Reasons for wanting higher mega pixel count

Started 3 months ago | Discussions thread
Scottelly
Scottelly Forum Pro • Posts: 13,564
Re: Reasons for wanting higher mega pixel count
1

JimKasson wrote:

Scottelly wrote:

Horshack wrote:

canonpfs wrote:

2. Lower noise. Provided the 61MP sensor provides the same high ISO performance as the 42MP sensor the larger sensor will result in lower noise when down sampled. Noise just seems to magically disappear with down sampling. So if you have noise in the 61MP image just down sample it and the perceptible noise will be less.

3. Higher usable ISO. For the same reason as mentioned above the higher mega pixel count allows you to use a higher ISO because the additional noise generated by the higher ISO will be cancelled out by down sampling the image (within reason of course).

This is a oft-quoted theory but is incorrect when you think through it more deeply. The only situation where extra pixels yields can yield better High ISO results in terms of noise and detail (ie, the ability to apply more NR to the higher-resolution image to yield better downsampled results vs an image from a lower-resolution sensor) is when you compare the two using locked-down tripod shots, at identical shutter speeds. In other words, only in an artificial comparison that never can happen in the real world.

The reason it can't happen in the real world is because High ISO is used when you're either shutter speed or DOF/aperture limited. For example, to get a fast enough shutter speed to avoid motion blur from a moving subject or shake blur from hand-holding a camera , or when you need to step down the aperture to get sufficient DOF. In both scenarios if there isn't enough available light to accommodate that faster shutter speed and/or smaller aperture then one must increase ISO in order to maintain the same pixel-level sharpness desired (ie, absence of undesirable blur).

As the pixel density increases one must increase the shutter speed a commensurate amount in order to maintain the same pixel-level sharpness. Pixel-level sharpness is necessary in order to take advantage of the detail advantage from downsampling a higher MP image to achieve better post-NR detail results vs a lower-resolution sensor. Otherwise the higher MP image will just be oversampling motion blur, yielding no detail benefit when downsampled.

Since a higher shutter speed is necessary for a higher MP sensor to achieve the same amount of pixel-level sharpness, the ISO must be increased on the higher MP sensor vs a lower MP sensor. And with that the presumed detail+noise advantage from a higher MP sensor suddenly disappears...because the amount of ISO increase necessary to keep per-pixel sharpness scales directly with pixel density.

As indicated, the only situation where you don't lose the presumed Higher MP advantage is in artificial test scenarios, where you lock both the higher MP and lower MP cameras on a tripod shooting a static, non-moving subject and with both cameras set to identical shutter speeds. In the real world you can't do this because, again, the only reason to use Higher ISO is to avoid blur, so if you have the ability to avoid blur at any shutter speed on a tripod then you would necessarily be using a lower ISO to begin with.

Wrong. I can think of a siuation off the top of my head in less than ten seconds, where what you're saying is incorrect. Take a shot of a night scene, for example. You might want to reduce the motion blur of people walking along a street, so you step up the ISO from 800 to 3200.

To reduce the motion blur, you'd increase the shutter speed. Whether or not you increase the ISO is a separate decision. At a base ISO of 800 with an a7RIII, I probably wouldn't increase ISO at all.

https://blog.kasson.com/?s=a7riii+shadow+noise

Jim

Good point Jim. I guess there's really no much point in pushing up the ISO past 800, huh?

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