Fast vs Slow Lenses on Mirrorless

[Wandalynn]

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.

[FloridaDrafter]

I haven't studied it intensively, but I don't think cameras, in general, open up the aperture to get focus. My thinking is, at wide open, DOF is razor thin, at times, so it would be hard to get focus that way. The R's use "Dual Pixel AF II" and is 100% phase-detection coverage across the sensor and is a "new animal" for the most part [edit] The way I understand it is each pixel has a twin so it gets a "stereo view" to aquire focus [/edit]. I think the only other Canon camera to do this is the 1DX mark III, but only in "live view".

What I can say for sure, is we have had the RF 100-400mm for a couple of weeks now and my wife has been using it on her R6, for the same reasons as you... It's light and produces great photos! She is disabled and just can't lug L glass around. I haven't used it yet on my R5, so I can't comment on that, but she reports (I just asked her) that AF is snappy in low light situations with no lag. Now, I have read that the R6 is better with low light, so I won't know how the RF 100-400 behaves on my R5 until I can give it a go. I plan on doing comparisons with my EF 100-400 L II, I know, apples to oranges, but dang, that will be a good test. So far, they look comparable as I have shot the same birds, same location, same distance, that she has, and TBH, I'm impressed with this budget RF 100-400 lens.

We shoot in low light, usually under tree canopies and thick brush to get birds, flowers, and insects.

Newton

[MikeSowsun]

All modern DSLR cameras focus with the aperture wide open. The aperture is always held wide open regardless of what is set on the camera.  The aperture only stops down as the photo is taken. 

If you want to see the effect of the selected aperture value, you can press the Depth of Field Preview button. The aperture will stop down to the selected value, the viewfinder darken, and there will be a change in the depth of field. 

All DSLRs work this way, and I assume it is the same for those newfangled Mirrorless cameras. 🙂

[FloridaDrafter]

Thanks, Mike. That makes sense. I should have known that since I use the DOF preview quite a bit for macro 🙂

Newton

[jrhoffman75]

Many of the RF lenses have smaller max apertures than DSLR lenses because of the ability of the sensor AF system to focus better than the DSLR systems at lower light levels. The majority of DSLRs are limted to f/5.6 for AF. The higher spec models, like the 1D series, can AF at f/8. Canon has releasesd RF lense with f/11 max aperture. But it is still going to be harder for the camera to focus at lower light levels compared to brighter levels.

The RF 100-400 is an f/5.6 max, whereas the EF 100-400 is f/4.5 max. 

[ebiggs1]

If it is possible to use and the fact slower, higher f-number, lenses are easier and cheaper to make, why not. Slower lenses are also smaller and usually much lighter. Canon feels that is the way to go. They can be good but have certain other issues beside AF functions like lens diffraction because of the tiny apertures. I personally would not be interested in a lens that only had a f11 max aperture.

[wq9nsc]

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 🙂

Rodger

[Wandalynn]

Florida Drafter, Thanks for your input. I agree that the AF is snappy and the IS is amazing on the RF 1-4 but since most of my subjects are tiny, I have more trouble getting the camera to focus on that particular point with the RF 1-4 vs the EF 1-4 ii L and was wondering if it was that extra stop advantage made that much difference. It's a terrific lens and I'm definitely not disappointed. The below image is one I took with the RF 1-4 of a damselfly that's about 1.25" long and is a 100% crop (it doesn't look quite as sharp though after being uploaded). They are about like photographing a piece of dental floss. I just have to work a little harder with getting the camera to focus on it with the RF than with the EF.

[ebiggs1]

" 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. 

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

[jrhoffman75]

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.