topic Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens in Gear Guide

Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

mglotspeich — Fri, 27 Jun 2025 00:22:05 GMT

Hello,

I have had the Canon APSC T2i camera for years. I purchased the EF-S 55-250mm f/4-5.6 IS STM Lens when I received the camera (10 years ago). I now have a full frame r6 Mark ii camera and I am looking at purchasing the RF 100-400mm f/5.6-8 IS USM Lens ($699.00). With the cropped sensor I was getting 400mm out of the EF lens. If I purchase the RF lens for my R6 mark ii will it be an upgrade? Or should I get the adapter and use the old lens on my full frame camera? Or should I purchase the 600mm f11 lens instead?

Thank you! Mike, St. Louis, MO

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

kvbarkley — Fri, 27 Jun 2025 00:26:34 GMT

You probably do not want to use the 55-250 on the R6. Putting the -S lens on a full frame R camera will make the camera use only an APS-C chunk of the sensor. Get the R lens, which *will* be an upgrade.

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

p4pictures — Fri, 27 Jun 2025 07:11:57 GMT

I would go for the RF 100-400mm F5.6-8 IS USM lens for your EOS R6 Mark II, and then you can add an RF 1.4x extender to get you to a 140-640mm F8-11 zoom should you need the reach of a 600mm lens. You have the most versatility with the zoom lens.

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

March411 — Fri, 27 Jun 2025 15:02:00 GMT

The RF 100-400mm and the R6 MkII are a great combination and adding the 1.4x if you can swing the expense would be a great play. I own this setup and it has provided many excellent captures.

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

ebiggs1 — Fri, 27 Jun 2025 15:15:44 GMT

I am sure you really already knew the answer and should buy the RF 100-400mm f/5.6-8 IS USM. The EF-S 55-250mm f/4-5.6 IS STM Lens was never 400mm, you do know that?

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

mglotspeich — Fri, 27 Jun 2025 18:59:30 GMT

Thank you for your advice!

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

mglotspeich — Fri, 27 Jun 2025 19:03:17 GMT

Thank you! With the RF 1.4x extender, wouldn't the RF 100-400 become a 140-560 mm?

Thanks again!

Mike

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

mglotspeich — Fri, 27 Jun 2025 19:06:29 GMT

Thank you, Marc for confirming your experience with the exact camera and lens with extender. Also, thanks for posting this beautiful photo! I will mainly be using the setup for bird photography.

Mike

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

mglotspeich — Fri, 27 Jun 2025 19:17:19 GMT

Hello. I do not understand why you are sure that I knew the answer. If I knew the answer, I would not waste anyone's time asking a question. And yes, I did know that the EF 55-250 was never 400mm. But I ALSO knew that when attached to my T2i APS-C cropped sensor it is an equivalent to a full frame (35mm) 88-400mm lens. If I use the EF 55-250 on my full frame camera (using the RF to EF adapter) I will be getting 400mm out of the lens. I was asking if I should just keep the EF lens instead of purchasing an additional RF 400mm lens.

Thanks.

Mike

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

kvbarkley — Fri, 27 Jun 2025 20:14:29 GMT

"If I use the EF 55-250 on my full frame camera (using the RF to EF adapter) I will be getting 400mm out of the lens."

At much less resolution.

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

March411 — Fri, 27 Jun 2025 20:40:30 GMT

If you have this information mglotspeich my apologies for being redundant.

To kvbarkley's point, the calculation for determining reduced megapixels using an EF-S lens on a full frame sensor. Verify the crop factor of the camera that you will mount the lens onto, for Canon it is typically is 1.6x (Canon does have several choices, the camera defaults to 1.6x). This means the sensor is 1.6 times smaller in linear dimensions compared to a full-frame sensor.

Square the crop factor, to calculate the reduction in area you need to square the crop factor. For a Canon camera it would be 1.6x1.6 = 2.56

Calculate the reduced megapixels by dividing the full-frame camera's megapixels by the squared crop factor.

Full-frame camera: 24 megapixels
APS-C sensor (Canon): 1.6 crop factor
Calculation would be 24 / (1.6 * 1.6) = 9.375 megapixels. The resulting image will have approximately 9 megapixels

The chart below is a general rule of thumb. It represents very good print quality being viewed at a close range. Size can be made larger but some detail may be lost.

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

kvbarkley — Fri, 27 Jun 2025 21:04:55 GMT

No calculations required. When you put an -S lens on a FF R camera, the camera automagically only uses an APS-C sized center chunk from the sensor. So the 24MP R6mkii becomes a 9MP camera.

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

mglotspeich — Fri, 27 Jun 2025 21:20:55 GMT

Thank you Marc. I was JUST researching this same thing on the internet. I have always known about the 1.6 crop factor but I was thinking that I will be losing megapixels shooting the EF lens on the full frame camera. My t2i (cropped camera) is 18 mp. The R6 Mark ii is a 24 mp. That is why I asked the original question. I was wondering if the full frame camera would lose the mp's when cropped. In other words, I would have more usable mp's on my cropped camera than using the same lens on my R6. Thank you so much for confirming. Now I see why I need to to get the RF 100-400mm.

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

March411 — Fri, 27 Jun 2025 21:27:17 GMT

My pleasure mglotspeich, glad it saved you some research time and helped you with your decision. The R6 MkII also performs well in low light and if you use noise reduction software you can push the body/lens a bit further in low light. Enjoy the new gear.

Re: Canon RF 100-400mm f/5.6-8 IS USM Lens vs EF-S 55-250mm f/4-5.6 IS STM Lens

mglotspeich — Fri, 27 Jun 2025 21:39:54 GMT

Thank you!