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Old May 8, 2008, 12:58 PM   #11
conor
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conor wrote:
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I have no idea why the EXIF data was lost on that image, but here's the bit you're asking about:

Focal Length: 28mm (using my 18-55mm kit lens)
ISO: 800
F Number: 4.5
Exposure Time: 1/20s
White Balance: Auto
Noise Reduction: Off
Flash Used: Not Fired

Yes, I shoot this environment often (40% of the pictures I have taken over the last two years were shot in a similar environment), and it's not a place where I really want to start changing a lens.

I lied, I was looking at a different picture from the same "set"

this one was...
Focal Length: 23mm (34mm for 35mm equiv.)
ISO: 800
F Number: 4.5
Exposure Time: 1/13s
White Balance: Auto
Noise Reduction: Off
Flash Used: Not Fired

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Old May 8, 2008, 1:03 PM   #12
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Well... that one is a tough call, since it appears be underexposed, too (even using a shutter speed of 1/20 second at f/4.5, which is really too slow not to have blur unless the subject is motionless. If you fill the frame more (move closer or use a longer focal length), or if your viewing size is larger, the blur from any subject movement is going to be even more obvious (since the movement would occur across a greater percentage of the frame).

A brighter exposure would have also required a slower shutter speed (meaning even more blur). My guess is that you really needed around 1/15 second for a bit better exposure of the subject under the lights, and you'd probably only get around 1/30 second or so at f/2.8 using ISO 800 for a better exposure. Even at ISO 1600, you're still going to be at around 1/60 second for proper exposure in that lighting (which is a bit on the slow side for a non-stationary subject, but still better than you're getting now).

I'd suggest shooting with an f/2 or brighter prime in that lighting, even with ISO 1600 if given a choice. Even then, you're probably going to have a relatively high percentage of blurry photos if you don't keep viewing/print sizes relatively small. You'll need to learn to time your shots so that you're taking them during natural pauses in the action (longer notes, etc.) for best results, even using a brigher lens.

If you wanted to try and get by with an f/2.8 zoom, I'd probably look at something like the Sigma 24-70mm f/2.8, Tamron 28-75mm f/2.8 or if you want a bit longer, go with something like a Sigma 50-150mm f/2.8 or 70-200mm f/2.8. But, I probably wouldn't try to get by with an f/2.8 zoom in that lighting if given a choice. You'd lose some flexibility using a fixed focal length prime. But, you should have fewer blurry photos using a brighter lens. You'll need to decide if the framing compromises with a prime versus a zoom are worth it or not (it's all a matter of balance).

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Old May 8, 2008, 1:14 PM   #13
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Don't I remember reading somewhere that that you have 50mm f/1.8? If so, did you try it there?

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Old May 8, 2008, 1:17 PM   #14
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JimC wrote:
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If you wanted to try and get by with an f/2.8 zoom, I'd probably look at something like the Sigma 24-70mm f/2.8, Tamron 28-75mm f/2.8 or if you want a bit longer, go with something like a Sigma 50-150mm f/2.8 or 70-200mm f/2.8. But, I probably wouldn't try to get by with an f/2.8 zoom in that lighting if given a choice. You'd lose some flexibility using a fixed focal length prime. But, you should have fewer blurry photos using a brigher lens.
As a newb, I need clarification on two points of that:

1) Is it a general rule of thumb that my 50mm prime at f4.5 will provide a brighter image than my 18-55mm at f4.5 (at 50mm)? (This would explain why one would actually WANT a prime lens!)

2) I guess I'm still not QUITE clear on the 1.5 crop factor. Assuming we compare my EXIF info against the mentioned lenses: I have 23mm and 34mm in the EXIF tag, the latter being the "35mm equiv". Assuming I compare that against the Tamron 28-75. Would the picture have been possible with that lens? (23 falls under 28, but 34 falls easily between 28 and 75).

My impression is that the answer is yes, as the Tamron lens is giving me numbers for a 35mm camera... but maybe I've missed the boat, the warf and the harbor all together
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Old May 8, 2008, 1:20 PM   #15
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JimC wrote:
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Don't I remember reading somewhere that that you have 50mm f/1.8? If so, did you try it there?
Yes,I have a 50mm f/1.8, noI havent tried it, yes I will, but I'm pretty sure I wont be "back" far enough... since the 50mm is (nearly) the same as my 18-55 all the way "in".
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Old May 8, 2008, 2:02 PM   #16
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conor wrote:
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1) Is it a general rule of thumb that my 50mm prime at f4.5 will provide a brighter image than my 18-55mm at f4.5 (at 50mm)? (This would explain why one would actually WANT a prime lens!)
No. Using a setting of f/4.5 with one lens gives you the same brightness as a setting of f/4.5 with a different lens (for exposure purposes versus AF purposes), and you'll need the same shutter speed for proper exposure with both, provided the lighting and ISO speed setting is the same.

But, you have wider available apertures, represented by smaller f/stop numbers when you use the 50mm f/1.8 (and you don't have to use a lens at the widest available aperture setting if you don't need to).

To put things into perspective, the aperture scale in one stop increments goes f/1.0 (with theoritically larger apertures available), f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22, etc.

With each one stop move to a smaller aperture (represented by larger f/stop numbers), for example f/2 to f/2.8 , you will need shutter speeds twice as long for proper exposure, given the same lighting and ISO speed. For example, you'd need shutter speeds 8 times as long shooting at 50mm and f/5.6 with your kit lens, versus shooting at f/2 with your 50mm f/1.8 for the same lighting and ISO speed (and f/5.6 would be the widest aperture available to your kit lens at 50mm).

That also means that with each one stop move to a smaller aperture (higher f/stop number), you lose half the light getting through the lens to the AF sensors and viewfinder (and to the main CCD when the mirror flips and shutter opens).

A brighter (a.k.a., faster) lens also helps a camera to "see" better for Autofocus Purposes. See this handy online exposure calculator to get a better idea of the relationship between light levels, ISO speed and aperture. Note that Film Speed in the calculator is the same thing as ISO speed:

http://www.robert-barrett.com/photo/...alculator.html

If you are trying to take photos in low light, you need higher ISO speeds and larger apertures (smaller f/stop numbers) in order to try and keep shutter speeds fast enough to reduce blur from camera shake and/or subject movement if you can't use a flash. Larger available apertures will also increase lens size, weight and cost.

Another tradeoff of using a larger aperture is that depth of field will be shallower.

Noise will also be higher as ISO speed is increased.

It's really not that complicated, once you have a better understanding of exposure.

In low light, you can use Av Mode (aperture priority) setting your aperture to a larger opening to let in more light (set it to smaller f/stop numbers), and the camera will automatically select the correct shutter speed for proper exposure. It would probably do the same thing in Auto modes if light was low, too. Then, use exposure compensation as needed for the desired exposure (brightness), using a +EV setting for a brighter exposure, or a -EV setting for a darker exposure. That will influence shutter speed using Av mode (brighter requires a slower shutter speed, darker requires a faster shutter speed).

If shutter speeds are still not fast enough, increase ISO speeds (each time you double the ISO speed, the camera can use shutter speeds twice as fast for the same lighting and aperture).

You can use tools like Noiseware or Neat Image when shooting at higher ISO speeds is needed to help reduce the appearance of noise if higher ISO speeds are needed to get a higher percentage of photos without blur.

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Old May 8, 2008, 4:28 PM   #17
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JimC wrote:
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conor wrote:
Quote:
1) Is it a general rule of thumb that my 50mm prime at f4.5 will provide a brighter image than my 18-55mm at f4.5 (at 50mm)? (This would explain why one would actually WANT a prime lens!)
No. Using a setting of f/4.5 with one lens gives you the same brightness as a setting of f/4.5 with a different lens (for exposure purposes versus AF purposes), and you'll need the same shutter speed for proper exposure with both, provided the lighting and ISO speed setting is the same.

Alright, that's the assumption I was under, which is the reason i bought my 50mm prime in the first place (ability to take pictures with a very shallow depth of field).

Are there any benefits to a prime lense versus a zoom when used at the same aperture and focal length? Less distortion in the lens? Less "problems" with flaring? "Nicer" bokeh?


I think this one's my last outstanding question

conor wrote:
Quote:
2) I guess I'm still not QUITE clear on the 1.5 crop factor. Assuming we compare my EXIF info against the mentioned lenses: I have 23mm and 34mm in the EXIF tag, the latter being the "35mm equiv". Assuming I compare that against the Tamron 28-75. Would the picture have been possible with that lens? (23 falls under 28, but 34 falls easily between 28 and 75).
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Old May 8, 2008, 4:39 PM   #18
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conor wrote:
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Alright, that's the assumption I was under, which is the reason i bought my 50mm prime in the first place (ability to take pictures with a very shallow depth of field).

Are there any benefits to a prime lense versus a zoom when used at the same aperture and focal length? Less distortion in the lens? Less "problems" with flaring? "Nicer" bokeh?
Yes, that's one of the reasons you want to use a lens with wider available apertures.

I can't help but feel with got some kind of disconnect here... You need to use a wider aperture (for example, f/2.8, f/2, f/1.8 ) if you want to take advantage of this lens type, so that you are letting in more light.

You can't expect to see any benefits as far as things like faster shutter speeds go for a given lighting and ISO speed if you're going to stop down the aperture to f/4.5.

In addition to how much light gets through, Depth of Field is controlled by the aperture you use (combined with focus distance and focal length) If you want a shallower depth of field for a given focal length and focus distance, you need to use a wider apeture setting (smaller f/stop numbers). Otherwise, there is no benefit from a depth of field perspective to one lens over another, if the focal length and aperture setting is the same between them. If you want a shallower Depth of Field, don't use f/4.5.

See this DOF calculator for more info on Depth of Field:

http://www.dofmaster.com/dofjs.html

Frankly, I'm confused at why you think you'd want to use f/4.5 with the brigher prime, if you need faster shutter speeds in low light, or a shallower depth of field for other reasons. Unless you use a different aperture, there's not gong to be any difference. That's the main reason you want a brighter lens (so that you can use a wider aperture opening, versus trying to use the same apertures you were limited to with a dimmer zoom).

Quote:
Are there any benefits to a prime lense versus a zoom when used at the same aperture and focal length? Less distortion in the lens? Less "problems" with flaring? "Nicer" bokeh?
You have to take each lens on a case by case basis. But, as a general rule, you'll have a bit sharper photo with most prime lenses when stopped down from their widest aperture opening, versus using a zoom wide open at the same aperture. Flare and more will need to be looked at on a lens by lens basis..

But, I think you're missing the main point of using brighter lens (so that you can use wider apertures than you had available with the zoom, with a shallower depth of field if you need one).

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Old May 8, 2008, 4:50 PM   #19
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JimC wrote:
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But, I think you're missing the main point of using brighter lens (so that you can use wider apertures than you had available with the zoom, with a shallower depth of field if you need one).
I completely understand the relationship between the three
-> Double the ISO, double the sensitivity (aka, double the light)
-> One full aperture step, double the light (more light = smaller number)
-> One full shutter stop, double the light (more light =slower shutter)

Each one has a "trade-off" (some may be desired, but thats a different debate)
Higher ISO = grainier image
Slower Shutter = more blur (subject motion or otherwise)
Wider Aperture =shorter depth of field (less stuff in focus)

The trick would be selecting the right balance between the three for the light available. For example, for high light, high motion, you'd probably pick a faster shutter first, then decide a balance between ISO and Aperture depending on your subject... etc.

I think I phrased my question poorly, I was trying to get a handle on why I might want to take my 18-55 lens off my camera to replace it with my 50mm, assuming that I was going to use an aperture that my 18-55 had been capable of, but you answered that one in closing:

JimC wrote:
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But, as a general rule, you'll have a bit sharper photo with most prime lenses when stopped down from their widest aperture opening, versus using a zoom wide open at the same aperture.
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Old May 8, 2008, 4:54 PM   #20
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For one thing, you'll have wider available apertures if you need them (so that you'll be able to achieve faster shutter speeds to help prevent blur from subject movement or camera shake for a given lighting and ISO speed).

You could also use wider apertures if you wanted a shallower depth of field for the same subject framing.

Your Autofocus sensors would also be able to "see" better in low light, giving you faster AF speeds and the ability to focus in lower light (for example, around 10 times as much light gets through to the Autofocus sensors using a 50mm f/1.8 versus the amount of light that gets through to the AF sensors using your 18-55mm kit lens at 50mm). That also means a much brighter viewfinder image. A camera will always Autofocus with the aperture wide open, stopping it down to the setting you want to use when taking the photo.

In addition, you'd probably have sharper photos for equivalent apertures, especially if you wanted to shoot with the aperture wide open using your 18-55mm (most lenses are not as sharp at their widest aperture settings).

You don't need to to use the widest available aperture setting with a brighter lens. You can always stop it down to a smaller aperture opening. But, you'd have wider available apertures if you need them.

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