Wednesday, November 12, 2014

Sony A7s High ISO Still Image Quality Review


"Better pixels, not more pixels" has been something of a counter-culture camera refrain in the camera community in the post-Nikon D800 era. For the most part, the manufacturers haven't truly given this to users. Fujifilm has held the line at 16mp for their APS-C cameras, and Nikon only marginally bumped the 12 megapixel count of the D3s to the 16mp D4/Df generation, but nobody has offered less pixels... until the Sony A7s, that is. 

Before we continue any further, it should bear repeating that the A7s' strength is primarily as a video camera. The ability to 4K at extremely low light levels sets it apart from all other videography tools, including the well received Panasonic GH4. This is what business types affectionately call "the unique selling proposition"... it's the only one of its kind, and it's good at what it does. (However, the inability of the A7s to record 4K internally is a hassle for some.) As a stills camera, the A7s is not so much unique as it is superlative. There are other low-light cameras for stills photography; the A7s just so happens to be the best. Or is it...?


Shutter Shock is Alive and Well


First a word about shutter shock. The following samples will involve the Sony A7s, the Nikon D750, the Nikon Df and for fun, the Nikon D810. The sensor in the Nikon D750 and D810 are variants of the ones used in the Sony A7 and A7r.

Like all mirrorless cameras, the natural shutter progression for the A7s goes like this:

  1. Shutter is open for live view streaming
  2. Shutter closes to clear the sensor
  3. Shutter opens to begin exposure
  4. Shutter closes to end exposure
  5. Shutter opens to re-initiate live view

That's two openings and two closings for ever exposure, twice what you would see on a DSLR. This is analogous to mirror-slap that you would see on Nikons and Canons, except that it is often worse on mirrorless devices because the camera body has less overall mass relative to the moving parts of the shutter mechanism. It doesn't seem to affect the A7 drastically, but it is something that you have to watch out for on the A7r. The A7s is not immune if you don't set up correctly:


A7s shutter shock - 1s

The above example is an exaggeration of the effect done by firing a shot with the camera held on a solid surface. Almost all cameras with mechanical shutters will display the vertical-motion blurring, but the A7s is quite pronounced compared to a DSLR. Fortunately, the A7s has a silent mode (electronic shutter) the eliminates this. You can get extremely crisp images with the silent mode turned on, but you also have some of the same downsides that you would get in video mode, such as the rolling shutter effect.

High ISO Noise and Image Quality



The following is an ad hoc demonstration of the camera's JPEG output quality through the ISO range using a set of pop refrigerators located across from the camera store. Pay attention to the legibility of the soda bottles for detail retention (or lack thereof) as the ISO value rises, speckling in the broad colour patches for overall image noise, as well as the harshness of the reflections and shadows as dynamic range decreases.

These samples are not directly comparable to similar samples found elsewhere in this blog because of variable ambient lighting conditions. (This is especially true of this image set, as the subject magnification is larger than that used in some of the other posts on this blog.) This set was shot at f/16 on 35mm lenses with center-weighted metering. The lenses used were:
 

High ISO noise reduction was turned off for all cameras. Though it is true that better noise reduction and detail retention is possible with RAW samples, the default out of camera output is sufficient in this case to demonstrate gross trends. Note that the Nikon Df exposes 1/3 EV brighter than the other two camera. This is generally true across most circumstances, regardless of metering mode. Click on images for 100% crop view.

ISO 6400
ISO 12800
ISO 25600
ISO 51200
ISO 102400
ISO 204800

There's a lot going on here, but if you look at it on the pixel level, yes, the A7s is the cleanest by far of the three cameras. It also visibly produces the smallest file size, though the difference between it and the Df is not as large as between it and the 24mp D750 (or regular A7 if you prefer.)

ISO 6400 is usually the crossing point for many people between casual shooting and increased attention to shooting technique and post-processing. The D750 is marginal here at this limit; going up more more stop doesn't hurt it when the there is not a lot of fine detail in the scene, but this is pretty much the practical limit. The Sony's cross-over point? It's somewhere between ISO 25,600 and 51,200, though for video, ISO 102400 would still be pretty workable. Going further, the highest settings on the Sony A7s and the Nikon Df are complete fictions. There is no possible way that ISO 409,600 (A7s) or ISO 204,800 (Df) are remotely usable for anything other than for impressionistic filter-effects.

This begs a question... at ISO 1600-3200, the 24mp Nikon D750, though noisier, displays ostensibly more detail. What else is 24mp? .... How about Sony's own APS-C sensor A6000. If you aren't shooting in ultra low light and the scene does not have extremes of dynamic range, the smaller sensor pedestrian A6000will outperform the thoroughbred A7s.

A7s vs D810: High ISO Detail Retention


If you compare the A7s vs a camera with vastly more pixels such as the A7r (or the Nikon D810 in this case), the high ISO advantage isn't as clear. On a per pixel level,m the A7s is better, but the simple fact of the matter is that the 36mp sensor cameras are collecting more pixel data, and therefore more detail. The D810 is no slouch at conventional higher ISO levels. At ISO 6400, D810 files downsampled to 12mpare crisper, cleaner, and show better edge definition. The cross-over point is roughly ISO 12,800 where noise and edge retention are roughly the same.

ISO 6400
ISO 12800

This has implications on what you can do with the files from these cameras at these ISO levels. With the A7r and D810, its important to remember that cropping into a high-ISO file will boost the perceived amount of noise in the file image. The A7s, however, might not even have that luxury; because it is capturing less detail, you will have less leeway to crop. This underscores an important point about shot discipline; it is always better to frame properly at the time of shooting rather than to try to fix it afterwards in post-processing. Certainly, having more pixels or having cleaner pixels mitigates some of the downsides, but the problems they don't eliminate all of the downsides.

Practical Considerations


Well, what can you do with all of this high ISO goodness? Sunny 16 conditions correspond to 1/125s at f/16 taken at ISO 100. A typical stroll through a brightly lit city center street at night is roughly 8EV less than that, which also corresponds to our arbitrary cutoff value of ISO 51,200 as being the limit to reasonably intact image quality. In other words, at this ISO level, you could theoretically still shoot at f/16 and 1/125s, as you would on a bright sunny day. That's a considerable amount of ISO boost by any means.

Concluding Thoughts


"Better pixels" instead of "more pixels"... while there is still some truth to that, in practice, its not so simple. Today we have more pixels, which are also better than they were before. In fact, if we look at it from a straightforward arithmetic point of view, more pixels = better pixels. When you consider that the 36mp sensor collects three pixels worth of information in the same sensor area that the A7s collects a single-pixel's worth of data, you can see why more pixels are better. It's (crudely) the same amount of light in the same amount of sensor area, but having more pixels means more data sampling and the ability to better capture colour information lost to the Bayer array.


Ordinary shooting circumstances don't require ultra-high ISO capabilities. Even dim indoor situations don't often require heroic efforts past ISO 6400. The A7s doesn't solve problem in mundane situations that couldn't be solved through other means such as fast aperture primes and image stabilization. What the A7s does allow is for new areas of photography in low light, particularly the ability to increase depth of field shooting hand-held instead relying on a lens help wide-open at maximum aperture.

In other words, the A7s at first glance seems to be the ideal small-size shoot-anything type of photographic camera, but for the price, this isn't an effective use for it. The opportunity cost is a measurable sacrifice in detail; even if you aren't using large file size or printing in large sizes, the files of the more main-steam full frame cameras offer more resolving power. However, if you are interested in exploring low-light photography without the need for a tripod, this is most certainly the best camera for the job as of 2014.



With thanks to Broadway Camera

2 comments:

  1. Nice review. So far I prefer the D810.

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  2. I am happy to find this post Very useful for me, as it contains lot of information. I Always prefer to read The Quality and glad I found this thing in you post. Thanks sony a7s review

    ReplyDelete