Lenses with fixed aperture
 

I'm shortly going to upgrade from a compact P&S (Canon S40) to a Canon DSLR
and have been pondering lenses. I find a lot of lenses which have a fixed
aperture, such as the well regarded 17-40mm f4 L lens. I'd have thought
that fixed aperture would be a bad thing...

One of the things about a compact camera is that changing aperture from one
end of the range (f2.8 for the S40) to the other (f8 for the S40) really
doesn't do a great deal in many cases. I was rather looking forward to
working, experimenting and learning with a system that allows a good range
of aperture adjustment. But now, in my quest for a small number (like 1 or
2) of quality lenses as a starting point, I find myself homing in on lenses
with fixed aperture.

Am I right to be concerned about this, or is having a single wide lens fixed
at f4 a good thing for reasons I don't understand?

--
The email address used to post is a spam pit. Contact me at
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Derek Fountain wrote:

I'm shortly going to upgrade from a compact P&S (Canon S40) to a Canon DSLR
and have been pondering lenses. I find a lot of lenses which have a fixed
aperture, such as the well regarded 17-40mm f4 L lens. I'd have thought
that fixed aperture would be a bad thing...

17-40 f4L is a fixed-aperture lens?!?!?!?!?!!?

Read a book or something on SLR & lenses. The f4 indicated is the
MAXIMUM aperture you can obtain for that lens. But it's not always
fixed. The only lenses that i know with fixed aperture are those in
cheap point&shoot cameras, and mirror lenses.

By the way, the "L" series are the pro lens series on Canon.

--
chidalgo

chidalgo wrote:
17-40 f4L is a fixed-aperture lens?!?!?!?!?!!?

Read a book or something on SLR & lenses.

I am, and I'm asking questions when confused. Thanks for the pointer though.

--
The email address used to post is a spam pit. Contact me at
http://www.derekfountain.org : a
href="http://www.derekfountain.org/"Derek Fountain/a

"Derek Fountain" wrote in message
...
I'm shortly going to upgrade from a compact P&S (Canon S40) to a Canon
DSLR
and have been pondering lenses. I find a lot of lenses which have a fixed
aperture, such as the well regarded 17-40mm f4 L lens. I'd have thought
that fixed aperture would be a bad thing...

One of the things about a compact camera is that changing aperture from
one
end of the range (f2.8 for the S40) to the other (f8 for the S40) really
doesn't do a great deal in many cases. I was rather looking forward to
working, experimenting and learning with a system that allows a good range
of aperture adjustment. But now, in my quest for a small number (like 1 or
2) of quality lenses as a starting point, I find myself homing in on
lenses
with fixed aperture.

Am I right to be concerned about this, or is having a single wide lens
fixed
at f4 a good thing for reasons I don't understand?

--
The email address used to post is a spam pit. Contact me at
http://www.derekfountain.org : a
href="http://www.derekfountain.org/"Derek Fountain/a

Fixed aperture, in this case, refers to the fact that the aperture does not
change when you zoom the lens, i.e., it will stay at f4, if you set it
there, whether you are at 17mm or 40mm, or somewhere in between. Of course,
you can set the aperture at anything from f4 to f22 (I believe) and have it
stay there. A wide aperture lessens your depth of field, so that
backgrounds can be pleasantly out of focus, diminishing distracting details
behind your subject. So, the 24-70mm f2.8L can, but doesn't have to, stay
at a maximum aperture of f2.8 throughout its zoom range, but the 28-135
f3.5-5.6 IS will be at a maximum of f3.5 at 28mm, f4 by 50mm, f4.5 at 70mm,
and f5.6 at 100mm. So you can keep the background more out of focus at the
long end with the 24-70 than you can the 28-135.
I know it's confusing, I have a "fixed aperture" 400mm (old Canon) FD mount
Spiratone "Baseball Bat" telephoto that is permanently set at f8.
By the way, small sensors like in the S40 give the lenses a greater depth of
field, so apertures don't make as much difference as they do with the larger
sensors of DSLRs, or so it seems.

--
Skip Middleton
http://www.shadowcatcherimagery.com

"Derek Fountain" wrote in message
...
I'm shortly going to upgrade from a compact P&S (Canon S40) to a Canon
DSLR
and have been pondering lenses. I find a lot of lenses which have a fixed
aperture, such as the well regarded 17-40mm f4 L lens. I'd have thought
that fixed aperture would be a bad thing...

One of the things about a compact camera is that changing aperture from
one
end of the range (f2.8 for the S40) to the other (f8 for the S40) really
doesn't do a great deal in many cases. I was rather looking forward to
working, experimenting and learning with a system that allows a good range
of aperture adjustment. But now, in my quest for a small number (like 1 or
2) of quality lenses as a starting point, I find myself homing in on
lenses
with fixed aperture.

Am I right to be concerned about this, or is having a single wide lens
fixed
at f4 a good thing for reasons I don't understand?

For a lens that is (for example) a 24-70mm 2.8 L, the "2.8" simply indicates
that the maximum aperture of 2.8 is available throughout the zoom
range...which is highly preferable to most consumer zoom lenses that end up
giving you a reduced max aperture as you zoom toward the tele end.
Remember, this ONLY refers to maximum aperture, and does NOT indicate a
limitation of adjusting to smaller apertures. On lesser lenses, you'll see
a range of apertures given, but that only indicates how much the **maximum**
aperture changes throughout the zoom range. Most zooms allow larger
apertures at the wide end of the zoom than at the telephoto end. You can
always reduce the aperture to whatever the limitations of the lens is
(usually anywhere from f22-f32, or perhaps f64, up to whatever it's max ap
is).

The only modern lenses (that I'm aware of) that have a TRULY "fixed"
apertures (CANNOT be changed) are mirror lenses--which are NOT a good choice
for most people, due to their strange rendition of background out-of-focus
elements, and highlights.

Derek Fountain wrote:
chidalgo wrote:

17-40 f4L is a fixed-aperture lens?!?!?!?!?!!?

Read a book or something on SLR & lenses.


I am, and I'm asking questions when confused. Thanks for the pointer though.

The fixed aperture at f4.0 simply means that the aperture is f4.0 at
17mm as well as 40mm. I do not know of a lens with that short a focal
length that is that slow. But I do not know everything.
The 17-40 fixed at f4.0 will also allow 5.6, 8.0 and more than likely f
11, and mabey f16.

"Derek Fountain" wrote in message
...
I'm shortly going to upgrade from a compact P&S (Canon S40) to a Canon
DSLR
and have been pondering lenses. I find a lot of lenses which have a fixed
aperture, such as the well regarded 17-40mm f4 L lens. I'd have thought
that fixed aperture would be a bad thing...

One of the things about a compact camera is that changing aperture from
one
end of the range (f2.8 for the S40) to the other (f8 for the S40) really
doesn't do a great deal in many cases. I was rather looking forward to
working, experimenting and learning with a system that allows a good range
of aperture adjustment. But now, in my quest for a small number (like 1 or
2) of quality lenses as a starting point, I find myself homing in on
lenses
with fixed aperture.

Am I right to be concerned about this, or is having a single wide lens
fixed
at f4 a good thing for reasons I don't understand?

The higher priced lenses like the Canon L 17~40 is not a fixed aperture
lens, it's a constant aperture lens. This means it stays the same through
the entire zoom range. Example when wide open at f:4 it stays f:4 all the
way from 17 to 40mm. If I set it at f:8 it stays a constant f:8 through the
entire zoom range. A less expensive lens can be f:3.5-5.6 so at 17 it's
f:3.5 but drops to f:5.6 at the 40mm end. The TTL light meter will
compensate for this, but if I switch to manual, like when I use a studio
flash I would have to correct for the offset as I zoom.

I hope I have explained this clearly, if not ask and I can try to simplify
it more.

Darrell Larose
Ottawa, Ontario

Skip M wrote:
Fixed aperture, in this case, refers to the fact that the aperture does
not change when you zoom the lens

This sentence straightens out my confusion. :o} Thanks to you and the others
who pointed out the (now obvious to me) concept.

--
The email address used to post is a spam pit. Contact me at
http://www.derekfountain.org : a
href="http://www.derekfountain.org/"Derek Fountain/a

"Derek Fountain" wrote in message
...
Skip M wrote:
Fixed aperture, in this case, refers to the fact that the aperture does
not change when you zoom the lens

This sentence straightens out my confusion. :o} Thanks to you and the
others
who pointed out the (now obvious to me) concept.

--
The email address used to post is a spam pit. Contact me at
http://www.derekfountain.org : a
href="http://www.derekfountain.org/"Derek Fountain/a

You're welcome!

--
Skip Middleton
http://www.shadowcatcherimagery.com