The idea that you should invest in glass first and foremost is a very popular one among photographers, especially professional ones.
And in most situation this is true.
However, going to extremes is never good and today we’ll see how it applies to top level lenses when they are attached to an APS-C bodies.
The main idea that people get here is something like this: “I have a good APS-C camera and if I buy one of the most expensive lenses available (designed for a full frame and NOT APS-C camera) I should achieve better image quality. As lenses tend to get less sharp at the edges and so by using full-frame lens on cropped sensor I will only utilize the central part of the image circle generated by the lens, leaving the softer edges outside of the picture frame”.
At first this seems to make sense. But why don’t we see what one of the best metrics on the internet– DxO mark – says about this?
And we’ll do. But first let’s go through the basics.
Let’s say you have a 24-70 mm f/2.8 full frame lens. And on a full frame camera it will give you exactly that: 24-70 mm focal length and 2.8 aperture.
But what will happen if you attach it to APS-C body?
The sensor will capture only a part of the light that goes through the lens, and this will lead to a behavior the same as 36-105 mm f/4.2 lens on a full frame camera. In other words, to get an idea what full-frame lens on APS-C bodies will lead to you should multiply all values by 1.5 for Nikon or 1.6 for Canon cameras. Many people take focal length change in consideration but very often forget about aperture change which may lead to certain level of frustration, especially among portrait photographers as they’ll get considerably less of background blur to what they might have expected.
But back to the subject.
But maybe it’s just an APS-C sensor that is the limit here? Well, let’s determine this. To do this let’s see what measure of sharpness will a kit 18-55 f/3.5-5.6 lens gives us:
As you can see, it gives us 8 perceptual megapixels. 8 perceptual megapixels from a $200 lens compared to 7 from a $1500 lens!
So, apparently lenses designed for APS-C sensors give you better sharpness than even Full frame lenses made of premium materials.
Shouldn’t it be exactly the opposite?
Not really. APS-C sensors have much higher pixel density than Full frame cameras and lenses designed to be used with them take this into account and focus all the light they gather onto that tiny spot crowded with pixels. With the full frames, that light is gathered and spread over a wider area with lesser megapixel density, so by using Full Frame lens on APS-C sensor you will only utilize a central part of image circle which already has lesser intensity, losing even more extra light (information) to nothing. It’s almost like doing a digital zoom.
But is decreased sharpness a trend or a one-time occurrence for only given above lenses?
Let’s see what we have in Nikon world:
So, Nikon’s 24-120mm f/4 is substantially less sharp on APS-C sized D7100. It’s 1/3 as sharp, even though the sensor resolution remains the same. But again, maybe it’s caused by D7100 sensor technology that cannot produce more details? And again the answer is no. We can clearly see this if we attach $800 Sigma 18-35 f/1.8 (27-52 f/2.7 in equivalent):
17 perceptual megapixels compared to 13 on Full Frame D610 paired with supper advanced lens. And the whole APS-C setup ended up costing us 2.5 times cheaper ($1300 instead of $3500)!
Indeed, this Sigma lens is a truly remarkable one and one of the best you can get for APS-C camera; I always recommend people who are serious about their photography to get it.
So, as we can see with real examples, bigger and costlier isn’t always better. In fact quite the opposite is true: to get the best results you need to see how thing work in combination and stick with the ones that were designed for each other and of course keep on reading informative web-sites!
Of course, sharpness is not the single characteristic of the quality of the lens, there are others such as contra light contrast, chromatic aberration, quality of bokeh, durability and so on, but
again this article is not a guide to the best lenses you can buy but an illustration to a common misconception found in photography world.