Making ev-bracketed exposures for noise-free output
and a few choice words about Sony's A7 cameras
This tutorial explains how to set up your camera to make ev-bracketed exposures that can be merged for essentially noise-free output.
Along the way I give some tips for getting the best possible ev-bracketed shots out of the Sony A7 family of cameras. I have the first version of the A7, but sadly only a few of the problems with the original A7 cameras have been addressed in the Mark II reissues.
Written March 2017.
Ev-bracketed exposures for "Zero Noise" photography
I don't like blown highlights. I also don't like noisy shadows. Fortunately, for reasonably static scenes shot raw from a tripod, Guillermo Lujik's "Zero Noise" technique produces clean shadows and intact highlights.
This tutorial describes how to set up your camera and tripod for making ev-bracketed exposures. If you've never done this before, or even if you have but your shots didn't line up, this article has step-by-step instructions for shooting from a tripod.
This tutorial also has advice for dealing with the Sony A7's crippled firmware. Only a limited number of the deliberately "firmware-crippled" features on the A7 have been "uncrippled" in the A7 MkII and MkIII, so many of the tips for the original A7 cameras also apply to newer A7 cameras.
Even if you don't have a Sony A7, you might find it interesting to peruse my list of complaints and associated links to articles about various shortcomings of the A7 line of cameras. One good thing about having purchased a Sony A7 is that now I have a long list of things to watch out for the next time I purchase a camera, which likely won't be a Sony even though in many ways I like my Sony A7 camera quite a lot.
A word about using tripods
If you are anything like me, the first time you tried to take a picture with your camera on a tripod, you decided that the results weren't worth the effort. If you were trying to make ev-bracketed shots, I bet the individual shots weren't exactly aligned with one another (even if the subject obligingly held still). Here's the cure:
- Get a good tripod.
- Get a good tripod head.
- Practice, practice, practice: At first setting up and adjusting the tripod will be cumbersome, awkward, and annoying. Then it will get easier. And eventually using the tripod will be so easy and so automatic that you won't even think about it. About this time you might have a new problem, which is that you'll be really annoyed in situations where you can't plant your camera on a tripod.
This tutorial is not the place to cover choosing a tripod and tripod head. But please consider this: A good photographer can take good pictures with a relatively inexpensive camera, and the process of taking the picture won't be all that different regardless of the cost or quality of the camera. This is not the case when using a tripod: A flimsy tripod is difficult to use, likely to break at the worst possible time, and won't keep the camera steady. And the wrong head can make framing a shot pure misery.
So make sure you actually try setting up the tripod and framing various shots (I mean seriously practice framing shots to your complete satisfaction) before you plunk down your money. To keep costs down considering buying used equipment. And be resigned to the fact that you might not really know what type of tripod and head is best for your personal style of taking pictures until you've spend some time actually taking pictures with your camera on a tripod.
In case you are curious, I have two tripods, a Gitzo G2220 that I mostly use outside, and sometimes indoors for framing awkwardly angled shots; and a Bogen 3033, purchased used, that I use almost exclusively indoors (where floors are more or less flat and perpendicular to gravity). I'd probably use the Gitzo all the time, but my particular (older) model isn't as stable as I'd like it to be.
My first tripod head was a relatively inexpensive ball head that absolutely refused to move smoothly, which made framing an image exceedingly difficult. Eventually I got rid of that awful piece of kit and got an Induro pan tilt that is a pleasure to use. One of these days I'd like to get a nice ball head (one that actually moves smoothly enough to allow for precise placement) for use outdoors on the Gitzo tripod.
Making the ev-bracketed exposures
If you've never made ev-bracketed exposures before, I recommend practicing at home before trying to do this out there in a real world shooting situation. This way you'll have a chance to get the camera properly set up and solve all the operational issues. The steps below assume you've already figured out where to put the tripod to frame the shot you want.
- Put the camera on a tripod.
- Put the camera on a tripod.
- Really, seriously, put the camera on a tripod!
Use the heaviest, most stable tripod that you have. If you are outside, make sure each leg makes firm contact with the ground — if the ground is soft, push down a bit until the tripod feet are somewhat settled into the ground.
- Attach a remote release to your camera.
Pressing the shutter with your finger will almost certainly move the camera a tiny little bit, making it next to impossible to make exactly aligned exposures, thus defeating the purpose of putting your camera on the tripod.
- Set the camera to save raw files.
Merging exposure-bracketed image files requires using image files that are scene-referred. This means the intensities in the image RGB channels are proportional to the intensities in the scene that was photographed. You can't successfully merge exposure-bracketed camera-saved jpegs because the in-camera processing used to produce "pretty" jpegs completely destroys the scene-referred nature of the data in the raw file.
Most camera sensors respond more or less linearly to light. But many cameras have menu options that modify what was captured by the sensor before the data is saved as a raw file. So go into your camera menu and disable anything that messes with a straight scene-referred capture. Especially eliminate fill light, HDR, and any other setting that's designed to lighten shadows or compress highlights or turn a high dynamic range scene into a low dynamic range raw file.
- Put your camera in full manual mode.
Making exposure-bracketed shots is no time to let your camera make your decisions for you.
- Set the lens aperture and focal point.
You want to make the ev-bracketed exposures by modifying the shutter speed, keeping the lens aperture (f-stop) and point of focus constant. I only use manual focus lenses, so I don't have any advice for what to do to keep your autofocus lens from doing something you don't want it to do when making ev-bracketed exposures.
- Disable automatic noise removal and automatic black frame subtraction . . .
. . . assuming your camera manufacturer was kind enough to allow this level of user control. As you are merging ev-bracketed shots, you don't want the camera messing with what the sensor actually captured.
Unfortunately for A7 owners, every time you use bulb mode on a Sony A7 camera, noise reduction and black frame subtraction are automatically activated. So for a 30-second shot, you have to wait an extra 30 seconds for the black frame subtraction before you can take the next shot. A minute-long shot? Wait an extra minute before you can take the next shot
Think about how long 30 seconds or a minute for automatic black frame subtraction is when the sun is going down and you are working to capture the best light — with a real camera you'd be able to deactivate automatic black frame subtraction and shoot your black frames separately as convenient. But Sony has programmed the A7 firmware to not provide user control over these very basic functions. Even my old Canon Rebel/400D DSLR provided such control (and unlike the Sony A7, the Canon Rebel made no pretensions of being a professional-level camera). But the so-called professional-level Sony A7 camera does not. Sigh.
- Use the camera's base ISO . . .
. . . unless your lighting conditions are so low that you must use excessively long exposure times. Though for most cameras (but not the Sony A7), I'm not sure there's any point in using higher than base ISO when making exposure-bracketed shots for the specific purpose of producing clean shadows and intact highlights.
For the Sony A7, anything longer than 30 seconds is excessively long. Why? With a Sony A7, for exposures longer than 30 seconds you have to use "bulb", and "bulb" on the Sony A7 does two nasty things:
- Applies some odd "sinusoidal" automatic noise removal that you can't disable even if you plan on shooting your own black frame, which would be a waste of time as the A7 will also automatically subract a black frame whether you want it to or not. Astrophotographers refer to Sony's "automatic noise removal that can't be disabled" as Sony's "star eating" algorithm, and they don't mean this as a compliment.
- When using the "bulb" setting, your already "pretend 14-bit" analog to digital camera that apparently is really a 13-bit AtoD camera, will only using 12-bit analog-to-digital processing, thus throwing away half of the captured information even before producing Sony's already artifact-ridden lossy-compressed 11-bit raw file. And no, on the original Sony A7 there is no option to save an uncompressed or losslessly compressed raw file.
Speaking of "base ISO", this is not necessarily the lowest ISO available on your camera as some cameras (including the Sony A7) include one or more ISOs that are actually lower than the base ISO (for example ISO 50, when the base ISO for the camera is really ISO 100). These "fake" ISOs are actually the result of mathematical manipulation of the digital readout from the sensor, and don't have anything at all to do with the sensor's native sensitivity to light.
- Decide how many stops apart you want your ev-bracketed exposures to be and set your camera accordingly.
Ideally, you want at least four full stops between the shortest and longest exposure. In a pinch, in a Low Dynamic Range scene, two exposures (base for the highlights, +4 for the shadows) will do the job. But depending on the scene dynamic range and the camera, you might get better results with a closer spacing between the exposures. And depending on the camera, there might be a limit to how many exposure-bracketed shots you can make:
For the Sony A7, for 0.7 stop increments you can take 5 bracketed exposures, but this only produces 0.7 times 4 additional shots equals 2.8 stops between the shortest and longest exposure. For anything more than a 0.7 stop increment, you are limited to only 3 bracketed exposures. So your only real options are:
- Make 5 exposures at 0.7 stop increments for a total of 2.8 stops.
- Make 3 exposures at 2ev increments, for 4 stops total.
- Make 3 exposures at 3 stop increments for 6 stops total. But for some cameras and scenes, 3-stop intervals might be too much for smooth transitions from one exposure to the next during the blending process.
- Make two sets of ev-bracketed exposures, which requires touching the camera to reset the base exposure shutter speed, which means that the camera is likely to move as a result of being handled between making the first and second set of exposures.
FWIW, I settled on making 3 exposures at 2ev increments, for 4 stops total, and so far this has been quite sufficient for producing noise-free blended results.
- Figure out the base exposure and set the shutter speed accordingly.
By "base exposure" I mean the lowest ev exposure you need to make in order to capture the highlights. To properly set the base exposure (that just avoids clipping the highlights) what you really need is a histogram of the raw file and/or "zebras" based on clipped channels in the raw file. But despite pleas from photographers, raw histograms and zebras based on raw histograms are not available in any digital camera that I know of. Instead digital cameras show you histograms and zebras based on the in-camera-saved jpeg that your camera would be saving, if you were asking it to save a jpeg instead of a raw file. So the best advice is to use UniWB and pick your picture style to minimize in-camera modifications.
Unfortunately, as far as I can tell from exerimenting, the Sony A7 doesn't actually allow to make a true UniWB custom white balance, or if it does I haven't figured out how. So to allow the zebras to be maximally useful for setting exposures, I use the following settings:
- Creative Style: Neutral
- Scene Mode: Standard
- Scene Capture Type: Standard
- Color Temperature: 4500
- Color Compensation Filter: -7
- Contrast: -3
- Saturation: -3
- Sharpness: -3
- Zebras: 100+
- Jpeg color space set to AdobeRGB1998
Even with the above settings, for my Sony A7 exposures based on the setting at which the zebras disappear are still almost a full stop underexposed. So I can (but usually forget to) safely add a half-stop to what the camera says is the maximum exposure to avoid blown highlights.
Sony does have the very nice feature — not found on all cameras — of allowing you to choose the order in which bracketed exposures are made. This nice feature means I don't have to figure out the base (lowest) exposure and then mentally add two stops to get to the middle exposure (which on many cameras is the first exposure that the camera makes when making automatic ev-bracketed shots).
- Decide whether to use continuous mode auto-bracketing.
I have my doubts about the utility of using continuous mode auto-bracketing on a tripod, as it seems to me that the camera doesn't have a chance to fully stop vibrating from the previous shot before the next shot is made. But I haven't searched for relevant tests done by other photographers, and I haven't made any tests myself to confirm whether this is actually a problem or not.
However, if you are using a Sony A7, the situation is clear-cut: Don't ever use continuous mode auto-bracketing! Instead use single shot bracketing, even though this introduces a time delay between frames, thus increasing the chance that something in the scene might move between frames (people, tree branches, and such). If you use continuous mode auto-bracketing with the Sony A7, your already "pretend 14-bit" analog to digital camera that apparently is really a 13-bit AtoD camera, will only using 12-bit analog-to-digital processing, thus throwing away half of the captured information even before producing the already artifact-ridden lossy-compressed 11-bit raw file for which Sony and the Sony A7 camera have been so justly maligned.
So for the Sony A7 the remote release is doubly important as otherwise you'll have to press the on-camera shutter release three times, once for each exposure, which will result in your ev-bracketed raw files being slightly misaligned with one another. What's the big deal with misaligned shots? Sadly, the misaligned files won't be "one pixel off" but rather more or less than one pixel off, and so even with a "move tool" that moves the image layer around one pixel at a time, it's not possible to get an exact realignment. Though I think maybe, just maybe, the align_image_stack utility can make sub-pixel adjustments. I'm not sure, still testing.
- Frame the scene to be captured and make any desired final adjustments to the camera and lens settings.
If you haven't already done so, you might want to use your camera's provided options (if any) to avoid DSLR mirror slap and mirrorless camera shutter slap:
- If you are using a DSLR camera, use the DSLR camera's "mirror lock up" option to avoid vibrations from mirror slap.
- If you are using a mirrorless camera such as the Sony A7, to avoid shutter slap you can use EFS Electronic Front Shutter. But apparently under artificial lighting this can cause various problems, and at very fast shutter speeds or when using manual lenses, can cause other problems. Sigh. This means don't use Electronic Front Shutter unless you've tested to make sure that your light source+lens+shutter speed are within the safe usage parameters.
- Then press the remote release to make your ev-bracketed exposures.
Unless you are using continuous mode auto-bracket (and with the Sony A7, don't use continuous mode auto-bracket!), you'll need to press the remote release once for each exposure. Often there will be a noticeable delay before you can press the remote release for the next exposure, so be patient as this takes a bit of getting used to.
Be careful when pressing a wired remote's release button as you don't want to jostle the camera by tugging on the wire. Also stay in one place while pressing the remote — if you move around, that supposedly solid surface you are standing on can slightly deform and also transmit vibrations, thus moving the tripod on which the camera is sitting.
If you haven't yet acquired a remote:
- When picking out a wired remote, make sure the wire is long enough to handle cases where you don't want to be near the camera when triggering the shutter (such as when photographing highly reflective objects).
- Also think carefully before getting a wireless remote, as sometimes there are limits as to how far, from what direction, and "through what" a wireless remote can trigger the camera shutter — personally I don't use wireless remotes.
- Transfer the bracketed shots from your camera to your computer for processing.
Links to technical information about camera limitations imposed by the Sony A7 firmware
Limitations on using the Sony A7 for bracketed and long-exposure shots are not inherent in the actual camera sensor but rather have to do with the supplied firmware. Sony could issue a firmware update to eliminate most or all of the above-listed firmware limitations on what by all rights should be (but certainly is not) a professional level camera. But so far Sony has ignored pleas from A7 users, and I expect they will continue to ignore pleas from A7 users.
- How to avoid turning your nominally 14-bit Sony A7, which is at best only a 13-bit camera, into a 12-bit camera
- Testing the Sony a7, part 1 — as of March 6, 2017 there are a very large number of technical studies of the various Sony A7 cameras on the blog.kassen.com website, starting with the original Sony A7 released in 2013, and continuing on up to the recently release MkIII. The blog.kasson.com site is well worth delving into (and yes, there are articles on cameras other than Sony cameras).
- Sony a7 noise floor in continuous mode Quoting from Kasson's excellent analysis (do read his reviews if you plan on purchasing or using any Sony A7 camera):
In single shot mode the a7 is a 13-bit camera. In continuous mode, it’s a 12-bit device. This characterization is only accurate at 13-bit raw values of 1000 or less; above that, the Sony raw tone curve reduces the resolution until near saturation, where it’s 11 bits. . . . With auto bracketing set in continuous mode, you get 12 bits. With auto bracketing set in single shot mode, you get 13 bits. (blog.kasson.com)
- Archive 2015 · A7RII 12-bit mode. How to avoid it. (fredmiranda.com forum)
- Continuous bracketing on the alpha 7 cameras (fredmiranda.com forum)
- Sony a7II 12-bit mode’s effect on shadows (blog.kasson.com)
- Sony lossy compression: what it is and when it causes obvious artifacts
- RawDigger: detecting posterization in SONY cRAW/ARW2 files
- Sony’s 11+7-bit LOSSY File Compression: 32-pixel Line Artifacts Clearly Visible on Star Trails (diglloyd.com)
- A tough test of Sony raw compression (blog.kasson.com)
- Sony a7II 12-bit mode’s effect on shadows (blog.kasson.com)
- Sony A7 II - new firmware test on DCRAW (compressed vs uncompressed RAW) — Guillermo Lujik's excellent analysis of the Sony a7 Mk2 lossless vs lossy compression — we lowly owners of the original A7 are stuck with lossy comprssion even though Sony could issue a firmware update if they chose to (luminous-landscape forum)
- Sony A7 issues with long exposures and night photography
- Various problems using Sony A7ii bulb mode when shooting fireworks (dpreview forum post)
- Sony A7S: Stars blurry in Bulb mode (community.sony.com)
- Sony A7RII lossless RAW — an excellent discussion of the very serious "firmware-induced" problems with trying to use a Sony A7 camera for astrophotography (cloudynights.com)
- Has the Sony a7 Star-eater-algorithm fixed? (cloudynights.com)
- Dumb a7R astro questions (contrary opinions regarding using the Sony A7 for astrophotography)
- Issues with Electronic Shutter and Electronic First Curtain
- Limitations of the electronic shutter function (phillipreeve.net)
- Sony A7 e-Front Curtain Shut. (helpguide.sony.net)
- When to use/not use Electronic First Curtain Shutter on A7II (dpreview forum post)
- Fastest EFCS shutter speed usable on the a7RII? (dpreview forum post)
- a7RII read noise with EFCS on and off, plus silent, continuous (blog.kasson.com)
- Base ISO and Fake ISOs
- Sony Alpha 7R II: Real-world ISO invariance study (dpreview article)
- Base ISO for Sony A7 (naturescapes.net)
- Sony a7: Is there more noise for ISO <100?(photo.stackexchange)
- ISO Invariance: What it is, and which cameras are ISO-less(improvephotography.com)
- Sony a7RII fake ISOs (blog.kasson.com)
- More information about raw processing in general and Sony A7 raw files in particular
- RawDigger Histograms, part 3: Overexposure Shapes (rawdigger.com)
The Sony histogram turns out to have an "empire state building" shape at the point of sensor saturation. The entire How to use RawDigger series, aptly subtitled "Everything You Always Wanted to Know About Raw (But Were Afraid to Ask)", is an absolute treasure trove of information about getting the most out of your camera's raw files.
- Links to read noise analysis of various camera models, including the Sony A7R Mk2 andA7S Mk2 (photonstophtos.net)
- RawDigger Histograms, part 3: Overexposure Shapes (rawdigger.com)
For whatever unfathomable reason, Sony crippled the firmware for the original A7 cameras, turning what should have been a professional-level camera into, well, a camera I wish I hadn't purchased. Sony refuses to issue appropriate and much-needed firmware updates to older model A7 cameras despite repeated requests from users and ample documentation of various problems that could be solved by firmware updates. And Sony is dolely out critically important changes to the firmware for newer A7 models at a depressingly slow rate. Sony, what's the deal here?