RAW File Bit Depth Changes with ISO

Let’s begin with the fact. The usable bit depth of your RAW file depends on the ISO used to shoot the image.

I discovered this while working on the RAW histogram feature in Kuuvik Capture. To make the RAW histogram usable, we have to scale the data coming from the RAW file. This scaling ensures that the left side of the histogram represents pure black and the right side represents pure white. Technically scaling is done by first subtracting the black level from each pixel, then mapping pixel data from the [0, white saturation] interval into the [0, 1] interval.

Black level is the value your sensor emits when no photons reach a given pixel. This is calculated utilizing a black masked area along the edges of the sensor (see my former post on this).

White saturation is the value from the given pixel when it’s completely full – that is more photons reaching the pixel will not generate a higher value. This depends on physical attributes of the sensor. We do a series of measurements for each sensor to determine its value. The higher the white saturation the more tones your RAW file contains.

What surprised me during the initial white saturation measurements is that with most of Canon’s cameras this value changed as I changed the ISO. Some cameras even present different white saturation in different exposure modes (Av and M for example).

The following graph shows the result from these measurements converted into usable bit depth for four cameras up to ISO 6400.

bit-depth-vs-iso-2For the mathematically inclined, usable bit depth is calculated with the formula:

\log_2 (w - b)

Where w is the white saturation and b is the black level.

The roughly 0.3 bit difference between the lowest and highest values doesn’t seem that large at first sight, but this means that you lose 15% of the tones at ISO 640 compared to ISO 800. To put it another way it’s a 1/3 stop difference.

Implications

Avoid non-full-stop ISOs.

The truth is that both ISO 500 and ISO 320 are exposed at ISO 400, putting a 1/3 stop “digital exposure compensation” value into the RAW file. For the ISO 320 setting this produces an overexposed image, which should be pulled down 1/3 stop. The downside is that you lose 1/3 stop of both tonal and dynamic range. The upside is that there will be less perceived noise, which can be helpful in some situations (and which is the basis of lots of false myths)

Avoid ISOs < 200 on crop-sensor Canons.

As you can see on the graph above, bit depth on these machines are less below ISO 200 than on or above it.

What about the 1D X?

Some of the 1-series bodies are not prone to the 1/3 stop bit depth loss. For example the 1D X starts to show this behavior at ISO 12800. The 1Ds Mark III produces the exact same bit depth at each ISO. And the 1D Mark IV works like the 5D Mark III.

So my practice is to use just full-stop ISOs and forget about ISO 100 on crop-sensor bodies.

How to Make Focusing a Tilt/Shift Lens Easier

The tilt movement is used in technical cameras as well as DSLR tilt/shift lenses to precisely adjust where the plane of focus is on the image. Focusing with tilt is a tedious process (described here and here), but the results always worth the time!

There was a big pain point in using DSLR T/S lenses: checking what you have done. The viewfinder isn’t enough for that with today’s high resolution bodies, so you have to zoom in and check different points on the image using magnified live view. The adjust either tilt or focus. Then check the points. Then refocus… I had some images where I spent more than half an hour on fine tuning focus!

I said “was” – as it was the case before Kuuvik Capture’s Split View feature came along. I’m using this since I was halfway into developing the first prototype, and man, it can save lots of time! No, it won’t think instead of you, but the ability to quickly and visually asses what you have accomplished is priceless. It is also a great tool for learning how to focus a tilt/shift lens.

So watch the video below, and if you are using a Canon EOS-1D X, 5D Mark III or 6D with any of Canon’s great tilt/shift lenses, then grab Kuuvik Capture’s beta now! It’s that good (OK, don’t believe me, try it for yourself ;)).

Canon EOS-1D X and 6D Impressions

Many thanks to Canon Hungary for kindly supplying test cameras for our project!

From time to time a bunch of cameras arrive at my desk for measurements and software compatibility testing. This is a double-fun exercise: besides learning a lot about cameras I have the opportunity to try out and photograph with almost all of them. Among the recent group borrowed from Canon Hungary, there was two cameras I was eager to try out: the EOS-1D X and the EOS 6D. Fortunately the testing period included a weekend, so I had a little more time to go out and play with both, and to compare them with my 5D Mark III and 1D Mark II (which I still have because it can’t be sold at any sensible price).

First and foremost: I would be hard pressed if I had to choose between the 1DX, 5D3 and 6D based solely on image quality. All three are capable of producing great images. You can’t go wrong with any of these. You can also find several reviews on the web doing all the pixel-peeping. So I will concentrate on handling and usability – both playing an important role in my camera selection.

In General

I have been an EOS-1 user for almost a decade, and I immediately felt home with the 1D X. Sure, it is bigger and bulkier than recent models, but at 1550g it’s still 20g lighter than the 1D Mark II. For me this weight dictates the use of the E1 hand strap.

It seems that only the 1-series Canons are designed for people having a nose. Having anything than a small and flat nose is a recipe for discomfort and greased LCD on all non-1 Canons. The 1D X being thicker reduces the distance the viewfinder protrudes from the body, so it’s slightly less convenient than previous models. In comparison: the 5D Mark III is bearable, but the 6D is awful: I can’t see the entire image in the finder without risking to break my nose…

Switching between the 1DX and the 5D3 is effortless: I was able to instinctively find all the controls as they were where they should be. Not so with the 6D. I found the omission of the joystick, the inconvenient selection dial and mixing picture taking controls with playback controls so much frustrating to use that I put down the camera just after half an hour and decided against buying one, despite holding it still feels good. I understand that it’s a sacrifice one has to make for reduced size/weight/price, but I’m rather carrying/paying more for something that’s a joy to use. If I desperately need a cheap/light backup camera then I might buy one, but at the moment I don’t feel that need.

The CF compartment door on the 1D X I tested was loose and emitted a squeaking noise every time I squeezed the body – and you have to squeeze it to be able to pick it up. I don’t know if it’s a problem with this given demo unit, but it’s not something that I experienced with previous 1s and definitely not something I would accept on a $6800 camera. Even the 5D3’s CF door was better.

Features I Miss

Although the 5D3 and the 1DX are from the same mold, there are a couple of pretty useful 1-series features I miss on the 5D3: eyepiece shutter (the 5D3’s plastic thing is a joke), the ability to save the whole camera configuration to a card and load it later, and the ability to lock up the mirror for several shots (which is pretty useful if you do brackets).

On the other hand, the RATE button introduced on the 5D3 and the truly silent shutter of both the 5D3 and the 6D (the silent shutter on the 1DX is pretty useless) are features that could find a home the 1DX.

Things That Need to be Done Fast

All four cameras were pretty responsive, but I was especially interested in a few things. The first was buffer clear time. Lots of people talk about the buffer capacity, but I found the time needed to write all images to disk more important. Even a smaller buffer with faster clearing could be useful. In this comparison the 1D2 lost by huge margin: using a 16GB SanDisk Extreme Pro card it was capable of taking 19 shots (on average) in a burst and it took 15 seconds to clear the buffer. During the last decade it proved to be inadequate more than a couple of times. The 5D3, with a 32G Lexar Pro 1000x card was able to capture 37 images before starting to slow down and the buffer cleared in mere 2.5 seconds! The 1DX was able to capture 58 shots in a 12 fps burst, and wrote them to the same 32GB Lexar Pro 1000x card in 7 seconds. Even the 6D was better than the 1D2: it took 21 frames and wrote them in 8 seconds to a 32GB SanDisk Extreme Pro SD card. In terms of fast capture and fast card writing both the 1DX and the 5D3 are wonderful.

I was interested in how fast these cameras drive a long telephoto (without and with teleconverters). I made no numerical comparisons, just how fast they felt. Surprisingly the 1D2 was the candidate for the fastest lens drive medal, but it has to correct what it did in a second round several times. Maybe its NiMH batteries could provide more power to the lens? The 1DX drove the 500mm f/4L IS quickly and precisely, even with teleconverters attached, so the aforementioned medal went to the 1DX. Lens drive is not where the 5D3 AF system shines. However, my experience shows that its AF system is far better than the 1D2 for tracking birds – even if it drives lenses noticeably slower than its big brother. Here the 6D pleasantly surprised me: lens drive was faster than the 5D3’s! So I sincerely hope that Canon would be able to squeeze out a 6D-equivalent lens drive from the 5D3 with its upcoming firmware update.

The last thing I tried was low light focusing – with the center point only. The 6D is the clear winner here – it was able to focus on features I was barely seeing! The 5D3 took second place, with a bit of hesitation (read: several seconds) before grabbing focus at the same spot where the 6D focused instantly. The 1DX hesitated even more, but was able to grab focus, but the 1D2 was unable to achieve focus in any of my tests.

Conclusions

Let’s start with the easiest one: the 6D impressed me with its low light focusing ability and speedy focus drive, but it was not enough to outweigh its shortcomings in the handling department. So as I mentioned I decided to skip this body for now.

The old 1D Mark II held against the competition pretty well, despite its 9 years in service. I changed my mind about selling it: I would get less for it than a medium level Montblanc pen costs, but its still a pretty usable and capable camera – up to ISO 800. It stays until it dies.

And now the big question: 1DX or 5D3? I bought the 5D3 at the time when the 1DX had no f/8 focusing ability. Would Canon introduce the 1DX with this feature I would end up with that camera, no question about it. But the 5D3 will get that feature in April, so again a tie. Now I see four decision factors:

  • Action-stopping ability (high fps, focus tracking and focus drive). It you need this go with the 1D X.
  • Size/weight. I would take my 5D3 to a vacation paired with my beloved light primes without any hesitation. Would not even think about that with the 1D X (been there, done that with the 1D2 – not again).
  • You get 22% more pixels with the 5D3, which is important for landscapes/architecture. I will do some print comparison between the 1DX/5D3 files in the coming weeks to see how much they differ at 40×60 cm print size.
  • And, of course, price. At $6800 I feel the 1DX a bit overpriced. At the vicinity of $5000 it would be an instant get for many people I believe.

Nowadays I do more landscape and architecture photography than birding, so the 5D Mark III serves me well. Metering and the quality of its files are well above previous generations. The only advantage of the 1DX from my point of view is its action-stopping ability. In all other aspects the 5D3 is a better choice. Should I feel the need for more than 6 fps and slightly faster focus I will grab one.

Oh yes, one way to avoid the above decision is to own them both 🙂

Red Veil in 5D3/1DX Viewfinder

This still annoys the hell out of me. I briefly mentioned the viewfinder illumination issue in my initial 5D Mark III impressions post. Today I shot with an 1D X (more on the camera in a later post), which has the exact same problem. OK, this is not surprising as the two cameras share (almost) the same auto-focus system.

To quickly sum it up: when anything (AF point or grid line) is illuminated in the 5D3/1DX viewfinder, then the entire screen glows in red. This is especially distracting if you have the grid lines turned on (which will produce a whole-screen red flash every time the AF system locks). The following picture shows what you see in the finder in complete darkness while selecting the focus point (body cap on). You’ll get the same amount of red light for AF confirmation.

1D X viewfinder with center AF point selected

A red veil over the finder image is clearly visible in usual conditions I shoot in, such as during golden and blue hours (but surprisingly the same amount of illumination is not enough to show anything in bright sunlight making the situation even more frustrating).

I like grid lines but hate to see them flashing, so I decided to swap out the ground glass in the 1DX with my Ec-D (gridded) screen from the trusty old 1D2. I thought that will cure the problem by avoiding grid line illumination. It wasn’t true success, as the red veil still remains when AF points are lit. Not s smack-you-in-the-face red flash, but still there and still distracting.

Just for comparison’s sake, here is a picture showing how the old-style Canon AF point illumination worked. Just the selected area was lit (actually there was a faint glow in other/unselected AF points, but my iPhone was unable to capture that).

1D Mark II viewfinder with center AF point selected

I still prefer the old-style illumination and consider the new system a huge step backward in usability, which plagues otherwise wonderful tools. Hope that Canon will find a better solution in their next generation cameras.