Sensor Plus Review

No one who has used one, or seen their output, doubts the image quality possible from a medium format back. Yes, DSLRs have now crept into MF back territory when it comes to resolution, but backs have kept pace, and while DSLRs now boast as high as 25 megapixels, the highest resolution backs such as thePhase One P65+, now offer 60 Megapixels. And, of course, there’s more to medium format digital than just magapixels.

In addition to their higher resolution medium format backs have other advantages, such as their lack of an anti-aliasing filter, which produces higher accutance, and also that they usually produce true 16 bit files, rather than the 12 bit or 14 bit files from DSLRs. There’s even the fact that they use CCD technology rather than the now almost ubiquitous CMOS sensors used in DSLRs, which some workers feel produces higher quality image files.

But, one image quality area where medium format backs have always fallen short is when it comes to high ISO. At ISO 50, 100 and sometime even 200 backs are untouchable. And for the type of shooters that mainly use these backs – studio, product, fashion, architecture, and landscape, this usually isn’t much of an impediment. But, let’s be honest – photographers can not live by low ISO alone, and whether its the type of assignment, the nature of the shoot, the need to work handheld, or changing weather conditions, sometimes ISO 200 simply isn’t high enough – especially in medium format where f/2.8 is a fast lens.

The new Phase One P65+ is a step in the right direction. I found that it is at least a stop cleaner at ISO 400 than its predecessor the P45+, and it also offers a usable ISO 800. This means that the P65+ is as clean at ISO 400 as the previous generation was at 200, which means that it’s clean enough for almost any application, with files hardly needing any additional noise reduction attention at all.

ISO 800 is usable. Not great, but certainly OK when one needs the extra speed to get the shot, and for some applications will be most welcome.

If that was as good as the P65+ got, then it would be a worthwhile but not remarkable achievement in the area of noise. But, beginning with the P65+ and the adoption of a proprietary Dalsa designed sensor, Phase One is introducingSensor+technology, something that has been in the works for several years and which is the subject of an international patent expected to be granted later this year.

Luminous Landscape Gallery Show Opening Reception. Toronto, March, 2009
Photograph by Chris Sanderson

Phase One 645 with 28mm f/4.5; P65+ with Sensor+ @ ISO 1600

100% Crop – Sensor+ Mode @ ISO 1600
(Don’t use this blow-up to judge resolution. The shot has hand-held at a slow shutter speed)

The above frame and its accompanying 100% enlargement tell the story. This is a typical "event" photograph taken under available light. At ISO 1600 in Sensor+ mode the P65+ delivers a clean and usable 15 megapixel file. Close examination does show a bit of aggressive noise reduction in some areas of very fine detail, but this is only when compared to the usual totally clean files that medium format backs provide at lower speeds, and is no more so than files from contemporary DSLRs at a comparable speed.


Sensor + Explained

The P65+ has 60 million pixels, each 6 microns in size. What Sensor+ does is "bin" four pixels together so that they effectively become one 12 micron pixel, while retaining the same image coverage. In doing so, though the resolution of the back is reduced by a factor of four to 15 megapixels, the sensitivity is consequently increased by a factor of four, which is two F stops.

Interestingly, since the P65+ does not have an AA filter, and typically doesn’t need one because of its extremely high resolution, one would think that with fat 12 micron pixels in Sensor+ mode moire might be a problem. Apparently that is not the case.

Before answering the question in detail of "does it work and how well" (yes it does, and very well) we need to understand that "binning" has been done before, but rarely with a colour matrix sensor and never very well. Monochrome sensors have been binned, but doing so with a Bayer matrix is a monumentally more difficult task because each adjacent pixel is of a different colour and has a quite different spectral response. Also, because binning reduces resolution by a factor of four, it really doesn’t make a lot of practical sense to attempt unless and until one has a sensor such as the one in the P65+, with 60 megapixels unbinned. The resulting 15 megapixel binned size is bigger than many pro DSLRs, such as the Nikon D3 and D700, and therefore quite usable for many commercial applications.

According to Phase One this technology is proprietary. Several years ago they approached both Kodak and Dalsa (the two makers of large sensors for medium format backs) and Dalsa agreed to work with them in developing a sensor which would include this technology. Apparently at this time this type of binning is limited to CCDs, not CMOS, so this combined with Phase’s patents means that we’re unlikely to see it from anyone else any time soon.

One trade-off though is that it turns out that Sensor+ and Phase’s ability with their Kodak sensor equipped backs to do very long time exposures (up to 1 hour) are mutually exclusive. It may be that some future iteration of this sensor technology can combine both, but not at this time. The P65+ is therefore limited to about 1 minute exposures.


Sensor+ Results

Shortly after I publishedmy P65+ reviewI got one of the first production Sensor+ backs for testing, and so the results reported on here should be representative of what users can expect.

In practice the technology appears to deliver exactly was theory would predict – two full stops of additional sensitivity. So while the P65+ is capable of producing extremely clean ISO 400 files at 60 megapixels resolution, with Sensor+ mode turned on 15 megapixel files at ISO 1600 are the result. In normal mode the back can be set to ISO 800, with reasonable results, and similarly with Sensor+ ISO 3200 is available at 15 megapixels with comparable visible but moderate noise.

Is the low noise as low as a D3 or D700, for example, at ISO 1600? No, but close. Quite close. Of course the D3 then goes on all the way to ISO 25,000. But, at ISO 1600 I judge the P65+ in Sensor+ mode to be comparable to most current mid-range DSLRs of similar resolution.

Which of course begs the question – why buy a $45,000 back to get images comparable to a $1,500 DSLR? Well, to ask that question is to miss the point that by pressing a button one can almost instantly switch between the two modes, allowing the photographer to shoot world-leading resolution at low ISOs, and contemporary DSLR resolution and quality images at high ISO, and of course, in 16 bit mode without an AA filter, which no DSLR can currently offer.

This series of images were taken with the P65+ in normal as well as Sensor+ modes
using a Phase One 645 camera and 75-150mm lens at 150mm.
The light was mixed window light to the left and tungsten from the right of frame. ISO 200 Normal Mode

(slightly soft due to shutter bounce) ISO 200 Sensor+ Mode ISO 400 Normal Mode ISO 400 Sensor+ Mode ISO 800 Normal Mode ISO 800 Sensor+ Mode NA ISO 1600 Sensor+ Mode NA ISO 3200 Sensor+ Mode

This is a problematic test for presentation on the web. With the exception of the ISO 800 normal mode and the ISO 3200 Sensor+ mode the noise differences are small and not easily seen, even in prints. Also, there is a huge size difference between the modes, and to present them in any kind of practical manner means reducing the Sensor+ images to the same size as the regular ones. But, this is obviously unfair since reducing the size of the regular 60MP files to the equivalent of 15 megapixels makes their noise much less apparent.

I have no good solution. So, below are 100% crops from the regular ISO 800 and the Sensor+ ISO 3200 frames, just to show how much, or how little, difference can be seen when they are notnormalizedin size. These are the P65+’s maximum ISO settings in both modes.

My recommendation is that if this is of more than academic interest to you to have a Phase One dealer visit your studio so you can do this comparison for yourself. No web test should be relied on when it comes to this level of expenditure.

ISO 800 @ 100% ISO 3200 Sensor+ @ 100%


Shooting Speed

I had determined that on a Phase One 645 camera (Mamiya AFDIII) the P65+ was capable of one frame every three quarters of a second in continuous shooting mode. In Sensor+ mode it produced almost 1.2 FPS, a 33% shooting speed improvement. But, it should be noted that with a fast card the P65+ in Sensor+ mode can write continuously to the card until it’s full. This means that in some situations it may actually outperform some DSLRs that can shoot faster, but then which have to pause to empty their buffer.


Early Adopters?

The P65+ started shipping in January, but the first cameras out of the gate did not have Sensor+ built in. Following a recent telephone discussion with Phase One’s chief technologist, Claus Mølgaard, I learned that the reason for the delay was the necessity to develop some new highly critical back calibration routines, ones even more rigorous than those previously used during the testing and calibration of new backs at the end of their hand assembly process.

In any event, Sensor+ is now in production and owners of P65+ previously shipped can returned their backs to the factory via their dealer for a free upgrade.

Please note though that the full capabilities of Sensor+ technology can only be achieved if you’re running Capture One 4.6.3 software, or higher.


Who is it For?

There are so many types of photographers and assignments that it’s hard to say who will find this technology of use. I imagine that high-end wedding photographers will now find that they can use the same camera system for formal portraits outdoors in the garden and then candid shots in the church, rather than having to switch camera systems. A fashion photographer working on location will be able to spontaneously move into the deep shade, or continue to keep working for another half hour as the sun goes down – again, without having to switch systems and while retaining the high image quality that 16 bit capture without an antiailiasing filter provides.

As for my type of shooting, landscape and nature, since I work on a tripod much of the time it’s less critical for me, but there are many times when the light gets low and I have to shoot hand-held that I wish for higher ISO capability, and would be more than willing to trade off resolution for sensitivity. Now I have that capability just a button press away.

In the end, few photographers will make the expensive purchase of a P65+ simply because it is capable of switching resolutions and sensitivity. But, I imagine that when photographers look at the competitive landscape, and weigh their alternatives, depending on their individual needs this new capability will be one of the factors affecting the balance.

March, 2009