Ming Thein - Bigger isn’t always better, or why you can’t see the difference most of the time

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Chris Dodkin
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Ming Thein - Bigger isn’t always better, or why you can’t see the difference most of the time
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Interesting blog entry from Ming Thein - more via the link

Bigger isn’t always better, or why you can’t see the difference most of the time

Ming Thein

  1. Larger sensors will require larger lenses for a given aperture and angle of view. The tradeoff is that you compromise aperture, which reduces physical iris opening requirements, the degree of optical correction required, and thus the amount of glass – this brings relative overall size down. But, your depth of field is effectively the same as the faster lens on the smaller format. And since you’re gathering less intensity of light, this has to be compensated for by the larger, more sensitive pixels. Net result in practice? 54x40mm, 50mm 1/100s f4 ISO 3200, or M4/3, 14mm 1/25s f1.4 ISO 100. These are practical numbers, taking into account a) minimum shutter speed to avoid camera shake; b) stabilisers; c) DOF. An ISO 3200 file from a full 645 100MP sensor is great, but then again, M4/3 files at ISO 100 are pretty clean, too. Which brings us to the next elephant:
  2. Real resolving power doesn’t climb linearly with pixel count; not even by area. Why? Because of the relationship between angle of view, real focal length and sensor size, and then depth of field, aperture and diffraction. It means that if you can get everything in focus in the previous scenario at say f2.8 on M4/3, you’re going to need f8-f11+ on 54×40. And beyond that, you start to actually lose resolving power at the focal plane as you stop down thanks to diffraction.
  3. Most of the time, stabilisers and faster apertures compensate for smaller sensors. One stop of light is one stop of light: if you gain two stops on the stabiliser, and another two on your lens, that’s four stops: or the difference between ISO 100 and ISO 1600.
  4. Newer sensors are good, but even the best of them are linear. This means that for every stop increase in sensitivity, you lose a stop of dynamic range. Your M4/3 sensor with 12 stops at ISO 100 will still have 12 stops in this situation; 54×40 with 15 stops has lost four or five of them: it’s a wash, or worse.
  5. Bigger is harder to make. This is a practical manufacturing limitation: larger sensors require more power to move and stabilise, and inertia means they won’t compensate as fast; the larger the sensor, the less effective IBIS seems to be given the power constraints in a compact body, and further compounded by size meaning resolution requiring more precise control control of a larger mass than a smaller sensor. Larger lenses require higher tolerances across much larger elements, plus mechanical strain on helicoids and the like.
  6. Higher resolution requires more precise AF. Since your effective DOF decreases proportional to the sensor’s circle of confusion, the actual DOF plane gets thinner and thinner as resolution increases (independently of sensor size): you need more precise mechanisms to focus the lens such that the focal plane and the sensor plane overlap perfectly, and the maximum resolving power is achieved. Moving things in decreasingly smaller increments is harder.
  7. The final kicker: more pixels aren’t even always visible. Assuming critical focus is achieved, you only see the difference in resolving power at the subject plane: the OOF areas do not require high resolving power even for decent transitions (just good color/luminosity bit depth). So if you’re a shallow DOF shooter, your gains from sensor size resolution, but access to lenses.

If you have sufficient light that ISO doesn’t have to be raised above base to achieve a hand-holdable shutter speed, you will see an increase in image quality proportional to sensor size providing depth of field is sufficient, and the technical factors (like focus accuracy) aren’t an issue.

Whilst earlier cameras were not very demanding on shot discipline and minimum shutter speeds – I remember the 12MP D700 being good down to 1/0.5x shutter speed, or about 1/40s-1/50s at 85mm – you’re deluded if you think that’s going to work with a 50MP body.

Correction: you’ll get 12MP of real resolution, because the limitation on resolving power is now your handshake or rate of instability. Raise the shutter speed to compensate (i.e. decrease the displacement of shake at the sensor plane to below the half a pixel width) and you soon find yourself having to raise ISO.

Practically, I need 1/200-1/250s with the 100mm and H6D-100c to consistently achieve 100MP worth of resolving power. Less than that still makes an image, and depending on how far you downsize – the results may still look okay. But 100 imperfect MP downsized to 20 decent ones may not view or print as well as 20 perfect ones; this is a function of visual acuity and how our eyes interpret hard edges and resolution.

In short: the absolute output quality is limited because we either hit the limits faster if we want to make the most of the resolution (thus limiting the shooting envelope).

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Your time is limited, so don't waste it arguing about camera features - go out and capture memories - Oh, and size does matter - shoot MF

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