aperture and distance

[MACollum]

I know how changing the aperture works, what it's supposed to do. Putting it into practice is another matter entirely. Does distance have anything to do with it? If you're trying to focus on an object close and want the background blurry, you'd use the smallest f number possible for your lens. What about when your subject is farther away? Would you close down the aperture? I'm having a really hard time understanding depth of field (putting it into practice anyway, I understand the concept).

I've been trying to use Av mode on my camera more so that I can understand more about aperture. The problem I'm having is that a lot of the time, the pictures come out more blurry than they should since the shutter speed isn't too low, though this could be a focus problem, I'm trying to determine that with my question. I'm wondering if I'm consistently opening up the aperture too much or if my focusing is just really crappy.

 

[Big Mike]

The basic principle is that your DOF is shallower when you use a wider aperture (smaller F number). That holds true at all distances.

The distance to your subject (assuming you are focused on your subject) will make a difference in the actual Depth of Field. The farther away you are, the deeper it is...the closer, the narrower.

You have the 50mm F1.8. Do a Google search and try to find a DOF chart for that lens...it should tell you what your DOF will be for a given aperture and distance.

 

[Hertz van Rental]

Or you could have years of fun doing the calculations:

f = f number
c = diameter of circle of confusion
F = focal length of lens
u = distance from lens to object (or to point of focus)

Distance from lens to nearest point in focus:
Fu(F+cf)/(Fsquared+ucf)

Distance from lens to furthest point in focus:
Fu(F-cf)/(Fsquared-ucf)

 

[MACollum]

Several charts are listed as 50mm but they're 35mm format. Is this where I use the crop factor to find the equivalent or am I making it too complicated?

 

[Mo's Photos]

Mindy,

It’s hard to say because there are several variables that could contribute to the problem you describe. However, I’m guessing by your description that your focus is not as good as it could be. Also, if you are handholding your camera and your shutter speed is slower than the reciprocal of your focal length (i.e. 1/300 when at 300mm on your Tamron lens) then you may be suffering from camera shake.

Hope this helps.

 

[MACollum]

Maybe I'm asking the wrong question. If I take a picture of something that's, say, 5 feet away with a 50mm lens, will I want to use a larger f/number to get any of it in focus?
​

 

[MACollum]

Mindy,

It’s hard to say because there are several variables that could contribute to the problem you describe. However, I’m guessing by your description that your focus is not as good as it could be. Also, if you are handholding your camera and your shutter speed is slower than the reciprocal of your focal length (i.e. 1/300 when at 300mm on your Tamron lens) then you may be suffering from camera shake.

Hope this helps.

I think you're right. I think my focus isn't too good. I try to use fast enough shutter speeds but it's pretty obvious that my hands aren't too steady. When I use the 50mm lens (which is almost always), I can never get away with 1/50 shutter speed. Any tips for focusing? I don't use manual focus because I have astigmatism. Maybe I should try since I'm usually wearing my sunglasses outside (they're prescription). I'm not having much luck with AF.

 

[Big Mike]

Maybe I'm asking the wrong question. If I take a picture of something that's, say, 5 feet away with a 50mm lens, will I want to use a larger f/number to get any of it in focus?

You are asking us...what you want?

It all comes down to what you want. If you want a shallow DOF, then use a large aperture. If you want more to be in focus...then use a smaller aperture. Sometimes it's a compromise that has to consider things like shutter speed and camera shake...but that is something that you have to prioritize for yourself...on each and every shot.

 

[Mo's Photos]

There seems to be two issues here. First is focus. Second is depth of field. Regardless of your aperture setting, you normally want to make sure that your main subject is in focus. However, if that subject is fairly long (think semi trailer truck), and your are standing close to the front with your 50mm lens pointing down the length of the truck then even with a small aperture setting (i.e. 16 or 22) portions of it may be out of focus. If you take the same shot with a wide open setting (i.e. 1.8) then you can guarantee that portions will be out of focus. However, if you are taking a picture of a person in front of the truck, and you are focusing on the person, then that person should be in focus regardless of the aperture setting. As I mentioned earlier there are many variables that can affect the focus.

I’ve found the XTi to be pretty good about getting things in focus, and when it can’t it will tell you. I like to adjust the AF points so that I only use one point in most situations instead of letting the camera choose which points to focus on. This helps me to ensure that the main subject is in focus.

 

[MACollum]

You are asking us...what you want?

It all comes down to what you want. If you want a shallow DOF, then use a large aperture. If you want more to be in focus...then use a smaller aperture. Sometimes it's a compromise that has to consider things like shutter speed and camera shake...but that is something that you have to prioritize for yourself...on each and every shot.

I know what I want to do, but I'm not yet sure of all the technical aspects that help me get what I want. The concepts are easy to understand, applying those concepts in the right way isn't always so easy.

 

[Big Mike]

Don't worry, just keep shooting and testing things out. Before you know it, this stuff will be 2nd nature for you.

 

[MACollum]

There seems to be two issues here. First is focus. Second is depth of field. Regardless of your aperture setting, you normally want to make sure that your main subject is in focus. However, if that subject is fairly long (think semi trailer truck), and your are standing close to the front with your 50mm lens pointing down the length of the truck then even with a small aperture setting (i.e. 16 or 22) portions of it may be out of focus. If you take the same shot with a wide open setting (i.e. 1.8) then you can guarantee that portions will be out of focus. However, if you are taking a picture of a person in front of the truck, and you are focusing on the person, then that person should be in focus regardless of the aperture setting. As I mentioned earlier there are many variables that can affect the focus.

I’ve found the XTi to be pretty good about getting things in focus, and when it can’t it will tell you. I like to adjust the AF points so that I only use one point in most situations instead of letting the camera choose which points to focus on. This helps me to ensure that the main subject is in focus.

I usually use the center point and recompose to cut down on changing points all the time. An exception is when I'm taking numerous shots of the same thing (usually for experimental reasons) and get tired of constantly recomposing.

I wish I could post some examples but many of the pictures I took yesterday were varying apertures. I was trying to determine if I'm having trouble with the focus (not holding the camera steady enough while focusing) or if I'm always choosing f/stops that are too small. I usually don't go over f/4 unless I'm taking a picture of something farther away than a few feet, which is almost never. I know that it's becoming a crutch for me and I'm trying to avoid it. But how can I if I don't understand how a small f/number affects focus at varying distances?

 

[MACollum]

Don't worry, just keep shooting and testing things out. Before you know it, this stuff will be 2nd nature for you.

That's what I was hoping for but I'm definitely getting discouraged by my lack of progress so far. It's so much easier to use P mode and I'm starting to slip back into doing that. If I'm having a problem with focus, then I don't feel so bad, but I'm not sure focus is always to blame. Especially since the camera does the focusing. It seems like the camera's not the one screwing up.

 

[Big Mike]

I think that you are over thinking this.

You should always be able to get your subject into focus...or at least, whatever you focus on, should be sharp and in focus. It doesn't matter how close or far away it is (as long as it's not really close, within the lens's ability to focus). It also doesn't matter the aperture you are using...there will always be a Depth of field...whether it's deep or shallow.

Adjusting the aperture to manipulate the DOF, should affect things that are not the same distance away as what you focus on. For example, lets say you are shooting a child in a field, with trees in the background. If you shoot at F2.0, and focus on the child...the child should be in focus and the trees in the background should be blurry. If you change the aperture to F16, (the shutter will need to be much slower) but the child will still be in focus, just like the first shot. The difference will be how the trees in the background look. They will certainly be more in focus, and they may be completely in focus.

It's the same principle if you are focusing on something closer or farther away...but the actual distance of your DOF will vary.

Maybe the problem you are having, is getting the subject into focus in the first place. If so, that is a probably a different issue, and we can help with that if you post some examples.

 

[THORHAMMER]

just go manual focus while you are at both ends of the aperature spectrum. GO to infinity on your focus and see if they come out sharp.
then you will know if its your shutter speed, or handholding etc....

Then the main problem will be solved and you can move onto howto control the focus's range