Railfan Technology > Autofocus: does it vary over the frame?

Knowing nothing about autofocus, I've always assumed
it moves the lens, or part of the lens, but the lens stays
in one spot for the length of the exposure. So there's still
just one distance from the lens that's perfectly in focus.
Is that always true, no matter how expensive the camera?

This pic (click on it to see it full size)

https://cdn.hasselblad.com/f/77891/9963x7472/5a76623560/manny-ortiz-x2d-xcd90v-1.jpg

was taken with a digital Hasselblad and a 90 mm lens.
Does the autofocus change during the exposure?
Part of the subject farther away from the camera,
part near, but all in focus?

Another question: is it true the 90 mm f/2.5 is
always at f/2.5? There's no diaphragm? How common is that?



Edited 3 time(s). Last edit at 07/12/25 12:43 by timz.

timz Wrote:
-------------------------------------------------------
> Knowing nothing about autofocus, I've always
> assumed
> it moves the lens, or part of the lens, but the
> lens stays
> in one spot for the length of the exposure. So
> there's still
> just one distance from the lens that's perfectly
> in focus.
> Is that always true, no matter how expensive the
> camera?
>
> This pic (click on it to see it full size)
>
> https://cdn.hasselblad.com/f/77891/9963x7472/5a766
> 23560/manny-ortiz-x2d-xcd90v-1.jpg
>
> was taken with a digital Hasselblad and a 90 mm
> lens.
> Does the autofocus change during the exposure?
> Part of the subject farther away from the camera,
> part near, but all in focus?
>
> Another question: is it true the 90 mm f/2.5 is
> always at f/2.5? There's no diaphragm? How common
> is that?

That lens stops down to f/32, which would be fairly typical.  I'm not aware of any modern camera lens with no diaphragm.  Some enlarging or process lenses might not have diaphragms.  The various Schneider enlarging lenses, very popular in film days, all had diaphragms.

Here is more than you wanted to know about that Hasselblad lens:

https://www.bhphotovideo.com/c/product/1725357-REG/hasselblad_cp_hb_00000746_01_xcd_90mm_f_2_5_lens.html/?ap=y&ap=y&smp=y&smp=y&store=420&lsft=BI%3A514&gad_source=1&gad_campaignid=1413797499&gbraid=0AAAAAD7yMh0K9CephykGnkuaAQCgTDFpT&gclid=Cj0KCQjwj8jDBhD1ARIsACRV2Tur7lAtb7zIq7oqG9f6ZM7IVJwS-Z0x4Z_zHQiAouKTjkBsEiQCqQMaArfbEALw_wcB

The picture is not all in focus.  Look to our left of the model, just above the table.  You can see the top of her pants/skirt definitely not as as sharply in focus.

Back to your original question...  Autofocus does not change during a single exposure.  Some of the newer digital cameras do focus stacking, which means multiple exposures focused at slightly different distances, then post processed to look like a single shot with much depth of field.  The Nikon Z6, Z7, Z8 and Z9 all do that.  Similarly priced cameras from Canon, Sony, Fuji, Hassleblad, Leica, maybe others, likely provide similar functionality, perhaps with less automation or requiring external focus control as well as post processing.

Autofocus is set for the exposure. Try an internet search on 'depth of field'.

> Some of the newer digital cameras do focus stacking, which
> means multiple exposures focused at slightly
> different distances, then post processed to look
> like a single shot with much depth of field.  The
> Nikon Z6, Z7, Z8 and Z9 all do that.

You push the shutter button, and the camera takes ... as many exposures as you've told it to, moving the lens a tenth of a millimeter between each one? How long do ten exposures take?

timz Wrote:
-------------------------------------------------------
> > Some of the newer digital cameras do focus
> stacking, which
> > means multiple exposures focused at slightly
> > different distances, then post processed to
> look
> > like a single shot with much depth of field. 
> The
> > Nikon Z6, Z7, Z8 and Z9 all do that.
>
> You push the shutter button, and the camera takes
> ... as many exposures as you've told it to, moving
> the lens a tenth of a millimeter between each one?
> How long do ten exposures take?

It looks like the Nikon Z8 will shoot as fast as five shots a second, so two seconds for ten exposures.  You can slow it down if, for instance, you need to let  a flash recharge.  

Z8 and Z9 will do 20FPS full frame raw, 30FPS full frame jpg, and 60 or 120FPS with smaller size files... Of course you can slow down from that.

But the focus setting doesn't change during a 30 frame per second run?

timz Wrote:
-------------------------------------------------------
> But the focus setting doesn't change during a 30
> frame per second run?

There should be some re-focusing between shots.

skyview Wrote:
-------------------------------------------------------
> Z8 and Z9 will do 20FPS full frame raw, 30FPS full
> frame jpg, and 60 or 120FPS with smaller size
> files... Of course you can slow down from that.

The 5 frames per second I mentioned above was specifically for focus stacking.  

So focus stacking at 5 fps but not 30?

timz Wrote:
-------------------------------------------------------
> So focus stacking at 5 fps but not 30?

That is how I read everything I've seen.