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Old Aug 5, 2006, 1:11 AM   #31
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For non-macro application the aperture settings of f/1.7 and f/2.8 are great if you are chasing short DOF. This allows you to isolate your subject from the crowd or from a busy background.

The subject become aparent in the photograph,as per this example.

300mm at f/4. ISO 400 1/320sec.


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Old Aug 5, 2006, 8:24 AM   #32
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oh cool, I didn't know that.

There, another piece of wisdom gathered Thanks crash!

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Old Aug 5, 2006, 9:05 AM   #33
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DigitalAddict wrote:
Quote:
Being a newbie I am yet to discover opportunities where a faster than F2.8 setting will be desirable for a non-macro lens (not sure about macro either).

Can you share some from your experience?


Large aperture (low f-number) = shallow/thin depth-of-field (sharpness from-to, counted in distance from camera).

Small aperture (high f-number) deep depth-of-field

Im macro, DOF gets so thin that it is usually uninteresting to use large apertures if you don't try to achieve a special effect. Therefore fast macro-lenses are not normally that desirable, you will seldom use the large apertures anyway.

Fast non-macro lenses allows you to do two things. First the thin DOF on purpose as demonstrated sowell and nicely by Crashman in the post above. Secondly it may allow you to take a photo hand-held in low-light conditions where a slower lens would have left you with nothing or just some motion-blurred coloured fields.

This was taken with my 50 mm FA 1.4 @f1.4, 1/20 sec, ISO400. No flash. No artistic masterpiece, but it demonstrates what a fast lens can do for you.

Kjell
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Old Aug 5, 2006, 11:05 PM   #34
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Clearly I have plenty to learn on the subject.Thanks for advice guys.

Nothing beats real life examples. It seems to me, the newbie, that the focal distance plays a role as well in getting the right DOF. Or not?

Would the sameDOF be possible on a 50mm vs 300mm with the same F4 aperture?

Hope not to bore you to death with my questions...
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Old Aug 6, 2006, 1:02 AM   #35
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No such thing as boring anyone here with questions about photography.

There is a ratio of DOF to the focal distance from camera to subject and the f stop.

I have seen a chart somewhere that points out how long the DOF is at certain focal distances.

Obviously at short focal distances the DOF is almost a sliver of focus. Thats why macro photography is so difficult. You have to shoot at small apertute to try and get the longest DOF and by doing that you have to drop your shutter speed right down.

Something to do with the 'rule of thirds' and I don't mean the composition rule. The DOF extends 1/3 in front of your point of focus and 2/3 beyond. So if your DOF is about 3 metres, when you focus on your subject then 1 metre in front will be in focus through to 2 metres beyond your subject. If you were to walk closer to your subject and re-focus then the DOF will reduce in depth.

The example below demonstrates how a simple change in F stop can increase your DOF.

Both shot at 70mm at 200 ISO. The left side is f/2.8 at 1/1000, the right side is f8 at 1/160.

You can see the left side DOF starts at the second sailor in and extends through past the officer but not to the second group of sailors. The right side image at f8 has a longer DOF and starts at the first sailor and extends pretty much to infinity (well not really but you would be hard pressed to see where it stops.) Look at the banners on the poles to see that they are in focus.

So opening up your aperture will shorten your DOF, being close to your subject will shorten your DOF as well. Longer focal lengths aparently also can have an impact.

If you want long DOF keep your shutter speeds down and your aperture closed to at least f8 and use longer lens to put some distance between your camera and your subject.

Crash

edit..

This is a cut and paste from http://en.wikipedia.org/wiki/Depth_of_field

Thus for a given film format, depth of field is calculated from three factors: the focal length of the lens, the f-number of the lens opening (the aperture), and the camera-to-subject distance. While it is commonly said that lenses of short focal length have greater depth of field than long lenses, this rule of thumb is not strictly true because it takes into account only one of the three factors. In fact, for a given subject framing and aperture, lenses of all focal lengths have approximately the same depth of field. This is because subject framing is dependent on two of the factors (focal length and subject distance), while aperture is the third. Once the three factors are set in a fixed proportion, the depth of field will be almost the same.

An example makes this easier to understand. Take a photographer using a 400 mm lens to shoot a subject (for example, a bird) 10 metres away. Assuming an aperture of f/2.8, the depth of field of this shot would be 10 cm. Should the photographer now switch to a 50 mm f/2.8 lens, the depth of field at 10 metres is now 7.62 metres. However, once the photographer has moved to 1.25 metres from the bird, being the distance required such that the bird fills as much of the frame as it did with the 400 mm lens at 10 metres, the depth of field is almost exactly the same as before, 10 cm. In some cases, though, the DOF of one lens may extend to infinity, or nearly so, and the other still finite, so the approximation can break down in such cases.



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Old Aug 6, 2006, 1:27 AM   #36
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You got me looking now.

An online DOF calculator

http://www.shuttercity.com/DOF.cfm

Examples and tutorials

http://www.photozone.de/4Technique/compose/seperate.htm

http://www.photozone.de/4Technique/compose/dof.htm

http://www.luminous-landscape.com/tutorials/dof2.shtml


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Old Aug 6, 2006, 6:56 PM   #37
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Thanks Crashman for those links, they are really worth a look for those of you who haven't already done so.

Kjell
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Old Aug 6, 2006, 7:05 PM   #38
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I don't know if any of you have reason to need information on the old screw mount lenses, but I ran across this site when I was researching a 50mm F4 macro SMC Takumar - Bruce

http://www.aplimited.com/pentax/pages/home.htm
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Old Aug 6, 2006, 11:23 PM   #39
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Since we have drifted into prime fast lenses in this thread let me point out something else that separates digicams from dSLRs, actual focal length.
- Most of us do not own lenses with focal lengths less than 18mm, this gives the equivalent of 27mm on a 35mm film camera because of the size of the digital sensor (APS vs 35mm, result 1.5X crop factor).
- The typical zoom digicam with a 1/2.7 sensor has a sensor so small that the crop factor is about 6X, this means that a 6mm lens results in the equivalent of about 36mm in 35mm film format. As a result a small digicam (I have a Fuji 2800Z in front of me as I type) may have an equivalent to 35-200mm but its actual focal length is 6-36mm.
- The shorter the focal length a lens has, the greater the DOF at the same marked aperture.
- Result of all the information above, digicams have tremendous DOF. This means that it is difficult to separate a subject from its background unless you are very close to the subject and very far from the background (or vise versa). This is part of the reason why digicam users are often disappointed when they purchase their first dSLR, they get pictures that do not have the sharp focus of their digicam, because the dSLR lens has a greater focal length thus it has less DOF, it is more prone to pictures where some of the subject is out of focus. The picture I posted above is a good example, to most it will just look out of focus.

Controlling depth of field is one of the primary reasons for owning a dSLR, it is also the reason many very good photographers prefer Av mode over program. In Av mode they can have control over DOF. Tv mode is useful if you need to stop, or exaggerate, action but if your subject allows it, Av gives more control over the look of the final image.

Check out the budget bins at your local book store for books on 35mm photography, they are usually considered obsolete and sold cheap, but all of the information about lenses, composition, lightingand basic photographic terms still apply with digital and are very valuable. Anything you can't find, ask here and someone will answer.

Ira
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Old Aug 7, 2006, 12:54 AM   #40
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Talk about revisiting my phtoography training from 26 years ago..well, there is nothing wrong with a good refresher of the ntus and bolts of how it all works, is there?

:O
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