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Prompted by my own answer to "Photographs cannot be taken" with manual lens and wireless triggers with off-camera flash & in reference to How do I take the right exposure quickly with Nikon D5300 and old AI-S Lenses? & others...

Is there a good reason that my camera, a D5500, can't use the exposure meter with a manual lens attached?
Of course, it cannot read any data from the lens without the appropriate connections, but why can it not just measure what it can see?
It can focus based only on 'what it can see' so why not also measure the light?

Does this also apply to higher-end bodies, or is it true for any manual lens on any body?

Tetsujin
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Is there a good reason that my camera, a D5500, can't use the exposure meter with a manual lens attached?

Not really. All Canon dSLR bodies are capable of stop-down metering when they fail to sense electronic communication from a lens and the higher-end prosumer Nikon bodies can do accurate metering with an adapted lens. But the D3x00 and D5x00 bodies have metering systems that are only set up to perform wide-open metering. And wide-open metering does have the benefit of giving you the most light to compose/see by when using the camera, regardless of the aperture you've set.

... why can it not just measure what it can see?

Actually, the problem isn't that it's not measuring what it can see; it's that it's making the assumption the lens is wide open while doing so. If you are using any aperture setting smaller than the maximum aperture, then the metering system has to compensate for that. And Nikon simply didn't program that into their entry-level bodies.

Does this also apply to higher-end bodies, or is it true for any manual lens on any body?

No, it only applies to the entry-level bodies. The prosumer models are capable of accurate metering with non-CPU lenses, and some can be programmed to know the focal length and max aperture of such lenses.

inkista
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As the body has no information from the lens about the diaphragm, it can't set time and ISO. Even you setting an aperture won't work, as the measurement is done supposedly with the lens wide open, and while you know the maximum aperture of the mounted lens, the body doesn't.

Focussing is different, as the body can reliably decide if the focus point is reached, no information from the lens is needed.

remco
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  • I'm in manual - I'm setting time, ISO & aperture. It doesn't answer my question of why the body cannot just measure the available light falling on the sensor. – Tetsujin Apr 19 '18 at 14:05
  • It probably can, but might have no way to report it decently – remco Apr 19 '18 at 14:19
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Of course, it cannot read any data from the lens without the appropriate connections, but why can it not just measure what it can see?

It can measure what it can see. But with no way to communicate with the lens it has no way of knowing how many stops difference there is between what it is seeing with the lens wide open, or even if the lens is currently wide open or already stopped down, and what the lens will be stopped down to when the camera's aperture lever pushes against the lens' aperture lever. With the D3x00 and D5x00 series cameras, Nikon has never seen fit to give the user a way to provide that information to the camera.

Michael C
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