DPReview:What is equivalence and why should I care?

I was arguing something along these lines other day. FF thoeretically is no better than M43. In reality, though it's far more flexible because M43 manufacturers don't make lenses to match the light gathering properties and focal lengths of those available on FF.

e.g To match the noise, angle of view and depth of field of a 50mm f1.4 on FF at 100iso would require the M43 camera to shoot at 50iso with a 25mm f0.7 lens.
 
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englishpremier said:
I was arguing something along these lines other day. FF thoeretically is no better than M43. In reality, though it's far more flexible because M43 manufacturers don't make lenses to match the light gathering properties and focal lengths of those available on FF.

e.g To match the noise, angle of view and depth of field of a 50mm f1.4 on FF at 100iso would require the M43 camera to shoot at 50iso with a 25mm f0.7 lens.
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I also argue this point all the time, especially when people change from a Canon crop with 17-55mm f/2.8 IS to a canon FF with 24-105mm f/4.0 IS. Why? And I see people doing worse, e.g. Nikon D600/610 with 24-85mm f/5.6, just what is the point?

With the Sigma 18-35mm f/1.8 equivalence is even easier to reach, not using a 24-70mm f/2.8 or fast primes on FF, then why not simply use an APS-C sensor and save a load of cash to put on better glass. And you can get a load of additional benefits staying on crop gear, e.g. the Nikon D7100 has a better focus system than the D600/610 and even compared to the D800/D4/5Dmk3/1Dx you have the advantage of the focus points covering almost all of the frame.



As you point out, once you get to fast prime then it gets harder to achieve, although even then that is mostly only on the wide to normal end. Shooting a 24mm prime at f/1.4 on FF getting close to your subject- you can't replicate that on a smaller sensored camera.


The other way people get caught out is some of the high end point and shoots with faster glass can give a shallower DoF and better light gather capability than camera with much larger sensors. E.g. the Panasonic LX7 at the tele end has a faster equivalent aperture than the Sony RX100 with the much larger sensor and crop camera using kit lenses.
 
D.P. said:
I also argue this point all the time, especially when people change from a Canon crop with 17-55mm f/2.8 IS to a canon FF with 24-105mm f/4.0 IS. Why? And I see people doing worse, e.g. Nikon D600/610 with 24-85mm f/5.6, just what is the point?
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Resolving power and flexibility..

The only way equivalence is some be-all-end-all is if we make an assumption that at all points lenses always outresolve the sensor, which they very much don't given constantly rising MP counts and very few lenses able to keep up.

I might have seen a fair argument for equivalence in the way you posit it say 6 years ago when 12MP was the cap but in today's context there are plenty of reasons for larger formats even before low light performance.
 
Regarding the lens argument that is also making the assumption that larger sensors always have lower photo-detector densities than APS-C sensors,which is not always the case. When it comes to low light abilities,its the amount of light gathered by the lenses and the individual photo-detector area(but the back end parts of the chip,and other technology can complicate things),which are probably important determining factors IMHO.
 
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