Chris Malcolm
Senior Member
If "reach" in this context is to be angle of view, then a changed sensor size changes angle of view in exactly the same way as does increasing the focal lenghth, or cropping the image down. Some cameras have an optional "digital zoom" facility where they'll crop down in camera. I've seen lots of arguments over the years about whether this actually constitutes a "real" change of reach or a fake one. Some cameras boast of being able not only to crop down in camera but then to interpolate up again to the full sensor size MP. They call this improved digital zoom things like "super zoom" or "smart zoom". More people seem willing to concede that this kind of "smart zoom" is a "real" change of "reach" as compared to plain vanilla digital zoom. Yet cropping with good interpolation back to the original MP in post processing on your computer, although identical in its effect on the image to an in-camera "smart zoom", is much more rarely regarded as a change of reach.
It's clear that the concept of long lens "reach" is confused and used in a variety of different ways. Since it's simply a vague popular metaphor with nothing in the meaning of the word to allow its definition to be tightened up to one particular unambiguous meaning the solution to the problem won't ever be found by arguing about the term. New more precise terms are needed.
I have a 500mm full frame lens. I use it to photograph distant small things which are often too small to fill the image. such as a small bird or the moon. I'm interested in getting the best detail resolution, such as feather detail or the craters on the moon. Do I get more detail by using the lens on an APS-C DSLR? Not if the APS-C camera puts the same number of pixels on the bird (or moon). But if I add 50% to the number of pixels in my FF DSLR, making it say 36MP from 24MP, I'll put more pixels on the moon (or bird), just as many as putting the lens on a 24MP APS-C DSLR would put on it. If the air was perfectly clear, and the lens was perfect way beyond the practical optical limits of lens perfection, that would give me a 22% increase in linear feather detail or crater detail. But my lens isn't perfect, and the air is rarely perfectly clear, so in practice the slight increase in detail resolution by upping my FF sensor by 50% of MP (or switching to an APS-C camera of the same MP) will only rarely be visible in the best conditions on a few particularly critical parts of the image.
I'd very much like more "reach" in the sense of getting more feather detail in distant small birds, or more craters on the moon. Let's say I'd like to get twice as much detail. If the air and lens were ideally (and impossibly) perfect I'd need a 1,000mm lens to do that. So let's say I'd actually need a 1,500mm lens to double detail. It would also have to have the same aperture as my 500mm lens, which means it would have to be VERY big and VERY expensive. Forget handholding, I might need help just to lift the thing off the ground onto a tripod. Which is why astronomers, who use such lenses, often build them onto a fixed support.
On the other hand I could achieve the same increase in detail resolution without changing the lens or image perspective by simply increasing the camera MP. Because we measure detail resolution linearly that means I'd have to increase MP by 400%. Because the lens isn't perfect we'd need say an extra 50% to make up for that loss and make it a real doubling of resolution. That meas increasing MP by 600%. So if my FF DSLR was 24MP I'd want to upgrade to a 144MP sensor (or on APS-C, a 96MP sensor).
"Reach" and "equivalent focal lengths" tell me nothing of what I want to know when thinking about what upgrades would best increase the detail resolution I'm getting from my longest lenses.That's what people are usually interested in when they talk about needing more "reach". They wouldn't be interested in a lens of twice the focal length which gave no more detail resolution. What I'm interested in are angle of view and arc seconds per pixel. As someone has pointed out, these are the terms already used in the astronomical community.
I'm glad, however, that so many people like the term "reach". It helps to persuade lots of people to keep buying slightly longer lenses and more sensor MP, even though they won't be able to see any diffrence in detail resolution, and that helps to keep the research and development teams of the camera makers going in the direction of more detail resolution.
