This Extender works great with existing Zoom Lenses. It can also save money by not having to buy additional zoom lenses. The only drawback, if not planning your shots, is having to take it on/off.

Using it on my 70-200 IS USM version 2. Was really surprised by how well it works. So glad I spent the extra money. Well worth buying if you have a need for it. Wish I would have had it sooner because of all the pics I took before owning it.

I bought this extender to complement the 70-200mm telephoto. It adds a lot of capability at a reasonable cost and is worth the investment

In wildlife photography, one seldom sees a distant object of interest filling the entire frame. Therefore, options to improve the picture often include: 1. cropping to the desired object size, or 2. using a longer lens, or 3. using an extender with a compatible long lens. In the comparison between 1 and 3, I used a a Cannon 5d Mark II and a EF100-400mm f/4.5-5.6 L IS USM.(Push/Pull Model) ISO 400, Aperture Pref., Daylight WB. set at f5.6 and 400 mm, and handheld for Case 1. For case 3 all the same except that an EF 2XIII extender and a heavy duty tripod was added and the f stop went to 11. In both cases, the resultant shutter speed was fast enough to eliminate any camera shake. When case 1 and case 3 photos of stationary birds at a sun lit feeder at a distance of about 25 yds were cropped to the same object size, not only was there NO IMPROVEMENt in case 3 but actually, the pictures became grainy, noisy, and and had color distortions on the edges of the objects. The Extender appeared to be well built and attached and detached easily. With the above combination, standard AF will not work because the max f is 11. However, the workarround of shooting in live mode, with AF on, AF driver set as Live and focussing by depressing the AF-ON button till the rectangle turns green is easy and gives an accurate focus but slightly slower than standard AF. Since the investment cannot yield an improvement for the particular use for which I purchased it, I returned the Extender.

waited a month to get it but it did not fit my lens. Ad did not say that few lens fit the extender

I bought mine several years ago and never got chance to use it. I just got a new camera and wanted to try it, but I am so disappointed, the autofocus doesn't work at all when used with EF100-400mm f/4.5-5.6 L IS USM on 5ds r.

Selected an EF 100-400 zoom to pair with this 2x extender thinking it would give better versatility than selecting a longer 500 or 800 lens. Sad and disappointed that EF signals are not passed through this extender from camera to lens. Looking forward to a firmware upgrade from Canon to address this.

Outstanding images with this in conjunction with my 300mmF2.8L. Frustrating that it does not work with the 100-400 F4.5L ll USM, while it did work with the original version of this lens, (although this was not consistent).

I was not prepared for the lack of sharpness using this adapter at 200mm

This extender when paired with a high optical quality lens can yield sharpness very close to one of the long primes. I've paired it with the 300mm F2.8L IS II lens and a 7d mark II body to get almost 1000mm at f5.6. Image sharpness is very close to what I get with a 600mm f4. AF speed is very fast. It seems slow but due to the design it doesn't have to move AF as far so the net time to acquire focus is about the same. I use this set up with the 300mm to hike with. I can take it places I cannot go with the 600mm f4. Note that it will magnify a poorly performing lens. I definately recommend this if you have a need for occasional long distance photography.