Tony Northrup has made a video on YouTube about mounting full-frame lenses to APS-C camera bodies and it’s inspired me to write this. If you are someone who shoots with an APS-C camera body, buying full-frame lenses for your APS-C camera body is a great idea and could potentially save you a lot of money. Plus full-frame lenses are usually higher quality.
What is your use case? Do you plan to upgrade?
The title, “TECHNICAL: Full-frame lenses on APS-C cameras is USUALLY bad“, makes no mention of image quality; Tony Northrup has alleged, with the title of his video, that it’s bad practise in general to mount a full-frame lens to an APS-C camera body. This claim is wrong.
The title should read “are” and not “is”. Tony Northrup struggles with the English language, for example he says things like “that works good” instead of “that works well”, etc. Something doesn’t work “good” — it is either good or it works well. At first you might think I am being a grammar nazi but a part of me wonders if the average viewer (including myself) misunderstands Tony Northrup’s video because his use of the English language is so poor. If I could understand what he truly means, perhaps his arguments might not be so far fetched? Unfortunately, I cannot debate the imaginary or what’s inside his head; I can only debate what’s said or written… Both of those are inaccurate.
To refute what’s written in the title… If you’ve got an APS-C camera and you plan to upgrade in the future, buying full-frame lenses will likely save you money long-term as you won’t have to sell your APS-C lenses and buy new full-frame equivalents when you upgrade. This should be a major reason as to why you consider full-frame lenses. Full-frame lenses are often sealed better and have superior build quality, amongst other things.
Image quality, lens design, etc.
Tony Northrup has discussed his methodology for judging the image quality. From what I can understand, he’s gone about this entirely the wrong way. He also seems a bit perplexed as to how you keep the lens in the same position to judge sharpness results.
Firstly, you should know that lenses change sharpness based on their aperture, focal length, and focus position. Focus position is changed by changing the aperture and focal length as well. The solution is to mount a manual focus prime lens to your tripod; do not mount your camera body to the tripod unless the sensor is in an identical position relative to the tripod mounting point. Leave the lens set at its widest aperture and do not touch its focus ring. When you want to compare different cameras, you can then swap them over leaving the lens in exactly the same place. An autofocus lens might work but you have to make sure it doesn’t change focus when the camera is powered on.
If you use the crop tool in photoshop, and crop into an image, on a 1:1 pixel level the quality hasn’t changed. That is the equivalent of a crop camera vs a full-frame camera. Assuming all things are equal, mounting a full-frame lens onto an APS-C camera body will not decrease the detail. The sharpest part of a lens is usually always in the middle of a lens; however, it’s somewhat unfair to argue APS-C is sharper in that regard as if the pixel density of the full-frame sensor is the same as the APS-C camera, it still has those sharp parts of the image. It is fair to say that the edge of a full-frame photograph looks less sharp than the edge of an APS-C camera though, taken with the same lens.
The only theoretical anti-argument to this is if APS-C cameras have slightly different microlenses on their sensor or the thickness of the glass covering the sensor is slightly different, and APS-C camera lenses are designed SPECIFICALLY to those specifications.
For you to test all of this accurately, you would have to use cameras with sensors that have an identical pixel density per square inch. Otherwise, you risk changing more than one variable. I’d also recommend mounting an APS-C camera lens on a full-frame camera set to APS-C mode. If Tony Northrup’s claims are accurate, the APS-C camera lens should lose detail on the full-frame camera just as the full-frame lens loses detail on the APS-C camera. If it does not, all you’ve really proven is that specific lens is sharper than the one you compared it to. I’d be impressed (and disgusted) if lower budget (i.e. APS-C) Sony, Canon, Nikon, Leica, etc. lenses are all sharper than their own higher budget models.