A good way to think about sensor sizes

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Andy873 Regular Member • Posts: 325
A good way to think about sensor sizes

I know not everybody understands equivalence, not even here, but no time to talk about that, will just assume everybody does.

So for example if we think a FF sensor with f/4 lens as having "light gather power" 1

then FF sensor with f/2.8 lens would have power 2

APS-C Sensor with f/2.8 lens would have power 1

M43 with f/2 lens would have power 1

m43 with f/4 lens would have power 1/4

Modern smartphone: 1/16

So now consider the standard FF sensor with f/2.8 zoom. That's a power level 2 combo with a 3x zoom range. We won't debate if this power 2 is "enough" image quality here, that's not the point, the point is, assuming you need this power 2, then FF is your best choice. Because even if there exists a 16-50mm f/2 lens for APS-C, it would be insanely expensive, large and heavy.

On the other hand, suppose we have another camera with MF sensor (0.7x crop) and 34-100mm f/4 lens. it would also have power 2, but the lens wouldn't be significantly cheaper than a 24-70 f/2.8, with the sensor being a lot more expensive.

So really, FF + f/2.8 is the most cost-optimal way to get a power 2 camera with 3x zoom. Obviously if we can to start from scratch maybe we can find something slightly different that's even more optimal, maybe a sensor that's 42mm across and f/3.5 is the most cost optimal, but we can't do that so we're stuck with that we have.

As you go down in power, the optimal combination of lens speed/sensor size changes. So for example APS-C with f/4 lens is probably the most optimal for power 1/2. Since F/4 zoom lenses aren't that expensive, and f/5.6 zoom won't be much cheaper, so you might as well save some money on the sensor. As for m43, well they'd be looking at f/2.8 zoom lens and those aren't cheap at all.

It's a similar situation with primes but the scales are shifted because primes are cheaper to make. You can get a FF with f/2 for a power of 4, there are certainly desperate attempts to try and match it with APS-C but I'm afraid they just won't make much money from a reasonably priced f/1 lens.

In practice, most people have a combo that makes the most sense, because most people are naturally seeking to be most cost-optimal. That means ff with 24-70 f/2.8 or 24-105 f/2.8. APS-C with 16-70/80/105 f/4.

The new a7c will be an interesting case, the new lens is more like an 18-45 f/4 so basically kit lens on aps-c but it's a much more expensive combo. It's an FF with just power 1/2. Where as for much less money I can get an APS-C with power 1/2 but more zoom range. I wonder if the fool frame marketing will be enough to sway the objective value disadvantage. I mean yea you can put a GM on your a7c but let's be honest a lot of people won't go much further than a cheap 50/1.8 beyond the kit lens. Speaking of which, you do get power 4 out of your 50mm, rather than power 2 like on aps-c, maybe that will make it worth it.

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