# Scale

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With Sony's recent announcement of the 61MP A7RIV, there has been much discussion of the impact those megapixels will have on a photographer's images. Invariably, one or more folks will say something to the effect of, "I don't need all those megapixels," and this will be the catalyst for an exchange on the subject of how many megapixels a photographer does need. Similar exchanges on the subjects of dynamic range, burst rate and other hotly debated camera specs often ensue.

Reading the threads, I am reminded of the discussion amongst amateur astronomers about aperture. As photographers know, aperture determines the light-gathering ability of an optic. In amateur astronomy, particularly amongst visual observers, the magnitude scale is used as a reference for the ability of a telescope to capture light and allow an observer to see objects of a certain brightness.

For example, a 60mm aperture telescope under a clear, dark sky may be described as allowing an observer to see stars as faint as about 12th magnitude. Each difference of 1 magnitude corresponds to a 2.512x change in brightness. As the surface area of an optic changes according to the square of its radius, we can apply the square root of 2.512 to determine the change in size of an optic required to produce a full-magnitude change in light-gathering. If a 60mm aperture has a stellar limiting magnitude of 12 for a visual observer under a dark sky, a 95mm aperture (1.585 is the square root of 2.512 and 1.585x60=95.1) will have a limiting magnitude of 13 for the same observer under the same sky.

If we simplify things a bit and say a 60% increase in aperture should allow an observer see objects one magnitude fainter, that 60% increase can serve as a benchmark for identifying a significant gain in performance between two telescopes. While there is no universal agreement among amateur astronomers on the minimum change in aperture that constitutes a "significant" change in performance, the is almost universal agreement that going a full magnitude deeper is significant.

At relatively small apertures, a full magnitude gain in light-gathering can be attained at a comparatively modest cost, both in terms of dollars and the physical size of the instrument:

• Naked eye (approx. 6mm) = 7th magnitude, FREE
• 60mm = 12th mag., about \$60
• ~95mm = 13th mag., about \$100
• ~150mm (6") = 14th mag., about \$300

However, beyond small aperture, entry level telescopes, the cost to attain an aperture delivering a full-magnitude gain can pretty quickly become quite significant:

• ~250mm (10") = 15th mag., about \$700
• ~400mm (16") = 16th mag., \$2,000+
• ~640mm (25") = 17th mag., \$15,000+

Depending on the optical design, precision of the figure of the primary optic, mount & drive system and quality of overall construction, one can easily pay \$10K or more for even a modest size "amateur" telescope. That acknowledged, if we limit ourselves to the "Fords" and "Chevys" of the amateur telescope universe, the cost in both price and size to go just two full magnitudes deeper from a garden variety 6-inch Dobsonian reflector is substantial. To achieve a similar gain in light-gathering from an 8-inch Dob to a 20-inch Dob, is at least an order of magnitude...high four-figures to low five-figures in price.

In the world of photography, what would you propose as a change in resolution (how many megapixels) that constitutes what most would reasonably agree to be, significant: a change in resolution yielding obvious results? What cameras would you use as examples illustrating the steps on a scale measuring changes of that nature? Are there any other performance areas (e.g. dynamic range?) which can be used to illustrate similarly significant changes in performance; changes also yielding obvious results?

I raise these questions because, as "large" as 61 megapixels sounds, the comparisons I've seen of photos made with the A7RIV, D850, A7RIII and 5Dsr show differences in performance but only at the pixel peeping level. Looking at full-size photos on a 24" computer monitor at screen resolutions, I'd be hard pressed to tell them apart. There is certainly no difference in resolution I would describe as, "obviously better."

What do you think? What's a scale delineating resolution changes that make an obvious difference in image quality? How about dynamic range or other criteria?

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Bill Ferris Photography
Flagstaff, AZ
http://www.billferris.photoshelter.com

Bill Ferris's gear list:Bill Ferris's gear list
Nikon D610 Fujifilm X-T20 Nikon AF-S Nikkor 16-35mm F4G ED VR Nikon AF-S Nikkor 70-300mm f/4.5-5.6G VR Tamron SP 24-70mm F2.8 Di VC USD +3 more
Canon EOS 5DS Canon EOS 5DS R Nikon D850 Sony a7 Sony a7R
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