pa.

amfoto 1

I thank you very much for this in depth and informative explanation. This is exactly what I needed. Many thanks.

bwana Loc: Bergen, Alberta, Canada

If you're using a mirrorless camera, you don't have to worry about it.

bwa

Crazytooth Loc: Suffolk, UK

I had an issue with my 600D and the 24-105L where the lens focused short, I sent the camera and lens to a Canon approved repair shop who tuned the the lens with the camera.
I have several other lens and this is the only one that had a problem.
The lens is my walk-about and was worth doing.