What is your standard prime?

Started 3 months ago | Discussions thread
sportyaccordy Forum Pro • Posts: 19,159
Re: What is your standard prime?

DutchMM wrote:

Martin_99 wrote:

DutchMM wrote:

That said, I have a theory that the faster the lens, the faster the AF. Because the AF systems in (at least) Sony bodies perform the AF with the aperture wide open.

Are you sure?

Quite unsure about most things, but the point of live view is that you can see what the photo will look like. And you can turn that visible effect off. And that suggests to me that the camera is holding the diaphragm open for AF purposes. Why else would it be a good idea?

So faster lens = more light = faster focus.

I think that this is only part of the truth. When camera have more light it maybe process the focus faster, but AF motor have to do the hard work. So if lens AF motor is slow, no amount of light will help.

I agree that the weight inside the lens has an impact - as well as to your earlier remark about more modern lenses having more powerful motors. Nevertheless you would have to agree that barring the Sigma 1.4 trinity, FF lenses (at least from 12 to 55mm) are typically at least a third of a stop faster than the available crop selection. Again - barring the Sigmas, which crop lens is faster than f1.8?

What I meant about the speed requirements of FF lenses is unrelated to Sony's enthusiasm (or lack of it) for the APS-C market. Pursuing my light = AF speed theory, I am guessing (I was a linguist, not a physics guru) that FF lenses having to put enough light on the extremes of the sensor means that they also have to put a lot more on the more central regions. And that this would provide much much more info for AF sensors than an equivalent crop lens. I mean, the inverse square law must apply inside a camera as well as outside?

There are a few confounding factors here

1 if there's ample light is easier to focus at a smaller aperture

2 it can be harder to acquire focus if depth of field is thin

3 faster glass often has more and heavier elements that slow autofocus down

It's definitely not cut and dry and is too situational to be a hard and fast rule

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 sportyaccordy's gear list:sportyaccordy's gear list
Sony a7 III Sony FE 24-70mm F4 OSS Sony FE 85mm F1.8 Samyang AF 35mm F1.8 FE Samyang AF 45mm F1.8 FE
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