08-28-2017 08:24 AM
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08-28-2017 08:45 AM
Your camera can accept EF-S and EF lenses or 3rd party lenses made for those mount systems. There is a price to pay when adding teleconverters because they cut the amount of light hitting the AF sensors & in many cases you loose AF. The very best source of info I know of for TC info is here (note 2 discussions about them).
08-28-2017 10:29 AM
Right off the top of my head. I don't think any Canon t-con works with any ef-s lens. So, that is OOTQ.
However, any of them work with your EOS 70D.
BTW, I think some of, if not all, the Kenko t-cons will work with ef-s lenses. I believe it to be such a bad idea I haven't tried to do it. MOST of the time extenders are not good. You give up too much for what you get. There are a few lenses that handle a t-con well but they are the exception and they are the expensive lenses.
08-28-2017 11:27 AM - edited 08-28-2017 05:22 PM
Traditional tele-converters will not work with the EF-S 55-250 IS STM.
You can use a teleside-converter. I use one for weight savings on long distance hikes. I am using Canon TL-H58 1.5X with the EF-S 55-250 IS STM. Since teleside-converters have a larger front lens (entrance pupil) than the lens they are mounted on, they do not change the f/stop the way traditional teleconverters do. This is not a budget option and I am only using it because weight is my number one priority.
Canon SL1, EF-S 55-250 IS STM, Canon TL-H58 1.5x, 250mm X 1.5 while maintaining f/5.6 AF.
Instead I would recommend you get the Canon EF 70-300mm f/4-5.6 IS II USM (Nano-USM).
08-28-2017 04:09 PM
I purchased the kit which includes a Canon EFS 55 to 250 mm IS lens. I am trying to understand options going forward. Can the 70D body only accept a EFS lens?
You can use ANY Canon "EOS" lens *except* for the Canon EOS "EF-M" lenses. EF-M lenses are designed to work with Canon's "mirrorless" model cameras.
This means you can use any Canon EOS "EF" lens or any Canon EOS "EF-S" lens.
Canon also makes some specialty lenses including the MP-E (special extreme macro photo lens) and a series of TS-E (tilt-shift lenses that correct for parallax distortions and allow you to adjust the angle of the focus plane) and these ALSO work with any Canon EOS body (including your camera).
You can also use any 3rd party lens (such as those by Sigma & Tamron to name just a couple of brands) PROVIDED they are designed for "Canon EOS" mount (because those same companies make lenses for other camera brands.)
As for the extenders... there is a list of lenses that are supported by each extender and you should only attempt to use an extender with a supported lens. There are a few reasons for this... one is to make sure it fits. But the other is that the extender changes the behavior of f-stop & focus and you want to make sure the lens actually works correctly when the extender is attached.
Canon's list of supported lenses for the 1.4x III and 2x III is located here:
If it isn't on that list, you should not attempt to use the extneder with the lens.
08-28-2017 05:55 PM
The image sensor is not used for focusing when you are shooting through the viewfinder. Autofocus is performed by a sensor in the bottom of the camera, which looks up towards the mirror.
It is the AF sensor that is affected when you use an extender, because of the reduction in light reaching it. The sensing elements on the AF sensor have a f/stop rating, which reflects their sensitivity to light. Your sensor needs f/5.6, or wider, to function reliably.
A 1.4x extender reduces light by one full stop, and a 2.0x extender reduces light by two full stops. The reduction in light passing through the lens, is similar to stopping down the aperture, or reducing the size of the lens aperture.
A 1.4x extender changes the effective focal length of a 100mm f/4 lens into a 140mm f/5.6 lens. Notice how both the focal length and f/stop have been multiplied by 1.4. A 2.0x extender will effectively change the lens to a 200mm f/8 lens.
The camera engineers know that reducing the light hitting the image sensor reduces its' accuracy and reliability. Through the camera firmware, if the camera sees an affective maximum aperture as high as f/8, then it will disable the AF system.
On that note, if you decide to buy an extender, be aware that you will most likely be manually focusing with most lenses, including the two that you have cited. If you do go for an extender, I suggest a 1.4x extender, instead of 2.0x extender.
08-29-2017 03:20 AM
A 55-250 IS already looks unsharp at 250mm. Adding an extender will just make a big impact of the image quality. You will, like everyone here already answered you, loose the viewfinder AF at smaller apertures than f/5,6. Live view AF will still work. 70D has dual pixel AF and that will even work better.
I have tried 100-400/4,5-5,6L IS together with x2 III. It gave me 800mm f/11. In daylight my M5 handled it without any issues. So did also my 7D and my 6D in Live view. The image quality is so bad that I don´t use the combination.
If you want to compare lenses against each other, check out The Digital Picture and press Lens Image Quality.