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Context

So I've recently watched a video about how Tony Nortrup is referencing that you should buy the lens that perfectly fits your image circle, to get the most sharpness out of it, or so I'm understand. I've looked into focal reducers which seem to shrink the image circle so that it "fits" perfectly on a smaller sensor, yielding the correct image circle size.

A post here states that:

There are some other bold claims made by the manufacturers of the speed booster. In addition to increasing speed by a stop they also claim the resultant image is sharper, which goes against conventional wisdom.

Question

My question here is, what is the conventional wisdom stated above (my guess is putting anything between the sensor and lens degrades quality?) and in practice does the sharpness of the lens, get closer to something you would get from a lens on a "correct" body, or is the gains marginal and the only real benefit is shallower DoF, like an FF?

I realize again this is highly suggestive depending on the adapter in question and lens but to bring it into context I'm curious if it's worth using a focal reducer on a Nikon AF 300mm f4 on my Sony a6000 is worth it, or is it better spent saving the money to get a FF body to "match" the lens? I realize the AF performance might not be the best so I'll ignore that, for this discussion.

Reference

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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.

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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).

  • I don't mind manual focusing and from what is stated the Canon AF is slow, turtles or slower they said. I might try it if I can afford it. – unsignedzero Feb 12 '16 at 23:00
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My question here is, what is the conventional wisdom stated above

Looking at the post you referenced, author Matt Grum describes the conventional wisdom:

However, the statement "when you add elements to a lens, you make the image worse" is not true absolutely...

He goes on to explain that it is indeed possible to add elements that improve lens performance.

in practice does the sharpness of the lens, get closer to something you would get from a lens on a "correct" body, or is the gains marginal and the only real benefit is shallower DoF, like an FF?

It's impossible to say in the abstract. There are several reasons that sharpness could increase:

  • more light on the sensor lets you use a shorter exposure, so less chance of motion blur

  • more light lets you use a smaller aperture, so you can shoot at the aperture where the lens performs best (say, f/8) instead of shooting wide open where the lens might be a bit soft

  • compressing the image to a smaller size might also make the image look sharper simply because the details are smaller

That last point is the one you're probably asking about, and it's also the one where it's really hard to know exactly what would happen. The additional element could also reduce sharpness, depending on how well it's made, how well it works with whatever lens you're using, etc.

I'm curious if it's worth using a focal reducer on a Nikon AF 300mm f4 on my Sony a6000 is worth it

This is almost certainly one of those cases where it's worth a try. It's getting harder and harder to find a good local camera store, but it's also exactly the reason to support the local guys instead of buying online, as you can walk into a store with your gear and inside 15 minutes have a pretty good idea of which choice will work better for you.

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I'm curious if it's worth using a focal reducer on a Nikon AF 300mm f4 on my Sony a6000 is worth it, or is it better spent saving the money to get a FF body to "match" the lens?

Why can't both be true? You linked to Roger Cicala's blog post about the Metabones at LensRentals.com – why not go ahead and rent one, and a FF body as well, to test them out? Then you can put them through the paces and see what you like and dislike about the formats, focusing speed, etc. You would have gained valuable experience and knowledge, all without the sunk cost of a FF body you might not like or want.

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