The Optec MaxField is designed for f/10 Schmidt Cassegrain Telescopes (SCTs) and will shorten the effective focal length to 33% of the starting value - hence it yields a f/3.3 system. It was not designed for other optical systems, and I have read reports that it works very poorly, if at all, with anything other than a f/10 SCT.
In my case, I am using the MaxField with an 8" Meade SCT of focal length 2000 mm, so the nominal reduced focal length should come in at about 660 mm. I measured it at 676.6 mm (by calibrating star fields in JIMSAIP. The measured pixel resolution came out to be 2.72 arcseconds per pixel which is larger than the suggested 2 arcseconds per pixel, but the results have been quite satisfactory. The field of view in the SBIG ST-7 is also 35x23 arcminutes - quite an improvement over the previous 17x11 arcminutes with an Orion focal reducer that yielded f/7 (1400 mm efl). For asteroid hunting, that is four times the sky coverage compared to the Orion focal reducer. With the Orion unit, I was using the 2x2 binned mode of the ST-7 which yielded a pixel resolution of 2.64 arcseconds/pixel, so the resolution didn't change much. Of course, the 4x improvement in sky coverage at the same (roughly) pixel resolution means image files four times larger which means more downloading time, more hard disk requirements and longer processing times. If I discover new asteroids at a 4x greater rate, though, it will be well worth it!
Another benefit of the MaxField compared to the Orion unit derives from the fact that the MaxField was designed to work with the extended spectral response of CCD chips. The MaxField produces much better focussed star images than the Orion unit - a fact I attribute to better color correction. That is not to criticize the Orion unit - to my knowledge it was never intended for CCD use. My star images are nice and round now where they hadn't been so before.
A third benefit is the shorter effective focal length makes guiding easier - which also contributes to rounder star images.
Lastly, but not least, the unit is well-made. I purchased the ST-7 adapter to be sure I wouldn't have any mechanical problems. It is reported that the spacing from the MaxField to the CCD chip face is critical and, as best I can tell, with the supplied adapter everything works as designed. The MaxField has a heavily built unit that screws into the telescope back and seats firmly. A second piece slides into the first one is aligned by a pin and held in place by thumbscrews that seat into a v-shaped groove. It appears to be very repeatable if, for example, one wanted to change filters (the MaxField accepts 48 mm filters). A separate adjustment, held by allen set screws allows you to rotate the camera for orienting the field of view. I like to orient the camera with the long axis parallel to E-W. It keeps my sky directions constant on my images and also helps with the self-guiding feature of the ST-7.
It has been reported that the color correction of the MaxField is not as good in the blue end, so a yellow filter is recommended. I purchased a No. 12 yellow filter (also a No. 21a red) but haven't had enough time yet to see what the practical effects/benefits are. I will update this part when I have more information.
(April 1998) I've now had a whole season with the MaxField and my enthusiasm for it is still high. I've learned that a flat field correction is a MUST. There appears to about a 5% variation in the field from center to corners and this shows up quite readily. In fact, it can be a problem when taking focus images with the ST-7 using the auto range function if there is a bright sky (as in moonlight or light pollution). A flat field correction takes out the variation nicely, however.
Regarding the filters, I did a little experimentation and found that the yellow filter causes a slight loss in limiting magnitude and the red filter a large loss of limiting magnitude, at least as far as asteroids are concerned. Asteroid light is, after all, the same as moonlight, ie, reflected sunlight. For deep sky objects, the red filter really cuts the background light (especially moonlight) without apparently hurting the deep sky image. I didn't do any quantitative or exhastive evaluations, but it appears to me that a red filter is a good idea for deep sky and stellar photography. I couldn't tell any difference in image sharpness (but I didn't look really closely) so I ended up not using any filter for asteroid work. All-in-all though, I am very happy with what I have seen so far and can give the MaxField my endorsement.