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Old Feb 26, 2008, 10:45 AM   #1
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As I'm learning more about the manual modes on my rebel, I'm reading the exposure book by Bryan Johnson. Apeture, ISO, and shutter speed are all related. So, fast forwarding to my practice shots. I have my kids running around, playing street hockey. I'm using my 50mm 1.8 lens to shoot with. I'm using AV mode. I alse have defined the custom 4-3 setting to lock the focus in.

My questions are the following. If I'm shooting on a bright sunny day, is there any reason why I would NOT want to set the apeture to 1.8? The apeture should blur my background. The shutter speed should freeze the action. What does the ISO do, or what purpose does it serve? Should I try TV mode to see what the camera thinks my apeture should be? I also set my focus to the center point. I'm assuming the right light should not blink. Does that mean it's out of focus? About 2/3s of my shots are out of focus.

thx.
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Old Feb 26, 2008, 12:10 PM   #2
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Well, there are a few factors in play here.

Quote:
If I'm shooting on a bright sunny day, is there any reason why I would NOT want to set the apeture to 1.8?
Yes - there is a good reason. Depth of field. You want to blur the background - which is good to do. But, you also have to realize that your 50mm lens really isn't designed to focus on subjects 30 feet away. 10-15 feet away is about the limit if you want sharp results. At 15 feet or less, the dof is very thin so your focus needs to be spot on to get clear shots. I would suggest using a value of 2.8 instead.

Quote:
What does the ISO do, or what purpose does it serve?
Aperture, Shutter speed and ISO work together to give an exposure. Aperture controls how much light is let in, shutter speed controls how long the light is let in and ISO determines how receptive the sensor is to that light. The freezing of motion is an AFFECT of a fast shutter speed. The blurred background is an AFFECT of the apterture. ISO, does not have a beneficial affect other than it's contribution to exposure. There is a negative and that is - the higher the ISO you use, the more noise your images will have. So it's always best to use the lowest ISO you can that still gets you a proper exposure AND gets you the shutter speed you need.

For what it's worth - outdoor hockey I would shoot for 1/800 shutter speed.

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Should I try TV mode to see what the camera thinks my apeture should be?
Not a huge point to this. The idea is to use the widest aperture you can get away with to isolate your subject and still have enough dof to get in-focus shots. That's why I suggest using 2.8 for an aperture setting.
Quote:
About 2/3s of my shots are out of focus.
That could be a result of many things. First, you didnt say - do you have the focus mode set to ai-servo? If not then you should.

Second - you really shouldn't be shooting at moving subjects past about 15 feet - the lens just wasn't designed for critical focus that far out (all lenses suffer focus and sharpness issues as you approach infinity focus on the lens).

third - if they are close and you're shooting at f1.8 you have a very small margin for error.

forth - that center focus point must be on an area of good contrast. So the middle of a white t-shirt or jersey is a poor choice of area to focus on (and for what it's worth, white in general is bad because it reflects ligiht and in my experience causes more focus issues than non-white areas).

Your technique is also important - you need to acquire your subject and track them for a second before firing the shutter.

It would be helpful if you posted some sample shots - ones without any editing done (just resized for the web, but no other alterations).









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Old Feb 26, 2008, 3:54 PM   #3
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Double post....
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Old Feb 26, 2008, 3:58 PM   #4
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I was thinking of weighing in on this, but John, you did a great job. Clear, concise, and easy to understand ~ Ron

One thing that might make it easier for a new photographer though. Use "av" or "tv" to set what is most important to you - apeture or shutter speed and let the camera do the rest. Some of the newer dslrs will even set iso for you. This is easier while you are trying to learn about your new hobby ~ R
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Old Feb 26, 2008, 5:00 PM   #5
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Ron - I would agree, manual is tough. If that's the case, AV is a safer mode (vs. TV)for two reasons:

1. The underexposure mentioned above - it's better to give up a little shutter speed than exposure and if the camera maxes out the aperture of the lens because your shutter speed is set to high in TV you're in trouble.

2. Even in good light there is no reason to restrict shutter speed to a specific value UNLESS you want motion blur (panning etc..) so even if your target shutter speed is 1/500, 1/1000 or 1/2000 wont hurt your photos. BUT, closing down the aperture WILL hurt your photos. You lose subject isolation which is bad. So if you artificially set shutter to 1/500 limit and yourusing a 2.8 lens, the lens might close down to 5.6. Vs if you used aperture priority and set it at 2.8 your shutter speed goes up to 1/2000 (instead of 1/500) so no negative affect. If you're trying to show some type of motion blur it's completely different. Otherwise there is no benefit to artificially putting a limit on your shutter speed. But there is always a benefit to controlling your DOF and keeping it wide open (unless and until you want more dof but at least you decide and not the camera).

Anyway, just my opinion. In the end, digital 'film' is cheap so try both ways and see which does better for you.
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Old Feb 26, 2008, 5:08 PM   #6
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Hi John,

That's why I like this forum. Good responses to my question.

Once I get the chance, I'll post my images. The kids that I was shooting, were only a few feet away. Will it be easier to get a sharp focus (more often) using 2.8, rather than wide open? I'm not sure I understand the DOF part of it.

-Vinnie
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Old Feb 26, 2008, 5:38 PM   #7
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kc571 wrote:
Quote:
I'm not sure I understand the DOF part of it.

-Vinnie
Vinnie,

Basically dof refers to how much in front of and behind your point-of-focus will remain in focus. DOF is controlled by sensor size, physical focal length of lens, aperture and distance to subject.

Here's a link to a calculator:

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

With a 50mm 1.8 lens on a drebel, at 10 feet and f1.8 your dof is:





Nearlimit
9.61ft

Farlimit
10.4ft

Total
0.81ft

So only about 8 inches to play with. Not much room for error. What this means is if the point your camera focused on is exactly 10 feet away, anything within 9.61 to 10.4 ft will be in focus. Outside of that things start to show blur - the farther outside that range the more blur.

Change aperture to 2.8 and now you get:





Nearlimit
9.39ft

Farlimit
10.7ft

Total
1.29ft

That's an extra 5.7 inches of DOF to play with.

I'm not saying this is entirely your problem but it's potentially a contributing factor.
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Old Feb 26, 2008, 6:56 PM   #8
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To add to what John wrote, you can use the dof calculator to figure what apeture to use to be safe. While learning I'd calculate what apeture will give you 2' or 3' in focus. That should allow plenty of room for error. As you get better, you can decrease the dof for more isolation. Or, if you want 2 kids in focus, you can increase the dof to suit. That's part of what makes photography so much fun - once you gain a little knowledge, you have so much control of the finished photo.
Ron
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Old Feb 27, 2008, 10:20 AM   #9
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A few random test shots...some I think ok, others not.

-V
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Old Feb 27, 2008, 10:22 AM   #10
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