Considerations for using a FF lens on MFT?

Question

I am wondering about the workability of using FF lenses, such as those designed for the Nikon F mount, on a Micro Four Thirds (MFT) body using an adapter.

One obvious consideration is that an adapted lens needs to have an aperture ring, since there will be no electronic aperture control. Also, there will be no auto-focus, obviously.

So, given these constraints let's imagine trying a lens like the Sigma 70mm F/2.8 macro. This lens is renowned for sharpness. So, if we mount it on an MFT will it be even more sharp and distortion free because we are only using the center of the glass? Will this extra sharpness be noticeable? I guess it is hard to know without having a distortion map of the Sigma and seeing whether the image circle in inside of the distortion wave of the lens, or just getting the lens and shooting a grid with it. However, I would like avoid buying the lens if it is not going to work and rentals are a hassle.

An additional caveat is that the field of view will narrow by a factor of 2, so the Sigma will shoot, I guess, like a 140mm in terms of crop factor and magnification.

One thing I notice in forum posts about using an adapted lens is that some people have complained that telephoto lenses sometimes do not focus when adapted and also sometimes have strange artifacts like blue dots or halos in the center of the image. On the other hand, users with macro lenses seem to be reporting that they generally work well. Is this a general rule, that doing a FF-to-MFT adaptation works better for macro lenses and worse for telephotos? Is there any way to estimate what lenses might not work without actually trying them?

Answer 1

It all depends on the specific lenses, cameras, and adapters in question.

One obvious consideration is that an adapted lens needs to have an aperture ring, since there will be no electronic aperture control. Also, there will be no auto-focus, obviously.

This is not as obvious as you seem to think it is. Many adapters do translate the aperture control and AF protocol from the camera's system to the lens' system with varying degrees of success. Some even do it very well.

Remember, all micro four-thirds cameras use an all-electronic connection between the camera and lens. Many photographers who adapt other lenses to micro four-thirds cameras choose Canon EF lenses precisely because the EOS system also uses all electronic communication between the camera and lens. This means there is no mechanical connection(s) to try and replicate when using EOS lenses adapted on µ4/3 - just data signals that must be translated.

So, given these constraints let's imagine trying a lens like the Sigma 70mm F/2.8 macro. This lens is renowned for sharpness. So, if we mount it on an MFT will it be even more sharp and distortion free because we are only using the center of the glass?

In the case of the Sigma 70mm f/2.8 Macro you probably won't 'gain' much, if any, sharpness using it on a cropped sensor. Why? Because the primary reason that particular lens (and other flat field lenses) are considered 'sharper' than some of their counterparts with a more curved field of focus is because they remain sharper to the edge of the field when imaging a flat target. Lenses with field curvature can be just as sharp at the edges as a flat field lens can, they just can't be as sharp in both the center and the edges at the same focus distance when imaging a flat test target.

Will this extra sharpness be noticeable?

What extra sharpness? See above.

An additional caveat is that the field of view will narrow by a factor of 2, so the Sigma will shoot, I guess, like a 140mm in terms of crop factor and magnification.

That may be considered a caveat or an advantage. It all depends on whether you'd rather obtain a narrower or wider FoV with any specific lens. This is exactly the same as non-adapted FF lenses used on APS-C cameras with the same native mount. Birders love the 300mm equivalent FoV obtainable using a 200mm lens on a 1.5X APS-C crop body. Landscape photographers don't like the 36mm equivalent FoV obtained with a 24mm lens on the same APS-C camera near as much (or at all).

This entire issue can be compensated for with a speed booster that effectively changes the optical power of the lens in the reverse manner to the way a teleconverter does.

There are additional considerations.

• Adding an adapter introduces an additional mechanical interface that can introduce small alignment errors between the optical axis of the lens and the plane of the camera's sensor. Variations of as little as 20 microns in flange distance from one side of the interface to the other can be seen in images made using fast wide angle lenses.
• Lenses designed in the digital age usually account for the thickness of the filter stack in front of the image sensor for the native mount system for which they are designed. The total stack thickness varies from one mount system to the next. This can affect the overall optical performance of the lens, particularly with regard to maximum sharpness. This is why Roger Cicala and his crew at lensrentals.com/OLAF now test lenses with a cover plate of the designated thickness for each lens mount system (or specific camera) on their optical bench to simulate the filter stack when testing lenses for a certain mount system.

Answer 2

To answer your questions..

Will a lens become sharper because it's being used on a m43 format? No. The lens sharpness is independent of the sensor. However the sensor's pixel density (megapixels) can sample more data which might show more data that was collected. However, diffraction will set in which will be around f5.6 to f8 depending upon the megapixels. Once stopped down beyond that aperture you'll lose some sharpness. You can read more about that here: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

Metabones does have a m43 adapter for Nikon lenses which can regain some of the lost light gathered by the lens. So you can operate with the lens closer to the original field of view. It also has an aperture dial so you can work with the G and E type AF-S lenses Nikon makes

When I've used adapters on the Fuji system I didn't notice any side effects like you mentioned. That may be due to buying a higher quality adapter. They aren't cheap but likely will give better results than the cheaper clones. So, no, it's not a general rule that macro lenses work best and telephotos the worst. I would not expect any issues but I can't think of any way to really try it without buying it.

Answer 3

Why not just adapt FF lenses on M43 bodies and look at the results? I have been doing this for years using the so-called "dumb adapters". I use adapters made by Fotodiox. I happen to have a sizable collection of legacy FF prime lenses using a variety of different mounts: M42, PK, FD, MD, Cine, OM, Nikon F-mount, C/Y, Pentax-110, and even EOS & Sony/Minolta A-mount. The FF lenses on the first 9 mounts all have aperture rings on the lens. The EOS & Sony/Minolta A (alpha) mount lenses require me to preset the aperture on the native camera before mounting the lens on the M43 - generally I will set those lenses wide open before mounting on the M43.

Generally I do not do this often using FF zoom lenses. The FF zooms are generally too big and heavy, and frankly cumbersome to use. FF prime lenses are not overly large and generally work quite well.

I have not experienced any systematic issues in mounting FF prime lenses on M43 bodies (Olympus & Panasonic Lumix). It's true I am restricted to manual focus, and the effective focal length is always doubled (because the field of view is reduced). But the focus is generally spot on (using manual-focus assist) and the specific characteristics of the legacy lenses used is retained. The FF focal lengths I have used include 18mm, 28mm, 35mm, 50mm, 55mm, 120mm, 135mm & 200mm. In the M43 format these eight focal lengths act as 36mm, 56mm, 70mm, 100mm, 110mm, 240mm, 170mm & 400mm.

That said, have a look. There are over 1,500 examples in this flickr album (at time of writing). If you see any optical issues with these images let me know:

Legacy Lenses on M43.