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fixed f-stop lens, how sharp?
I am new to dSLR, so I was studying up on some stuff. I ran across this question when I read about the aperture:.

According to Wikipedia, a lens is sharpest at around f/8 ish (varies with lens). But since I want to do portrait photography, I am looking at the Canon f/2.8 24-70mm IS lens. Since it's fixed at f/2.8, how can it give sharp images? Reviews I read says it's a sharp and good lens... With my kit lens, the f-stop changes as I zoom in and out..

Just wondering, maybe that's why that lens cost $1200 USD while the other lens are cheaper.....

Comments (16)

RE>Since it's fixed at f/2.8, how can it give sharp images? <.

It is not fixed at f2.8..

This stuff gets confusing..

With that lens, the MAXIMUM aperture is f2.8 regardless of what zoom position / focal length you choose..

And at whatever focal length you choose, you can select any aperture you want, from f2.8 to something like f16 (maybe 22)..

Most zoom lenses have a variable maximum aperture, so when "fixed" is used, it in in contrast to variable..

BAK..

Comment #1

So, still, at f/2.8, I won't be getting the sharp images? I need to be at like f/8 still?..

Comment #2

4honor wrote:.

So, still, at f/2.8, I won't be getting the sharp images? I need tobe at like f/8 still?.

Short answer: don't worry about it..

Long answer: there are a lot of other factors you should learn first before considering the sharpness drop for 24-70 from f8 to f2.8 when used for portraits ..

Comment #3

The rule of thumb is that the optimal aperture is around 3 stops down from wide open. What's really happening is that the increase in depth-of-focus masks residual aberration as we stop down until we arrive at the point where increasing diffraction masks further reduction in aberrations..

The lens in question is expensive because it takes a lot more glass to make a 70mm f/2.8 lens than a 70mm f/4.5 lens. Of course it's also a Canon and an IS lens and those factors contribute to the price..

Note that I used the term optimal. What is "sharp" depends on the end use of the image. Sharp photos depend on a lot more than just stopping down to some "optimal" aperture. Is the the DOF sufficient (compositionally dependant)? Is the shutter speed high enough to stop camera/subject motion? At a low enough ISO so that noise/noise reduction doesn't wipe out detail? Situation - hand holding the camera, ISO 800, 70mm which will yield the sharpest image - f/5.6 at 1/4 sec. or f/2.8 at 1/15 sec? Answer - depends or even none of the above...

Comment #4

Have a look at this test of your lens:.

Http://www.photozone.de/...184-canon-ef-24-70mm-f28-usm-l-test-reportreview.

Even wide open at f/2.8 it out-performs cheaper lenses at their optimum aperture. Centre performance is always better than edge, and for portraits your subject is usually towards the centre of the frame and defects at the edge / corner are irrelevant because that area is out of focus anyway..

Short answer: don't worry about it. Your lens is a very good one.Mike..

Comment #5

4honor wrote:.

I am new to dSLR, so I was studying up on some stuff. I ran acrossthis question when I read about the aperture:.

According to Wikipedia, a lens is sharpest at around f/8 ish (varieswith lens). But since I want to do portrait photography, I amlooking at the Canon f/2.8 24-70mm IS lens..

There is no Canon 24-70 F2.8 IS lens..

Comment #6

Oh boy, are you trying to scare away the newbie? .

Well too bad, cause I want to experiment and learn! Anyways, I understand your explaination and it's very helpful. I guess I will just try taking pictures at different aperture settings and compare the results....

Still, I want that $1200 lens... ..

Comment #7

Oh yeah, oops, I was also thinking about the 70-200mm f/2.8 one, which had the IS version... which I also want... together about $3000... Anyone has $3k pocket change?  ..

Comment #8

4honor wrote:.

So, still, at f/2.8, I won't be getting the sharp images? I need tobe at like f/8 still?.

Let's put it this way. It'll give you infinitely sharper images at f/2.8 than the 17-85mm f/3.5-5.6 at f/2.8..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #9

A couple of points....

"Fixed" normally refers to a lens that can not be removed from the camera body. A zoom lens that does not change it's maximum aperture as the FL is varied is referred to as a "constant" aperture lens..

For portrature, you OFTEN don't want a sharp lens! In fact, there are special, expensive portrature lenses that intentionally blur the image..

Many sages say that the perfect FL for portrature is longer than 70mm...usually around 100mm. The reason is that the perspective changes as the FL changes and it's "better" at around 100mm. Shorter FL's tend to make the face "bulge" a bit..

A good way to save $$$ is to buy lenses from one of the 2nd tier (Sigma, Tamron, & Tokina). Each of these brands include a RANGE of lens qualities. For example, Sigma calls it's high quality lenses "EX", in much the way that Canon labels theirs "L"..

That 70-200 lens you want can be used to take portraits. I find that 120-150mm is a good FL as it renders the background OOF with nice bokeh..

Charlie DavisNikon 5700, Sony R1, Nikon D300HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/'Experience: Discovering that a claw hammer will bend nails.Epiphany: Discovering that a claw hammer is two tools...'..

Comment #10

Chuxter wrote:.

For portrature, you OFTEN don't want a sharp lens! In fact, there arespecial, expensive portrature lenses that intentionally blur theimage..

Using a soft-focus lens. The look you get is often referred to as the "softcore porn" look. Those dreamy slightly distorted images that let you ignore minor imperfections..

Many sages say that the perfect FL for portrature is longer than70mm...usually around 100mm..

...on 35mm. The 35mm range is typically from 85mm (full length) to 135mm (head shot). On APS-C that translates roughly from 50mm to 85mm..

The reason is that the perspective changes as the FL changes.

You are mistaking cause and effect, but the idea is right. Perspective is affected by distance, not focal length. However to fill the frame as you move further back you need a longer focal length. 85mm to 135mm on 35mm lets you get the shots you want at a distance of about 10 to 15 feet, which gives a pleasing perspective. Long enough to flatten out features, short enough to not make the subject look like a cardboard cutout. Plus you can usually walk back 15 feet in any studio, and you'll be able to communicate with the subject without resorting to a bullhorn..

That 70-200 lens you want can be used to take portraits..

On APS-C a lens starting at 50mm would be better. Sigma makes a nice 50-150mm f/2.8, and Tokina a 50-135mm f/2.8..

I find that120-150mm is a good FL as it renders the background OOF with nicebokeh..

120-150mm on APS-C is a bit long for a classic portrait length. OTOH many 35mm shooters did portraits up to 200mm, so it's in the same ballpark. At any given focal length, aperture and subject distance you'll get the same amount of out-of-focus blur regardless of the lens. Bokeh refers to the subjective quality of the out-of-focus areas, which depends on the lens..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #11

Nickleback wrote:.

Chuxter wrote:.

For portrature, you OFTEN don't want a sharp lens! In fact, there arespecial, expensive portrature lenses that intentionally blur theimage..

Using a soft-focus lens. The look you get is often referred to asthe "softcore porn" look. Those dreamy slightly distorted imagesthat let you ignore minor imperfections..

Many sages say that the perfect FL for portrature is longer than70mm...usually around 100mm..

...on 35mm. The 35mm range is typically from 85mm (full length) to135mm (head shot). On APS-C that translates roughly from 50mm to85mm..

Perspective is not affected by the size of the film or sensor..

The reason is that the perspective changes as the FL changes.

You are mistaking cause and effect, but the idea is right.Perspective is affected by distance, not focal length..

No..

However tofill the frame as you move further back you need a longer focallength. 85mm to 135mm on 35mm lets you get the shots you want at adistance of about 10 to 15 feet, which gives a pleasing perspective.Long enough to flatten out features, short enough to not make thesubject look like a cardboard cutout. Plus you can usually walk back15 feet in any studio, and you'll be able to communicate with thesubject without resorting to a bullhorn..

That 70-200 lens you want can be used to take portraits..

On APS-C a lens starting at 50mm would be better. Sigma makes a nice50-150mm f/2.8, and Tokina a 50-135mm f/2.8..

I find that120-150mm is a good FL as it renders the background OOF with nicebokeh..

120-150mm on APS-C is a bit long for a classic portrait length. OTOHmany 35mm shooters did portraits up to 200mm, so it's in the sameballpark. At any given focal length, aperture and subject distanceyou'll get the same amount of out-of-focus blur regardless of thelens..

Huh? Not even close!.

Bokeh refers to the subjective quality of the out-of-focusareas, which depends on the lens..

Yes..

Charlie DavisNikon 5700, Sony R1, Nikon D300HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/'Experience: Discovering that a claw hammer will bend nails.Epiphany: Discovering that a claw hammer is two tools...'..

Comment #12

Chuxter wrote:.

Nickleback wrote:.

...on 35mm. The 35mm range is typically from 85mm (full length) to135mm (head shot). On APS-C that translates roughly from 50mm to85mm..

Perspective is not affected by the size of the film or sensor..

I never said it was. I said that perspective is affected by distance, not focal length..

However to shoot from the same distance and get the same field of view, on APS-C you would use a shorter focal length than you would on a 35mm camera..

You are mistaking cause and effect, but the idea is right.Perspective is affected by distance, not focal length..

No..

Got anything to back that up? Here's a very simple example which shows that perspective when shooting with a 33mm lens and 80mm lens is the same when taken from the same distance, but perspective changes when you move closer:.

Http://www.dpreview.com/learn/?/Glossary/Optical/Perspective_01.htm.

For more on this....

Http://en.wikipedia.org/wiki/Perspective_distortion_(photography).

120-150mm on APS-C is a bit long for a classic portrait length. OTOHmany 35mm shooters did portraits up to 200mm, so it's in the sameballpark. At any given focal length, aperture and subject distanceyou'll get the same amount of out-of-focus blur regardless of thelens..

Huh? Not even close!.

What exactly are you referring to as "not even close"? The 35mm vs APS-C ballpark? The same amount of out-of-focus blur regardless of lens if everything is held constant (focal length, aperture, subject distance, and I forgot to mention enlargement)?.

Bokeh refers to the subjective quality of the out-of-focusareas, which depends on the lens..

Yes..

Finally, something we agree on..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #13

Chuxter wrote:.

..Many sages say that the perfect FL for portrature is longer than70mm...usually around 100mm..

No problem with that. Most people say 80 to 135 full frame (Around 60-85 on a Nikon 55-80 on a Canon APS-C).

The reason is that the perspectivechanges as the FL changes and it's "better" at around 100mm. ShorterFL's tend to make the face "bulge" a bit..

You are confusing cause and effect in your explanation. Distance affects perspective and TO FILL THE FRAME you need different focal lengths at different distances..

That 70-200 lens you want can be used to take portraits. I find that120-150mm is a good FL as it renders the background OOF with nicebokeh..

Whatever floats your boat. There is no right length..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #14

I was not reading or writing well!.

Charlie DavisNikon 5700, Sony R1, Nikon D300HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/'Experience: Discovering that a claw hammer will bend nails.Epiphany: Discovering that a claw hammer is two tools...'..

Comment #15

Chuxter wrote:.

I was not reading or writing well!.

No problem, been there and done that!.

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #16

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