Showing results for 
Show  only  | Search instead for 
Did you mean: 

Fast vs Slow Lenses on Mirrorless


Since some of Canon's RF lenses are slower than most typical EF lenses and I know it is said that that's not an issue since the mirrorless camera sensors are better able to handle higher ISOs but I have a question about that. Am I misinformed in thinking that when any camera auto-focuses, for a split second, the lens aperture is opened to the max to let in as much light as possible? If so, then what's different with mirrorless in that regard?


I'm asking this because my new RF 100-400 seems to struggle a little with gaining focus so I have to spend a little more time with a subject than when I use my EF 100-400 L ii. Or maybe I'm just spoiled by the "on point" AF capability of the EF. It's not really fair to compare those two lenses. And, I am very happy with the new lens--when it does zero in on my usually small subject like a bee, the picture is sharp (not like the L of course)--and it's just more work getting there. But due to its super light weight, I can carry it on long walks and not miss shots I normally would without it. I'm just curious about how the AF functions. I'm using an R5.




The camera initial focus is going to occur with the lens wide open.  Even when a camera can still AF with a "slow" lens, the AF is going to be slower but this is somewhat offset by the less critical focusing with the smaller aperture glass due to the greater depth of field around the critical focus point.


From a cost and weight standpoint, there is a rationale for making these narrow aperture lenses and a big driver for mirrorless cameras is the reduced complexity and thus cost of manufacture so the two somewhat fit together.


Like Ernie stated, there are a lot of things a wide aperture lens can do that a narrow aperture cannot so it isn't just about focusing.  The slowest lens I own are 400 and 800 f5.6 primes.  I use the 400 f5.6 when hiking but it is a big step down in speed AND quality from the EF 400 f2.8 I use for sports.  Outside of macro glass, I rarely have a lens stepped down to the f11 range and I would feel very limited with a telephoto of that type.  I might change my mind when the fast glass starts feeling too heavy 🙂



EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video

" I might change my mind when the fast glass starts feeling too heavy"


I use to carry two 1 Series camera bodies with big lenses all the time and not really think about it. Now I think I about it. Smiley Surprised

I am still not interested on a f11 or even a f8 lens. Had them in the past! 

EOS 1DX and 1D Mk IV and less lenses then before!

Despite what Canon has in its marketing material, I wouldn't consider the RF lens for sports/action or birds in flight. but I suspect that the difference in cost between $13K for the 800mm EF lens and $900 for the 800mm RF lens, coupled with the apparently very good IBIS for the mirrorless bodies would make that lens attractive to wildlife photographers. Many wildlife shots are basically still shots so fast shutter speed isn't essential.



John Hoffman
Conway, NH

1D X Mark III, Many lenses, Pixma PRO-100, Pixma TR8620a, LR Classic

Are using any type of filter? UV filters are not really needed in a digital camera. CPL filters can sometimes wreak havoc with a phase detect AF system. ND filters are becoming obsolete as the mirror less cameras have shutter speeds hitting 1/32,000 or even faster.

Use a clear filter or nothing, at all.
"The right mouse button is your friend."

I am using a B+W clear filter although probably don't need it since I have the hood--I just automatically put it on there. I should try shooting without it though--thanks for that suggestion. I thought there was a bit of a haze on the picture but I couldn't get totally perpendicular to the bug for even focusing so assumed that was the problem instead of the filter. Thanks.

The reason for the “clear” filter is protection. Nothing else. If you are shooting in a safe place remove it. If you are outside I would use it most of time. The hood does not provide enough protection by itself.
Some lenses require a filter to complete their weather sealing.
EOS 1DX and 1D Mk IV and less lenses then before!
National Parks Week Sweepstakes style=

Enter for a chance to win!

April 20th-28th