Question about a camera spec I saw...

[jedirunner]

Hey all,

I was looking over comparison specs between 5dMIII and D800 and noticed a comparison point that I didn't know how to interpret. On one camera, the AF only goes to f/5.6 and on the other AF goes to f/8. I can't be interpreting this right, as I interpreted that to mean "auto focus doesn't work at narrower aperture than f/5.6" (or f/8 for the other).

Is that how that spec is supposed to be interpreted? When does or does not that particular spec matter to someone?

(I'm not in the market for either of these cameras ... just trying to understand what the spec means).

Thanks!

Kevin

 

[analog.universe]

That refers to the maximum aperture of the lens, not the aperture you choose when you take the shot. When you shoot a dSLR, the aperture remains wide open until you actually click the shutter, and then it stops down.

 

[jedirunner]

That refers to the maximum aperture of the lens, not the aperture you choose when you take the shot. When you shoot a dSLR, the aperture remains wide open until you actually click the shutter, and then it stops down.

So why would the specs on a camera body state "AF to f/5.6"? How does the lens aperture relate to the AF capabilities of the camera?

Thanks,

Kevin

 

[tacticdesigns]

I think that means that if you have a lens on the camera that at the focal length you are at, the lens is only able to have a maximum of f5.6 or f8 or something slow like that, the Nikon D800 will still be able to have some of its autofocus points be cross focus types.

Which is good because on older cameras, I guess you had to be at least at f2.8 or faster.

 

[KmH]

That refers to the maximum aperture of the lens, not the aperture you choose when you take the shot. When you shoot a dSLR, the aperture remains wide open until you actually click the shutter, and then it stops down.

So why would the specs on a camera body state "AF to f/5.6"? How does the lens aperture relate to the AF capabilities of the camera?

Thanks,

Kevin

The smaller the maximum lens aperture, the less light there is available for the auto focus module to use.

Half the light the lens gathers gets deflected by the 50% transparent main mirror up to the viewfinder. The other 1/2 of the light that goes on through the 50% reflective main mirror hits a secondary mirror behind it that deflects that light down to the AF module in the bottom of the camera.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. - File:E-30-Cutmodel.jpg - Wikipedia, the free encyclopedia

 

[jedirunner]

Thanks everyone. That makes more sense now. Kind of an "oh, duh" which maybe should have already just occurred to me, but yeah.

Thanks,

Kevin

 

[Tiberius47]

That refers to the maximum aperture of the lens, not the aperture you choose when you take the shot. When you shoot a dSLR, the aperture remains wide open until you actually click the shutter, and then it stops down.

So why would the specs on a camera body state "AF to f/5.6"? How does the lens aperture relate to the AF capabilities of the camera?

Thanks,

Kevin

It doesn't.

What it means is that with f8, less light is getting into the lens than it is with f5.6. So a camera that can autofocus at f8 can focus in less light than a camera that can only focus at f5.6.

As Analog Universe said, the lens is wide open until you actually take the photograph. However, some longer lenses close down to f5.6, and if you use a teleconverter, that can cost you a stop or two of light as well, so you might only be able to get the equivalent of f8 or even f11, depending on your lens and set up. In this case, a camera that can focus with less light is very desirable.

It also makes it a bit easier to autofocus in low light conditions.

 

[Derrel]

Hey all,

I was looking over comparison specs between 5dMIII and D800 and noticed a comparison point that I didn't know how to interpret. On one camera, the AF only goes to f/5.6 and on the other AF goes to f/8. I can't be interpreting this right, as I interpreted that to mean "auto focus doesn't work at narrower aperture than f/5.6" (or f/8 for the other).

Is that how that spec is supposed to be interpreted? When does or does not that particular spec matter to someone?

(I'm not in the market for either of these cameras ... just trying to understand what the spec means).

Thanks!

Kevin

Kevin--the minimum aperture at which an AF system will function is one of the better indicators of the raw, brute strength of the AF system and its AF module. An AF system that will function down to apertures as small as f/8 mans that the AF system will be able to focus with slow lenses paired with teleconverters, for example. This specification you have brought up is more important than the number of AF points. The f/5.6 limit is and was and will often be "hit" with MANY lenses and many teleconverters....Nikon's new f/8 minimum aperture setting is the best specification on AF limits that has EVER been made. That means the D800 will be able to focus with a lens+ tele-converter combination that admits only HALF as much light as the new 5D-III will require to achieve its autofocus.

 

[Helen B]

Don't get confused between the two limitations to the type of AF used by SLRs. There is an aperture limitation, which is being referred to here, and a brightness limitation, which is different. The aperture limitation is to do with how the AF system detects out-of-focus lines - the smaller the aperture the harder it is for the system to work properly. Those of us who used proper manual focus SLRs will remember problems with part of the focusing prisms/microprisms blacking out with lenses that have a small-ish maximum aperture (it's to do with the maximum difference in angle of arrival of the light rays from a point, not how bright they are). The same happens with the common type of AF used behind a semi-silvered mirror. It is an electronic version of the split prism type of rangefinder spot in the middle of an SLR viewing screen. This limitation is independent of brightness.

The AF system also has a brightness limitation which, although it is affected by the lens' aperture, is a different matter - it is simply whether or not enough light is reaching the AF sensors. That limitation will usually be expressed as an EV at ISO 100 with a certain aperture lens.

If it was only brightness that was the problem, a camera that could focus at f/5.6 but not f/8 at a particular lighting level could focus at f/8 at twice that lighting level.

 

[jedirunner]

Wow, some great information has come out of this thread. Very educational for me. I have a lot to learn, and every time I think I started to get a grasp on one idea, I find 3 more that I don't yet fully (or at all) understand. This forum is a great place to hang out!

Thanks again everyone, and not just for this answer, but for all the answers to all the questions out here.

Kevin