DSLRUsers.com

http://www.photoclubalpha.com/2009/07/10/the-sigma-70-200mm-f2-8-ex-dg-hsm-macro-ii/

I accepted his views and I was a bit confused at this comment:

"They tested the Sigma on a Nikon body. Nikon does not use f/2.8 AF sensors – all their sensors work at a virtual aperture of f/5.6. Therefore, the focus is always correct for f/5.6 only. Yet they found the f/2.8 results apparently agreed with the AF, which is impossible using the Nikon body! Sony A700 and A900, Canon 5D, 5D MkII, and all 1D series have f/2.8 sensors which means they focus perfectly at full aperture."


Should we get to the bottom of this or is this just some random bias?

even if it's true, Doesn't the D700 focus faster than the 5D?

Sounds like bollocks to me. AF works before the lens is stopped down so it will always work at the maximum aperture of the lens regardless of what you have it set to.

Oh, and changing the f/stop doesn't affect focus, only depth of field.

I thought F/stop did have some bearing on A/F, the more light let in the better a/f sensors work?

MATT

I did hear somewhere else that the Nikons Depth of Field preview only works out as far as about f4. once you go wider than that, there is no change in the DOF shown in the viewfinder. This was explaining why the 85 1.4 shows more DOF in the viewfinder at f1.4 than it does in the photo. This certainly appears to be true with mine. If the AF is at the viewfinder end of the works, the same cropping (if that is the word) may affect the AF sensor too.

MATT wrote:I thought F/stop did have some bearing on A/F, the more light let in the better a/f sensors work?

Correct, but that related to the maximum aperture of the lens as this is where the lens sits when the camera does its AF. i.e. the larger the maximum aperture of the lens, the better the AF should be, especially in low light. e.g. an f/1.4 lens should result in better AF in low light than an f/4 lens.

This further shows that the comments by the reviewer are off the mark as they are pretty much suggesting the exact opposite.

Mr Darcy wrote:I did hear somewhere else that the Nikons Depth of Field preview only works out as far as about f4. once you go wider than that, there is no change in the DOF shown in the viewfinder. This was explaining why the 85 1.4 shows more DOF in the viewfinder at f1.4 than it does in the photo. This certainly appears to be true with mine. If the AF is at the viewfinder end of the works, the same cropping (if that is the word) may affect the AF sensor too.

Not sure why it would. The DOF preview button does basically the same thing that the shutter release does - stop the lens down from the maximum aperture to the chosen aperture. When you use AF, the lens does not get stopped down and stays wide open. This is especially true in AF-S mode. I guess if AF-C there is the potential that the AF is still working at the time the shutter is released and the lens stopped down, but I suspect that as the mirror goes up at the same time, the AF system wouldn't be working at that point.

a bunch of inaccurate BS posted on a sony board ? nooooooooooooooooooo....i dont believe it

I would agree that ....THIS IS UTTER BULLSHIT!! I am sure there must be something somewhere that will back this up. I would have a look but I dont have time atm....let me get back to you on this.

All,

Not sure about where the Nikon camp stands on this one but I know I did read in a Canon whitepaper on their CMOS chips, that depending on the model of camera, it employs one of a few AF systems.

From the 5D whitepaper (pg. 9) available here

"The center AF point has a special hybrid design. With f/2.8 or faster lenses, focusing is
a two-step process. First, the f/5.6-sensitive cross-type sensor components are used to
focus. When focus is almost achieved, a switch is made to the f/2.8-sensitive vertical
line-sensitive sensors for high-precision focusing. Additionally, the center AF point’s
f/5.6-sensitive, horizontal line-sensitive sensors each have two lines with individual pixels
arranged in a zigzag pattern, making a total of four lines for horizontal line-sensitive
focusing at the center. With this improvement, the center AF point can now do a better
job of detecting difficult-to-read, low-contrast subjects, thus improving the overall
performance of the autofocusing system. All six Supplemental AF points are f/5.6-
sensitive. The two Supplemental AF points directly above and below the center are also
f/2.8-sensitive when a lens of f/2.8 or brighter is used."

Perhaps that's what the original poster was getting at..? I may also be waaaay out-of-my-league. If that happens to be the case, I prefer to have "my foot" served with BBQ sauce. Isn't that the Aussie way?

ATJ wrote:Oh, and changing the f/stop doesn't affect focus, only depth of field.

Generally true, but I understand it can, and it's called focus shift.

There's a discussion of it here in a review of the Nikon 50mm f1.4G by the British Journal of Photography.

Regards,
Murray

Who says you can't teach an old dog new tricks? I have learned something today.

From looking at the BJP and my old Manual of Photography I now know about focus shift. It can occur due to spherical aberration in the lens. Both the BJP and the Manual suggest that it doesn't occur in all lenses - i.e. it can be corrected.

The original posted information could have an element of truth to it, although I have no idea if Nikon bodies do or do not cater for focus shift in lenses. I guess we can assume, though, that the Sigma 70-200mm f/2.8 does indeed suffer from focus shift.

Interestingly, this new information (well, new to me) may explain why some people have had focusing issues when taking shots with the lens wide open. I had always assume it was a combination of focusing error and narrow depth of field but could be caused by focus shift (assuming the Nikon AF doesn't cater for it).

Nice to know i can be useful sometimes.

There might also be another effect. When researching to purchase the Nikon 50mm, I discovered this review of the Sigma 50mm that is quite damning in it's autofocus reliability. Unlike most reviews which are "lab" reviews conducted in good light, this was a field reveiew conducted in low light.

Since it's quantifiable, most reviews measure focus speed which is defined as the time it takes the lens to go from near to far focus. This is not usually a practical criterion because the question is usually whether a lens tends to hunt in a particular range. Most reviews commented on the focusing accuracy of the Nikon. The Nikon also has a much larger focusing throw (manual turns from close focus to far) than the Sigma. This is pure conjecture on my part but I can't help wondering whether the very length of that throw means that the Sigma is designed for speed and the Nikon for accuracy. Perhaps the smaller throw makes it qucker for the focusing motor to operate but also more difficult to determine exactly where to stop in low light.

Regards,
Murray

Murray,

Inquiring minds (mine, mostly!) want to know... Was I off the mark?

I suppose in addition to that, the review you posted, was done with a Canon (though not specified which model). It think I might venture a guess that the results might be affected by that as well. All of my reading has suggested that Nikons generally "focus better", but there always seems to be the allusion that this may be related to the increased number of focus sensors (vs. Canons). In short, they track better, across the range of sensors.

Does anyone have any other insights on this? Can someone point to me to some technical data sheet on the various iterations of Nikons Multi-CAM AF system?

Must do more Google-ing, or Yahoo! search, or Bing (as appropriate)...

I found this:

51-area autofocus The D3X is the fourth Nikon digital SLR to utilize a Multi-CAM 3500-based autofocus system (it's officially called Multi-CAM 3500FX in Nikon's full-frame cameras).

The AF system is comprised of 51 AF areas, including 15 cross-type AF areas (they operate as cross-type with lenses whose maximum aperture is f/5.6 or faster). The 51-area grid forms a wide rectangle across the frame, with minimal spacing between each AF area.

http://www.robgalbraith.com/bins/multi_page.asp?cid=7-9318-9761

If the day ever comes when I can’t focus, either automatically or manually, then that will be the time for me to worry about this kind of review

Hi H

Well, I don't know what aperture Nikon autofocus works at but it's only going to make a difference is there is focus shift in the lens. That in turn will be less significant the futher you have the lens stopped down and the wider then angle is. Probably you can test for that by focusing and exposing at different apertures, then changing the aperture and taking another exposure.

The current generation of Nikon cameras you would think must be much better at autofocus than the Canon because they have many more sensor sites. Probably this is particularly true of the D300 where the focusing sensors cover much of the frame. It's true the reviewer that slated the autofocus of the Sigma 50mm was comparing it with Canon lenses and bodies and maybe that wouldn't help but my guess is that it wouldn't greatly change the comparison between the Sigma and other lenses of either manufacturer.

I also agree that this sort of thing is only an issue if you're seeking to buy a lens and suspect there may be an effect there, or if you have trouble focusing and can't work out why.

Regards,
Murray

aim54x wrote:I found this:

51-area autofocus The D3X is the fourth Nikon digital SLR to utilize a Multi-CAM 3500-based autofocus system (it's officially called Multi-CAM 3500FX in Nikon's full-frame cameras).

The AF system is comprised of 51 AF areas, including 15 cross-type AF areas (they operate as cross-type with lenses whose maximum aperture is f/5.6 or faster). The 51-area grid forms a wide rectangle across the frame, with minimal spacing between each AF area.

http://www.robgalbraith.com/bins/multi_page.asp?cid=7-9318-9761

Nah, a different issue. That's more to do with getting enough light through the lens to AF. i.e. if the lens' maximum aperture is f/5.6, the camera will focus. Now, most lenses have maximum apertures faster than f/5.6, but this comes into play with things like teleconverters. for example, take an f/4 lens and put a 2x TC on it and you won't be able to focus with the cross-type AF areas.