what about focal reducer or speedbooster?

When using lenses designed for bigger sensors thanks to a mount adapter, the resulting illumination circle is bigger than what is needed... the light is absorbed by the mount adapter i.e. it is lost... although only the best parts of the lenses are used: the lenses borders, which are known to be softer, are not used. An interesting idea is to use a speedbooster, also named focal reducer, which is concentrating the illumination circle to a smaller one that match the actual camera sensor size. Metabones proposed it first, but much cheaper ones have been proposed by Zhongyi (LensTurbo), Pixco, Viltrox, etc...

Using a focal reducer with lenses for 24 x 36 sensors on a APS-C camera will increase the aperture by 1 stop (for instance, when it is written 1:8 on the aperture ring, you have to understand 1:5.6, and if the lens maximum aperture is 1:2, the actual maximum aperture is equal to 1:1.4!). The angle of the focal lens is also changed: it remains the same than the one obtained for a 24 x 36 sensor. No need then to multiply by a scale factor to get the 24 x 36mm equivalence. If full frame lenses are used with a micro four third sensor, aperture is increased by 2 stops.

Manufacturers use to suggest that, when using focal reducer, the resulting image quality is better than without focal reducer because of the bigger image concentrated on a smaller surface. Nevertheless, there is extra glass... that might affect image quality.

Let's do some experiments before concluding. I am going to use a Fujifilm X-T2 camera with an APS-C sensor and 2 full frame lenses: an ukrainian Helios 44-2 1:2 focal length 58mm, with a Zhongyi Lens Turbo II M42-FX, and a (excellent) Micro-Nikkor AI-S 1:2.8 focal 55mm with a Pixco NG-FX focal reducer. The Zhongyi is famous although the Metabones speedbooster is more: here, they are compared. The conclusion is that they are closed but with a little advantage in the image quality for the Metabones one. However the price had discouraged me from buying it (from 300 to 500€).

I used a tripod and took the following shots with lamp on (yellow square) and off in order to appreciate the resulting flare. Then, I focused on the upper left corner.

• Helios 44-2 1:2 58mm with a Zhongyi Lens Turbo II M42-FX

The picture excerpts have been ranked according to image quality from left to right and top to down. 2 apertures have been tested: 1:5.6 and 1:2.0, without focal reducer ("helios-xx") and with ("helios-red-xx"). Yellow square means the lamp was on.

The ranking is:

1. 1:5.6 no lamp

2. 1:5.6 lamp

3. 1:5.6 no lamp with focal reducer

4. 1:5.6 lamp with focal reducer

5. 1:2.0 no lamp

6. 1:2.0 lamp

7. 1:2.0 no lamp with focal reducer

8. 1:2.0 lamp with focal reducer

The Zhongyi Lens Turbo II M42-FX decreases the image quality and increases the sensitivity to flare. The loss of quality is more important at large aperture. However, it may be argue that the comparison is not fair because one stop should be removed for a fair comparison. Indeed, 1:8 → 1:5.6 with focale reducer and 1:2.8 → 1:2.0. Let's see the results (lamp off).

The ranking is now:

1. 1:5.6

2. 1:8.0 with focal reducer

3. 1:5.6 with focal reducer

4. 1:2.0

5. 1:2:8 with focal reducer

6. 1:2:0 with focal reducer

Increasing by one stop improves slightly the image quality must not suffiently to reach the image quality without focal reducer.

• Micro-Nikkor AI-S 1:2.8 55mm with a Pixco NG-FX focal reducer

Same experiment that before but testing at aperture 1:5.6 and 1:8 (right hand side is with lamp on, and off on the other side, top is without reducer and down with).

Here also the focal reducer decreases the image quality but it is not sensitive to flare.

• upper left corner, lamp on: 1:5.6, higher image quality

• upper right corner, lamp off: 1:5.6, higher image quality (no change)

• lower left corner, focal reducer and lamp on: 1:8.0 (eq. 1:5.6), lower image quality

• lower right corner, focal reducer and lamp off: 1:8.0 (eq. 1:5.6), lower image quality (no change)

Maybe the extremely expensive Metabones focal reducers are better but does it worth to pay such a price? Although the decrease in image quality, focal reducers behave properly when shutting down the aperture and, together with a simple mount adapter, you can get 2 focal length with an old lens.