40D & f/2.8 Conundrum

Background: The 40D has a special center AF point with, at the time it was introduced, the world’s first high-precision diagonal cross-type AF sensor. According to Canon this diagonal sensor functions with lenses with maximum apertures of f/2.8 or larger.

The Problem: Most lenses, even the “L”s, with maximum apertures of f/2.8 are noticeably softer wide-open than similar lenses that start at f/4.0.

The Conundrum/Question: Would I rather have a standard cross-type AF point through a sharp lens, or a high-precision AF point through a softer lens?

Note 1: Canon cameras Autofocus only through a lens's maximum aperture.

Note 2: This thread assumes that a quality f/2.8 lens is not as sharp wide-open as a quality f/4.0 lens when wide-open. If you disagree, do some research and you’ll find that this is almost universally true when comparing lenses of equal cost and/or quality.

Pre-work: I have a 40D and my only lens with a f/2.8 or faster lens is the 50mm f/1.8 II which is terribly soft at f/1.8. It is rather sharp at f/2.8 but I can’t seem to force it to f/2.8 when focusing. I did try setting the aperture to f/2.8, then holding down the “DOF preview” button but the autofocus doesn’t work with the DOF preview button held down.

I am feeling a need for a quality f/2.8 lens just to take advantage of this “feature” in the 40D. Then, I stop myself and ask, “Is this just a marketing ploy to get folks to buy more lenses?”

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.

what scoobs said. essentially the camera will autofocus with your widest aperture, and then stop down the lens when you release the shutter. so regardless, if you're using a lens that's f/2.8 or faster, you'll be able to utilize this feature on your camera.

The 50mm f/1.8 is a cheap lens anyways, with really slow focusing- I wouldn't compare it to a 17-55 or 24-70L as far as focusing speed.

Don't mix focusing aperture and shooting aperture. Even if the lens is inherently soft wide open there will still be a distance, i.e. the focus distance, which is relatively sharper than than all the other distances. The AF only has to find the point where the relative sharpness is best (smallest coc or at least within the range of permitted tolerance). The shorter depth of focus of a fast lens makes this easier. Then when shooting the diaphragm closes down.

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.

That's exactly my point.

My question is still unanswered.

Rubberhead wrote in post #5133607
That's exactly my point.

My question is still unanswered.

It has been answered. I dont understand why you are trying to stop down the lens to focus. Like everyone said 3 times. If you have a lens that is 2.8 and even if you are shooting an image at f8 it still remains wide open to focus until you press the shutter button then it closes up for the shot. I think you are confusing yourself

tzalman wrote in post #5133009
Don't mix focusing aperture and shooting aperture. Even if the lens is inherently soft wide open there will still be a distance, i.e. the focus distance, which is relatively sharper than than all the other distances. The AF only has to find the point where the relative sharpness is best (smallest coc or at least within the range of permitted tolerance). The shorter depth of focus of a fast lens makes this easier. Then when shooting the diaphragm closes down.

Rubberhead wrote in post #5133607
That's exactly my point.

My question is still unanswered.

That's because you're making it more complicated than it needs to be. Reread tzalman's answer above, especially the part I bolded.

IQ isn't as important for AF as it is for image recording; all the AF mechanism needs to be able to do is to zero in on the "best" contrast offered. It doesn't have to be "perfect" contrast, just the point where it starts getting worse no matter which way you go. Think of the camera's AF mechanism as you adjusting the diopter setting. You don't just wheel the diopter adjustment one way and stop when it's sharp. You rock back and forth across the sharpest point as you zero in on it.

So no, it isn't a marketing ploy; there are many better reasons to buy f/2.8 lenses than "improved AF". But if you've got one of those fast lenses and shoot a lot in low light, that improved AF at f/2.8 sure is nice to have.

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.

alt4852 wrote in post #5132954
what scoobs said. essentially the camera will autofocus with your widest aperture.

tzalman wrote in post #5133009
Don't mix focusing aperture and shooting aperture.

05Xrunner wrote in post #5133650
I dont understand why you are trying to stop down the lens to focus. Like everyone said 3 times. If you have a lens that is 2.8 and even if you are shooting an image at f8 it still remains wide open to focus until you press the shutter button then it closes up for the shot. I think you are confusing yourself

Jon wrote in post #5133656
That's because you're making it more complicated than it needs to be.

Jon,
Please understand that I’m not trying to be hardheaded or argumentative. I am well aware of that a Canon camera focuses with the lens wide open yet every respondent started by telling me something that is plainly stated in my original post.

I understand tzalman’s post. Unlike the others, he, at least, posted something that wasn’t already in my original post. That doesn’t change the fact that he was still telling me something that I knew. Yes, I understand feedback loops and that autofocus works by maximizing image contrast within the AF area through a feedback loop.

Your point about the diopter setting does help me spress my question in a different way, if you will indulge me:

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.

Never mind - I know someone with a 70-200mm f/2.8L. I can set up a focus test between that lens and my 70-200mm f/4L IS.

I’ll post the results.

Rubberhead wrote in post #5132667
I am feeling a need for a quality f/2.8 lens just to take advantage of this “feature” in the 40D. Then, I stop myself and ask, “Is this just a marketing ploy to get folks to buy more lenses?”

lose the lame font tags please.

f/2.8 allows 2x light vs f/4 and certainly is no marketing ploy.

plus the f/2.8 lens stopped down to f5.6 will focus better than a f4 lens at f4.

your argument is very silly.

The other point you're overlooking is that at f/2.8 you're actually using a different AF sensor pattern in the center spot than at slower apertures. If you remember back in the MF days, camera focusing aids would "black out" at small apertures. You could wiggle your eye around and if you got just the right position and were able to hold it long enough, you might be able to focus with a slow lens. In fact, cameras with interchangeable focusing screens offered special screens for fast and slow lenses, where the focusing aid (microprism or split image) was designed for that aperture.

The f/2.8 "high sensitivity" AF aid won't work properly at apertures slower than f/2.8. Like a focusing screen designed for a "fast" lens, it'll "black out". None of the AF points will AF reliably at apertures slower than f/5.6. This is why, even if you "tape" a TC, or have a non-reporting TC, lenses may "hunt" if the actual aperture of lens+TC is slower than f/5.6. It just can't see well enough to focus.

Rubberhead wrote in post #5134188
Jon,
Please understand that I’m not trying to be hardheaded or argumentative. I am well aware of that a Canon camera focuses with the lens wide open yet every respondent started by telling me something that is plainly stated in my original post.

I understand tzalman’s post. Unlike the others, he, at least, posted something that wasn’t already in my original post. That doesn’t change the fact that he was still telling me something that I knew. Yes, I understand feedback loops and that autofocus works by maximizing image contrast within the AF area through a feedback loop.

Your point about the diopter setting does help me spress my question in a different way, if you will indulge me:

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.

Never mind - I know someone with a 70-200mm f/2.8L. I can set up a focus test between that lens and my 70-200mm f/4L IS.

I’ll post the results.

Why are you talking about vision and diopters?
No, its not a marketing ploy. Ideally, youd want a camera to have as wide an aperture as possible when focusing so that it can tell a bigger difference between whats out of focus and whats in focus. I really dont see what exactly it is youre failing to understand but I hope you figure it out.

Thanks Jon. That does help.

chinch wrote in post #5134206
lose the lame font tags please.

f/2.8 allows 2x light vs f/4 and certainly is no marketing ploy.

plus the f/2.8 lens stopped down to f5.6 will focus better than a f4 lens at f4.

your argument is very silly.

Sorry the font tags are there because I cut-and-pasted from Word. For some reason the version of Word I use a work includes these where as my home version doesn't. I use Word as a spell checker.

Secondly, you bother to call my argument silly, but you answered with one of the mostly basic elements of technical photography. Then you top that by making a statement that makes no sense.

Are you sure that you're not the one with the plastic knife at a gunfight?

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.