What I look for in a lens.
How many sharp pixels is enough?
Given that more of my images are viewed on electronic devices, I need them to be at least as sharp and detailed as those devices. I also need my images to be future proof for a while. There is no point in shooting for HD (2MP) quality if smartphones are already beyond that resolution and people are starting to upgrade to 4K or UHD smart TVs. Add talk of 8K devices in the next couple of years then it is clear that I need at least a resolution of 3840 x 2160, (8.3MP) for 4K compatibility, or ideally 7680 x 4320, (33MP) for 8K compatibility, of sharp pixels detail from my images.
Loss from aspect ratios.
Considering that the 4K and 8K display aspect ratio is different than the sensor aspect ratio, it means some of the pixels will be cropped away, reducing the usable pixels on the sensor slightly. Then I may want to be able to crop slightly to re-position the subject/s better for each aspect ratio. That also reduces pixel count even further, but let’s call that bad composition technique and ignore that for a moment.
So if I’m using a Canon 6D or 70D with 20MP resolution and absolutely perfect technique I’ll have 16.8MP after cropping to UHD 4K. That means I will need to use a lens that gives me at least 49.2% sharp pixels throughout most of the image frame. That leaves no margin for error or composition adjustments in post. And you’ll find that you quite often come up short and cannot use the image. Anything more than this minimum is a bonus and makes things a lot easier.
To get to 8K images of 33MP, I’d need to use a camera with at least 39.1MP to cater for aspect ratio differences assuming an absolutely perfect lens that gives me 100% sharp pixels. Such a lens does not exist. That means I cannot use the Sony or Nikon full frame cameras as they do not deliver a high enough pixel count. Even on the Canon 5DSr with 50.3MP I would get only 42.5MP after aspect ratio adjustments and I’d need a lens that delivers at least 78.1% sharp pixels across most of the frame.
Most lenses deliver between 50% and 90% image sharpness. Obviously if I’m going to stick with 4K I can use almost any lens out there as long as I use a sensor with 20MP or more. If I’m doing HD images, then I only need 2MP and can sell all my gear and use my mobile phone instead but 8K is a different beast all together.
Minimum and ideal sharpness
Based on the calculations above, if I’m buying a lens, I consider 49% sharpness as minimum and 78% or higher as ideal. Let’s call that 50% and 75% to make it easier to remember. Perhaps one of the manufacturers will release an 80MP or 100MP sensor soon that will mean I do not have to care about sharpness so much, but then so will display device resolutions increase, and I’ll care about sharpness again.
Go to the next page to find out what affects the lens sharpness.