Please see Making Sharp Images for extensive coverage of all aspects of lens performance and techniques for maximizing sharpness.
A reader emails to ask:
I'm hearing more buzz about a 36 megapixel D800 from Nikon. If the sensor resolution - and resulting small pixel size - are true, what are your thoughts about the demands it will place on lens selection?
Let’s just suppose that Nikon dishes out a delicious Nikon D800 with a 36 megapixel sensor (I’m just sayin’).
First of all, does such resolution even make sense? Yes it does, because while that’s 50% more pixels than the Nikon D3x, I also have full confidence that sensor and electronics technology has advanced enough to mitigate the per-pixel quality challenge— evidence of that is apparent with Canon’s new 1D X, which opted for fewer pixels, but very low noise; the alternative was likely not chosen due to market requirements (“18 megapixels is enough” when pixel quality is high).
Existing smaller sensor DSLRs already have photosite sizes similar to or smaller than a 36MP full-frame camera would have; the Nikon D7000 pixel pitch scaled up to full-frame would be a 37MP sensor. We’re already there in terms of pixel density.
Optimal resolution from a 36MP sensor will require impeccable technique, and this subject is addressed in every aspect in Making Sharp Images.
Not just about resolution
A sensor with 36 megapixels can be thought of as just a higher sampling density, suitable for down-sampling to 24MP to eliminate digital artifacts
So even if technique and lenses are not scaled up to match the higher pixel density requirements of a 36MP sensor, the lens still delivers the same image to it— so the higher pixel count can be used to produce a higher quality 24MP (or 18MP or 12MP) finished image from a 36MP sensor. This technique is covered in DAP. It works very well to eliminate digital artifacts on the finest of details.
Camera and lens tolerances
Camera and tolerances are already too sloppy. Unless tolerances are tightened substantially, 36 megapixels with a misaligned lens mount and sensor (yes, from the factory) is going to mean uneven sharpness, even with a perfect lens. For that matter, viewfinder optical path and sensor optical path are already outside the required tolerances on many camera bodies (e.g. the two paths do not match in distance). As well, the Nikon implementation of Live View has got to be fixed after being algorithmically broken for 4 years now (banging the mirror is an absolutely moronic implementation). Canon’s “Mode 2” Live View is perfect. See MSI.
As demonstrated in MSI for both Canon and Nikon f/1.4 lenses, today’s DSLRs have focus consistency and accuracy adequate for perhaps an eight (8) megapixel camera. It is hopeless for a 36MP camera. Yes, such cameras can be accurate with some lenses a good part of the time, but nowhere near enough to be relied upon at wide apertures. One can forget about optimal results with autofocus at f/1.4 and f/2 and f/2.8, except under ideal conditions with some luck. This means using Live View with a loupe, or having really good eyes and a perfect camera viewfinder, or just shooting more and getting lucky.
Which leads us to lenses— what are the requirements for a lens that can be considered not just good enough, but excellent for use with a 36 megapixel sensor?
The following issues are key, assuming exacting camera tolerances and excellent technique.
- High resolving power with high contrast = high MTF. The fine pixel pitch of a 36MP sensor demands high MTF for the finest details.
- Low field curvature — designs with significant field curvature become even more problematic than they already are, yet stopping down to mitigate field curvature will kill contrast from diffraction (both topics addressed in MSI). Fast lenses, even the most expensive ones with the red dot on them, have pronounced field curvature. With some lenses, focusing at infinity requires f/8 to produce edge to edge sharpness— with an 18MP camera!
- Sharpness over the whole frame — here, even new designs have their limits when the edges and corners are approached. The best lenses still do not deliver full sharpness across the frame, especially in the wide angle realm. There are a few exceptions, but these are lenses with slow maximum apertures.
- Minimal color aberrations. While lateral chromatic aberration can be corrected in software, it remains highly undesirable, and can impede Live View focusing when strong enough. Axial chromatic aberration is much more problematic because it interferes with focusing, making exact focus ambiguous, as well as polluting the resulting image with lower contrast and/or purple fringing.
- Accurate focus. This means using Live View with a loupe. And it means that manual focus lenses with manual focusing helicoids are even more useful than ever before. Autofocus lenses are difficult to work with because of overly sensitive and very limited “throw” for fine adjustments.
Here I will nit-pick on Zeiss lenses for Canon and Nikon, which are my preferred lenses. These comments are strictly in the realm of obtaining maximum sharpness, sharpness is only one aspect of a satisfying image.
Challenges in the context of a 36-megapixel DSLR—
- Zeiss 18mm f/3.5 Distagon — chromatic errors need improvement, field curvature at infinity is too strong, cyan color cast towards the corners.
- Zeiss 21mm f/2.8 Distagon — should carry forward nicely, but will likely show some limits that are not apparent at 21 or 24MP, including some traces of chromatic errors.
- Zeiss 25mm f/2 Distagon — perhaps the best 24/25mm lens available, but still not strong enough at the edges and corners, interacting with mild field curvature. Central 3/4 of the frame should perform really well on 36MP sensor. Outstanding color correction might nonetheless show a trace of weakness on 36MP.
- Zeiss 25mm f/2.8 Distagon — field curvature and chromatic errors need improvement, but otherwise very sharp and contrasty, will hold up well overall. Not well corrected for close range, yet I love shooting this lens!
- Zeiss 28mm f/2 Distagon — field curvature and chromatic errors need improvement. Edges and corners could disappoint on 36MP.
- Zeiss 35mm f/1.4 Distagon — will hold up well on 36MP, but as shown in my Guide, there can be some apparent color errors at close range (or something akin to it which makes focusing harder), and mild field curvature.
- Zeiss 35mm f/2 Distagon — great sharpness, but chromatic errors are too high for a 36MP sensor, and edges/corners might be more of an issue.
- Zeiss 50mm f/1.4 Planar — excessive field curvature (which is also the case for just about every 50mm lens of the dozen I own). Focus shift from spherical aberration is already a challenge for a 24MP sensor.
- Zeiss 50mm f/2 Makro-Planar — field curvature as edges and corners are approached, which is odd for a macro lens. And limiting for full-frame sharpness.
- Zeiss 85mm f/1.4 Planar — focus shift from spherical aberration already makes it a challenge when focusing wide open and shooting at f/2 - f/5.6, especially at close range.
- Zeiss 100mm f/2 Makro-Planar — outstanding sharpness, but needs better axial color correction to reduce purple halos in the f/2 - f/4 range, as well as background color cast.
There you have it— even top-grade glass has plenty I’d like to see improved! Don’t take these comments as suggesting there are better alternatives. If there were, I’d be shooting them! The question is whether there is a middle ground between a $25,000 cine lens and a lens for a DSLR that is not too large and heavy (make a lens large enough and expensive enough and most optical issues can be addressed). Note that the recent Zeiss designs (21/2.8, 35/1.4 and 25/2) have already become more highly corrected than predecessors, with correspondingly higher prices. I would like to see Zeiss take that approach still further, even if it meant greater cost, since owning fewer better lenses is preferable to more lesser lenses.
One reader already has misconstrued my comments above— when I say the Zeiss lenses are my “preferred” lenses, I mean just that. I am not making any claim that they are always the sharpest— they aren’t. Both Canon and Nikon have some excellent lenses also (and some duds). But the image that results from a lens, the way it draws is not something that comes down to a single sharpness value; it is a mistake to think that sharpness is the sole criterion for choosing a lens. By way of example, the Zeiss 25mm f/2.8 Distagon is poorly corrected at close distance, not very sharp, and yet it might still be my favorite lens for what I’d term “artistic” work. Ditto for the Zeiss 85mm f/1.4 Planar, and this is by design (class computation for portraiture). An artist doesn’t use just one brush, nor should any photographer looking for a particular style of rendition.