Fri Jan 15, 2021 9:48 am
EDITED AND CORRECTED.
The Speedbooster appropriate to your camera would have been the best bet. The position of the optical cell in the Speedbooster body has to be absolutely spot-on or the results may be less than desirable.
The sharpness result from the 0.71x optical cell should not be bad but as your small sensor is still seeing through the centre of the lens/speedbooster combination, any softness in the image may be greater than the image yield by the correct Speedbooster for your camera.
Another responder mentions having have tested three separate Speedboosters and had the same poor result. The chances of three different speedboosters having been mismanaged by a previous owner or gone faulty are pretty low.
If it does not work for you then yes, it is probably best to walk away but I would not give up on it so soon. There are a few variables which may interact additively to cause a problem.
The speedbooster optical cell may be incorrectly adjusted. This is highly likely if yours was bought used and adjustment trimmed to a particular camera/lens combination.
Some stills-camera zoom lenses are not and never can be parfocal. They will never hold focus through the full range of the zoom movement.
The lens may no longer be correctly collimated (some call it backfocused) if mount tails have been swapped and maybe shims lost or a bit of grit or swarf gets in between the faces. This may not be the case with your zoom lens. Do you know its history from new?
If you had access to several different speedboosters to test and the result remains poor, then this following comment will be irrelevent.
The optical cell within the 0.71x speedbooster can be inadvertantly reversed and still work but with poor results. I am prompted to this thought by descriptions of speedbooster cells being "rotated". The backfocus is adjusted by screwing the cell in and out of the body of the speedbooster, ie "rotating".
If however "rotating" has been interpreted as "reversing" the orientation of the optical cell within the speedbooster body by unscrewing it out completely then installing it back-to-front, then it is going to work poorly.
If the optical cell is correctly screwed into the speedbooster body, a small serial number printed on the optical cell should be closest to the sensor, not to the lens tail.
To most successfully adjust the optical cell, use the lens on which you know the focus marks to be accurate.
Position your camera so that the focal plane (sensor) is an accurately measured distance from a focus chart or target. There should be a focal plane mark on the camera body somewhere.
Set your lens focus to that measured distance. I use the 1 metre or 1.5 metre marks.
Light the focus chart or target and set your camera shutter speed so that the lens iris has to be wide-open and the lens depth-of-field as shallow as it can be.
If the lens barrel has to be turned for sharp focus and the focus number in the barrel reads higher (closer) than the number you selected as the real distance, you may have to screw the speedbooster optical cell inwards (clockwise). This direction is when you view the speedbooster from the front.
If the focus number is lower, then you may have to screw the optical cell outwards (anti-clockwise)
Screw the optical cell inwards or outwards only in one-quarter turn increments to find the spot where the lens image is in sharp focus at the measured distance between the focus chart or target and the camera focal plane mark.
Use the largest and sharpest screen monitor you can find. A Siemens-style (slices-of-pie) focus chart is the easiest to use. If you only have a resolution bar chart like a Lemac card, then the camera's tendency towards aliasing may be your best friend as sharpfocus will really pop as a colour shimmer. That's all well and good so long as you do not have an anti-aliasing filter installed.
For an alternative focus target, you can use a sharply defined distant outdoors object such as a power pylon with your lens set to infinity focus, your very widest lens such as a 14mm f2.8 with known good focus marks and enough ND filtering selected to force the depth-of-field to its narrowest with a wide-open iris. This is not as easily accurate but doable.
If the backfocus is really off, take care when adjusting, and avoid screwing the optical cell too far rearward or you may contact the IR filter glass in the camera and crack it. The 0.71x optical cell rests furthur forward than in other speedboosters with a more powerful focal reduction so chances of cracking the IR filter are lesser but still possible if it has already been so badly adjusted you don't know which way is what.
With some speedboosters, the optical cell rocks slightly in the speedbooster body when a small lockscrew is tightened after adjustment. this lockscrew pushes on the cell fro the side at the sensor-end and may skew it ever so slightly and throw focus off on one side-edge.
My personal preference after finding correct backfocus is not to use the lockscrew but to add a small dob of water-cleanable white bathroom sealer on the face of the body across onto the thread.
This material can be easily picked away with a sharp pointer like a dart-end or a tiny screwdriver but holds well enough. Do not use silicone bathroom sealer (smelly stuff), threadlocker or any sort of glue which will wick in along the threads and lock the cell in for good.
I will check the method as it happens that I need to readjust the optical cell in the "Big" URSA after cleaning. If necessary I will correct this post. Meanwhile. please heed the good advice of others smarter and more competent than I who respond here.