Understanding autofocus, and tips for all cameras

_7048556 copy
Levitation (and prefocus). Nikon D700, 85/1.4 G

A side effect of the ever-increasing resolution of today’s cameras is that autofocus must necessarily get more precise, too. The Nikon D800/ D800E issues have shown that even a small misalignment or miscalibration in the focusing system can basically cripple the camera into resolving at a far lower level of performance than it would be capable of under ideal circumstances. Short of using manual focus and magnified live view for everything – I would still recommend doing this for critical work, and I do it when working under any controlled lighting situation since I’m more likely to have the time and be using a tripod – it is therefore highly beneficial to pay closer attention to exactly what is going on when the camera acquires focus.

_7038950 copy
A situation where fast, reactive autofocus can help. Nikon D700, 300/4 D

For DSLRs, SLTs and some mirrorless cameras (the Nikon 1s and Sony NEX-5R and NEX-6), a phase detection system is used. This involves taking some of the light from the subject area, passing it through a beamsplitter and comparing the difference in phase of the output; a CCD is used to measure light intensity as a function of position, and the lens is moved until light from both arms of the beamsplitter is coincident upon a single point. This entire module constitutes the AF sensor array that’s either located at the bottom of the mirror box (DSLRs) or embedded in certain specific photosite locations (mirrorless cameras). If you select a specific AF point, then the camera uses only the sensors corresponding to the location of that point; if you let the camera pick, it will usually sample all points to find which is the closest subject covered by the AF sensor array, and focus on that.

_3035242 copy
En masse. Nikon D3, 70-300VR

Phase detect autofocus is fast and generally does not require racking the lens back and forth – otherwise known as ‘hunting’ – because the sensor is able to tell whether the light is positively or negatively out of phase, and thus in which direction to move the lens in order to correct this and bring the light coincident, thus achieving focusing. The precision of focus depends on several factors: firstly, the resolution of the AF sensor; secondly, the alignment of all secondary optics involved in transferring the light to the AF sensor – specifically, the main and submirror assemblies; any microlenses involved; thirdly, the alignment of the AF and imaging sensors (both must be perfectly perpendicular to the lens mount); fourthly, any calibration data the system requires to establish a perfect zero or null position; and finally, the ability of the lens’ focusing groups to move precisely in small increments that maintain perfect alignment with the optical axis.

_8013045 copy
Turning on the inside. Nikon D800E, 28-300VR

Focusing with wide angle lenses is generally less precise with this method because the differences in phase are a lot less; to complicate things, the lens itself may have optical limitations in its design, introducing field curvature, coma etc – all of which can send potentially misleading data to the AF sensor, resulting in incorrect focus. It also doesn’t help that subjects tend to be a lot smaller, and not filling the AF boxes completely. (It’s also worth noting at this point that the AF boxes themselves are an indication of where the sensor grid lies, but there’s no documentation covering precisely where the active areas are located. For greater precision, perhaps the sensors should be crosses instead of boxes.)

_3030456bw copy
Precisely a fast as a speeding bullet. Nikon D3, 24-70

For moving subjects, phase detect systems either continuously change the focus distance, depending on the instantaneous phase information received at the AF sensor, or alternatively employ a predictive algorithm and multiple focusing points in order to track the subject. The most sophisticated systems also employ information from the metering sensor in order to track the subject by color. None of these systems are infallible, and can be fooled by objects of a similar color or larger size coming between the camera and subject – for instance if your subject happens to duck behind something. Although the level of processing power and sophistication of these systems has significantly increased over the past years; I have yet to see any autofocus system that can 100% reliably track an erratically moving subject – especially if it leaves the area of the frame covered by the autofocus sensor array.

_7032151bw copy
Cyclist, Nepal. Nikon D700, 24/1.4

I’m sure you can now see why the challenge of achieving perfect focus gets more and more difficult as sensor resolution increases: if any one of these is out of tolerance by a very small margin, you’re not going to have a sharp image.

Most mirrorless/ CSC cameras, compact fixed-lens cameras and DSLRs in live view all use a much simpler method of focusing – contrast detection. This involves moving the focus point of the lens back and forth to test which direction delivers the highest contrast. The camera will then iterate this process until highest contrast is achieved; although hunting has been minimized with the new generation of contrast detect cameras; it is still necessary to rack focus back and forth simply because there is no way for the camera to know which direction in which to move the lens. Because of this contrast detect autofocus will always be slower than well-implemented phase detect autofocus, with all other things being equal. However, it will also be more accurate simply because the imaging sensor is used to determine the point of optimal focus, and there are far fewer potential issues with tolerances and alignment of components.

_780_IMG_2312bw copy
Point of view. Canon IXUS 100 IS

It is also worth noting that the size of the sensor actually plays an important part in determining just how fast contrast-detect autofocus systems can be; this is because larger sensors have shallower depth of field for a given field of view and aperture, requiring more movement of the focusing groups within the lens in order to determine where the point of highest contrast (and correct focus distance) lies. This is especially noticeable when comparing a compact camera to a DSLR; compounding this is the fact that small sensor cameras require much shorter real focal lengths to achieve the same angle of view; this results in extended depth of field for a given angle of view, requiring less focus precision because any potential errors can be covered up by increased depth of field. The slow focusing of DSLRs in live view mode is not due to the lens’ focusing motor speed; the same combination often is capable of delivering blazingly fast results when used with the regular phase-detect system.

_M224792 copy.jpg
The instant of terror when you’re base jumping and not sure if your chute is going to open or not. Nikon D200, AI 500/4 P

There’s one added method that used to be common in older cameras, but is now only to be found on some of the Ricoh compacts: active phase detect. This uses an infrared beam to light the subject, and the reflected light is measured by two phase detection sensors on the front of the camera to assist the contrast detect system. It can greatly speed things up, but range is limited because it requires active illumination from the camera – and the power of these secondary lights is always limited.

Now that you have some understanding of how autofocus systems work, let’s talk about some tips to maximize the accuracy and speed of your camera.

_M80_DSC3155 copy.jpg
Through the flood. Nikon D80, 17-55/2.8

All cameras

  • Don’t let the camera pick the focus point for you. Unless you are shooting and erratically moving subject which you cannot follow manually selecting the focus point; always shoot in single point mode and pick your focus point carefully to be over your subject. Many cameras also weight the metering in favor of the focus point; it is therefore important to ensure that it corresponds with your subject – it is almost always what you would want to have correctly metered anyway.
  • Make sure your subject is larger than your focus point. If it isn’t; you need to either move closer (this also becomes a compositional issue) or focus on something at the same distance which presents a larger target.

Phase-detect cameras (DSLRs, Sony NEX-5R, NEX-6, Nikon 1)

  • The camera will always focus on the closest object underneath the focusing point. It may sometimes be fooled by a higher contrast structure – for example, a barcode instead of a blank piece of paper immediately behind it – but in general it will pick the closest subject providing it completely covers the focusing point.
  • High-contrast subjects (again, like barcodes) make ideal autofocus targets. It is also worth noting that some autofocus points are sensitive to detail in one direction only; i.e. horizontally or vertically, and not both directions. (Cross type points are sensitive to detail in both directions; but these are generally only found at the center point, or distributed around the AF-sensor arry only on high end cameras.) It is therefore important to find a suitable target for your camera – a QR code rather than a barcode, I suppose.
  • Use continuous autofocus, unless you are shooting a static object with the camera on a tripod. This is because any small motion of either you or the subject can be enough to move the plane of focus away from the intended point; this is especially critical with fast, shallow depth of field lenses. With continuous autofocus, the camera is always focusing right up to the point of image capture. The one exception to this, is slow, or wide angle lenses. Smaller format cameras are a bit less sensitive to this issue because they have more depth of field for a given angle of view, which tends to compensate for any errors in the focusing system.
  • Try to avoid focusing at the center and recomposing your image where possible, because there are potential issues with field curvature – especially at the edges and corners of wide angle lenses. Use the autofocus point that is either directly over your subject or closest to it in order to minimize any potential issues with the lens’ design.
  • Assign a button to locking focus (AF-L) to use in conjunction with continuous autofocus; this saves you having to switch to single autofocus with static subjects. Alternatively, decouple focusing from the shutter button by assigning an AF-ON button that activates focusing when pressed; I don’t use this method as it requires you to press two buttons to shoot; I prefer to minimize the number of controls that must be attended to especially in fast-moving situations.

IMG_0940b copy
Panning in the rain. Apple iPhone 4.

Contrast detect systems (DSLRs in live view, compacts, CSCs, mirrorless cameras)

  • Once again, do not let the camera pick the focus point for you; select it yourself. If anything, cameras that use contrast detect systems tend to be far more flexible in where you can put your focusing point; this is because they use the entire area of the imaging sensor.
  • Avoid continuous autofocus. This seems counterintuitive in light of my advice for phase detect cameras, however this is because continuous autofocus on a contrast detect system is constantly hunting back and forth around the point where expects to subject to be; imagine a car trying to follow a curve that the driver can’t see until it’s almost immediately in front of him – the path (here, the focusing distance) will be erratic and not match the curve exactly. This tends to result in a very low hit rate. It also helps that contrast detect cameras tend to either have an alternate system to deal with moving objects (in the case of DSLRs); or employ much smaller sensors that are very forgiving of minor focusing errors or changes in subject position due to their extended depth of field.
  • If you have to use continuous autofocus because your subject is moving; there are two other alternatives. The first option is to set your camera to maximum contrast (for obvious reasons) – the live view image is usually a preview of your current camera settings and will match the JPEG output. If you’re shooting raw; your file will not be affected by in camera processing. The second option is an old trick using the days of manual focus photography; it’s called ‘trap focusing’. First, decide on your composition and where your subject must go in order to complete it; ensure your shutter speed is high enough to prevent motion blur of the subject; finally, choose single autofocus and prefocus the camera at that position, releasing the shutter when the subject is in the intended position. One added advantage of this technique – especially for compact cameras – is that it significantly reduces the shutter lag to the point where it is very easy to release the camera at the precise moment you intend. Note that if you cannot get a high enough shutter speed; then you will need to pan through with the subject in order to only blur the background out and keep the subject sharp; this is a combination of panning and trap focusing techniques and works best when the subject is moving across your field of view; it is pretty useless if the subject is coming towards you.
  • Some cameras have a continuous pre-focus or full-time autofocus option that is always adjusting the lens based on whatever subject happens to be under the focusing point at the time. This is generally a good option if you absolutely must reduce shutter lag and are unable to pre-focus. However, note that the system can also be fooled, most notably by moving the camera around rapidly – especially if you are not pointing it at anything in particular. It is also an enormous drain on (usually already short) battery life because the lens’ focusing groups are constantly in motion so long as the camera is switched on.

It is worth practicing all of these techniques until they become second nature; you’ll be surprised by both the increase in your keeper rate, as well as the improvement in acuity and sharpness at the individual pixel level. It is just one of the many elements of shot discipline; which is critical in achieving the highest possible image quality from your camera. You’ll also be surprised at just how much more responsive your camera has seemingly become. MT

____________

Visit our Teaching Store to up your photographic game – including Photoshop Workflow DVDs and customized Email School of Photography; or go mobile with the Photography Compendium for iPad. You can also get your gear from B&H and Amazon. Prices are the same as normal, however a small portion of your purchase value is referred back to me. Thanks!

Don’t forget to like us on Facebook and join the reader Flickr group!

appstorebadge

Images and content copyright Ming Thein | mingthein.com 2012 onwards. All rights reserved

And the Nikon D800 autofocus saga continues (with some comments on specific lens performance)

This post is a quick update to my D800 autofocus issues, as well as commentary on the specific performance on some of the more popular lenses people have been asking about. I don’t have time to post crops, but I think most of you would trust that I know what I’m doing.

I spent the morning at Nikon. Ostensibly, to collect my replacement D800, and a PC-E 85/2.8 Micro. However, it turns out the replacement D800 exhibits the SAME autofocus issue – namely, with wide angle lenses, the center and right side AF points yield in focus images, the left side bank is way out. This is especially obvious with the 24/1.4 G. We also tried their NPS loaner demo units and their D800E sample. The results were mostly the same – all of the D800s showed identical results. The D800E was a bit better, but still noticeably soft on one side. It gets worse: I’ve had a number of emails from people with cameras in the same serial number block – below 1000 – and the 24/1.4, who are finding the same thing. Apparently it is a serious issue, because my NPS rep told me that HQ has asked for updates and is looking into it on the production line.

Perhaps it was my 24/1.4 sample that was the issue – nope, because it works fine on a D3x, D4 and D700; we tried another 24/1.4 which showed consistent results – that rules out lens problems. I think we can also rule out sensor alignment problems as I don’t see any odd shifts in the focal plane when focusing using live view.

The upshot is that it will take them two days to diagnose the problem, and possibly longer to fix. Since it’s only an issue with wide angles, and not an issue with anything above about 50mm, I elected to keep this body for the time being – I’m only using it in the studio with the 60/2.8 G Micro, and now the 85/2.8 PCE. It looks like I will be reviewing both E and non-E after all – I’ve elected to take a D800E for the replacement unit.

There were more surprises in store, though – specifically, with lenses.

Summary of Nikkors tested so far on the D800:

AFS 14-24/2.8 G: Not good at 14mm; obvious corner sharpness issues. Displayed AF issues at 24mm. Center is sharp. T stop is probably closer to f4 than f2.8. Average to good performer.

AFS 24-70/2.8 G: Sharp everywhere in the range, at every aperture. No AF issues, even at 24mm. Excellent performer.

AFS 70-200/2.8 G VR II: Sharp everywhere in the range, at every aperture. 85mm setting better than the 85/1.4 G at f2.8, and comparable to the 85/1.8 G at f2.8 (yes, you read that right. The 85/1.8 G is better than the 85/1.4 G.). No AF issues either. Excellent performer.

AFS 24/1.4 G: Sharp everywhere except extreme corners at every aperture if you live view – remains an optically amazing lens, but now even more fiddly to use thanks to the AF issues. Three copies all displayed left-side softness on the D800, but not on other bodies. Cautiously, I’d say excellent performer, to be confirmed once I have a properly working body.

AFS 28-300/3.5-5.6 G VR II: Sharp everywhere if you close down the aperture on stop. Microcontrast not great, but serviceable. Overall global contrast is good. Color a bit odd. Good to very good performer. No AF issues, probably covered by depth of field and small apertures.

AFS 35/1.4 G: No good wide open. Center is okay, both sides are not good – even after AF fine tune. Not recommended. The 24-70 performs much better at 2.8 than the 35/1.4 does at the same aperture. Slight AF issue noticed, same as 24/1.4.

AFS 85/1.4 G: Inconsistent. Wide open displays LCA and LoCA at edges. Nowhere near as good as it was on the D700/ D3/ D3s. Stopped down to f2.8, it improves, but only to about the same level as the 70-200/2.8 II wide open. Note T stop is pretty high for this lens though – probably 2/3 stop more than the 70-200/2.8 II, and half a stop more than the 85/1.8 G for the same aperture. Good to very good stopped down. Honestly, I’m not liking this lens very much anymore.

AFS 85/1.8 G: Incredible. Sharp everywhere at every aperture, no LCA or LoCA. Bokeh is neutral, not quite as good as the 85/1.4 G. Surprising considering this lens has no ED glass, Nano coating or aspherical elements. It’s honestly an optical masterpiece, and very, very cheap. If you need an 85mm and don’t have the 85/1.4 G already, I’d suggest buying one of these. Performance at wide open at f1.8 is better than the 85/1.4 G at 2.8; it matches or slightly exceeds even the 60/2.8 G Micro at the same distances. You’re probably wonder what’s the catch: two things; T stop and build quality. T stop is half a stop down on the 85/1.4 G for the same aperture, and it’s light and plasticky. Still weather sealed, though. Excellent plus performance, no visible AF issues.

PC-E 85/2.8 Micro: This is the only lens of the group tested that could best the new 85/1.8 G, and by the slightest of margins (or maybe both lenses out resolved even the D800E sensor and we’d need something even higher density to see the difference). Global contrast is a little lower than the 85/1.8 G, but micro contrast has more bite and structure to it – reminds me of the Zeiss macros. Excellent plus performance. (I took this one home, after relieving my credit card of some of its available balance. Look out for a full review in the future once I get a chance to shoot it in the studio.)

PC-E 24/3.5: A truly excellent piece of glass. Matches the performance of the 85/2.8, but at 24mm. Shame about the small aperture, though. Handily focuses to about 20cm – which is about 3cm from the front element of the lens. I’d say sharpness performance of this and the 24/1.4 G at f4 are about the same, however the micro contrast structure of this lens is almost Zeiss-like in detail. Excellent plus performance again. And whoever said it won’t mount is wrong – it mounts and offers full movement just fine, but you must zero all of the movements before trying to mount it, and there are certain orientations that work better than others (big knobs vs small knobs near the prism etc.)

AFS 60/2.8 G Micro: Although this was my reference standard on the 12MP FX bodies, it’s performance clearly isn’t up to the D800’s demands: I’m seeing plenty of longitudinal CA (especially in the bokeh) that wasn’t there, or almost negligible, on the D700. It’s sharp already at f2, but not critically bitingly sharp til f4-5.6; your working aperture range is somewhat limited because diffraction kicks in noticeably by f16, and it’s unusably soft by f25. I’d say f22 is probably best reserved for emergencies. This is the main reason I got the 85/2.8 PCE: lack of depth of field control. I’d put it in the good-to-excellent range.

A word on the D800E: I didn’t have a lot of time with it, but from what I can see, there is a slight but noticeable difference in fine micro contrast, as well as sharpness and resolving power. It seems to offset diffraction to some extent. However, file sizes will be even bigger, and lens demands even higher. Recommendation: use with caution, requires controlled circumstances to get the most out of it (tripod or studio lights, low ISO).

Conclusion: If you plan on getting the most out of your D800/D800E, you’re going to have to rethink your lens lineup. What worked brilliantly for me on the D700 – as in I felt I couldn’t get any more image quality out – isn’t working on the D800. And there are a lot of surprises here; not all of them good – the 85/1.4 G and 24/1.4 G are good examples of this. It seems that one has to now choose for a lens set optimized for studio work (or slightly brighter light conditions) – 24-70, 70-200, 85/1.8G, 85/2.8 PCE – with the compromises that brings for available light work, especially now that you’re going to require more shutter speed to handhold and the sensor loses out a stop to the D700 at the pixel level – or run two sets of lenses. This obviously isn’t ideal, or cheap. I feel the latter route is likely the way I’ll have to go – probably with the 85/2.8 PCE for the majority of my studio work, and a Zeiss 21/2.8 or 24/3.5 PCE for architecture and interiors.

Am I happy with feeling like a bit of an expensive guinea pig? Not one single bit. I think this latest push in resolution has brought up manufacturing tolerance and QC issues that were never previously noticeable. But at least a) it works under a known range of conditions, and more importantly NPS here deserves credit for doing their best to rectify the situation, and at least provide me with a working solution in the intermediate period (D3x on extended loan for high-res WA work).

It’s not ideal, but when you get everything right, the D800 is capable of delivering pretty darn amazing image quality. The trouble is, once you’ve seen it, you really don’t want to give it up – even if it is a colossal pain to achieve. Of course, none of this will be news to seasoned medium or large format shooters – but for anybody expecting to go from a DX consumer body, or even 12MP FX, to D800 and get pixel-level crispness across the frame, there’s going to be something of a steep learning curve to climb. MT

Check back for more updates once my D800E replacement body (finally) arrives at the end of the month.

____________

Visit our Teaching Store to up your photographic game – including Photoshop Workflow DVDs and customized Email School of Photography; or go mobile with the Photography Compendium for iPad. You can also get your gear from B&H and Amazon. Prices are the same as normal, however a small portion of your purchase value is referred back to me. Thanks!

Don’t forget to like us on Facebook and join the reader Flickr group!

appstorebadge

Images and content copyright Ming Thein | mingthein.com 2012 onwards. All rights reserved

Update on Nikon D800 focusing issues

It seems that I’m going to have to redo all of my focus tracking and lens evaluation tests: my AF sensor module is misaligned. I’ve come to this conclusion after a) trying out a number of wide angle lenses, all of which exhibit a soft left side when the AF system or focus confirmation dot reports achieving focus; and b) sending it in to Nikon locally to diagnose. Curiously, the problem is only exhibited with wide angle lenses (telephotos are mostly fine), and live view confirms it’s an AF sensor issue not an imaging sensor issue.

The bad is that it’s serious enough to require a new camera because the calibration is a very high precision adjustment that takes time (and unclear if it can be done locally); the good news is that NPS Malaysia made the situation right: a couple of hours after reporting the problem, I met with a representative who informed me a new D800 will arrive next Wednesday, and I’ve been loaned a D3x until the replacement arrives – at which point it will just be swapped out. Kudos to NPS for handling the problem well though – that’s how professional service should work (though a new camera on the same day would be even better…)

Still, a bit of a shame, because I planned to use the D800 for its first commercial shoot this weekend – looks like I’ll only be using it for the stop-down telephoto shots. MT

Coda: It’s interesting just how sluggish the D3x feels when shooting in 14-bit raw mode – just 1.8fps, with what feels like a good 200ms or more of lag between hitting the button and the shutter firing. It’s very noticeable if you put the camera back into 12 bit mode, after which it feels responsive and snappy like it should. Even the menus feel a little laggy, like the camera’s processor is working very hard. The D800, by comparison, runs happily along at 5fps in 14-bit mode – 3x the frame rate with files that are 50% larger, for a total of 450% more data. You don’t feel any lag, and zoomed in images – even 14 bit raw – are very fast to navigate around. Faster than the D700, even. I don’t know how many other people have noticed this, but it’s pretty darn impressive, in my opinion.

____________

Visit our Teaching Store to up your photographic game – including Photoshop Workflow DVDs and customized Email School of Photography; or go mobile with the Photography Compendium for iPad. You can also get your gear from B&H and Amazon. Prices are the same as normal, however a small portion of your purchase value is referred back to me. Thanks!

Don’t forget to like us on Facebook and join the reader Flickr group!

appstorebadge

Images and content copyright Ming Thein | mingthein.com 2012 onwards. All rights reserved

How-to: Improving your autofocus performance with the AF Fine Tune setting

Over the last few days and in the light of focus criticality with the Nikon D800, I’ve had a huge number of questions specifically regarding focus precision, and whether there’s anything we can do about lenses that are obviously off. Specifically, how does the AF fine tune setting work, and how does one calibrate their lenses?

1. Make sure your camera has AF fine tune. Any of the newer Nikons or Canons offer this feature, and perhaps other brands, too.

2. Find a suitably well-lit target that you and the camera can focus, and preferably one that’s flat but slightly off-plane to the camera, so it offers a variety of different focus distances. An ideal test chart is a world map on a wall, but with the camera to a slight angle to the wall (i.e. sensor plane is not perfectly parallel.)

3. Use a sturdy tripod, and set single AF. This is to rule out camera shake or false-AF movements from the camera attempting tracking.

4. Perform this test with the aperture set wide open. Stopped down, depth of field will hide focus errors.

5. If you have live view (I believe most cameras with AF fine tune do), then use magnified live view to achieve critical focus and take one shot. This will be your benchmark image – the sharpest that the lens can possibly deliver, when focused perfectly.

6. Switch back to single AF, and viewfinder mode.

7. Defocus the lens between attempts: this is important, otherwise the camera may be trying to move the focus gearing in too small an increment to overcome backlash.

8. Let the camera focus normally, and take a shot.

9. Compare the image from step 5 with the image from step 8 – if they’re the same at 100% size, then you’re fine – move on to the next lens.

10. If the images from 5 and 8 aren’t the same, then adjust the AF fine tune setting – say by 5 notches (if you have +/- 20 notches like the Nikons) and repeat 7, 8, and 9. If it’s better, keep going – do another 5-notch adjustment in the direction that created your improvement. If worse, then go back in the opposite direction. Reduce the adjustment amount changes and keep going. Basically, you want to iterate the process until you match your viewfinder-AF image with your live view focused image.

11. Save the adjustment value – if you can’t save camera settings to a card, you might want to write it down somewhere just in case.

12. Repeat for all of your other lenses.

A special note for zooms: most AF fine tune functions don’t let you input separate values for different focal lengths, so you’ll just have to try both ends of the zoom range and see which is worse; then run steps 1-12. Try to find a compromise setting that works acceptably well for all focal lengths in the range. Or, if you use one particular end of the zoom more than the other, you might want to weight your adjustment to this end of the range. MT

More D800 autofocus observations

After a couple more days of testing, I’ve got more observations on the D800’s autofocus system:

1. I think we’re reaching the limits of accuracy for CAM3500FX, and in fact, any phase detect based AF system. There are just too many parts that have to be precisely perpendicular and in exact alignment to achieve focus accuracy – the AF sub mirror assembly, the AF sensor itself, and the main imaging sensor. If any of these is out of plane by a few microns, then you’re going to see some softness. We’re now getting enough resolution that the planarity of the lens mount relative to the sensor becomes an issue – to say nothing of perfect alignment of optical elements. I believe there was an article posted a while back on the Luminous Landscape about shimming a sensor and how much resolution improved by both on-center and especially in the corners of the frame.

2. Future AF systems will have to be hybrid – i.e. use some form of contrast detect or phase detect embedded into the imaging sensor in order to work around these limitations. It doesn’t however solve the problem of mount planarity or lens element alignment.

3. There are some things you can do as a photographer to counter these limitations, chief of which is use live view for critical focusing, or stop down – or better yet, both. Live view eliminates problems of AF sensor/ sub mirror alignment. Stopping down covers slight sensor misalignment with depth of field.

4. AF fine tune is an absolute must to get the most out of the AF system.

5. Bad news for manual focus fans. I did my mirror alignment and calibration this morning – it was almost perfect from factory, which is a first; however, my joy died after removing the focusing screen. The focusing screen in the D800 is a different size to anything Nikon has yet produced. Worse still, it’s the largest one I’ve ever seen, so you can’t even cut something down to fit – it’ll just drop out. This is a real shame; I can only hope a third party produces replacement screens for MF aficionados.

6. Finally, lenses you thought were fantastic on the previous 12MP FX cameras may now only be mediocre or average on the D800 – you have been warned. MT

A quick note on Nikon D800 autofocus…

Up to this point, I’d been shooting the camera with the same autofocus settings I used on the D3 and D700 – which share the same CAM3500FX AF module. I think I just discovered why the AF system doesn’t seem to be as precise as before.

Previously, I used single point AF-S for static subjects, and 51-point dynamic 3D tracking AF-C for everything else. I could lock on with the center point, focus and recompose, and everything would be fine. It seemed like a good starting point for the D800.

Turns out I was wrong. Single point AF-C is MUCH more accurate and slightly faster than 51-point dynamic 3D. It’s solved a good number of my AF issues. Remains to try it out tonight when the light gets low to see if performance is improved under those conditions too. MT

____________

Visit our Teaching Store to up your photographic game – including Photoshop Workflow DVDs and customized Email School of Photography; or go mobile with the Photography Compendium for iPad. You can also get your gear from B&H and Amazon. Prices are the same as normal, however a small portion of your purchase value is referred back to me. Thanks!

Don’t forget to like us on Facebook and join the reader Flickr group!

appstorebadge

Images and content copyright Ming Thein | mingthein.com 2012 onwards. All rights reserved

Follow

Get every new post delivered to your Inbox.

Join 27,534 other followers

%d bloggers like this: