Do focal reducers improve sharpness on APS-C, or just restore full-frame field of view?
Asked 2/10/2016
12 views
2 answers
0
I’m trying to understand whether a focal reducer (speed booster) can actually improve image quality when using a full-frame lens on an APS-C camera. I’ve seen claims that reducing the image circle to better match the smaller sensor can make the result sharper, but that seems to conflict with the usual idea that adding extra optics tends to reduce quality.
What is the “conventional wisdom” here? Is it simply that adding glass between lens and sensor usually makes the image worse, like a teleconverter often does?
In practice, does a focal reducer make a full-frame lens on APS-C perform more like it does on a full-frame body, or are any sharpness gains usually small, with the main benefit being a wider field of view and about one stop more light? For example, would using a focal reducer with a Nikon 300mm f/4 on a Sony a6000 be a reasonable alternative to moving to a full-frame body?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
3
A focal reducer doesn't follow the conventional wisdom that a teleconverter reduces image quality because it works in a completely opposite way from a teleconverter.
Are you familiar with why "crop factor" exists? A full frame ("FF35") lens projects a circle about 43mm in diameter into a camera. A FF35 sensor is 36x24mm and so records most of that circle. An APS-C sensor is only 24x16mm and so records a smaller portion of that that circle - the effect is the same as if you'd used a 36x24mm sensor and then cropped the image (thus the name). Illustration
Teleconverters and focal reducers work on a related principle. Teleconverters enlarge that projected circle to greater than 43mm so that even a 36x24mm sensor is only recording a small portion of it (thus giving a narrowed field of view, as if you'd cropped the image). Focal reducers do the opposte: shrink the projected image circle to less than 43mm so even a 24x16mm sensor is recording most of it.
The thing is, a lens has a finite amount of resolution. It can only record so much detail. Lets say a particular FF35 lens can record 43 units of detail, or X per millimeter across that 43mm circle. A teleconverter which doubles the size of the image circle "spreads that detail out" to only 1/2 X per mm, while a theoretically perfect focal reducer which halves the size of the image circle "squeezes that detail together" to 2 X per mm. The same can be said of the amount of light hitting the sensor, which is why a TC darkens your image and a FR brightens it.
--
There are too many factors to say whether you should get a new camera. One major thing to remember is that your Nikon will have no autofocus or auto-aperture using this lens on a SpeedBooster. The only ways to have an autofocus 300mm [equivalent] on your A6000 are superzooms, the FE 70-200/4 and the upcoming FE 70-200/2.8. The first option is slow aperture and generally very weak in performance, the second is oversized and said to be only pretty good, and the last is pricey and won't be out for a while. There's also the possibility of a Canon 300mm, as Canon retains AF with the SpeedBooster (though apparently it's very slow).
Originally by user48183. Source · Licensed CC BY-SA 4.0
user48183
10y ago
0
Generated from our catalog & community — verify before relying on it.
The “conventional wisdom” is basically: adding extra optical elements often risks reducing image quality. But that is not an absolute rule. As noted in the referenced discussion, added elements can sometimes improve performance.
A focal reducer is not like a teleconverter in effect. A teleconverter spreads the lens’s image over a larger area, while a focal reducer concentrates the image circle onto a smaller sensor. That restores a wider field of view and gives about one stop more light.
Whether sharpness improves depends on the specific lens and adapter, so there’s no universal answer. Any perceived sharpness gain may also come indirectly from the extra light, which can let you use a faster shutter speed or a different aperture, reducing blur from motion or suboptimal settings.
So yes, a focal reducer can make a full-frame lens on APS-C behave more like it would on full frame in framing and light transmission, but it won’t automatically equal a native “matched” setup in every respect. For a Nikon 300mm f/4 on a Sony a6000, the practical value depends on the exact adapter and your priorities. If you’re unsure, testing or renting both a focal reducer and a full-frame body is the safest way to decide.
Recommended products
UniqueBot
AI10y ago
Your Answer
Related Questions
Does a full-frame camera with a 1.4x teleconverter outperform an APS-C body without one?
Does a focal reducer make depth of field shallower on APS-C?
How do I match APS-C and full-frame field of view when choosing Sony FE primes?
What happens when you mount an APS-C lens on a full-frame camera?
Does Nikon’s 1.5x APS-C crop factor mean its sensor is meaningfully closer to full frame than Canon’s 1.6x?