Lens speed: Not always the best choice

People buy fast lenses for 2 reasons:
-The want the speed to capture action.
-They want the shallow DOF afforded by such lenses.

For for the first reason, speed is not always the best choice.
Shooting at a higher ISO sometimes works out better than using a lens
wide open and once you determine that it does for a specific lens,
then there is generally no reason to shoot a specific subject at the
wider aperture. The benefits are better image quality (fewer image
aberrations), possibly more accurate focus and the potential to use a
much cheaper lens. The downside is noise and giving up shallow DOF.

Here is an example of a shot at 800 ISO with a lens wide open and 1600
ISO with the lens stopped down. Shutter speeds were the same, so you
aren't giving that up.

The trick is to determine which lenses fall into this category.

http://www.pbase.com/andersonrm/image/114808342/original

 

Except that ISO 1600 at f/4 <> the same exposure as ISO 800 at f/2

David

 

A lens has to be pretty awful not to work well centrally at f8.0. But
some lenses are amazing wide open. Nikon's 14-24mm f2.8. Check out
the output from an Olympus 35-100mm f2.0, it's pretty amazing.

Panasonic G1.

 

True. Which means that the example is even more compelling because
the 1600 ISO shot was underexposed but still looked better after NR.
In that case, the lens wide open was no benefit at all, and you are
better off going with a higher ISO.

 

They need to take pictures in dim lighting.

 

And which cameras. Cameras seem to vary quite a bit in their performance
at higher ISOs. But it does seem like the more recent ones are getting
better at that.

 

For many people, the choice of a fast lens has little to do with the
two reasons you mentioned. The real reason is that fast lenses
focuses faster and better. That's a big advantage. If you're
shooting at f8, a f2 lens will still usually outperform an f4 lens
because of the difference in focusing. You'll miss the picture if
your lens is searching for a focus point.

In dim light you might have to switch over to manual focus, which
really isn't a big deal, but many people don't like doing it.

 

What's the f/2 lens?

--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam

 

If somebody claims that a $400 camera has optics that are as good as
those in a $2000 lens then they are a liar or a crackpot.

 

Specific examples... ?

 

If the optical design playing field was level. Which it most certainly
isn't in this case.

Cost depends on the optical problems the lens has to solve. A DSLR
lens has to deliver its intended quality while leaving enough room
between sensor and rear lens element to accomodate the mirror. That
adds a lot to the cost of lenses of shorter focal lengths.

 

Q.E.D. Your'e a liar AND a crackpot who didn't even notice that the
web page you cite refutes your stupid claim.

 

And if you have a tiny sensor then you certainly can compromise the
lens quality to match the sensor's abilities.

 

For many people, the choice of a fast lens has little to do with the
two reasons you mentioned. The real reason is that fast lenses
focuses faster and better. That's a big advantage. If you're
shooting at f8, a f2 lens will still usually outperform an f4 lens
because of the difference in focusing. You'll miss the picture if
your lens is searching for a focus point.

In dim light you might have to switch over to manual focus, which
really isn't a big deal, but many people don't like doing it.

Good response.

The autofocus system generally works much better (increased accuracy) and
quicker with faster lenses .

Some cameras cannot take advantage of their advanced autofocus
sensors/systems, with slower lenses.

 

You hope. Otherwise you end up with one of the innumerable cases of
back or front focus we see people talking about. At least some added
DOF might save a shot with a slower lens if that condition exists.

 

In your case it may take 10 years.

Nobody has said that it is, but lying bout other people is common with
people who really have no way of justifying themselves.

And much slower.

More missed shots.

 

Ahh, I see now. So if I took my cheap 50mm f/1.8 lens and glued a
stop in the aperture lever channel so that it could never open any
further than, say, f/2.8, I now have a diffraction limited lens that
is so far superior to any P&S lens it'll make your heart spin.

Of course, the same is true for *any* DSLR lens, i.e., I can turn them
into a diffraction limited lens by removing the ability to open up
wider and limit myself in the same way that P&S manufacturers have
limited you. At least a DSLR gives me the *choice* to decide whether
I want a faster lens at the expense of softness or whether I want to
stop it down a little and have superior image quality and maybe bump
up the ISO a little to deal with that. A P&S doesn't give you that
choice. I guess they figure the typical P&S user isn't sophisticated
enough to deal with options.

You're stuck with a slow lens and a camera that doesn't have the
ability to bump up the ISO to deal with a slow lens like a DSLR can.

Steve

 

And they are correct.

 

Plus if you stop down past f/5.6 on many of those, it's all downhill
from there. DX will go to about f/11, FX to about f/16 with the same DOF
as P&S at about f/5.6.

--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam

 

And with that, we now know you have no idea what you're talking about
and are self inconsistent. If your magical diffraction limited P&S
lens could open far enough, it would no longer be diffraction limited.
So by your definition above, P&S lenses are not diffraction limited
either, despite your claims otherwise elsewhere.

Of course being limited to ASA25 and ASA64 stopped pros from getting
shots they were after. That is unless you're so limited in your
creative ability that you only ever want to take pictures of things
sitting still or in bright sunlight or with a super flash. In that
case, you're right. And the fact that you have no creativity is
insight into why you don't feel limited by a P&S.

Steve