Are you getting found?
Create a FREE profile in under 2 minutes to grow your network and get work.   Sign up now!

What are parfocal lenses and why do we video folk want them?

The confusion between parfocal and varifocal lenses kicked in along with DSLRS starting to shoot video, a lens is a lens, right? To the surprise of many, there are significant differences between a lens made for photography, and a lens made for videography. An important difference is how well the lens maintains it's focal point when you zoom in or out. We are of course talking about zoom lenses only, prime lenses have no relevance here.

So what is varifocal?

Varifocal means that as you change the focal length(zooming in or out) of a zoom lens, the focus does not stay the same, it varies. The focal point moves with the zoom action. This is how most photography lenses work. Why? Blazing fast autofocus, that's why. A modern DSLR can auto focus in a split second to get that shot in focus right after zooming. So being parfocal is for the most part negated in photography because autofocus takes care of it. DSLRs are built for photography, and their video capabilities are still just an added feature to their primary function as a stills camera. It's because of that, that manufacturers justifiably didn't have a reason to design the lenses to be parfocal.

What is parfocal?

Parfocal simply means, as you zoom in or out with your lens, the focus will not change, the focal point remains stationary. A parfocal lens allows you to zoom all the way in, get your focus, and then zoom out to get your final framing and know for sure that your subject is in focus.

All(or should be all) fixed lens video cameras are parfocal, and likely a good number of fixed lens photo cameras are parfocal as well. Notice a connection? Pun intended... Designing a lens is a science, and adding more requirements adds more complexity and hence why parfocal lenses can be larger and/or more expensive than varifocal ones.

Fixed lens cameras such as the pictured RX10 are typically parfocal for a number of reasons, for one, the sensor and optics are typically smaller(RX10's is bigger than usual) which leads to already greater DOF and since they are permanently fixed together, they can be precision calibrated in the factory to work perfectly together. Interchangeable camera lens's on the other hand have to work on any camera with that mount and typically have larger sensors.

┬áThe big ENG b4 zoom lenses are parfocal and have a backfocus adjustment, which allows you to adjust the focal flange length(the distance between the lens mount, and the camera sensor). It needs to be spot on for a lens to be truly parfocal. You won't find a backfocus adjustment on a photography lens, again because it's already 'close enough' for the stills autofocus mechanism to finish the job with haste. 

Here's a way to relate backfocus to photography: have you ever owned a photo lens which seemed to autofocus incorrectly even though your focus point is right where you want it? Well in most newer DSLRS like the 5D you can microadjust the autofocus to correct this. It is just a way for you to tell the camera that the lens is focusing too far forward or backward and dial in the correction for it. That's pretty much photography autofocus's version of a backfocus adjustment, but it's dealing with the autofocus mechanism, it doesn't make a lens parfocal. It's the same concept though, a forward-backward adjustment is needed to get the desired results. It's not to say that a lens without backfocus adjustment can't be parfocal, many higher end lenses may not be advertised as parfocal, but since they are built to such a high standard many of them actually end up being parfocal or close enough to it that using a smaller F stop will effectively make it parfocal in certain shooting situations.

And that brings us to how to deal with varifocal lenses when using them for video. 

If you have a varifocal lens but really really want/need it to be closer to parfocal, there are some simple methods of doing so. They do have tradeoffs though, the big one is shallow depth of field and large apertures: You'll have to avoid them. You basically have to fight against the percent of focal point change by increasing the depth of field enough to negate it. If your lens goes really out of focus when you zoom, there probably isn't much you can do, these methods will only help lenses which are fairly close to being parfocal. 

Use smaller F Stops

As i just mentioned above, using a smaller F stop can help because it of course will widen the depth of field. If you have a lens the seems just a little bit varifocal, try stopping down to as much as f/16, and if it's still not quite sharp, see if a little added sharpness in editing can bring it the rest of the way. If at all possible, avoid closing down your aperture any farther than f/16, because you'll start to degrade your image from diffraction.

Be wary of distance markers.

On most lenses you get feet and meter markers which are supposed to tell you where your focal point is. They are great for parfocal lenses, but be extra suspicious of them with varifocal lenses. Think about it: the focal point shifts when you zoom, but the markers don't move, so there is obviously a percent of error going on. I usually use the distance markers as just another verifier of focus, if for some reason i don't have some of the other ideal tools for checking focus and i suspect my shot might be a tad soft, i'll glance at the distance marker to see if im in the right ballpark. It really helps to have a decent judge of distance.

Looking at your distance markers will also tell you how long your focus throw is (how much rotation the focus ring has between macro and landscape) Lenses built for photography have very short throws because it speeds up autofocus, but we as video people prefer longer throws for more fine-tune-ability. Realize also that really short throws means that even a tiny shift in the focal point can move it that much farther from where you want/think it to be. Example: two identical lenses, but one has 1 degree of rotation between 10 and 20 feet, and the other has 10 degrees of rotating between 10 and 20 feet. If both are equally varifocal, then the same slight shift in focal point will be 10 times worse on the lens with a shorter focus throw.

The farther your subject, the more parfocal it may act.

As with lens physics 101, the farther your focal point AKA your in focus subject, the wider the depth of field. We're talking about maximixing the DOF to get around varifocal limitations, and having your subject farther away/using a wide shot/wider field of view will widen your DOF. This of course is not ideal when we're talking about doing long zooms while recording,  because when you zoom in, you'll be narrowing your DOF. You'll have to test each lens to see the limitations, you might need to avoid the longest end of the zoom range, and do a little punch in in editing. As an example if you have an 18-250 lens, it might be best to avoid that 150-250mm range, as it's where the DOF will be the narrowest and more likely to show varifocal effects. The basic idea here is to be more conservative with your zooms if you want to try and get a usable shot out of a lens which is varifocal. For some lenses it just flat wont work, your mileage shall vary.

Diiiisclaimer! notice how i said "the more parfocal it MAY act?" Well, the funny thing about some varifocal lenses is that the focal point shift might not necessarily be directly in relation to the zoom. So as you zoom, the focus might get worse, but then BETTER! and then worse, etc... This throws a nice wrench into our efforts to get them to act parfocal, but the methods i've listed are worth a try nonetheless to see what your lenses are capable of.

Finally, don't expect a lens to be parfocal.

I see this more and more often, because people learn about parfocal and either dismiss a varifocal lens as crap or otherwise have some expectation of a lens to be parfocal. This again is due to either a misunderstanding of photo vs video lenses, and/or unjust expectation. You get what you pay for rules 90% of the time, and expectations of more than that i think of as insensible. 

Added on 2014-04-23 by
Darren Levine

Darren Levine

Stimulus Video

DP/Videographer, Video/Film Editor
Similar Recent Articles
Recommended Products and Services