Manual Focus Of Digital Cameras With Fly-By-Wire Lenses Versus Film Cameras With Manual Focus Lenses

Man on the moon (click to enlarge)
X-T1, 16-55mm f/2.8 lens @ 55mm ; 1/280th sec. @ f/5.6; ISO 800

A friend recently asked me about how well I thought the current focus-by-wire, electronically controlled and digitally designed autofocus lenses focused in manual mode versus focusing the old mechanical manual focus lenses.  In short, there is a huge difference in manually focusing each and I think there are at least a few reasons why, manufacturer intent, lens design and camera design.

Modern autofocus lenses are designed for autofocus with manual focus as a secondary thought, in my opinion.  The lens manufacturers put all emphasis on fast, smooth, accurate autofocus with as few and as lightweight moving parts as possible.  
It takes power, both in motor strength and battery capacity, to move those lens elements so they design focusing mechanisms to minimize movement and weight.  They design their lenses to support that.  Manual focus for them, in my opinion, is a nuisance and I bet they wish they could manufacture lenses without any ability for manual focus at all, but there would be such a backlash from user's that they can't do that—yet.  Manual focus is an afterthought.  I predict, in the future, you will see lenses from the big camera manufacturers which have no ability to manually focus.

The modern lenses are electronically controlled, focus-by-wire, and many don't have any reference as to where you are on the focus scale until you turn on the camera and start to actually focus the lens, either automatically or manually.  They don't have hard stops for infinity or a lens' closest focus point, thus not giving the user a point of reference (forcing you to have to visually look at the scale, if there is one) so one can quickly rotate the focus ring to achieve infinity focus instantly.  In fact, you can infinitely rotate the focus ring since it is not mechanically connected to the focusing mechanism.

Many manufacturers provide the ability to change the direction of infinity focus, others don't. Some lenses from the same manufacturers allow you to do this while other lenses don't. Some lenses focus more quickly than others.  On some electronic lenses you can go from infinity to its closest focus point in 1/4 turn.  Others require over 1 full rotation, and still others several full rotations of the focusing ring to go from infinity to the lens' closest focusing distance.  Talk about being confused when under pressure to focus quickly? Focusing with modern electronic lenses doesn't give one any cues unless you are actually looking at a scale or through the viewfinder.

50mm f/1.4 all electronic focusing and aperture control
lens (photo courtesy of B&H Photo)

Additionally, some lenses will go to a "resting" position when you turn your camera off. That means that if you are focused at 10 ft., then turn your camera off, the lens may automatically be at infinity when you turn your camera back on.  Other manufacturers allow the lens to stay at its last focus points between power cycles.  Also, I believe some autofocus cameras even return the lens to a specified focus point (maybe infinity?) between every shot!  Which is which?  You better know if speed and accuracy is important.

The manufacturers do build in some electronic "helpers" for us find precise manual focus, however.  Two are focus peaking and focus magnification.  Focus peaking provides visual colored (red, white, blue; user selectable) edges to the places in your scene that are "in focus" or maybe more accurately within the approximate depth of field of your image.  I don't find this method very accurate as many more things appear to be in focus than just the object on which I am focusing.  This method doesn't give me a very high confidence level.  From what I have read, Leica seems to have the best implementation of focus peaking.

The second, and to me better "helper," is focus magnification.  The manufacturers have given us a feature that allows anywhere from a 2X to 10X magnification of the part of the image on which we are focusing, thus giving us a much larger and clearer view to confirm we have correctly focused.  I tend to use this feature and find it very accurate.  Time consuming to use but accurate.  However, I also find anything more than about 4X to 6X too magnified for my liking as it takes my focus point too much out of context of the entire scene.  Also, trying to use a 10X magnification while handholding a lens is difficult since any camera/lens movement is also magnified by 10X.

Again, neither of these two "helpers" changes the fact that the focus ring is not mechanically connected to the lens' focusing mechanism and therefore doesn't really give us a positive feel when focusing.

I find myself, either with magnification or with focus peaking, rocking the focus ring back and forth, back and forth, to ensure I have exact focus.  I can achieve it with the modern lenses, make no mistake, but it takes me much longer than it did with mechanical lenses that were mechanically coupled and direct.

There are a few exceptions to the way in which most modern autofocus lenses are constructed and I am grateful for those few manufacturers for providing us with this alternative construction.  For example, Olympus and Fuji construct several of their lenses with the ability to pull the focusing ring back toward the camera which then mechanically couples the focusing mechanism with the ring.  This, in effect, turns the lens into an old fashioned mechanically coupled manual focus lens.  Perfect engineering and excellent design as well as serving all of their customer needs.  Thank you Fuji and Olympus!  A gift.

Again, modern electronically controlled autofocus lenses are not designed to easily manually focus and I don't see the manufacturers doing anything to improve that fact.

Prior, all manual focus lens , as above and is entirely
mechanically controlled (photo courtesy of Amazon.com)

As for the old manual focus only, mechanically coupled lenses of yesteryear, they were designed only for manual focus and have all of those attributes mentioned above that have been left out of autofocus electronic lenses.  The focusing ring stops moving at infinity (some slightly beyond to compensate for heat absorption).  The focusing ring stops moving at minimum focus so you can easily and quickly rack the lens to its closest focusing distance. If, for example you want to make a macro photo and want to get in as close as possible, you can quickly rack the focus to its closest point then physically move forward to a flower, small creature or other small object, and immediately know where your closest focusing distance is for maximum magnification.  How many star trail and astronomical photographers would love to have an infinity stop so they could easily set their lenses to infinity in the darkness of the night without having to look?

Manual focus lenses, being mechanically coupled, stay at the last focus point at which you left the lens.  They always turn in the same direction at the same relative speed. There is no hunting for focus. They feel good.  You can feel the lens' internals moving.  Typically, with old manual focus lenses, the amount you have to turn the focusing ring is much shorter than with autofocus lenses.  However, that shorter distance does not diminish accuracy, but enhances focus speed.

Illustrations of several types of focusing screens that could
be found in older, manual focus, film-based cameras
(click to enlarge)
Notice the "helpers" to assist in achieving fast, accurate
manual focus

The way cameras are designed also influences ease of focus with manual lenses and makes manually focusing with autofocus lenses more difficult.  The primary reason is the type of ground glass focusing screens provided.  The top illustration, from www.focusingscreen.com, shows what typical manual focus camera bodies included for ground glass focusing screens.  They were bright, easy to see through and had "helpers" as well.  Two most common were a "split image focusing screen" and a "microprism lens" focusing screen.  The split image, either horizontal or diagonal, works the same way as a split image rangefinder on a camera such as a Leica M.  A vertical line or edge would be offset when not in focus and come together to make one solid vertical line/edge when in focus. Easy to see and bring quickly into focus.

The microprism lens would make an out of focus area look "crystalline" (for lack of a better word) and unclear when out of focus, but when brought into focus, the crystalline look would disappear and the image would appear clear and sharp.

Some focusing screens had a combination of a split-image rangefinder surrounded by a microprism, which helped focusing even more.

Typical modern digital single lens reflect focusing screens
(click to enlarge)
Nothing but autofocus sensor points (which may in and of themselves
not be accurately positioned) to assist you in manually focusing a lens

Additionally, there were plain ground glass focusing screens that, if an image was out of focus, it was very, very obvious and when brought into focus would "snap" into focus as it was often referred.  In other words, if the image was in correct focus it was easily detected through the viewfinder.  Modern digital SLR viewfinders are not nearly as bright, contrasty and crisp as the old film camera viewfinders.  Again, the cameras and lenses are optimized for autofocus not manual focus.

The second illustration shows what a modern digital SLR's focusing screen looks like.  Do you seen any "helpers" that assist you in manually focusing?  None.  Additionally, I believe most autofocus focusing screens are plastic, not ground glass. I don't know if that makes a difference, but it is a difference.

To sum it up, manually focusing autofocus lenses in today's modern computers with a lens we call cameras is harder than it should be.  The camera and lens manufacturers make camera bodies and lenses to meet the needs of the market today.  The market today is autofocus, auto everything, piles of electronic features and little call by consumers for manual mode for any part of the photographic process.  Manual focus is used by scientists, technical photographers and a relatively few other photographers when compared to the general photographic marketplace.  So it is understandable why today's cameras don't make it easy for you to manually focus either an old manual lens on a modern autofocus camera or manually focus a new electronically controlled lens on a digital camera.

To achieve as much accuracy as you can in manual focus, you just have to fully understand your gear, know which way to rotate the ring to get to infinity as a reference point, then practice, practice, practice.

Thanks for looking. Enjoy! 

Dennis A. Mook 

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