40D & f/2.8 Conundrum

chinch wrote in post #5135299
read above captain obvious.

Comical.

thanks for the laughs but this is pointless. o&o

Just stick with your f4 lenses. You don't want to fool around with that crappy 2.8 glass.

Tapeman wrote in post #5135615
Just stick with your f4 lenses. You don't want to fool around with that crappy 2.8 glass.

Almost a classic - Lol

AdamLewis wrote in post #5135274
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.

But it doesn't.

Tapeman wrote in post #5135615
Just stick with your f4 lenses. You don't want to fool around with that crappy 2.8 glass.

That's all I was after - a little reassurance. The rest of ya'll need to take a lesson from Tapeman.

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PacAce wrote in post #5135711
But it doesn't.

The argument still stands. If the phase difference decreases as a picture comes more into focus, then it stands to reason that a lens with a smaller aperture while focusing is subject to a smaller relative difference between out-of-focus and in-focus due simply to the fact that the narrower aperture results in a longer DOF. Like you said, its simple physics. If you were focusing at F99, the phase difference between OOF/IF would be extremely small when compare to the phase difference during focusing at F1

So, the DOF (or lack thereof) does aid in focusing. It should also be noted that also does the doubling of light. Why else would lenses have trouble focusing in low light?

I just dont understand what youre trying to stand on when you say " BTW, DOF or the doubling of light at f/2.8 vs f/4 has no impact on the precision of focusing." Aside from excess light making it easier to focus, its also apparent that it allowed for the use of more accurate sensors.

the smaller your aperture number "2.8", the smaller your target is when focusing. The larger your aperture number "8", the larger your target is when focusing.

Ever watch The Patriot?

"Aim small, miss small."

The less space you're trying to focus on, the faster/easier and more acurate your focusing will be.

Scarter275 wrote in post #5137917
the smaller your aperture number "2.8", the smaller your target is when focusing. The larger your aperture number "8", the larger your target is when focusing.

Ever watch The Patriot?

"Aim small, miss small."

The less space you're trying to focus on, the faster/easier and more acurate your focusing will be.

Huh? This makes no sense to me whatsoever. Care to explain what you mean?

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.

The shallower your DOF, the less space there is to confuse the camera on what to focus on. Meaning it should focus faster and more accurately since it's focusing more-so on a single, smaller point.

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.

He's right you know. The amount of light makes no difference, otherwise if you doubled the amount of light in a room, an f/4 lens would focus just as well as an f/2.8 lens would if you didn't increase the amount of light. It really has nothing to do with DOF at all...if it did, macro lenses would see an improvement in their AF the closer you got to your subject (narrow DOF and all that).

But this isn't how it works, whilst low-light makes it harder for the AF to work, that has nothing to do with aperture size, but with how the AF system works. There is a fundamental difference between how AF works on an f/2.8 lens than there is with a slower one, it's not a quantitative one contingent on the amount of light.

Cross-type AF has nothing to do with all that stuff, and has everything to do with the actual ratio of the diameter of the front element relative to the focal length (i.e. the f/stop!).

I'm sorry to sound harsh, but PacAce is the only one in this thread that knows what he's talking about. The OP question makes no sense, because you can stop an f/2.8 lens down to f/4 to get that 'sharpness', and in most cases it'll beat the f/4 lens wide open, whilst still focusing at f/2.8. You can't open an f/4 lens up wider to take advantage of the cross type AF...

perryge wrote in post #5138236
He's right you know. The amount of light makes no difference, otherwise if you doubled the amount of light in a room, an f/4 lens would focus just as well as an f/2.8 lens would if you didn't increase the amount of light. It really has nothing to do with DOF at all...if it did, macro lenses would see an improvement in their AF the closer you got to your subject (narrow DOF and all that).

But this isn't how it works, whilst low-light makes it harder for the AF to work, that has nothing to do with aperture size, but with how the AF system works. There is a fundamental difference between how AF works on an f/2.8 lens than there is with a slower one, it's not a quantitative one contingent on the amount of light.

Cross-type AF has nothing to do with all that stuff, and has everything to do with the actual ratio of the diameter of the front element relative to the focal length (i.e. the f/stop!).

I'm sorry to sound harsh, but PacAce is the only one in this thread that knows what he's talking about. The OP question makes no sense, because you can stop an f/2.8 lens down to f/4 to get that 'sharpness', and in most cases it'll beat the f/4 lens wide open, whilst still focusing at f/2.8. You can't open an f/4 lens up wider to take advantage of the cross type AF...

Amen.

ROFL, i can't believe this thread made it this far for such a question! although this type of behaviour is not uncommon here at POTN =)

Perry and Leo -
Canon says that the tolerance for AF is that it focuses anywhere within the depth of focus. This sounds to me like saying that a lens with smaller dof will be held to tighter tolerances. What am I misunderstanding?

perryge wrote in post #5138236
I'm sorry to sound harsh, but PacAce is the only one in this thread that knows what he's talking about. The OP question makes no sense, because you can stop an f/2.8 lens down to f/4 to get that 'sharpness', and in most cases it'll beat the f/4 lens wide open, whilst still focusing at f/2.8. You can't open an f/4 lens up wider to take advantage of the cross type AF...

Sorry to sound harsh myself but you don't understand the question. It is a rather simple question, actually. Here it is again:

Rubberhead wrote in post #5132667
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?

In addition, some honest research will show you that an f/2.8 will not beat an f/4.0 lens wide open for sharpness. Now, will a f/2.8 lens beat an f/4.0 lens if both are at f/4? That's a very different question with very different results.

I'll use the Canon 70-200mm f/4L and f/2.8L for my comparison. See the attached link for a direct, head-to-head comparison of sharpness of these two lenses wide open aperture and at 200mm.

http://www.the-digital-picture.com …omp=0&FLIComp=4​&APIComp=0

There is no doubt that the f/4L is sharper wide open than the f/2.8L. Hence, my question.

I will say that a couple of folks in this, longer than it should have been, thread have helped answer my question. Although, I don't know that the answers are complete.