Hi Simon,
actually I wanted to write down a week ago already a comparison between these lenses, but have not had the time yet. I will probably move this conversation to the Zeiss lens section later this week.
Yes, the N70-200 is optically superior vs. the N70-300 and the same is valid for the N28-80 vs. the N24-85. The differences are in certain areas bigger then in others. I was discussing this last December with Zeiss in Oberkochen before I bought my lenses, and I guess they know what they are talking about. But you can see it also in the MTF charts and in real life photography, if you use both side by side. But I have to aknowledge, that the differences are only slightly - but visible.
Before I go into detail, one comment on the different prices and glasses. The use of certain modern glasses does not guarantee by itself to achieve optical improvements. Depending on the zoom-range of the lenses, the more complicated and more expensive lens design is already needed to be on par in image quality with other zooms, which has a narrower zoom-range. BUT The major reason for the more expensive lenses N24-85 and N70-300 is according to Zeiss Germany the implementation of the Dual Focus mechanismen, what produces a lot more problems in design and production of these lenses, therefore the higher price.
I made testshots (handheld, fuji velvia, Nx&N1) side by side with all 4 zooms and saw differences with a 4x loup on the light table. But to be more objective let us look at the MTF data (see the download section), which are giving a quiet good impression where is what better. The differences, that I am referring to are visible, if you compare both lenses to each other directly side by side, which is not really how people make pictures in general
You will never regret to use a N70-300 or N24-85, the optics are just great.
1. N70-200 vs. N70-300.
Every lens design follows certain task which shall be achieved with this lens. The N70-200 was designed for the best results in the center of the image. The N70-300 was designed with a more consistent performance til the corners of the image, above all til 15mm. The N70-200 outperforms the N70-300 in the center of the image til ca. 8-10mm, depending on the Zoom range significantly. Not at 10 lp/mm, but at 20 lp/mm and at 40 lp/mm.
If you look at the different curves, the higher is not always the better. As stated in the Zeiss article available in this Forum, there are some "rule of thumbs" at what point you can really see the difference in image quality. Depending on the lp/mm there are different percentages to look at. For the 10 lp/mm you only need an improvement by 5% and you will see the quality improvement in the final picture, for the 20 lp/mm you need more differences (10%) so that it is visible for the human eye in your picture and for the 40 lp/mm you even need 20% improvement to be able to detect it in in the picture.
As described above you can sse in the center of the image the differences. After 15mm from the image center the 70-300 is better then the N70-200. This is because of the different aim in this lens-design. Distortion and Vignetting is with both zooms more or less the same. I find the quality of the N70-300 outstanding if you think at the wider zoom range and the implementation of the Dual Focus mechanism. It just weights to much (1070g) for the aperture 4.0-5.6, compared to 620g for aperture 3.5-4.5 of the N70-200.
2. N28-80 vs. N24-85
This is more complicated. Depending on what is important to you, there can be different "winners". Additionall we do not have the MTF data at the exact same zoom-range, which makes it actually unfair to compare only by this. The N24-85 has at 24mm an outstanding correction of the distortion. The N28-80 has a bigger distortion by numbers (!) at 28mm then the other at 24mm. As you can read in my other article about common pitfalls in interpreting MTF data, you will see, that this is not comparable in reality like this.
At 50mm distortion is almost at zero with the 28-80, with the 24-85 slightly more then +1 at 20mm height, which is totally o.k. At 80mm/85mm the 28-80 has again an advantage with same center image quality but a more consistent line towards the corners. In general the height of the curves is almost the same throughout the picture with both lenses. The main difference is the gap between sag. and tan. lines for each lp/mm. To describe it very basic the closer they are, the better the image quality.
For vignetting there is basically no difference between the two lenses. So both lenses are excellent and again for the wider zoom range of the 24-85 and the dual focus mechanism an outstanding result.
I know this sound all very technical, but this is the only way to compare in a objective way the differences.
Dirk