f/2.8's have more accurate AF?!?!

[RbrtPtikLeoSeny]

Hey everyone, I've heard about this a few times, but I'd like to make sure....
Do lenses with larger apetures have more accurate AF or something?

[tim]

Yes. The cross type sensor at the centre focus point operates at F2.8 or wider. Another thing about wide aperture lenses is they get more light in so AF is easier - with my 50mm F1.4 it can AF in amazingly low light.

[RbrtPtikLeoSeny]

Huh, no wonder! I've been noticing my 70-200 f/4 hunts a lot indoors where there isn't much light. So, even at it's smaller apetures, it'll af faster? Like, a lens with a max apeture of f/2.8 will focus faster at f/8 than an f/4 lens at f/8?

[Tom W]

Huh, no wonder! I've been noticing my 70-200 f/4 hunts a lot indoors where there isn't much light. So, even at it's smaller apetures, it'll af faster? Like, a lens with a max apeture of f/2.8 will focus faster at f/8 than an f/4 lens at f/8?

Yes, though the difference will probably only show up in low light situations. The faster the lens aperture, the more light the AF system has to work with.

[tim]

Huh, no wonder! I've been noticing my 70-200 f/4 hunts a lot indoors where there isn't much light. So, even at it's smaller apetures, it'll af faster? Like, a lens with a max apeture of f/2.8 will focus faster at f/8 than an f/4 lens at f/8?

Yes. Remember while focusing the lens is at its maximum aperture, it only stops down to F8 for the fraction of a second it takes to take the photo.

[Croasdail]

Yes - any f2.8 lens will be brighter through the view finder - and for the AF. Generally it is true that 2.8's focus "easier" because they have more information to work with - but this in not always the rule. I have 4 lenses I use regularly, Canon 300 2.8 L, Sigma 70-200 2.8, and 17-40 L and with none of them is focus speed an issue - none ever hunt. I also have a tamron 28-75 2.8, and it will hunt on occasion. I do a lot of late afternoon shooting of sports, or transitioning to night lighting, and that was the only reason I opted not to go with the 200 f4 canon. With the two bigger lenses, I never have a time when the camera refuses to shoot because it did not have focus lock. Thats what I don't like about the pixel peeking comparisons - it doesn't matter if lens x is 5% sharper if the camera won't fire at the critical time because it hasn't locked onto focus. Cheers!

[ScottE]

If you focus manually you will find that a large aperture lens is easier to focus because it has shallower depth of field. The out of focus parts appear more blurry. Similarly, it is easier for auto-focus to find focus.

Also if you manually focus a short lens and a long lens you will find the long lens easier to focus, again because of the shallower depth of field.

Scott

[ttmatsu]

Just to make sure the original poster understands, it's not the lens that is doing the autofocus. He has a 20d which has a "high precision" center AF for lenses f2.8 and larger so as the previous replies have all stated, f2.8 and larger lenses allow the camera to autofocus more accurately due to increased light and the use of the high precision AF point.

[Tom W]

Just to make sure the original poster understands, it's not the lens that is doing the autofocus. He has a 20d which has a "high precision" center AF for lenses f2.8 and larger so as the previous replies have all stated, f2.8 and larger lenses allow the camera to autofocus more accurately due to increased light and the use of the high precision AF point.

Yes, there are 2 different reasons here. The 20D communicates with the lens and when an f/2.8 or faster lens is detected, it uses the high precision mode on the center AF point. Also, the brighter lens allows the AF system to work in darker situations because of the increased brightness, regardless of the improved precision. Two positive effects from the faster lens.

[CyberDyneSystems]

More light allways means faster AF.. larger aperure let's in more light...

[RbrtPtikLeoSeny]

Woah, glad I started this thread! I knew faster lenses made a difference, but I didn't know there was this much meat to it all. It's also evident in my current lenses. I tried shooting an indoor graduation over the weekend with my 70-200mm f/4 lens. I missed a lot of shots because the darn thing wouldn't AF! And when it did AF, sometimes it wouldn't AF accurately, and faces would be OOF. I even tried putting the center AF point right on the center of their faces and it still couldn't get it right. I'd have to say 30% of my shots came out OOF.:evil: Terrible low light lens!!!

I'm guessing that's normal for an f/4 lens right? Really wished it worked better in low light because I really love this lens. I guess this would justify upgrading to the
70-200 f/2.8L IS within the year then huh?

Love this hobby. There's so much to learn.:lol:

[condyk]

I'm guessing that's normal for an f/4 lens right? Really wished it worked better in low light because I really love this lens. I guess this would justify upgrading to the
70-200 f/2.8L IS within the year then huh?

Hate to say I told you so all those weeks ago, but ... :lol: :lol: :lol: :p

Yes, it's still a great lens when used within it's capabilities.

[cc10d]

When you want to focus in lower light situations, pick the area with the highest contrast that you can that is within the area you want focus to be on. Lock focus, Then recompose if needed.

Also note the afor mentioned feature of the 20D, It changes to a more precise focus mode when used with f 2.8 or faster lens and the center focus point.

[DavidEB]

I'm no engineer, but I suspect that the increased focus speed at f2.8 is not just a function of the amount of light but also the decreased depth-of-field. When an out-of-focus image strikes the focus sensor, it perceives low contrast. The contrast increases as the lens is focused. A large-aperature lens increases the rate of increase in contrast per unit movement of the lens.

But I don't understand why f2.8 is a magic threshhold.

[Tom W]

But I don't understand why f2.8 is a magic threshhold.

Because Canon designed it that way. And they sell f/2.8 lenses, which they would like you to buy. :)

[csnudelman]

I was so sure I was going to get a 70-200L f/4 to go along with my 24-70L f/2.8. Then after discovering about the subject you addressed I decided to put off getting the 100mm macro and use that money to add to and get the 135L f/2. I hope to soon get the 1.4X exrender which will give me a 196 f/2.5 as well. Why not the 70-200L f/2.8? Just way too heavy for me.

[RbrtPtikLeoSeny]

Yea, I know condyk. Don't rub it in.:wink:

And csnudelman, I've been looking at that 135 f/2.0L. Looks pretty sweet! That and it's price is more within reason. I also like the black lens with the red stripe design too! Got my 17-40L and just completely fell in love with the look of it. Not so sure about the white lenses anymore.

Anyway, my original plans were to pay off all the equiptment I've bought in the last month, (what you see in my sig) and then buy a 100-400L. Then I looked at it's apeture range... f/4.5-5.6...:confused: If I were to buy it, I'd sell my 70-200 f/4 to cover some of the cost, and then if this 100-400L can't perform well for sports shooting, I'd have a semi useless $1400.00 piece of equiptment.....

So, now I'm sort of leaning towards a 70-200 f/2.8L IS with a 1.4x t-con for a 280mm reach. be about $2,000, then the sale of my current 70-200 would cover about $500.00 of it, bringing it to a grand total of $1500.00. Not bad right? Then I'd have a great indoor lens, great sports lens, great up close wild life lens, ect. Seems like the smarter of the two buys..... hmmmm..... :confused:

Sorry about the long post.... I like to talk a lot.:lol:

[cdhender]

I'm going through the same thought process now but I've yet to buy a 70-200.

I've thought about the f/4, but like you said, what if I need low light or it's gloomy out? The 70-200 2.8's are nice but they're also heavy, and I'd want to use them to walk around with. Also, like you said, they're white.

Then I've thought about going the prime route with a 135 (which I'm getting regardless, b/c I MUST have this lense) and a 1.4 tele. But if I only get this, I think I'll miss the range of the zoom.

Who knows what I'll end up doing? Certainly not me.

[blue_max]

My thougths have been to get the 70-200f4 and supplement it with the primes for low light situations. This gives me the lighter weight zoom when I expect to have plenty of light and the primes when even the 2.8IS would struggle. Best of both worlds?

I really intended to sell the zoom when I got the primes, but it's quality really rivals the primes and I will see if I use it. It is still a bargain lens to be sure.

I have four lenses so far, with a fifth on the way (all second hand) and I can honestly say that none of them have focussing issues with my 10d that are not attributable to operator error.

I don't feel the need for the 2.8IS, but faster is always better in every respect apart from weight and cost.

To sum up, the 70-200f4 focusses very well in reasonable light and that is what it is good for.

Graham

[RbrtPtikLeoSeny]

Mmm yea, the 70-200 f/4 with primes is a good combo, but I'm too lazy for all that. I like one lens that does what I need it to do rather than a few lenses that can play a part in what I need them to do. Ya know?

Neway, I was thinking about the 135 f/2.0 and 200 f/2.8 primes since it's basically the same price as the 70-200 f/2.8L IS, but hmmmm........... really like the IS....... and don't mind the weight or size. I like the feel of big heavy lenses. It's amazing how many options are out there!!

Anyway, the 70-200 f/2.8L IS performs pretty well with a 1.4x tele right? Maintains it's fast AF and sharp image quality?

Do the 135 and 200mm primes work just as well with t-cons?

[Jesper]

So, now I'm sort of leaning towards a 70-200 f/2.8L IS with a 1.4x t-con for a 280mm reach. be about $2,000, then the sale of my current 70-200 would cover about $500.00 of it, bringing it to a grand total of $1500.00. Not bad right? Then I'd have a great indoor lens, great sports lens, great up close wild life lens, ect. Seems like the smarter of the two buys..... hmmmm..... :confused: Ok, now for some bad news again, I don't know if you already know this....: An 1.4x TC will eat up 1 stop of light. So your 70-200 f/2.8 with 1.4x TC effectively becomes a 140-400 f/4. So no f/2.8, but still a larger aperture than the 100-400 f/4.5-5.6.

[Tom W]

Ok, now for some bad news again, I don't know if you already know this....: An 1.4x TC will eat up 1 stop of light. So your 70-200 f/2.8 with 1.4x TC effectively becomes a 140-400 f/4. So no f/2.8, but still a larger aperture than the 100-400 f/4.5-5.6.

Actually, with the 1.4X teleconverter, the 70-200/4 becomes a 98-280 f/4 lens. The 2X will give you a 140-400 f/5.6 lens.

[csnudelman]

Actually, with the 1.4X teleconverter, the 70-200/4 becomes a 98-280 f/4 lens. The 2X will give you a 140-400 f/5.6 lens.

Actually, with the 1.4X TC f/2.8 becomes closer to f3.5 than f/4

[Tom W]

Actually, with the 1.4X TC f/2.8 becomes closer to f3.5 than f/4

Actually, no. You lose 1 stop on a 1.4X teleconverter. F/4.

[csnudelman]

Adding a 2X TC gives 1/4 the area of coverage as before or loss of 2 f/ stops. Adding a 1.4X TC gives 25/49 the area of coverage as before and thus a loss of .99999999999999999 f/ stops. I was wrong, sorry.

[Tom W]

Adding a 2X TC gives 1/4 the area of coverage as before or loss of 2 f/ stops. Adding a 1.4X TC gives 25/49 the area of coverage as before and thus a loss of .99999999999999999 f/ stops. I was wrong, sorry.

That's OK - I was going to do the focal length/aperture diameter = f/stop math, but you've reached the same conclusion through different means. No big deal - just don't want people looking for that 1/3 stop months from now. :)

[csnudelman]

Yeah, but ya gotta agree that the 1.4X dosen't make a f/2.8 a f/4 but in reality a f/3.99999999. And to think I used to brag about my 9th grade education.

[RbrtPtikLeoSeny]

I know it loses an f stop with the t-con, but that doesn't bother me. I'll take it off when I don't need it. I'm really only worried about the loss of image quality with the t-con. That and possible loss of AF speed and accuracy.

[raylks]

Having a large aperture would allow a more precise focus is right if we are talking about low light situation. If the screen looks brighter as in large aperture, then it is easier for AF sensor and human eyes to identify the subject so we have a faster and more precise AF.

[Tom W]

I know it loses an f stop with the t-con, but that doesn't bother me. I'll take it off when I don't need it. I'm really only worried about the loss of image quality with the t-con. That and possible loss of AF speed and accuracy.

the 1.4X teleconverter doesn't take much away from the image - just a tick off compared to the bare naked lens. The 2X is best left to the primes in my experience anyway.

[csnudelman]

Huh, no wonder! I've been noticing my 70-200 f/4 hunts a lot indoors where there isn't much light. So, even at it's smaller apetures, it'll af faster? Like, a lens with a max apeture of f/2.8 will focus faster at f/8 than an f/4 lens at f/8?

Just consider the additional problems you may have with an even smaller effective opening like f5.6 or f/8.

[MarkH]

I'm no engineer, but I suspect that the increased focus speed at f2.8 is not just a function of the amount of light but also the decreased depth-of-field. When an out-of-focus image strikes the focus sensor, it perceives low contrast. The contrast increases as the lens is focused. A large-aperature lens increases the rate of increase in contrast per unit movement of the lens.

But I don't understand why f2.8 is a magic threshhold.

As I understand it a wider aperture lets in more light, which makes it easier for the camera to AF - The more aperture the better.

Also - the wider the aperture (smaller f number) the shallower the DoF and the easier to tell if something is in focus or not. The more aperture the better.

And thirdly - On several Canon bodies there are 2 modes for the centre AF sensor, with the high precision mode available with f2.8 or better lenses. I think that f2.8 was chosen because it would include all of Canon's top zooms and primes, and of course the precision is just not possible with f5.6.

Also consider that lenses like Canon's 70-200 models have very good contrast, that will help AF as well. Actually it is the better contrast at max aperture that counts here. A lens that is sharp at f4, but a little soft at f2.8 will not focus as well as a lens that is sharp and contrasty at f2.8.

So f2.8 or better is the magic aperture for the 3 advantages to better AF, but generally wider aperture lenses are better for the first 2 reasons - regardless of the f2.8 thing. i.e. f4 lenses like the 17-40 or 70-200 are better at AF then a similar f5.6 lens would be, but not as good as a f2.8 would be. A f2 lens will be better than a similar f2.8 lens, despite the fact that they both get the high precision mode on centre AF point and both work extremely well - it's probably only in very low light that you could tell any difference.

When shooting at f8 with a f5.6 lens, your camera will focus at f5.6 then stop down by 1 stop for the actual photo - anything in focus at f5.6 will be in focus at f8, with a little margin for error. When shooting at f8 with a f2.8 lens, your camera will focus at f2.8 then stop down by 3 stops for the actual photo - anything in focus at f2.8 will be in focus at f8, with a big margin for error, and the focusing was done with a higher precision to start with.

However well your camera can focus with a f5.6 lens, it should be able to focus at least as well with a f2.8 lens in 1/4 as much light - for a start the lens lets in as much light at f2.8 as it would with f5.6 in 4 x the light. You just can't argue with the fact that in poor light there is no way the wider aperture is not going to make a big difference. With a cheaper lens from a different brand that doesn't AF as well even though it is f2.8 - imagine how bad it would be if it was f5.6.

[cdhender]

When shooting at f8 with a f5.6 lens, your camera will focus at f5.6 then stop down by 1 stop for the actual photo - anything in focus at f5.6 will be in focus at f8, with a little margin for error. When shooting at f8 with a f2.8 lens, your camera will focus at f2.8 then stop down by 3 stops for the actual photo - anything in focus at f2.8 will be in focus at f8, with a big margin for error, and the focusing was done with a higher precision to start with.


I did not know this. Thanks!

[RbrtPtikLeoSeny]

Very useful information MarkH! Thanks for that!

[MarkH]

Very useful information MarkH! Thanks for that!

It's amazing how Knowledgeable you can become if you hang around here for a couple of years and read as much as you can.

Before I bought my 10D 2 years ago I knew that the bigger the f number, the smaller the aperture and that the big apertures + faster shutter also meant less Depth of field. But that was about the extent of my knowledge. I didn't even understand the f numbers properly.

When I got my 10D I decided that I wanted to know about these f numbers and what they meant. Now I know that f4 has half the diameter aperture of f2 and therefore 1/4 as much light gets through. I also know that the stops are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 44, 64, etc. so each number is the square root of 2 multiplied by the previous number, or for simpler maths each number is 2 x the number before the previous one.

It's really as easy as reading some threads here, following some links and reading through them, trying stuff with your camera, asking questions here and reading some more. After a while it doesn't seem so complicated.