40D & f/2.8 Conundrum

I won't be reading all of the responses, but here's mine. A 2.8 lens may be softer at 2.8 than an f/4 lens is at f/4, but that's just it. Focus is made at max ap., which means that focus will be faster and more accurate with the 2.8, no matter what ap you're shooting at.

Rubberhead wrote in post #5134188
I have 20/15 vision and have never needed glasses or contacts so I don’t have any trouble setting the diopter on my camera or the dipoter ring on my binoculars. However, if I was to put on my wife’s glasses and set the diopter, I would, at best, see a very blurry image and I doubt I would be able to set the diopter as well as I could if I used my 20/15 vision even though all I had to do would be to chose the setting that gave me the less blurry image.

If it makes you feel any better, I (think) I understand the question you're trying to answer, and I think your analogy makes sense. In essence, you're asking if it is better to have more light (via a wider focusing aperture), or to have a sharper image (ie. using a lens with a smaller maximum focusing aperture), available to the camera to make a focusing decision.

So far I think all of the responses have contained correct facts but haven't completely addressed this question. I can't claim evidence one way or another, but I think you have to take in to account the difference in available light in your diopter analogy, as your eyes naturally adapt their 'aperture'. I think that having a full stop more available light for focusing at f/2.8, and doubling the number of contrast sensors at that aperture, more than compensates for the increased sharpness available from a f/4 lens, especially considering the AF is looking for the 'best' contrast at the centre AF point and not necessarily optimal 'sharpness' throughout the whole image.

The thing that makes focusing with an f/2.8 lens much more precise than using when using a lens with a smaller aperture has to do with the physical difference in the size of the aperture from the perpective of the camera. At f/2.8, the AF sensor pair used to detect phase shift differences, and hence focusing, can be places farther apart from each other. If you used an f/5.6 lens or small lens, the sensor pair used for the f/2.8 lens would be space too far apart for both to be visible through the f/5.6 aperture because the aperture is smaller. Therefore the AF sensor pair used for focusing with f/5.6 and smaller lenses are spaced closer together to allow focusing. But, since the AF pair are spaced closer together, focusing precision suffers. So, no, it's not a marketing gimmick. It's just a matter of physics.

BTW, DOF or the doubling of light at f/2.8 vs f/4 has no impact on the precision of focusing.

PacAce wrote in post #5134649
BTW, DOF or the doubling of light at f/2.8 vs f/4 has no impact on the precision of focusing.

None the less, an f/2.8 lens is noticeably more accurate.


2.8 ftw.

thepepperman,
Yes you do understand and thanks so much for helping explain the exact question I was asking.

Thank you too, for helping to answer that question.

This sounds right to me, I also noticed unfortunately the 40D doesn't auto focus on a lens with a max aperature of f/8. I tried a 500mm f/4 with 2x teleconverter, no AF. But works fine with the 1DMKIIn. So the camera does use the max aperature to focus.

Scoobs wrote in post #5132815
Hi,

Think you maybe confusing things a bit here. The max aperture of the lens is used to achieve autofocus. The lens then closes the aperture down to what ever you or the camera sets for its exposure at the point of shooting.

All lenses, be it a f1.4, a f2.8 or f4 etc focus at their max aperture. Your 50mm will focus at 1.8 which will allow the more senstive focus on your 40D

So I'd use a f2.8 lens or faster to have the enhanced focus but may not use my lens wide open unless i needed to due to lack of light etc.

Ronald S. Jr. wrote in post #5134702
None the less, an f/2.8 lens is noticeably more accurate.


2.8 ftw.

I never said it wasn't. What I did say was that the accuracy of the f/2.8 has nothing to do with the DOF or the amount of light going through the f/2.8 lens compared to, say, an f/4 lens.

PacAce wrote in post #5134649
BTW, DOF or the doubling of light at f/2.8 vs f/4 has no impact on the precision of focusing.

Wouldn't the stronger signal from more light and the more rapid change in focus from shallow DOF make the phase shift detection more precise? (Maybe not... I'm just thinking out loud. Then listening to what I thunk and typing it.)

all of which is totally moot - like the OP's inquiry - because you can always stop down the fast lens to f/4 or higher. so the grandstanding Is this just a marketing ploy to get folks to buy more lenses? is just soooooo ignorant it's beyond discussion and ignores the way the AF system works and has been designed (lenses that allow more light <f/4 will AF better, all things equal, regardless to what f-stop is shot at).

nevermind the slow lenses won't work on non-1 series cameras with extenders if the F is above f/5.6

either way, carry on...

thepepperman wrote in post #5134634
If it makes you feel any better, I (think) I understand the question you're trying to answer, and I think your analogy makes sense. In essence, you're asking if it is better to have more light (via a wider focusing aperture), or to have a sharper image (ie. using a lens with a smaller maximum focusing aperture), available to the camera to make a focusing decision.

So far I think all of the responses have contained correct facts but haven't completely addressed this question. I can't claim evidence one way or another, but I think you have to take in to account the difference in available light in your diopter analogy, as your eyes naturally adapt their 'aperture'. I think that having a full stop more available light for focusing at f/2.8, and doubling the number of contrast sensors at that aperture, more than compensates for the increased sharpness available from a f/4 lens, especially considering the AF is looking for the 'best' contrast at the centre AF point and not necessarily optimal 'sharpness' throughout the whole image.

Rubberhead wrote in post #5134615
AdamLewis,
I've got an idea how to communicate my issue but my kid has a ball game in a few minutes so I'll have to try this evening.

I guess the best way I can describe it is that focusing at larger apertures instead of small ones is like measuring with a micrometer or a yardstick. The camera can more accurately find the exact position the lens needs to be in if its using a large aperture. Why else do you think the cameras focus slower with TCs on? Or why do you think XXD cameras cant even focus a lens thats not faster than F5.6?

chinch wrote in post #5135206
all of which is totally moot - like the OP's inquiry - because you can always stop down the fast lens to f/4 or higher. so the grandstanding Is this just a marketing ploy to get folks to buy more lenses? is just soooooo ignorant it's beyond discussion and ignores the way the AF system works and has been designed (lenses that allow more light <f/4 will AF better, all things equal, regardless to what f-stop is shot at).

nevermind the slow lenses won't work on non-1 series cameras with extenders if the F is above f/5.6

either way, carry on...

That is not correct. An f2.8 lens stopped down to f4 will focus at f2.8.

PacAce wrote in post #5134649
BTW, DOF or the doubling of light at f/2.8 vs f/4 has no impact on the precision of focusing.

I have to beg to differ on this one. If the camera hunts around until it finds what is the most "in focus", having something with a very small DOF eliminates a wide range off possible other values. If the camera assigns a focus number to how in-focus a picture is, you should see that a chart of the focus numbers as a lens racks front to back would be steeper on the wide aperture lens than the narrow aperture one. Just as in math, when the chart is steep like that, its easier to identify where the max would be.

chinch wrote in post #5135206
all of which is totally moot - like the OP's inquiry - because you can always stop down the fast lens to f/4 or higher. so the grandstanding Is this just a marketing ploy to get folks to buy more lenses? is just soooooo ignorant it's beyond discussion and ignores the way the AF system works and has been designed (lenses that allow more light <f/4 will AF better, all things equal, regardless to what f-stop is shot at).

nevermind the slow lenses won't work on non-1 series cameras with extenders if the F is above f/5.6

either way, carry on...

The irony is the ignorance in not knowing that a lens always focuses at its max aperture. If you take a picture at F32, the camera still focuses at F2.8

we know.

what cretan would want to "focus at f4" and deactivate the 5D hidden AF points, and more accurate sensors on every body?

AdamLewis wrote in post #5135281
The irony is the ignorance in not knowing that a lens always focuses at its max aperture. If you take a picture at F32, the camera still focuses at F2.8

read above captain obvious.