This is not an article. A good article is like a five course meal, leading you through various tastes until you get to the big point of the whole thing: cheesecake. But I had a few things I wanted to talk about that weren’t an article in themselves, just blurbs, really. So I planned this one to be more like a buffet. Just go through the line and sample what you want.

That was the plan, anyway. I think what I ended up with was really more like a school cafeteria: go through the line and the big hairy lady slaps down a few various mystery dishes on your plate. If you’re really hungry, some of it might be edible. If you don’t eat yer meat, you can’t have any pudding. How can you have any pudding if you don’t eat yer meat?

Most people will already know about half of this stuff. Which means that most people won’t know about half this stuff. And I doubt any of it will change the way you shoot. But if you’re hungry for some lens knowledge, some of it’s probably edible.

Floating Elements Don’t Float

I get asked about once a week what a floating element does (and a bit less frequently if that’s what’s rattling inside when a lens is shaken). Most lenses have poor performance at their closest focusing distance. Center sharpness may be good, but aberrations and corner softness increase when you’re shooting closeups. Floating elements (Nikon calls it close range correction) are lens elements outside of the primary focus group that change position when the lens is focused on a close object, correcting aberrations and improving close up performance. If you’ll be shooting things 20 feet away a floating element is probably not an advantage, but if you like close up work a lens with a floating element will work much better for you. Almost every macro lens has them, but many other lenses do too.

_A wide angle lens with 2 focusing groups, giving a floating element._ _And a telephoto lens with a single focusing group._

Fixed Apertures Aren’t Always Fixed

At least they aren’t with macro lenses. Focusing on very close objects reduces the effective aperture of the lens, so even if the lens is supposed to be f/2.8, it may not be when you focus very closely. The actual promise of fixed aperture is “throughout the zoom range you can get f/2.8 — unless you focus really closely”. If you shoot Canon you might not notice it because the Canon cameras will still report f2.8 (or whatever the aperture is supposed to be) but automatically adjust ISO or shutter speed, or whatever variable they need to in order to maintain proper exposure. Nikon and other brands will actually report that your f2.8 lens has a maximum aperture of f/3.5 or f/4 when you focus closely.

Variable Apertures Vary More Than You Might Expect

I bet you know your f/3.5 to f/5.6 lens goes from f/3.5 to f/5.6. The manufacturers will tell you that in a heartbeat. But try to find out where it changes and you may be interested in the results. Here’s an example: Lets look at similar lenses with slightly different apertures, the Sigma 50-500 OS f/4.5 – 5.6 and the Sigma 50-500 f/4 – 5.6. OK, so the old one has half a stop wider aperture at 50mm. But look at the point in the zoom range where the lens hits its smallest aperture, f6.3:

  • 50-500 OS f6.3>220mm
  • 50-500 f6.3>420mm

Don’t get me wrong, I’d rather have the excellent OS (optical stabilization) than the half stop of light at 400mm most of the time. But some cameras have decreased autofocus accuracy when the aperture decreases beyond f5.6, so it’s certainly possible that the old lens would focus more accurately on your camera at 400mm.

It’s not a big deal, but it is interesting. For example, if you compare 70-300 zooms, Nikon’s is at f5.6 by 130mm while Canon’s stays under f5.6 until 200mm. The Olympus’ 70-300 gets all the way to 260mm before it hits f5.6. And one generalization: if your lens is f3.5 to whatever, it will probably be f3.5 only at the very widest end of the zoom. Move it 2mm out from that and the aperture will change.

More Elements Aren’t Always Better

Today’s lenses are marvels of design with low dispersion and aspheric elements, supersecret submolecular whatchamacallit coatings etc. I notice more and more the marketing people touting stuff like 63 elements in 45 groups!!!!! in their brochures. One thing we sometimes forget is that every air-glass interface in a lens is going to reflect a fraction of light (usually less than 1%, often as little as 0.2%). That stray light bounces around in the lens and can cause flare, loss of contrast, and ghosting (reflected sunspots in the picture).


The top lens above has 23 elements, the one below it has 12. Below are the flare tests done by Bryan Carnathan at The Digital Picture for those same lenses, 23 element lens on the left, 12 element lens on the right.

Of course lens coatings help minimize flare and the manufacturers are quick to tell you the new versions of some lenses have “supernano” or “subwavelength” coatings that are better than old coatings. They aren’t quite so quick to mention that usually just a single surface in the lens has the new coatings. The other lens surfaces are coated the old way.

_Only the back of the front element in this lens actually has the new subwavelength coating. The other 15 glass-air interfaces have standard coating._

It doesn’t mean lenses with more elements always have more flare and ghosting. But there is a general tendency that they do. Older lens designs with older coatings have the same tendency. So an old lens design with old coatings and lots of elements can have major problems shooting with the sun in the field.

One other point about all that reflection that may be happening: reflected light isn’t getting to the sensor, so the f/# (which is a theoretical calculation) may be quite different than the T/# which measures actual transmission. An f/1.4 lens may, or may not, actually let f/1.4 worth of light hit the sensor. (Actually, it definitely won’t, but more on that later.)

Third-Party Lenses are Different in Different Mounts

This one is going to cause me to get emails, I can tell you that right now. It’s one of those things that is so obvious nobody sees it, and fanboys are going to argue with it, because fanboys don’t let facts stand in the way of a good argument. For starters, the flange-to-sensor distance is different on a Canon camera compared to a Nikon camera compared to a Sony, etc. So theoretically there could be some optical differences in the angle that the image rays impact the microlenses on the sensor. But lets forget about that (because I have no idea if it really makes a difference). Lets focus on focusing.

Every camera manufacturer has a set of electronic algorithms built in to their cameras that talk back and forth with electronic chips in the lens when it autofocuses. When Tamron or Sigma or Tokina make a lens, they have to reverse engineer these algorithms and then make electronic chips. Certain chips (or firmware on the chips) are for talking to Canon cameras, different ones for Nikon cameras, etc. It’s not only possible that the electronics in the lens may work better with one mount than another, it is an absolute fact.

For example, a few years ago (2003 or so) existing Sigma lenses stopped talking to newly released Canon cameras, but they kept working fine with other brands. When the Nikon D3x was released the Sigma 120-300 wouldn’t autofocus with properly with it, although other Sigma lenses worked fine. More recently, the Tamron 17-50 VC and 60 Macro lenses were found to only be able to use the center autofocus point on Canon 50D and 60D cameras, and to not use any cross-type sensors. The reason in this latter case was apparently that Tamron used a “lens ID number” that told the camera it was actually an old, out-of-production Canon lens.

We see it all the time: certain third party lenses seem great on one mount, and have all kinds of focusing issues on another mount. It’s not that one brand of lenses is better than another overall. My point is, when you want to ask someone what they think about a third party lens you’re about to purchase, make sure you ask someone who shoots it on your camera. Otherwise their advice is meaningless.

in my experience, Canon cameras seem to have more trouble with third party lenses than other camera brands do. That’s an impression formed with thousands of lenses and hundreds of camera bodies to test them on, but still, its just my impression and I haven’t seen anyone else say it is so.

Aperture and Focal Length Numbers Aren’t Totally Accurate

Most of you probably know this. But the focal lengths printed on the lenes are approximations. Prime lenses are usually pretty accurate, but zooms may off by at least 5%. So a 50-500mm zoom may actually be 53-475mm. Its not a big deal but it is noticeable sometimes.

What might be important to a lot of people is close-focus, focal-length change that occurs particularly with lenses having rear focusing groups. (Rear focusing groups are all the rage these days because you’re only moving the little elements at the rear of the lens to focus, hence faster autofocus with small autofocus motors.) But when you focus a zoom lens on a close object, the movement of the elements actually shortens the focal length.

So a 70-200 zoom may be 200 fully zoomed when focusing at infinity, but only 150mm when focusing on something nearby. This effect is particularly noticeable on the new Canon and Nikon 70-200 f2.8 zooms and the Canon 70-300 IS L zoom.

Similarly the transmission of light by a lens is never quite up to what the f/stop says it is, the f/# is a theoretical calculation that doesn’t take into account the number of elements, coatings, and other things that might affect light transmission. Just looking at a number of 85 f1.4 lenses, for example, DXO Mark found they were all actually T/1.6 (T being the actual, not theoretical, light transmission). Again, probably not something that really matters to people.

Circular Apertures Aren’t Always Circles

I’ve yet to see a lens whose marketers say “straight aperture blades making a pentagram resulting in harsh out-of-focus highlights”. They either have a blurb that says “circular aperture for pleasing out-of-focus highlights” or they say nothing at all. But even when they say circular, they lie a LOT. Take a look at the apertures of your lens stopped down just a bit: some really are circular, many others are octagons, heptagons, whatever.

_Not picking on Canon (once again they are just willing to make more information readily available), everyone does it. But the apertures above are both described as “circular” in the brochure. Not so much._

By the way, on cameras that don’t have mechanical levers to let you see the aperture with the lens dismounted, you can see them by mounting the lens, setting the aperture to f/5.6 or so, hitting the depth-of-field preview button on the camera, and the removing the lens without releasing the depth-of-field button or turning the camera off.

Teleconverters are Lenses Too

Pity the poor teleconverter. People assume, for reasons I can’t understand, that it’s just a magical wizard’s tube that makes the image larger with no other effects. Teleconverters are actually quite complex little creatures with 3 to 9 lens elements inside:

_Diagram of a Canon 2X teleconverter._

Never forget, when you mount a teleconverter you are using a lens to look through a lens. You can’t do that without some change in image quality.

In Summary

Do any of these things make a difference? Probably not, or at most not often. But not a week goes by that I don’t answer at least one email about each of these topics, usually more.
But I saved the answer to the most frequent email I get for last, as a prize for those who’ve read this far.

Active Stabilizers (IS, VC, whatever) Rattle

The IS group is always near the base (camera end) of the lens, just in front of the rear elements. If you shake the lens, you can hear the IS elements rattle if they aren’t locked down. Why don’t they lock down? Usually because you removed the lens without turning off the camera. Put the lens back on the camera, take a shot, turn the camera off, and remove the lens. For most brands, this makes the rattle stop because the IS system was locked down when the camera turned off (its not true for every IS system, just most).

And while I don’t know for sure, I can see no benefit to shaking your IS system around and making it rattle. As my mom used to say whenever we discussed any plan I had: “No good could come of that. No good whatsoever”. So turn your camera off before removing your IS, VR, VC, Mega OIS, whatever they call it lenses.

This article is also available in German.