Cheap or vintage lenses, filters and poor focus can easily downgrade your 4K camera's performance to HD or worse!
was recently privy to a conversation in which someone first expressed an interest in the saleability of 4K production, then moved on in the next breath to an enthusiastic discussion of classic lenses and diffusion filtering. The creative direction was obvious: the production in question is set in the 1930s, and a crisp, sharp, modernistic style would clearly have been inappropriate. But artistic concerns aside, there might be a need for a reality check here, because while it's been normal for decades to produce material for 2K-ish output – by which we mean 35mm for the big screen – higher resolutions than that are only now becoming common, and not only at the high end where people are best equipped to deal with them.
A 4K digital image, as we've said many times, is sharper than most people are used to seeing from the traditional 35mm photochemical chain. It's sharper than almost all current movies. It's about as sharp as a moderately-good DSLR image. The inevitable result of this is that the demands on the rest of the imaging chain – from filters to lenses to focus pullers and all the rest – are consequently higher. This should be obvious, but I have discussions where it's overlooked all the time. So, if there was any doubt, let's be clear: shooting your movie on lenses which survived the Russian revolution probably isn't going to get you a 4K finish, no matter what camera is behind them.
Of course, a large amount of this is political. Early attempts at 4K cameras were more 4K in name and output resolution than they were in actual resolving ability, and this is an article intended for people who like to peer scowlingly at monitors while wearing magnifying spectacles as opposed to those for whom 4K is a soundbitten selling point. You could, if you wanted, take the position that you're somehow resolving the softness and aberration of old glass, or the glow of a diffusion filter, more faithfully with more pixels. Even so, some of the traditional advantages of higher-resolution shooting – the option to crop and stabilise and reframe – are still denied to filmmakers whose cameras are capable of more than their glass, so it's worth keeping in mind.
Let's consider a few issues which can reduce the effective resolution of cameras:
Motion blur is still blur. Unless the camera manages to describe a path of motion separately in two perpendicular axes within the same frame – which is possible in circumstances of extreme vibration but probably fairly rare in practice – this will not have the same effect as soft focus or a postproduction kernel blur. But the ease with which the precision of a 4K frame can be disturbed by motion blur is alarming, and handheld photography, even with short shutter times, is a good way to compromise perceived resolution.
Any piece of glass placed between a sensor and the scene will reduce resolution because there is no such thing as zero-diffusion glass. As a purely practical matter, the effect is usually small, but it's not difficult to find someone willing to claim that their pet brand of filter offers a better compromise than the competition. All the same, the real issue here is diffusion and low-contrast filters, effectively all of which compromise resolution to at least some degree above the baseline.