Now that we have discussed the ins and outs of MTF charts, we can consider how to use them to select a lens. In the previous blog I laid out the dilemma that I was faced with. As a starting point I went to the Canon site and considered the MTFs of two lens that I knew from personal experience perform excellently: the EF-S 18-55mm f/3.5-5.6 IS II (This is the IS version of then lens that I discussed at length in a previous blog) and the EF 70-200mm f/4L USM, which is a lens that I am in love with. The 70-200mm is a bargain among the Canon L Lens series at ~$700 and performs fabulously, as long as you are willing to forgo image stabilization. I use a monopod almost all the time with that lens and results are amazing. If you click on the two hyperlinks for these lenses you can see their MTFs. This gives you a perspective of what good is, and all of the other lenses that I was considered did not have as good MTF performance.
So I thought that I had the issue settled. Buy the EF-S 18-55mm for the great price of ~$199. But then I took a look at the MTF for the EF-S 18-55mm f/3.5-5.6 IS STM. The STM feature enables continuous focusing when you are doing video. Actually, I have no interest in video but the superior performance of this lens in terms of sharpness really makes it, at ~ $249, worth the extra $50. As I’ve tried to emphasize, sharpness is not everything, and a review in PC Magazine, while rating the lens very highly, does describe some of its flaws: small amounts of barrel distortion at wide angle and edge darkening.
All this said the proof is ultimately in the forest! There is nothing like a good tree in the forest picture, with its myriad spatial frequencies to put a lens through its sharpness paces. In Figure 1, I show a picture of a fallen tree in the woods near my house. Note how well the pine needles in the background are resolved. (Note that I like to use a very small amount of Adobe Photoshop’s “Smart Sharpen” feature to crisp things up, although this is not really necessary. I have done so here). Also I took this image handheld to test out the IS feature As a result of all of this, I am declaring the EF-S 18-55mm f/3.5-5.6 IS STM a best buy Canon lens in my book.