[Note: before moving on with this post, a comment on stupid spell-checkers... my blog writer (and even Microsoft Word!) insists that "adaptor" is a mis-spelling. Not so... "adaptor" is a device that adapts one thing to another that would otherwise be incompatible, while an "adapter" is a person that adapts to new situations or environments... I've seen countless instances of mis-use... I fear that even educated users are deferring to software, assuming that it's always correct. The amount of flat-out wrong instances in both spelling and grammar in most major software applications is actually scary...]
Okay, now for the good stuff… While the iPhone (in all models) is a fantastic camera for a cellphone, it does have many limitations, some of which have been discussed in previous articles in this blog. The one we’ll address today is the fixed field-of-view (FOV) of the camera lens. Since most users are familiar with 35mm SLR (Single Lens Reflex) – or, if you are young enough to not have used film, then DSLR (Digital SLR) – and have at least an acquaintance with the relative FOV of different focal length lenses. As a quick review, the so-called “normal” lens for a 35mm sensor size is a 50mm focal length. Anything less than that is termed a “wide angle” lens, anything greater than that is termed a “telephoto” lens. This is a somewhat loose description, and at very small focal lengths (which leads to very wide angle of view) the terminology changes to a “fisheye” lens. For a more detailed explanation of focal length and other issues please see my original post on the iPhone4S camera “Basic Overview” here.
Overview
The lens that is part of the iPhone4S camera system is a fixed aperture / fixed focal length lens. The aperture is set at f2.4 while the 35mm equivalent focal length of the lens is 32mm – a moderately wide angle lens. The FOV (Field of View) for this lens is 62° [for still photos], 46° [for video]. {Note: since the video mode of 1920×1080 pixels is smaller than the sensor size used for still photos (3264×2448) the angle of view changes with the focal length held constant} The fixed FOV (i.e. not a zoom lens) affects composition of the image, as well as depth of field. A quick note: yes the iPhone (and most other cellphone cameras) have a “zoom” function, but this is a so-called “digital zoom” which is achieved by cropping and magnifying a small portion of the original image as captured on the sensor. This produces a poor quality image that has low resolution, and is avoided for any serious photography. A true zoom lens (sometimes called ‘optical zoom’) achieves this function by mechanically changing the focal length – something that is impossible to engineer for a cellphone. As a rule of thumb, the smaller the focal length, the greater the depth of field (the areas of the image that are in focus, in relation to the distance from the lens); and the greater the field of view (how much of the total scene fits into the captured image).
In order to add some variety to the compositional choices afforded by the fixed iPhone lens, the only option is to fit external adaptor lenses to the iPhone. There are several manufacturers that offer these, using a variety of mechanical devices to mount the lens. There are two basic divisions of adaptor type: those that provide external lenses and the mounting hardware; and those that provide a mechanical adaptor to use commonly available 35mm lenses with the iPhone. One example of an adaptor for 35mm lenses is here, while an example of lens+mount is here.
I personally don’t find a use for adapting 35mm lenses to the iPhone: if I am going to deal with the bulk of a full sized lens then I will always choose to attach a real camera body and take advantage of the resolution and control that a full purpose-built camera provides. Not everyone may share this sentiment, and for those that find this useful there are several adaptors available. I do shoot a lot with the iPhone, and found that I did really want to have a relatively small and lightweight set of adaptor lenses to offer more choice in framing an image. I researched the several vendors offering such devices, and for my personal use I chose the iPro lens system manufactured by Schneider Optics. I made this choice based on two primary factors: I had prior experience with lenses made by Schneider (their unparalleled Super Angulon wide angle for my view camera), and the precision, quality and versatility of the iPro system. This is a personal choice – ultimately any user will find what works for them – but the principles discussed here will apply to any external adaptor lens. As I have mentioned in previous posts, I am not a professional reviewer, have no relationship with any hardware or software vendor (other than the support offered as an end user), and have no commercial interest in any product I mention in this blog. I pick what I like, then write about it.
I do want to point out however, once I started using the iPro lenses and had some questions, that I received a large amount of time and assistance from the staff at Schneider Optics, particularly Niki Mustain. I would like to thank her and all the staff that so generously answered my incessant questions, and did a fair amount of additional research and testing prompted by some of my observations. They kindly made available an internal report on iPro lens performance, and the interactions with the iPhone camera (some of these issues to be discussed below). When and if they make that public (likely as an application note on their website) I will update this blog with a comment to point to that, in the meantime they have allowed me to use some of their comments on the general technology and limitations of any adaptor lens system as background for this post.
Technical specs on the iPro lens adaptor system
This particular system offers three different adaptor lenses (they can be purchased individually or as a set): Wide Angle, Telephoto and Fisheye. Here are the basic specifications:
As can be seen from the above details, the Telephoto is a 2X magnification, doubling the focal length and halving the FOV (Field of View). The Wide Angle changes the stock medium wide-angle view of the iPhone to a “very wide” wide angle (19mm equivalent – about the widest FOV provided by most variable focal length** 35mm lenses). The Fisheye offers what I would consider a ‘medium’ fisheye look, with a 12mm equivalent focal length. With fisheye lenses generally accepted as having focal lengths of 18mm or less, this falls about midway between 6mm*** and 18mm.
**There is a difference between “variable focal length” and “zoom” lenses, although most use the term interchangeably not being aware of the distinction between the two. A variable focal length lens allows a continuous change of focal length, but once the new focal length is established, the image must be refocused. A true zoom lens will maintain focus throughout the entire range of focal lengths allowed by the lens design. Obviously a true zoom lens is more difficult (and therefore costly) to manufacture. Typically, zoom lenses are larger and heavier than a variable focal length lens. It is also more difficult to create such a lens with a wide aperture (low f/stop number). To give an example, you can purchase a reasonable 70-200mm zoom lens for about $200 (with a maximum aperture of f5.6); a high quality zoom lens of the same range (70-200mm) that opens up to f2.8 will run about $2,500.
Another thing to keep in mind is that most ‘variable focal length’ lenses are not advertised as such, they are often marketed as zoom lenses, but careful testing will show that accurate focus is not maintained throughout the full range of focal lengths. Not surprising, as this is a difficult optical feat to do well, which is why high quality zoom lenses cost so much. Really good HD video or cinemaphotography zoom lenses that have an extremely wide range (often used for sports television – for example the Canon DigiSuper 80 with a zoom range of 8.8 to 710mm) can cost upwards of $163,000. Warning: playing with one of these for a few days will produce depression and optical frustration once returning to ‘normal’ inexpensive zoom lenses… A good lens is simply the most important factor in getting a great image. Period.
*** The extreme wide end of fisheye lenses is held by the Nikkor 6mm/f2.8 which is a masterpiece of engineering. With an almost insane 220° FOV, this is the widest lens for 35mm cameras of which I am aware. You won’t find this in your local camera shop however, only a few hundred were ever made – during the 1970s – 1980s. The last time one went on auction (in the UK in April 2012) it sold for just over $160,000. The objective lens is a bit over 236mm (9.25″) in diameter! Here are a few pix of this awesome lens:
actual image taken with 6mm f2.8 Nikkor fisheye
Ok, back to reality (both size and wallet-wise…)
Here are some images of my iPro lenses to give the reader a better idea of the devices which we’ll be discussing further:
The 3-lens iPro kit fully assembled for carrying/storage.
An ‘exploded view’ of all the bits that make up the 3-lens iPro system.
The Fisheye, Telephoto and Wide Angle iPro lenses.
Front view of iPro case mounted to iPhone4S, showing the attached tripod adaptor.
Rear view of the iPro case mounted on iPhone4S.
Close-up of the bayonet lens mounting feature of the iPro case.
2X Telephoto mounted on iPhone.
WideAngle lens mounted on iPhone.
Fisheye lens mounted on iPhone.
Basic use of the iPro lens system
The essential parts of the iPro lens system are the case, which allows precision alignment of the lens with the iPhone camera, and the detachable lens elements themselves. As we will discuss below, the precision and accuracy of mounting an external adaptor lens is crucial to good optical performance. It may seem trivial, but the material and case design is an important overall part of the performance of this adaptor lens system. Due to the necessary rigidity of the case material, once it is installed on the iPhone it is not the easiest to remove… I missed this important part of the instructions provided: you must attach the tripod adaptor to the case body to provide the additional leverage needed to slightly flex the case for removal. (the hole in the rear of the case that shows the Apple logo is actually a critical design element: that is where you push with a finger of your opposite hand while flexing the case in order to pop out the phone from the case).
In addition to providing the necessary means for taking the iPhone out of the case if you should need to (and you really won’t: I found that this case works just fine as an everyday shell for the phone, protecting the edges, insulating the metallic sideband to avoid the infamous ‘hand soaking up microwaves dropped call iPhone effect’, and is slim enough that it fits perfectly in my belt-mounted carrying case), the tripod mounting screw provides a very important improvement for iPhonography: stability. Even if you don’t use any of the adaptor lenses, the ability to affix the phone to a tripod (or even a small mono-pod) is a boon to getting better photographs with the iPhone. Rather than bore you with various laws of physics and optic science, just know that the smaller the sensor, the more a resultant image is affected by camera movement. The simple truth is that the very small sensor size of the iPhone camera, coupled with the light weight and small case size of the phone, means that most users unconsciously jiggle the camera a lot when taking an image. This is the single greatest reason for lack of sharpness in iPhone images. To compound things, the smaller the sensor size, the less sensitive it is for gathering light, which means that often, in virtually anything but direct sunlight, the iPhone is shooting at relatively slow shutter speeds, which only exaggerates camera movement.
Since the EXIF data (camera image metadata) is collected with each shot, you can see afterwards what shutter speed was used by the iPhone on each of your shots. The range of shutter speeds on the iPhone4S is from 1/15 sec to 1/2000 sec. Any shutter speed slower than 1/250 sec will show some blurring if the camera moves at all during the shot. So, whenever possible, brace your phone against a rigid object when shooting, particularly in partial shade or darker surroundings. Since often a suitable fence post, lamp pole or other object is not right where you need it for your shot, the ability to use some form of tripod will often provide a superior result for your image.
The adaptor lenses themselves twist into the case with a simple bayonet mount. As usual with any fine optics, take care to avoid dropping, scratching or otherwise damaging the delicate optical surfaces of the lenses. The telephoto lens will most benefit from tripod use (when possible), as the narrower the angle of view, the more pronounced camera shake is on the image. On the other hand, the fisheye lens can be handheld for most work with no visible impairment. A note on use of the fisheye lens: the FOV is so wide that it’s easy for your hand to end up in the image… take some care and practice with how you hold the phone when using this lens.
Optical issues with adaptor lenses, including the iPro lens system
After using the adaptor lenses for a short time, I found several impairments in the images taken. Essentially the artifacts result in a lack of sharpness towards the edge of the image, and color fringing of certain objects near the edge of the frame. I went on to perform extensive tests of each of the lenses and then forwarded my concerns to the staff at Schneider Optics. To my pleasure, they were open to my concerns, and performed a number of tests in their own lab as well. While I will discuss the details below, the bottom line is that both myself and the iPro team agrees that external adaptor lenses are not a perfect science, particularly with the iPhone. We must remember, for all the fantastic capabilities that this device exhibits… it’s a bloody cellphone! I have every confidence that Schneider (and probably other vendors as well) have made every effort within the scope of practicality and budget for such lenses to minimize the side-effects. I have found the actual optical precision of the iPro lenses (as measured for such things as MTF [Modulation Transfer Function - an objective measurement of the resolving capability of a lens system], illumination fall-off, chromatic and geometric aberrations, optical alignment and contrast ratio) are excellent – particularly for lenses that are really quite inexpensive compared to their quality.
The real issue lies with the iPhone camera system itself: Apple never designed this camera to interoperate with external adaptor lenses. One cannot fault the original manufacturer for attempting to produce a piece of hardware that offers good performance at a reasonable price within a self-contained system. The iPhone designers have treated the totality of the hardware and software of the camera system as a fixed and closed universe. This is typical of the way that Apple designs both their hardware and software. There are both pros and cons to this philosophy: the strong advantage is the ability to blend design characteristics of both hardware and software to mutually complement each other in the effort to meet design criteria with a time/cost budget; the disadvantage is the lack of easy adaptability in many cases for external hardware or software to easily interoperate with Apple products. For example, the software development guidelines for Apple devices are the most stringent in the entire industry. You work within the framework provided, or you don’t get approval for your app. Every app intended for any iDevice must be submitted to Apple directly for testing and approval. This is virtually unique in the entire computer/cellphone industry. (I’m obviously not talking about the gray area of ‘jailbroken’ phones and software).
The way in which this design philosophy shows up in relation to external adaptor lenses is this: the iPhone camera is an amazingly good camera for it’s size, cost and weight, but it was never designed to be complementary to external lenses. Certain design choices that are not evident when images are taken with the native camera show up, sometimes rather glaringly, when external lenses are coupled with the iPhone camera. One might say that latent issues in the lens and sensor design are significantly amplified by external adaptor lenses. This issue is endemic to any external lens, not just the iPro lenses I am discussing here. Each one will of course have its own unique ‘fingerprint’ of interaction with the iPhone camera, but the general issues discussed will be the same.
As usual, I bring all this up to share with my readers the best information I can find or develop in the pursuit of what’s realistically possible with this great little camera. The better we know the capabilities and limitations of our tools, the better able we are to make the images we want. I have taken some great shots with these adaptor lenses that would have been impossible to create any other way. I can live with the distortions introduced as a compromise to get the kind of shot that I want. The more aware I am of what the issues are, the better I can attempt (while composing a shot) to attempt to minimize the visibility of some of these artifacts.
To get started, here are some example shots:
[Note: all shots are unretouched from the iPhone camera, the only adjustment is resizing to fit the constraints of this blog format]
iPhone4 Normal (no adaptor lens)
iPhone4 WideAngle adaptor lens
iPhone4 Fisheye adaptor lens
iPhone4 Telephoto adaptor lens
iPhone4S #1 Normal
iPhone4S #1 WideAngle
iPhone4S #1 Fisheye
iPhone4S #1 Telephoto
iPhone4S #2 Normal
iPhone4S #2 WideAngle
iPhone4S #2 Fisheye
iPhone4S #2 Telephoto
The above shots were taken to test one of the first potential causes for the artifacts in the images: the softening towards the edges as well as the color fringing of bright areas near the edge of the image (chromatic aberration). A big potential issue with externally mounted adaptor lenses for the iPhone is lens alignment. The iPhone lens is physically aligned to the sensor as part of the entire camera assembly. This unitary assembly is then inserted into the case during final manufacture of the device. Since Apple never considered the use of external adaptor lenses, no effort was made to ensure perfect alignment of the camera assembly into the case. As can be seen from my blog on the iPhone hardware (showing detailed images of an iPhone torn apart), the camera assembly is simply pressed into place – there is no precision mechanical lock to align the optical axis of the camera with the case. In addition, the actual camera lens is protected by being installed behind a clear plastic window that is part of the outer case itself.
What this means is that if the camera assembly is tilted even very slightly it will produce a “tilt-shift” de-focus effect when coupled with an external lens: the center of the image will be in focus, but both edges will be out of focus. One side will actually be focused a bit behind the sensor plane, the other side will be focused a bit in front of the sensor plane.
The above diagram represents an extreme example, but you can see that if the lens is tilted in relation to the image sensor plane, the plane of focus changes. Objects at the edge of the frame will no longer be in focus, while objects in the center of the frame will remain in focus.
In order to eliminate this probability from my tests, I used three separate iPhones (one iPhone4 and two iPhone4S models). While not a large sample statistically, it did provide some certainty that the issues I was observing were not related to a single iPhone. You can see from the examples above that all of the adaptor lens shots exhibit some degree of the two artifacts (defocused edges and chromatic aberration). So further investigation was required in order to attempt to understand the root cause of these distortions.
Since the first set of test shots was not overly ‘scientific’ (back yard), I was advised by the staff at Schneider that a brick wall was a good test subject. It was easy to visualize the truth of this, so I went off in search of a large public test chart (brick wall…)
WideAngle taken from 15 ft.
Fisheye taken from 15 ft.
Telephoto taken from 15 ft.
To add some control to the shots, and reduce potential errors of camera movement that may affect sharpness in the image, the above and all subsequent test shots were taken while the iPhone was mounted on a stable tripod. In addition, each shot was taken from exactly the same camera position (in the above shots, 15 feet from the wall). Two things stood out here: 1) there was a lack of visible chromatic aberration [I think likely due to the flat lighting on the wall and lack of high contrast edges, which typically enhance that form of artifact]; and 2) the soft focus artifact is more pronounced on the left and right sides as opposed to the top and bottom edges. [More on why I think this may occur later in this article].
WideAngle, 8 ft.
Fisheye, 8 ft.
WideAngle, 30 ft.
Fisheye, 30 ft.
Telephoto, 30 ft.
WideAngle, 50 ft.
Fisheye, 50 ft.
Telephoto, 150 ft.
Telephoto, 150 ft.
Telephoto, 500 ft.
The above set of images represented the next test series of shots. Here, various distances to the “test chart” [this time I needed even a larger 'chart' so had to find a 3-story brick building...] were used in order to see what effect that may have on the resultant image. A few ‘real world’ images were shot using just the telephoto at long distances – here the large distance from camera to subject, using a telephoto lens, would normally result in a completely ‘flat’ image with everything in the same focal plane. Once again, we continue to see soft focus and chromatic aberrations at the edges.
Normal (no adaptor), auto-focus
Normal, Selective Focus
WideAngle, Auto Focus
WideAngle, Selective Focus
Fisheye, Auto Focus
Fisheye, Selective Focus
Telephoto, Auto Focus
Telephoto, Selective Focus
This last set of test shots was suggested by the Schneider staff, based on some tests they ran and subsequent discussions. One theory is that there is a difference in how the iPhone camera internal software (firmware + OS kernel software – not anything a camera app developer has access to) handles auto-focus vs selective-focus. Selective focus is where the user can select the focus area, usually with a little square that can be moved to different parts of the image. In all the above tests, the selective focus area was set to the center of the image. In theory, since my test images were flat and all at the same difference from the camera, there should have been no difference between auto-focus or selective-focus, no matter which lens was used. Careful examination of the above images shows an inconsistent result: the fisheye showed no difference between the two focus modes, the normal and telephoto looked better with selective focus, while the wideangle looked best when auto focus was applied.
The internal test report I received from Schneider pointed out another potential anomaly, one I have not yet had time to attempt to reproduce: using selective focus off-center in the image. This usage appeared to generate results that would be unexpected in normal photographic work: the area of selective focus was sharp, most of the rest of the image was a bit softer, but a mirror image position of the original selective focus region was once again sharp on the opposite side of the image. This does seem to clearly point to some image-enhancement algorithms behaving in an unexpected fashion.
The issue of auto-focus methods is a bit beyond the scope of this article, but some considerable research shows that the most likely methodology used in the iPhone camera is passive detection (that is certain – there is no range finder on an iPhone!) controlled lens barrel or lens element adjustment. There are a large number of vendors that support this form of auto-focus (and here, I mean ‘not manual focus’ since there is no mechanical focus ring on cellphones… – the ‘auto-focus’ can either be entirely automatic [as I use the term "auto-focus" in my tests above] or selective area auto-focus, where the user indicates a region of the image on which the auto-focus is concentrated. One of the most advanced methods is MEMS (Micro-Electrical Mechanical Systems) which moves a single optical element within the lens barrel, another popular method is the ‘voice-coil’ micro-motor which moves the entire lens barrel to effect focus.
With the advances brought to bear with iOS5, including face area recognition (the camera attempts to recognize faces in the image and focus on those when in full auto-focus mode), it is apparent that significant image recognition and processing are being done at the kernel level, before any camera app ‘gets their hands on’ the camera controls. The bottom line is that there may well be some interactions between the way in which the passive detection and image processing algorithms are affected by an unexpected (to the iPhone software) presence of an external adaptor lens. Another way to put this is that the internal software of the camera is likely ‘tuned’ to the lens that is part of the camera assembly, and the addition of a significant change to the optical pattern drawn on the camera sensor (now that a telephoto lens adaptor is attached) alters the focusing algorithm in an unexpected manner, producing the artifacts we see in the examples.
This issue is not at all unknown in engineering and quality control: a holistically designed system where all of the variables are thought to be known can be significantly degraded when even one element is externally modified without knowledge of the full scope of design parameters. This often occurs with after-market additions or changes to automobiles. One simple example is if you change the tire size (radius, not width) the speedometer is no longer is accurate – the entire system of the car, including wheel and tire diameter, was part of the calculus for determining how many turns of the axle per minute (all the speedometer mechanism actually measures) are required to indicate X amount of kph (or mph) on the instrument panel.
Another factor that may have a material effect on the focus and observed chromatic aberration is the lens design itself, and how an external adaptor lens may interact with the native design. Simple lenses are often portions of a sphere, so called “spherical lenses.” Such a lens suffers from significant optical aberrations, as not all of the light rays that are focused by a spherical lens converge to a single point (producing a lack of sharp focus). Also, such lenses bend different colors of light differently, leading to chromatic aberrations (where one sees color fringing, usually blue/purple on one side of a high contrast object and green/yellow on the opposite side). Most high quality modern camera lenses are either aspherical (specially modified shapes that deviate away from a perfect spheroid shape) or groups of elements, some of which may be spherical and others aspherical. Several examples are shown below:
We know from published literature that the lens used in the iPhone4S is a 5 element lens system with at least several aspherical elements. A diagram released by Apple is shown below:
iPhone4 lens system [top] and iPhone4S lens system [bottom]
As a final example, using a more real-world subject, here are a few camera images and screen shots that demonstrate the challenge if one attempts to correct, using post-production techniques, some of the errors introduced by such a lens adaptor:
Original image, unretouched but annotated.
The original image shows significant chromatic aberrations (color fringing) around the reflections in the shop window, grout lines in the brickwork on the pavement, and on the left side of the man’s shirt.
Editing using Photoshop ‘CameraRaw’ to attempt to correct the chromatic aberrations.
Using the Photoshop Camera Raw module, it is possible to manually correct for color fringing shifts… but this affects the entire image. So a fix for the edges causes a new set of errors in the middle of the image.
Chromatic aberrations removed from around the reflections in the window…
Notice here that the color fringing is gone from around the bright reflections in the window, but now the left edge of the man’s shirt has the color shifted, leaving only the monochromatic outline behind, producing a dark gray edge instead of the uniform blue that should exist.
…but reciprocal chromatic edge errors are introduced in the central portion of the image where highly saturated colors abut more neutral areas.
Likewise, the green paint on the steel column has shifted, revealing a gray line on the right of the woman’s leg, with a corresponding shift of the flesh tone onto the green steelwork on the left side of her leg.
final retouched shot after ‘painting in’ was performed to resolve the chroma offset errors in the central portion of the image.
To fix all these new errors, a technique known as ‘painting in’ was used, sampling and filling the color errors with the correct shade, texture and intensity. This takes time, skill and patience. It is impractical in the most part – this was done as an example.
Summary
The use of external adaptor lenses, including the iPro system discussed here, can offer a useful extension to the creative composition of images with the iPhone. Such lenses bring a set of compromises with them, but hopefully once these are known, careful choice of lighting, camera position and other factors can be used to reduce the visibility of such effects. As with any ‘creative device’ less is often more… sparing use of such adaptors will likely bring the best results. However, there are shots that I have obtained with the iPro that would have been impossible with the basic iPhone camera/lens, so I happy to have this additional tool.
To close, here are a few more examples using the iPro lenses:
Fisheye
WideAngle
Telephoto
