What I look for in a lens.
The ideal lens
DXOmark has done us a huge favour to put so many lenses to the test and publish their results. Their sharpness rating is said to be the average sharpness over the lenses aperture and focal length ranges. Therefore, if a lens is terrible at one or more of its extremes, wide open, fully stopped down or at maximum focal length, it will score much less even though it may be pretty good in the sweet spot. In my opinion, the ideal lens is capable of producing sharp 8K images across most of its aperture and focal length ranges. Very few lenses today fall within this ideal category. According to DXOmark, only 4 lenses they have tested on the 5DSr achieve an average sharpness greater than what is needed to produce sharp 8K images over most of its range, and almost two thirds of lenses fail to meet sharpness requirements for 4K on a 20MP sensor. Those top performers are:
Canon EF 300mm F2.8L IS II USM – 90% sharp
Carl Zeiss Apo Sonnar T* 2/135 ZE – 82% sharp
Carl Zeiss Apo Planar T* Otus 85mm F14 ZE – 82% sharp
Sigma 50mm F1.4 DG HSM A – 80% sharp
But DXO’s PMP (Perceived Mega Pixel) score does not tell the full story. Take the wedding photographer favourite Canon 70-200mm F2.8L IS II USM for example. According to DXOmark, the average sharpness rating achieves 33PMP on a Cannon 5DSr (66%) sharpness. But if you look at the lens profile, wide open it gives about 70 to 75% sharpness across the frame. Once you stop it down a bit and zoom in a little, the sharpness profile is even better than that of the Sigma 50mm listed above, giving you about 75 to 80% sharpness across the frame. Yes, the Sigma 50mm is pretty sharp across the frame for most apertures while the Canon 70-200 is less so, but the Canon is a lot sharper than 66% most of the time. Don’t let a few bad measurements from the extremes of settings fool you, it is indeed an exceptional lens.
Let’s look at one of the worst performing lenses, the Tamron AF 28-300mm F/3.5-6.3 XR Di VC LD Aspherical [IF]. It has a huge focal length range making it one of the most versatile lenses out there today. According to DXOmark it only delivers 10PMP on the 50MP Canon sensor.
That is terrible! If this score from DXO is to be believed, then it equates to only 20% sharpness. However, if you actually look at the sharpness profile you will see that wide open, you get an average of about 55 to 60% sharpness across the frame.
Very close to the sharpness of some of the very best lenses available. You’d really have to take a close look to see the difference. Even at its worst combination of focal lengths and aperture, this lens’ sharpness profile still showed almost 70% sharpness in the center of the frame going down to about 40% on the edges and corners. If you were to average that out, you’d still get a good 55% or so sharpness across the frame.
That is good enough for this lens to be used at any aperture and focal length setting on a 20MP sensor, and still deliver 4K images. How DXO came up with the 10PMP, I don’t know, their data does not support such a claim. Here is how DXO give their scores:
The DxOMark resolution score shows sharpness performance of a lens-camera combination averaged over its entire focal length and aperture ranges. You can find out more on their website.
Fair enough, what it does show is that some lenses have a definitive sweet spot where they work best, making them more than acceptable for normal use, while other lenses work really well over a larger range of focal length and/or aperture combinations, making them exceptionally convenient and easy to use as an ideal lens.
It is therefore crucial that you study each lens’ technical properties in detail by taking a closer look at the measurement details and technical specifications. Look at sharpness profile, aberrations, distortions, vignette, stabilisation, focus speed, focus noise, focus breathing, etc. and understand how it will fit into your shooting style or needs, before committing to buy it.
Final thoughts and conclusion on the next page.