Hey warp, found some super interesting stuff last night about magnification and sensor size. Turns out that a smaller sensor doesn't give a better DOF is all things are constant, it's other reciprocals that change to seem like this. I can't find the link as it was last night but I did copy the text.
It's from that stacking guy I linked to already, turns out he teaches algebra.
"I agree. There's been a lot written about sensor size, both as postings in our forum and as some long web pages appearing on other sites. But the information is scattered around, most of it's hard to read and interpret correctly, and there's a fair smattering of outright errors.
Here is my version of the short story, for your consideration and review.
Sensor size does not affect image quality if you're talking about equivalent images. That means same illumination, same camera position, same field size, same exposure time, and same DOF. That is, the pictures look the same even if the subject is moving.
The larger sensor allows to collect more light, giving less noise, but only by changing to a non-equivalent image by using a wider aperture, exposing longer, or using brighter illumination. The larger sensor also requires the use of longer lenses (to get the same field size at the same camera position), which allows the use of a larger diameter aperture to reduce DOF and increase sharpness.
On the other hand, the smaller sensor naturally comes with a shorter lens, which allows to easily get in closer, which gives more of a "wide-angle macro" appearance and also works better with auto-focus.
Which one works better depends on what you're doing. If you have time to set up and can afford either a longer exposure or brighter light, then you can get a quieter picture at same DOF from the larger sensor. If the smaller sensor gives you too much DOF even when it's wide open, then you need the larger sensor with its longer/wider lenses. If you need to work fast and easy, and the DOF and noise of the smaller sensor are acceptable, then the smaller camera is better.
There are other issues such as ability to change lenses and ability to shoot through eyepieces.Perhaps the most confusing aspect is that to get equivalent images, different sensor sizes require different settings for ISO and f-number. The smaller camera's f/8 is roughly equivalent to the larger camera's f/22, while the smaller camera's ISO 64 roughly matches the larger camera's ISO 400. This confusion often leads to false hopes that the larger camera will give more DOF because it provides bigger f-numbers. It won't."
and Precarious, finally I have found an explanation of the halos or radial smearing and it's perspective change from the start and end of the stack due to the movement of the pupil in relation to the sensor and the subject/focus plane. It moves on to describe rigs that would minimize this involving telecentric lenses and finally the theory clicked with what I understood about the airy disc and mechanics of why the DOF changes with aperture.
Again, sorry for not linking but it's too juicy not to post, trying to just post the essential nuggets pertinent what we discussed already.
"To avoid radial smearing, all images in the stack have to be converted to a common scale even in the out-of-focus areas. Consider a detail that is equally sharp in two adjacent frames. You're showing a change in magnification of roughly 0.4% (0.004) from frame to frame. If that detail is at frame center, no problem. But if it's at frame edge, say 1000 pixels out, then it'll move 4 pixels from one frame to the next.
The scale change can be reduced by using a longer lens. It can be reduced all the way to zero by using strange optics called "telecentric on the object side". That's a useful technique for stack-and-stitch with deep subjects (example HERE), but otherwise it's not worth the trouble. Stacking forwards versus backwards is a minor difference in most cases. Generally it is safest to start with whichever end has the smallest field of view, so that all the other images will cover the whole area when they get rescaled. If you start with the end that has the largest field of view, then the others only cover part of the area when they get rescaled. That produces streaky margins, which of course can be cropped off, but in rare cases it can also degrade the accuracy of alignment.
It is not a good idea to have frames out of sequence. Most of the software packages work by aligning frames 1 and 2, then 2 and 3, then 3 and 4, and so on. This works well when sequential frames are very similar. But if say frames 2 and 4 were adjacent focus planes, while frame 3 is very much different, then it can happen that this stepwise alignment leaves 2 and 4 not fitted well with each other, resulting in echos or streaks.
Sometimes it works, sometimes it doesn't."
