If I manually focus on something, then change the focal length, it seems
that the subject is still in focus. This is handy, but can it be relied
on? Is this something which is a given, or have I just been lucky so
far? Perhaps it depends on the camera/lens? I'm using an EOS 400d with
the stock lens (for now - thinking of an upgrade but I'm in no hurry).
_.@._ (Poldie) wrote in news:g55r03$en3$1@registered.motzarella.org:
> If I manually focus on something, then change the focal length, it
> seems that the subject is still in focus. This is handy, but can it
> be relied on? Is this something which is a given, or have I just
> been lucky so far? Perhaps it depends on the camera/lens? I'm
> using an EOS 400d with the stock lens (for now - thinking of an
> upgrade but I'm in no hurry).
Trying to remember back to the days when I actually bought lenses for
my SLRs, there are "true zooms" and "varifocal" lenses.
A "true zoom" will maintain focus as you change focal length, a
varifocal lens will not. Zoom lenses from different makers maintained
focus in various proprietary ways, often moving the internal elements
around with complicated cam actions. Zoom lenses were sometimes called
"parfocal". Some zooms tried to maintain focus throughout the focal
length and focus distance ranges, but didn't always make it with every
combination.
I don't know if there's any legal constraint on what kind of lens gets
to be called a zoom lens, or if you might get a varifocal lens with
that label.
If your camera/lens combo is auto-focus, are you sure that's not what
you're seeing?
Bert Hyman wrote:
> _.@._ (Poldie) wrote in news:g55r03$en3$1@registered.motzarella.org:
>
>> If I manually focus on something, then change the focal length, it
>> seems that the subject is still in focus. This is handy, but can it
>> be relied on? Is this something which is a given, or have I just
>> been lucky so far? Perhaps it depends on the camera/lens? I'm
>> using an EOS 400d with the stock lens (for now - thinking of an
>> upgrade but I'm in no hurry).
>
> Trying to remember back to the days when I actually bought lenses for
> my SLRs, there are "true zooms" and "varifocal" lenses.
>
> A "true zoom" will maintain focus as you change focal length, a
> varifocal lens will not. Zoom lenses from different makers maintained
> focus in various proprietary ways, often moving the internal elements
> around with complicated cam actions. Zoom lenses were sometimes called
> "parfocal". Some zooms tried to maintain focus throughout the focal
> length and focus distance ranges, but didn't always make it with every
> combination.
>
> I don't know if there's any legal constraint on what kind of lens gets
> to be called a zoom lens, or if you might get a varifocal lens with
> that label.
>
> If your camera/lens combo is auto-focus, are you sure that's not what
> you're seeing?
I'm a little new to this, so I'll have to read up on some of the stuff
you just wrote (thanks!). But I'm talking about manual mode. In
auto-focus then yes, it'll do it automatically when I half-press the
shutter. It's just that the other day I was doing 15-30 second
exposures of a candle in the dark, which required manual focus, and it
was then that I noticed it.
On Jul 10, 3:23*pm, Poldie <_.@._> wrote:
> Bert Hyman wrote:
> > _.@._ (Poldie) wrote innews:g55r03$en3$1@registered.motzarella.org:
>
> >> If I manually focus on something, then change the focal length, it
> >> seems that the subject is still in focus. *This is handy, but can it
> >> be relied on? *Is this something which is a given, or have I just
> >> been lucky so far? *Perhaps it depends on the camera/lens? *I'm
> >> using an EOS 400d with the stock lens (for now - thinking of an
> >> upgrade but I'm in no hurry).
>
> > Trying to remember back to the days when I actually bought lenses for
> > my SLRs, there are "true zooms" and "varifocal" lenses.
>
> > A "true zoom" will maintain focus as you change focal length, a
> > varifocal lens will not. Zoom lenses from different makers maintained
> > focus in various proprietary ways, often moving the internal elements
> > around with complicated cam actions. Zoom lenses were sometimes called
> > "parfocal". Some zooms tried to maintain focus throughout the focal
> > length and focus distance ranges, but didn't always make it with every
> > combination.
>
> > I don't know if there's any legal constraint on what kind of lens gets
> > to be called a zoom lens, or if you might get a varifocal lens with
> > that label.
>
> > If your camera/lens combo is auto-focus, are you sure that's not what
> > you're seeing?
>
> I'm a little new to this, so I'll have to read up on some of the stuff
> you just wrote (thanks!). *But I'm talking about manual mode. *In
> auto-focus then yes, it'll do it automatically when I half-press the
> shutter. *It's just that the other day I was doing 15-30 second
> exposures of a candle in the dark, which required manual focus, and it
> was then that I noticed it.
Back in the day when I used film cameras without auto-focus I found
that my Canon zoom lenses did a pretty good job of maintaining focus
over their range, and still do. But, because I'm a belt and
suspenders kind of guy, when I had time I'd touch up the focus after
zooming. These days I rely almost exclusively on the auto-focus (my
eye sight and reflexes aren't as good as they were 35 years ago). In
a situation where I had to use manual focus with a zoom lens I'd
probably revert to my old habit.
If you're using the kit lens the zoom range isn't that great, and even
wide open the depth of field isn't that shallow. Your manually
focused images are probably within acceptable, if not precise, focus
over the zoom range of the lens. Having said that, I'd personally
refocus after zooming.
On Jul 10, 3:23*pm, Poldie <_.@._> wrote:
> Bert Hyman wrote:
> > _.@._ (Poldie) wrote innews:g55r03$en3$1@registered.motzarella.org:
>
> >> If I manually focus on something, then change the focal length, it
> >> seems that the subject is still in focus. *This is handy, but can it
> >> be relied on? *Is this something which is a given, or have I just
> >> been lucky so far? *Perhaps it depends on the camera/lens? *I'm
> >> using an EOS 400d with the stock lens (for now - thinking of an
> >> upgrade but I'm in no hurry).
>
> > Trying to remember back to the days when I actually bought lenses for
> > my SLRs, there are "true zooms" and "varifocal" lenses.
>
> > A "true zoom" will maintain focus as you change focal length, a
> > varifocal lens will not. Zoom lenses from different makers maintained
> > focus in various proprietary ways, often moving the internal elements
> > around with complicated cam actions. Zoom lenses were sometimes called
> > "parfocal". Some zooms tried to maintain focus throughout the focal
> > length and focus distance ranges, but didn't always make it with every
> > combination.
>
> > I don't know if there's any legal constraint on what kind of lens gets
> > to be called a zoom lens, or if you might get a varifocal lens with
> > that label.
>
> > If your camera/lens combo is auto-focus, are you sure that's not what
> > you're seeing?
>
> I'm a little new to this, so I'll have to read up on some of the stuff
> you just wrote (thanks!). *But I'm talking about manual mode. *In
> auto-focus then yes, it'll do it automatically when I half-press the
> shutter. *It's just that the other day I was doing 15-30 second
> exposures of a candle in the dark, which required manual focus, and it
> was then that I noticed it.
Back in the day when I used film cameras without auto-focus I found
that my Canon zoom lenses did a pretty good job of maintaining focus
over their range, and still do. But, because I'm a belt and
suspenders kind of guy, when I had time I'd touch up the focus after
zooming. These days I rely almost exclusively on the auto-focus (my
eye sight and reflexes aren't as good as they were 35 years ago). In
a situation where I had to use manual focus with a zoom lens I'd
probably revert to my old habit.
If you're using the kit lens the zoom range isn't that great, and even
wide open the depth of field isn't that shallow. Your manually
focused images are probably within acceptable, if not precise, focus
over the zoom range of the lens. Having said that, I'd personally
refocus after zooming.
Poldie <_.@._> wrote:
>If I manually focus on something, then change the focal length, it seems
>that the subject is still in focus. This is handy, but can it be relied
>on?
No.
>Is this something which is a given, or have I just been lucky so
>far?
"Poldie" <_.@._> wrote in message
news:g55ul4$86m$1@registered.motzarella.org...
> Bert Hyman wrote:
>> _.@._ (Poldie) wrote in news:g55r03$en3$1@registered.motzarella.org:
>>
>>> If I manually focus on something, then change the focal length, it
>>> seems that the subject is still in focus. This is handy, but can it
>>> be relied on? Is this something which is a given, or have I just
>>> been lucky so far? Perhaps it depends on the camera/lens? I'm
>>> using an EOS 400d with the stock lens (for now - thinking of an
>>> upgrade but I'm in no hurry).
>>
>> Trying to remember back to the days when I actually bought lenses for
>> my SLRs, there are "true zooms" and "varifocal" lenses. A "true zoom"
>> will maintain focus as you change focal length, a
>> varifocal lens will not. Zoom lenses from different makers maintained
>> focus in various proprietary ways, often moving the internal elements
>> around with complicated cam actions. Zoom lenses were sometimes called
>> "parfocal". Some zooms tried to maintain focus throughout the focal
>> length and focus distance ranges, but didn't always make it with every
>> combination. I don't know if there's any legal constraint on what kind of
>> lens gets
>> to be called a zoom lens, or if you might get a varifocal lens with
>> that label. If your camera/lens combo is auto-focus, are you sure that's
>> not what
>> you're seeing?
>
> I'm a little new to this, so I'll have to read up on some of the stuff you
> just wrote (thanks!). But I'm talking about manual mode. In auto-focus
> then yes, it'll do it automatically when I half-press the shutter. It's
> just that the other day I was doing 15-30 second exposures of a candle in
> the dark, which required manual focus, and it was then that I noticed it.
Zoom lenses are supposed to be "parfocal," meaning the lens will stay in
focus as it changes focal length (in other words, as you zoom in or out).
Whether a zoom lens will stay *perfectly* in focus is another question, and
it's usually a good idea to check the focus after you've done any zooming
after focusing manually.
A varifocal lens generally will not stay in focus as it is "zoomed," and has
to be refocused with any change in focal length. Some lenses are called
zooms but are really varifocals, but most lenses for cameras of SLR type
such as yours are real zooms. There have been exceptions but they're not
common.
On Jul 10, 3:21*pm, Poldie <_.@._> wrote:
> If I manually focus on something, then change the focal length, it seems
> that the subject is still in focus. *This is handy, but can it be relied
> on? *Is this something which is a given, or have I just been lucky so
> far? *Perhaps it depends on the camera/lens? *I'm using an EOS 400d with
> the stock lens (for now - thinking of an upgrade but I'm in no hurry).
>
> Cheers.
There are a lot of lenses around, so it is not possible to make an
unqualified statement but most these days do a pretty good job of
holding focus.
Keep in mind that if you focus at extreme longest focal length and
then back off, you are unlikely to notice any slight shift in focus
because fine details are lost in lower angular resolution, so the old
rule of thumb is focus fully zoomed in, then back off to desired focal
length. I have not had a problem in recent years doing this with
several new lenses.
focus, then change focal length 
Linear Mode
