Focus / Aperture

[pcassity]

I once an read an article explaining how a camera will always focus at the largest aperture of the lens that is attached to the camera, regardless of the f stop that is selected. . For example a 400 2.8 lens will focus at 2.8, even if a smaller aperture is selected. It went on to explain it is why a lens with a large aperture will always focus faster and more accurately than a lens with a slower, smaller aperture. I was trying to to explain this concept on another forum but wasn’t real successful in making my case. Does anyone have further information or maybe a different explanation? Thanks! 

[paddler4]

Yes, the less light, the harder it is to focus. So for many decades, SLRs have left the aperture wide open until you hit the shutter (or the depth-of-field preview button), when the aperture will briefly close to the selected value. That's all there is to it.

Some fully manual lenses don't have this feature, and one has to manually close the lens down after focusing.

I don't know how early this feature became standard in SLRs, but I know it was standard by the 1960s.

[Dustin McAmera]

A brighter image is easier to see. Stopping down also gives you depth of field, which means you won't be able to see the exact point of correct focus so precisely when stopped down. I think it's lack of depth-of-field in a very wide-aperture lens that would give it an advantage for precise focusing.

That's human focusing covered.

Does the same reasoning mean auto-focus systems need to be at full aperture? I think so: it still makes sense that differences in focus will be more easily detected in a brighter image. And depth of field will increase 'micro-contrast' in slightly out-of-focus areas; the measure used in that system of AF reflects what we see.

[pcassity]

1 hour ago, paddler4 said:

Yes, the less light, the harder it is to focus. So for many decades, SLRs have left the aperture wide open until you hit the shutter (or the depth-of-field preview button), when the aperture will briefly close to the selected value. That's all there is to it.

Some fully manual lenses don't have this feature, and one has to manually close the lens down after focusing.

I don't know how early this feature became standard in SLRs, but I know it was standard by the 1960s.

Thanks! What brought be to bring up the subject in the first place was how a teleconverter would affect this process. For example, my newly purchased z400mm is 4.5. With the 1.4x it becomes a maximum 6.3. Does the it still acquire focus at 4.5 or at 6.3 when using the TC?

[Ed_Ingold]

 

There's no perfect solution.

SLR and DSLR camera generally have an automatic diaphragm, which is fully open until the time of exposure. The aperture ring sets a f/stop to which the diaphragm closes. This can lead to problems, because the focal point of many lenses shifts with the aperture.

Sony mirrorless cameras initiate focus with the aperture wide open, but finish with it closed to the preset value. In manual focus mode, the aperture is always closed to the preset. While this eliminates focus shift, it shifts from phase detection, which is fast, to contrast detection, which is much slower and tends to hunt. In high speed shooting (multi frame), the aperture closes and stays closed.

 

[Ed_Ingold]

2 minutes ago, pcassity said:

Thanks! What brought be to bring up the subject in the first place was how a teleconverter would affect this process. For example, my newly purchased z400mm is 4.5. With the 1.4x it becomes a maximum 6.3. Does the it still acquire focus at 4.5 or at 6.3 when using the TC?

The diaphragm opening is the same, with or without a teleconverter. But the relative aperture (f/stop) is one stop smaller due to the extra focal length.

[Niels - NHSN]

8 hours ago, pcassity said:

Thanks! What brought be to bring up the subject in the first place was how a teleconverter would affect this process. For example, my newly purchased z400mm is 4.5. With the 1.4x it becomes a maximum 6.3. Does the it still acquire focus at 4.5 or at 6.3 when using the TC?

There is no 4.5 when you are using the TC.

The 4.5 lens is effectively 6.3 with the TC. The lens is still acquiring focus at the largest aperture opening, which is now 6.3.

Edited October 17, 2022 by NHSN

[pcassity]

Niels, I understand that the lens becomes a 6.3 with the TC. But my original understanding was as Ed described  - "The diaphragm opening is the same, with or without a teleconverter." Which I take as meaning that the diaphragm continues to open at 4.5.

[q.g._de_bakker]

25 minutes ago, pcassity said:

Niels, I understand that the lens becomes a 6.3 with the TC. But my original understanding was as Ed described  - "The diaphragm opening is the same, with or without a teleconverter." Which I take as meaning that the diaphragm continues to open at 4.5.

The size of the hole itself is the same. The size of the hole relative to the focal length has changed because the focal length changed.

 

In photography the relative size is the important thingy.