You may not be aware that most of the lenses made in the past to fit various brands of 35-mm film SLR cameras (we'll refer to them as "film cameras" for short) can be used with E-1, E-300, E-330, and E-500 (or any other Four Thirds models which may show up in the future). Here I am offering some technical background about that, as well as some practical advice.

Related articles on this site:

• My Quest article, Using Legacy Lenses on Digital SLRs, a general introduction to the subject (actually, it makes sense to read that article before this one);
• Using OM Zuiko lenses on E-1 and E-300; general remarks and John Foster's observations on a long list of lenses;
• The 50 mm, F/1.8 Zuiko "standard" prime lens used on E-System cameras — remarks and image comparisons.
• Using macro bellows to get really close, with examples shot with some old Exakta lenses.

Mechanical coupling

First of all, the lens has to be mounted on the camera body. E-System cameras use the Four Thirds standard bayonet, providing both the mechanical coupling and the digital interface between lens and body. Obviously, the Four Thirds standard is compatible only with the Four Thirds lenses.

The mechanical connection can be, however, provided with use of lens adapters. Such an adapter is a short metal ring with a Four Thirds male bayonet at the rear, and the female lens mount (bayonet or screw-on) at the front.

This looks simple enough, but there are some limitations. The lens will focus properly only if the distance between its rear flange and the image plane (film or digital sensor) is right, i.e., the same as in the camera for which the lens was designed. This value is often referred to as flange-back or register.

The flange-back differs from system to system. If the lens was intended for a system with flange-back greater than that of the body on which the lens will be used, fine: the adapter thickness can provide the extra spacing. If not, tough: even if you could mechanically match the lens to the body, it will not focus all the way to infinity. (Actually, for some camera/lens combinations you can see a disclaimer: "suitable only for close distances".)

In case of E-system cameras, we are lucky: the Four Thirds flange distance is less than for any of popular manual-focus SLRs (this is possible thanks to smaller frame and therefore mirror size). Here is a table, showing some of the values in millimeters

(An extensive list of flange distances and mount sizes has been compiled by W.J. Markerink, and re-published in a more readable format with some additions by Stephen Westin.)

The Four Thirds value is a privileged information, unconfirmed officially but trusted; other values are compiled from various sources. (See here for more on the difference between the OM and Four Thirds values.)

The smaller flange-back distance gives Olympus (and Four Third cameras in general) an edge in terms of usability of other maker's lenses on their cameras; at this moment adapters are available for all lens standards listed above.

Digital cameras by other makers allow you to use their respective film system lenses directly, although often with limitations. This means that they inherited flange-back values from their film ancestors, and this, with very few exceptions, excludes the option of cross-system lens use.

Lens adapters

Olympus sells the MF-1 adapter ($100 in the U.S.), allowing for use of lenses made for the OM series cameras, being made from early Seventies to late Nineties. (I'm writing about it in the OM Zuikos page).

The same, or a very similar, adapter used to be available as MA-1; from the photographs both models look identical. This version used to be, until late 2005, given away upon request, to registered owners of Olympus SLRs on some markets; some of those did not have any designation markings except for "OM FOURTHIRDS ADAPTER" and "MADE IN JAPAN".

A Japanese company makes adapters for a number of other lens mounts: Nikon F, M42 screw mount (Pentax/Practica), Yashica/Contax RTS, Leica R, Pentax K, Exakta, and, again, Olympus OM.

Note of 2007: There is an outfit named Cirrus Adapter, offering a whole range of Four Thirds adapters at reasonable prices at their eBay store. I haven't dealt with them, however, so I cannot tell how well are these made.

The Exakta adapter

You have to be warned, however, that there are two limitations with this adapter:

• It offers only the internal bayonet, not the external one (which is used on just a few large and heavy lenses). This is by design, but I'm not happy with it, especially that the outer flange would not interfere with mounting of lenses with the more common inner bayonet. In a piece costing $175 I would expect full Exakta lens compatibility.
• A minor design error makes it impossible to use some of the later Zeiss lenses, including the 2.0/50 Pancolar, 2.8/50 Tessar, and some others. I believe that all late, automatic Zeiss lenses in black (or striped) finish may be affected by this problem. Mr. Gandy of Camera Quest is aware of the problem and I hope it will be rectified with the maker.

April, 2006: No news if the problem was addressed; if you want to use your black Zeiss lenses, check with the distributor first. I think that using a Dremel tool to take off a small piece of the bayonet lip should help, but I haven't tried that yet.

Focal length and field of view

The Four Thirds camera sensor, used in the E-1 and E-300 cameras, has a linear size almost exactly half of the standard 35-mm frame. (This is exactly true in terms of the frame diagonal, and slightly varies for image sides, as Four Thirds has a different aspect ratio. Risking a slight simplification let's just use the value of two.)

This means that when a lens of a given focal length, F, is mounted on a Four Thirds camera, the part of the optical image used by the sensor will be one half of the size of such part used by the same (or just the same F) lens mounted on a 35-mm camera. The camera's field of view is reduced by half (linear; the area is reduced four times).

(Some writers refer here to the "image angle", but this, while approximately OK for longer lenses, is not true for shorter ones. If you paid attention in your high school math classes, you remember the difference between an angle and its tangent function value; if you haven't, just take my word for it.)

To get the same field of view on a film camera, you would have to mount on it a lens of the focal length twice that value, i.e., 2F. (I'm usually referring to the latter as EFL, or Equivalent Focal Length, in terms of 35-mm film.)

Note: All I'm saying here is also applicable to digital cameras with APS-sized sensors, except that the factor of 2x I'm using all around the place has to be replaced with 1.5x.

This means that, for example, that any 50 mm lens mounted on an E-system body will have a field of view similar to that of any 100 mm lens mounted on a film camera.

The lens aperture, defined as the ratio of the entrance pupil diameter to the focal length, obviously does not change.

Lens resolution

There is one important difference, though. The recorded image (from film or digital sensor) has to be enlarged to be viewed (as a print, or on a screen). Because of smaller original size, the enlargement factor for an E-system is twice as large as that for a film camera. This means that all flaws of the image will be also enlarged twice as much; among others, the effective resolution of a lens used on a Four Thirds body is only half of that for the same lens used with on a film camera.

We may put it differently: to achieve the same final image quality, a lens designed for the Four Thirds system has to be twice as sharp (in terms of absolute resolution) as a lens intended for a film camera.

A lens for a film-based system may, for example, have a central resolution of 60 lpm (lines per millimeter), and for many purposes this may be considered satisfactory, if not stellar. When this lens is mounted on a Four Thirds camera, however, the image flaws are magnified twice as much, so the resolution (while still being 60 lpm) becomes equivalent to only 30 lpm (with the same print size).

For nitpickers only: the same argument holds when expressed in terms of the MTF (Modulation Transfer Function), where different image enlargements can be translated into rescaling the horizontal axis of an MTF response-vs-frequency plot by a factor of two.

Now, camera and lens makers are not here to save the world, whales and all; their basic goal is to earn money by making us buy their products. Making lenses of higher resolution costs more, so most lenses provide just enough resolution for their intended use. The latter varies with the target market: lens resolution requirements of a professional photographer will be higher than those of an entry-level amateur, as the latter usually needs smaller prints and has generally lower expectations.

What does this mean for you?

The second half of the preceding section boils down to two simple points:

• Whatever you do, your images made with an non-4/3 lens on an E-System camera will never be as sharp as when the same lens is used on a film camera. (This assumes that the lens is the bottleneck of the process, not camera shake and the medium, i.e., film or sensor, resolution.)
• If you are choosing such a lens to be used on your E-1 or E-300, limit the choice to high-quality lenses. These usually will be prime lenses, i.e., ones with fixed focal length, not zooms.

What may help, is the fact that a digital camera uses only the central part of a "non-digital" lens image, which is usually the sharpest. Not always: advanced lens designs may trade off some center resolution to gain more resolution in the outer regions.

It may happen that, even with the 2x factor in effective resolution loss, a good film-camera lens will be better than a middle-of-the-road ZD (Zuiko Digital) or third-party one, designed for the Four Thirds system. The only real way to tell is to try.

Depth of field

A general introduction to depth of field (DOF) and its peculiarities for digital cameras as compared to film ones is given in my DOF article, so I will not be repeating myself here, giving you just the bottom line instead:

When any lens of a focal length F is mounted on a Four Thirds body and stepped down to aperture (F-number) A, its effective depth of field will be the same as that of a lens with focal length 2F stepped down to aperture 2A, and working on a film camera.

This is an ugly, but accurate, sentence, and I wasn't able to make it any friendlier. Just in case, read it once again. Slowly.

This is good news if you want to maximize the depth of field (macro, landscape), but bad news if you want to keep it shallow (portrait).

For example, if you mount a 50 mm, F/1.4 lens on the E-1 and use it wide open, the DOF will be the same as when you mount a 100 mm, F/2.8 lens on a film camera. While the depth of field is still quite shallow, making the setup suitable for portrait photography (not to mention the perfect focal length), it is not as shallow as it would have been in a 100 mm, F/1.4 lens. The light-gathering capability is still defined by the actual aperture of F/1.4.

What are you giving up?

The Four Thirds system introduced an entirely new, fully electronic standard of communication between the lens and the camera body. Obviously, this system is 100% incompatible with any lens-camera coupling system existing before, regardless if the lens adapter you are using to mechanically secure the lens on body.

What this means is that there is no communication in either direction between your E-system camera and the lens. In particular, you will be facing three limitations:

• The camera must be set to manual focus, and you will have to focus manually with the lens focus ring.
• After the image has been composed and focused at full aperture (the latter for best accuracy), you have to step the aperture down to the desired value by hand.
• The camera's autoexposure system will work, but only in the aperture-priority mode; the release button has to be half-pressed (or AEL switch used) only after the lens has been closed down to the working aperture.

For some kinds of photography these are not really painful limitations, especially if you shoot wide open, but the necessity to step the lens down manually may be a disadvantage in action photography.

Exposure accuracy

We grew to expect that through-the-lens metering is usually accurate and dependable; after all, the camera measures the same light which is used to form the image or a film or digital sensor, right? Wrong!

During his extensive trials of OM Zuiko lenses on the E-1, John Foster observed that the aperture-priority autoexposure was often wrong at the widest apertures, resulting in overexposed images. We were both puzzled by that, but John stands by his results. As soon as I've put my hands on a 1.8/50 OM Zuiko, I was able to confirm the same effect at F/1.8, disappearing at F/2.8. What gives?

Then I noticed one more thing: closing the aperture down from F/1.7 to F/2.8 (about 1.5 EV, or almost a factor of 3x in terms of light amount) was not causing any noticeable darkening of the image seen on the viewfinder screen. Moving down to F/4 and beyond was, as expected, making the viewed image darker.

Hallelujah! Suddenly all seems clear (or almost so). The fine print below is intended for investigative minds; is you want just the bottom line, skip that discussion.

The optical image on the screen is almost identical to that on the sensor, with one exception: there is a finite-size mirror in the optical path. For a mirror large enough, this difference would have no effect. As it is, the mirror effectively plays a role of yet another, rectangular (or, more accurately, trapezoidal) aperture, cutting off the peripheral parts of the light "beam". Mind it, not just the light falling beyond the sensor, but also within it, as that aperture is still quite far from the image plane (i.e., the screen). The mirror swings off the light path during the actual exposure, therefore this additional aperture effect does not affect the recorded image.

The E-1 and E-300 measure the light off the viewing screen. The screen receives less light than the image sensor, so the camera assumes the scene to be darker than it actually is — and overexposes. At smaller apertures the whole light beam fits within the mirror, and the effect does not occur. Simple.

The Zuiko Digital lenses, designed for small-sized sensors, do not cause this problem, as the light beam is already narrow before it strikes the mirror. While Olympus was touting this feature in promotional materials, stressing its importance for the performance of sensor's photosites, it also has this extra effect.

I have noticed that the autoexposure error also depends of the diameter of the rear lens element; the larger it is the more AE problems you may expect, ant those may be not limited to just the widest apertures.

The good news is that, for a given lens and aperture, the over- or underexposure seems to be constant, not depending on any other factors (a slight dependency on focused distance can be safely neglected). Therefore the workaround is simple: for every lens you need to know the exposure correction, applicable for every aperture. Sometimes this will kick in only at widest lens openings, sometimes it will keep changing in the whole aperture range. This correction (applied on top of any others you may want to use, of course) can be established experimentally by taking a series of test pictures at varying compensation values; actually this is faster done than described.

For example, the aforementioned 1.8/50 OM Zuiko requires just -0.3EV compensation at F/1.8 and no compensation at smaller apertures (i.e., F-numbers from 2.8 upwards), as tested on the E-300 with matrix metering.

On the other hand, with the classic Zeiss 1.5/75 Biotar, the E-500 very slightly overexposes at F/1.5, stays even between F/2 and F/4, and then overexposes increasingly up to almost 1 EV at the lens fully stepped down to F/16. (This was checked with center-weighted metering.) In this particular case I have set the WB by reference before the first exposure, and the colors were consistent in the whole aperture range. By the way, the lens is wonderfully soft at F/1.5, and this, together with the shallow depth of field, makes it great for portrait shooting.

To be exact, the correction depends on mirror size. If the E-1 has a larger mirror than the E-300 or E-500, it will require less adjustment. I haven't measured the mirrors in these cameras, therefore I cannot drive any conclusions here.

The above discussion is based on an educated guess, and consistent with my own results (E-300) and those obtained by a number of people I know, most prominently John Foster (E-1), who checked a wide range of OMZ lenses. At the same time, I received a set of comprehensive image samples by Peter Klein (E-1), and his results seem to show the opposite trend.

According to Olympus, spot metering will also be unreliable with non-4/3 lenses; use the ESP or center-weighted mode. Actually, having used both these modes on the E-300 and E-500, I can say this still depends on lens and aperture used; this is not just a problem of spot metering.

White balance

Similarly to the exposure, the auto WB system in E-System cameras goes bananas at the widest apertures, especially for short-to-medium focal lengths. With many of my lenses, especially up to 75 mm, it provides results which are clearly off-mark, mostly in the yellow direction. The effect seems to go away aft F/2.8 or so.

I would recommend, therefore, using manual WB with manual lenses fitted on Four Thirds cameras. Actually, I'm in favor of using manual WB in almost all kinds of shooting, regardless of lenses, so this will not be a problem for those who follow this advice.

Focusing

As already mentioned, in addition to being obvious: you will be limited to manual focusing only, period.

The Olympus E-System cameras have a manual focus confirmation feature: the AF indicator blinks when the AF sensors determine the image is in focus. Unfortunately (and ironically!), this feature works only with AF lenses, so you will not get any help here.

Digital SLRs, as a rule, have finders with much less magnification than film cameras. This is also true about Olympus models. While in the E-1 and E-300 the image size is exactly the same as in models from other makers using APS-sized sensors (except for Pentax, offering slightly more magnification), in the E-500 and E-330 the size is an extra 10% smaller. Either way, this makes manual focusing problematic, at least for me.

While an eyepiece magnifier or a magnifying angle finder makes manual focusing easier, all this makes me rather skeptical regarding the usability of MF lenses on digital SLRs.

More and less useful lenses

Remember the 2× focal length magnifier. This means that your prized 14 mm lens will have an EFL of 28 mm on the Four Thirds body; nothing to write home about, with all limitations and inconveniences discussed above.

The widest-angle lenses are quite difficult and expensive to design and make. They also may have to sacrifice some of the central sharpness to achieve more acceptable results on peripheries of the full frame. They may also show more distortion, even in the central area used by the Four Thirds frame.

This may mean that such a lens, while working OK on your E-System camera, may provide results not better than your standard zoom set to 14 mm. If I am skeptical about using wide-angle lenses from film cameras on Four Thirds (or, for that mtter, APS-C) bodies, that's why.

On the other hand, a "normal" (50 mm) lens will become a moderate telephoto (EFL of 100 mm) on your digital camera; these lenses also offer widest apertures and usually, the highest resolution. You may obtain surprisingly nice results.

Last but not least, long MF lenses may simply have no equivalent (at least not in a non-exotic price range) in the Four Thirds mount, while still providing the necessary extra resolution, at least when stepped down. They can also often be bought quite cheaply, therefore you will probably be using them regardless of what I may say.

Advice

Adding all this up, here is some advice for those who would like to use their "older" lenses on Olympus E-system cameras.

• Carefully select the lenses you want to use; choose only the sharpest, as they will have to provide twice the resolution for which they were designed. This usually means limiting yourself to non-zoom lenses. (Sharpness is not all; if you like the results, use the lens anyway!)
• Make sure that you can live with stepped-down aperture priority metering. If not, forget it. Remember that most lenses will perform best (here: satisfactorily) when stepped down one or two F-stops from widest aperture.
• For every lens, shoot a series of test images under controlled conditions (preferably artificial light), at various apertures. For widest apertures the images may be overexposed. At these apertures apply negative exposure compensation until the image matches, exposure-wise, those taken with aperture closed down; write down these corrections and use them as needed.
• Use manual white balance setting (which is generally a good idea), especially when shooting at wide apertures.

Have fun.