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A good friend of mine came to me the other day with some questions about lenses. She’s a professional action sports photographer and was seeking advice on the best way to get a reach of about 400mm in a lens package that was still relatively lightweight and portable. I’ve shot a lot of the same kind of work she was specifically talking about so I immediately recommended using a 300mm f/2.8 with a 1.4x extender.
Her response surprised me:
“I’ve never had much luck with sharpness with extenders”
I know this is a feeling that a few people have, and yet I also know many people who rely on them for their work and wouldn’t leave the house without them (myself included). Why is there such a great divide?
This seems like a perfect opportunity to dig a bit deeper, bust a few ‘extender myths’ and show you guys how and when best to use them. There’s definitely a few do’s and don’ts when it comes to extenders and unfortunately the internet forums are awash with people who swear they know better, even though they’ve never actually used the things. I think a lot of the bad press that extenders get is based off old lens pairings and old technology, with people re-hashing the same “well Dave said….” stories from many years ago. Let’s set this straight, starting from the beginning…….
What Are Extenders?
Extenders, also often called teleconverters, are optical accessories that attach to the rear end of a lens to increase the focal length. The two most common varieties are 1.4x and 2x extenders, though Nikon also makes a 1.7x version as well. As you’ve probably guessed, the number represents the factor by which the focal length gets multiplied. A 70-200mm lens with a 2x extender becomes a 140-400mm lens for example.
There’s no such thing as a free lunch though! Using an extender will reduce the maximum aperture of your lens. A 1.4x extender will cause your maximum aperture to be reduced by one stop and a 2x extender will cause a two-stop decrease. Nikon’s unique 1.7x extender slots in the middle with a one and a half stop decrease.
- A 300mm f/2.8 lens with a 1.4x extender would become a 420mm f/4 lens
- A 300mm f/2.8 lens with a 2x extender would become a 600mm f/5.6 lens
You might want to refer to our f-stop reference chart which will help you visualize the various f-stop changes on your lenses, or check out the table below for some common lens examples.
NB: Not all lenses are compatible with extenders. You should always refer to the lens specifications on the manufacturers website before purchasing one. In general, all telephoto prime lenses will be compatible, as will tilt-shift lenses and sometimes macro lenses. Wide-angle lenses are not compatible with extenders, not because you shouldn’t use them, but because you physically can’t use them due to the rear element design of wide-angle lenses. You’ll notice from the photos of the actual extenders, that there is a protruding element that sticks into the back of the lens barrel. Some lenses, wide-angles in particular, have a rear lens element that’s too close to the lens mount to allow the extender’s protruding element to fit into the lens.
Advantages Of Using An Extender
The obvious answer to this is that it gives you a longer focal length, but the REAL answer to this is that it gives you a longer focal length at a lesser cost and weight! Long lenses are inherently big and expensive so using an extender can be a great way to get extra reach from your lens without breaking the bank or your back. To return quickly to the conversation with my friend that started all of this, I recommended she used a 300mm with a 1.4x extender because that combination weights about half what a 400mm f/2.8 prime lens does. In her specific situation, weight was a big factor as she had to carry the lens all day whilst skiing.
The second reason you might want to consider an extender is for the added versatility when used with larger prime lenses. Big primes, like a 300mm or a 500mm are wonderful for their purpose, but you can be limited in the type of shots you can get if you are not able to reposition yourself. Take shooting from a safari vehicle for an example. You spend all day tracking an animal and then when you find it, the vehicle parks up so you can observe. With a prime lens you are really left with only a couple of possible composition options and a vertical and horizontal shot. Using an extender will double the number of potential different shots you can get if you are stuck shooting from one spot! That’s important if you are shooting for an editorial or stock client because more layout options are always welcomed. If you are shooting field sports like football then you can also get added reach when the play moves to the opposite end. This is distinct from using a longer lens in the first place because once play moves back to your end again, you want the option to ‘zoom out’, by removing the extender. Obviously not something you can do with all sports, since you don’t want to be trying to change things in play, but for sporadic action like football it can work.
The third reason you might want to use an extender is to get to a focal length that’s not available natively in any other lens. If you used a 600mm lens with a 2x extender, you would have a 1200mm reach, something that’s not possible to achieve any other way. Even a 600mm with a 1.4x extender gets you to 840mm which is a little longer than an 800mm prime (the longest lens being made at the time of writing this article, apart from highly specialist limited manufacture lenses).
So to summarize the advantages, an extender gives you:
- A longer focal length (obviously)
- A better focal length/weight ratio
- A better focal length/price ratio (there are some exceptions)
- Double the compositional options
- A way to get to focal lengths that aren’t achievable with a prime lens on its own
Disadvantages Of Using An Extender
As I mentioned earlier, use of an extender does reduce the maximum aperture of a lens (explanation why further down). Based on the rules of the exposure triangle this means that you might need to shoot with either a slower shutter speed, or a higher ISO if your aperture was within the lost range before you attached the extender. In my experience, there’s a few common scenarios where this often comes into play and the first one is shooting sport indoors or under lights in a stadium. In those scenarios, light is at a real premium and you also need a fast shutter speed to stop the sporting action. A 70-200 f/2.8 is a common lens for basketball shooting, but adding a 1.4x in a dark sports hall can really cause a struggle for light. The same goes with the venerable 400mm f2.8 on the football pitch at night. NFL games have pretty well lit stadiums but it can be another situation altogether at a college game. These are a couple of times when it might be good to avoid using the extender due to the one stop aperture reduction. On the other hand, if you are shooting a static subject either with a tripod, or at light levels suitable for easy hand-holding, then the one stop reduction (with a 1.4x) won’t make the slightest bit of difference to you.
For most people, the biggest disadvantage is the loss in image quality and this is where most of the anti-extender online commentary is centered. It’s not totally misplaced, because there is a reduction in quality, but how much is it?
Simplistically speaking, an extender is just like a magnifying glass that enlarges the central portion of your lens’ field of view and projects that onto the camera’s sensor. Since it’s only projecting a smaller central part of the original lens, vignetting is actually reduced when using an extender. Unfortunately, all of the flaws in the original lens become magnified so this means that chromatic aberrations become more apparent, the image becomes softer and there is a reduction in contrast. A 2x extender will therefore always deliver a softer image because it is magnifying the flaws by 2x instead of 1.4x. The actual optical quality of the extender itself doesn’t have as big a part to play in this whole thing as most people would imagine. It’s the original lens’ optical quality that pays a huge part! If the lens didn’t have any flaws then there would be none to magnify, right?
This explains, for the most part, the split in opinions over the use of extenders. In fact it’s the very reason my friend was put off using them. When I questioned her further, she told me she had used one on a 70-200 zoom, but she’d not tried one on a prime supertelephoto lens, a lens that is natively MUCH sharper than a zoom lens. Most people’s assumption is that the image degradation is entirely due to sub-par optics in the extenders, and they forget to think about the quality of the original lens. As the saying goes….. “garbage in, garbage out, magnified by 2”………. Well ok, so I added that last bit myself, but you get the picture!
The final disadvantage of an extender is that it will cause a reduction in focus speed. With most lenses it’s not perceivable in single shot mode without dedicated testing, but it can be seen in AI Servo tracking accuracy. If you’re shooting static objects then it probably won’t be an issue, but if you are shooting sports or wildlife then it can cause a reduction in your keeper rate. How much, depends a lot on the model of the original lens, the camera model and the extender model. Due to all the possible variations, it’s hard for me to state exactly what would or would not be possible to shoot, but you can take a look at some of my examples to realize that there’s still a ton of possibilities……. tack sharp cars coming at me at 200mph for example…..
AF systems depend on light for their accuracy so by reducing the maximum aperture of the lens, we reduce the available light for the AF sensor. In-turn, this means that a 2x extender will have a greater effect on the AF speed. From my experience, for many people the 2x crosses a usability boundary with AF speed reduction and very fast paced sports like motor racing. I’m quite happy with the speed of a good lens and a 1.4x extender, but a 2x puts me into an area when I see my keeper rate drop below a threshold that I’m comfortable with. I appreciate this this is a bit of a generalization since we all have our own thresholds but it’s usually AF speed reduction that stops me using a 2x extender rather than an image quality reduction, and that fact alone might surprise a few people.
To summarize the disadvantages, an extender:
- Reduces AF tracking speed and accuracy
- Increases chromatic aberrations
- Decreases lens sharpness
- Decreases image contrast
Get Your Technique Down!
It’s often underestimated how much your long lens technique needs to improve when you start using extenders, particularly with longer telephoto lenses. There’s a radical difference between getting a sharp image with a 300mm and a 600mm, if you’ve just added a 2x extender for example. I believe this is another reason why extenders can have a bit of a bad reputation, so it’s worth paying attention to this.
The generally accepted starting point for shutter speed selection for a sharp image, is 1/focal length. That means if you add a 2x extender, you’ll want to double your shutter speed. Long lens technique is a whole article for another day, but keep in mind that you’ll want an elevated shutter speed or potentially a monopod or tripod to combat shake from handholding.
Common Lens And Extender Pairings
This table gives you the common available options but before buying an extender you should always consult the manufacturers specifications to verify your intended combination will work. With regards to most tilt-shift lenses, the manufacturers tend not to specify them as ‘officially’ compatible because correct aperture values are not passed to the camera, but they should work optically with no trouble. There’s also a few lenses out there for which the third-party Kenko extenders (1.4x, 2x) are able to work, where official ones do not. Canon’s 100mm f/2.8 L IS Macro for example.
A couple of spaces in the table have the value N/A for the Nikon only 1.7x extender where Nikon does not make a lens of that focal length.
Extender Usage Tips For Maximising Image Quality
There’s a few things that can be done in order to maximise the image quality you get from a lens/extender combination. Essentially we just need to follow the steps that we normally would when thinking about maximising image quality from a lens. The sharper we can get that original image, the less flaws will be magnified. The main thing is that we want to stop the lens down to increase sharpness and reduce chromatic aberration. Whenever I use an extender, I always try and stop down the aperture at least one stop from where I probably would have had it without the extender. The change in depth of field that this causes, is mostly cancelled out by the focal length extension anyway so there’s usually little to no difference in the overall look of the image, apart from the fact that you are zoomed in further of course. Since I shoot a lot of sports and wildlife where I need to maintain a fast shutter speed, I usually have to increase my ISO to compensate for the smaller aperture that I need to stop down to.
Since contrast is reduced, you also want to avoid using extenders in flat, low-contrast light. If you are able to reposition yourself and you’re struggling with the contrast, move until the sun is further off to one side where it will create more contrast.
In terms of AF speed and accuracy it all comes down to light levels. Try to avoid using extenders in very low light situations, especially the 2x ones. With wildlife photography it’s good to take this into account because much of the action can be at the very beginning or end of the day when light levels are very low indeed. If your plan is to always rely on using an extender then you might want to re-think it because in those low-light moments, the AF speed will be more greatly affected. This is one of the big reasons that I always tell people to buy a lens that suits their shooting without an extender, and use the addition of an extender for ‘bonus’ shots. I’d never recommend buying a lens with the sole intention of always having an extender mounted to it.
Why 1.4x and 2x?
Whilst researching this article I realized all of a sudden that I knew why extenders are typically presented in 1.4x and 2x formats. It hadn’t really occurred to me before but suddenly it was obvious! This does probably fall beyond the bounds of interest to some of you but I’m including it because it interested me to know the answer so there must be a few folks out there who’d also like to know.
F-stop is defined as focal length/diameter and it’s for this reason that the maximum aperture is affected when we use an extender, it’s got absolutely nothing to do with less light being passed through the extenders. If you double the focal length in that equation, with the diameter remaining the same, you can see that the f-stop will be affected. In my article, Understanding Aperture, we talked about the progression of f-stops being in multiples of 1.4 (actually the square root of two, but close enough). Each increase in f-stop is 1.4x the previous one (2.8, 4, 5.6, 8, 11 etc.). The reason that 1.4x is chosen for the extender then is that it’s this multiplication of focal length in that equation that gives an exact 1-stop difference in maximum aperture! I’d always assumed that 1.4x was an arbitrary number that someone has chose at some point, but now I can see that it was actually chosen because in that equation, multiply the focal length by 1.4 changes the f-stop by one stop. Multiplying the focal length by 2x changes the f-stop by two stops because 1.4*1.4 is 2! It’s that same progression again. The powers that be could have chosen any values for the the extenders, but by using 1.4x and 2x, they deliver a one-stop and two-stop reduction in aperture which is just a nice easy way to deal with it! That’s really all it is!
Using more than one extender at the same time is generally not accepted to provide acceptable image quality, but once again it depends somewhat on your intended use for the images. With excellent long lens technique, I believe it’s certainly possible to achieve what I would call ‘decent’ quality images of static subjects from a pair of stacked 1.4x extenders. Note the use of the word ‘static’ though, since AF speed takes a big hit when you try this.
Canon 200-400 using both the built-in 1.4x extender AND a second 1.4x extender.
Teleconverters vs Cropping
Can you simply crop in on your image instead of using an extender? Well…. a simple crop would have the same effect, up to a point. Without extending the lens, you aren’t getting the added compression and stronger out-of-focus background that comes from telephoto images though. Less noticeable if you are thinking of using a 70-200 rather than a big tele prime though. Another much bigger problem though is that you can’t accurately frame an image in your viewfinder if you always plan to crop it later. You’re essentially removing a huge part of the artistic process and I can’t wrap my head around why anyone would want to do that. It’s also a pain in the ass to have to crop hundreds of images and the reality is that you probably wouldn’t bother doing them all.
Now, from an image quality standpoint, what people want to know is whether the degradation caused by an extender, is less that that caused by cropping and then re-enlarging that crop to the original pixel dimension. I’ll hold my hand up says I’ve never performed a direct side-by-side…… but only because I think it’s not worth the bother. There’s no way that the extender is going to be a worse option than a crop and enlargement. Photoshop, and programs like OnOne Perfect Resize do a great job, but it’s always noticeable. Oftentimes when I look at my extender images I can’t even tell if I used one or not when it’s a good lens.
Is there a quality difference between using an extender or an aps-c sensor with a given lens to extend your telephoto capability (assuming the same ratio)?
This question came in via my personal Facebook page. The answer is yes, there is definitely an optical quality difference because the use of a cropped sensor doesn’t result in any of the image degradations we discussed earlier. None of the lenses flaws are magnified in any way, all that’s happening with a crop sensor is that a smaller area of the lens is being used to capture the image, the central part. This is if we just consider optics though. It get’s a little trickier when you consider that full frame sensors are inherently higher quality than APS-C ones, and whilst that has nothing to do with lenses and extenders, it should be taken into account.
To totally ignore the possibilities that an extender can give you would be a mistake. I hope that some of the sample images on this page can reinforce this notion, although web resolutions can’t be an ultimate judging point, you’ll just have to trust me that the pixel peeping results are very good and that I’ve satisfied clients all over the world with images created with an extender. You can’t just slap one on and expect to keep shooting the way you were without it though. A few simple adjustments in aperture and shutter speed will go a long way to maintaining the quality of your images.
Where To Buy
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