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Old Jan 18, 2008, 11:14 AM   #1
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I have a 50mm 1.8 for the first time in my life, and I must say I love it, but I fail to calculate the correct field of views in urgent cases, like at the zoo last Sunday.

The tigerwas coming straight to me but as I used 1.8,his face wasbeautifully sharp, but of course the body was not sharp. So in this case I would have liked the full body but not background. Maybe 2.8 would have been better.

Is there any rule of thumb, weighing the depth of target?

Thanks a lot

Phil
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Old Jan 18, 2008, 12:34 PM   #2
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More than any manual calculation, which you usually will not have time to do without losing the image most of the time, you'll learn more about when to use certain apertures by simply using the lens in as many different situations and learning through experience. Set up some scenes at home and shoot them at f1.8, 2.8, etc., and shoot from various distances as all those variables will change your depth of field. The great thing about digital is you can do all this yourself within minutes and learn more than carrying some sort of depth of field calculation/chart that you never have time to refer to when you are trying to capture a subject like you were last week.
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Old Jan 19, 2008, 6:53 AM   #3
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Greg has it right: experiment and find out for yourself. Reading won't hurt, but experience is key. If you are not afraid of a bit of optical mathematics, google "circle of confusion" (with quotes).

As Greg said, distance to prime subject is one key variable. Another is the size of the final print/screen. A range of distances at a specific f/stop could be just fine for a 4x6" print but a complete failure at 16x20". Or perfect for a 100x100 pixel avatar and garbage when filling a 1440x900 screen.


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Old Jan 19, 2008, 7:48 AM   #4
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Field of view refers to "width" or more commonly focal length or even more commonly what people think of when they say "zoom length" Field of view is typically measured in degrees.

What you are referring to is Depth of Field, that is how much of the scene is in focus. As was said, as aperatures increase (the f numbers get smaller), the plane of the image that is in focus gets smaller. Larger Fstops result in more of the scene being in focus. For maximum DOF and sharpness you'll typically shoot at f8-f11 on most lenses. Greater DOF is possible with smaller aperatures but due to diffraction, softness begins to creep in.

As was said your best bet is to practice and shoot the same scene at different aperatures to see the various effects. You'll quickly learn what works best to achieve the effect you're looking for. I wouldn't get into the complicated math, although it is interesting reading. The advice you've been given in previous posts is sound.
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Old Jan 19, 2008, 10:38 PM   #5
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philgib wrote:
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Is there any rule of thumb, weighing the depth of target?
See... http://www.dofmaster.com/dofjs.html...

(quoted last October by JimC, the friendly local moderator round here )
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Old Jan 20, 2008, 2:27 AM   #6
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Alan T wrote:
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philgib wrote:
Quote:
Is there any rule of thumb, weighing the depth of target?
See... http://www.dofmaster.com/dofjs.html...

(quoted last October by JimC, the friendly local moderator round here )
I assume the tiger was in a cage :lol:
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Old Jan 20, 2008, 6:19 AM   #7
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philgib wrote:
Quote:
I have a 50mm 1.8 for the first time in my life, and I must say I love it, but I fail to calculate the correct field of views in urgent cases, like at the zoo last Sunday.

The tigerwas coming straight to me but as I used 1.8,his face wasbeautifully sharp, but of course the body was not sharp. So in this case I would have liked the full body but not background. Maybe 2.8 would have been better.

Is there any rule of thumb, weighing the depth of target?

Thanks a lot

Phil
Fast prime lenses are nice, but the situations where one wants to use them wide open are limited.

To have the tiger's body in focus, while blurring the background, you would need to know the distance. Since it was moving, that value would change continuously. I know I would be unable to solve even a simple equation while keeping the subject in focus and framed . My suggestion would be to stop down tof/8 or f/16 and just keep shooting.

In a non-zoo situation, though, I would be more concerned with caliber than aperture.

brian
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Old Jan 20, 2008, 6:37 AM   #8
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VTphotog wrote:
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Fast prime lenses are nice, but the situations where one wants to use them wide open are limited.
If I had a dSLR, which I don't, I'd do what I used to do with my film SLR. That isto have an idea where the hyperfocal distance is for a moderate aperture, i.e., the focus setting for which everything from a bit nearer than thatjust to infinity will be in reasonable focus. So I could set that instantly, shoot & hope for the best, and then run away.
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Old Jan 20, 2008, 9:22 PM   #9
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Thanks for your comments :lol:

The small calculator at http://www.dofmaster.com/dofjs.html is great, many thanks for the tip.

Of course I bought a 1.8 for using wide apertures, so I only have to try to memorize not all apertures, just the small numbers like,

on 1.8, a 50mm shows
at 10m DOF : 3m,
at 5m DOF : 1m,
at 2m DOF : 0.25m
at 1m DOF : 0.03 m !

I will learn the same for apertures up to 2.8 and should be ready to experiment as suggested.

After a few weeks, I hope I will have all numbers in my head so that I can experiment as suggested.
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