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Author Topic: depth of field and d60  (Read 1840 times)
Dale_Cotton
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« on: February 26, 2003, 08:07:36 AM »
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A DOF scale correlates each aperture with an estimate of the DOF that will be available on either side of the focal point at that aperture. The problem is that a zoom lens introduces a new variable: focal length. The DOF scale would have to be a sliding scale that changed as you changed the focal length. For example, the DOF for f/8 will be very different when the lens is at 70 than when it is at 200. See here.

Notice the word estimate above. Even if you have a prime lens with a very authoritative-looking, very precisely marked DOF scale, the meaning of the scale is entirely dependent on the choice the manufacturer made for a certain variable called the circle of confusion. See here and here for more on that. In general, DOF scales on lenses are way to forgiving for today's standards of print sharpness. Many photographers routinely use the DOF reading from one or two stops wider aperture to compensate. IOW, look at the photo of the DOF scale on the Canon 50 lens in Norman's tutorial. If you wanted the DOF for f/11 you would read the distances opposite the f/8 or even the f/4 marks instead.

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does the depth-of-field preview button compensate?
Yes, that's the intent. When you look through your finder normally you are seeing what the lens sees when it is at its maximum aperture. For the lens you mention, the 70-200, the max aperture is f/2.8. You are seeing the DOF that f/2.8 gives. When you take the picture however the aperture stops down to the f/stop you've selected. The DOF preview button simply causes the aperture to stop down while you hold down the button. While you do that you are seeing the DOF that results from the selected f/stop, which is the same thing that will be recorded on film.

The downside is that when you press the DOF button you are narrowing the lens aperture and thus reducing the amount of light that makes its way to the finder. You'll have to wait a few seconds for your eye's iris to dilate before you can see clearly in the reduced light. One aperture is opening to compensate for another aperture closing!
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Dale_Cotton
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« Reply #1 on: February 27, 2003, 07:49:14 AM »
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First off the author of that quote is making a presumption: that you are interested in using shallow DOF, as in putting the background of a portrait out of focus. Since most landscape photographers are most often looking to maximize DOF, we could turn the quote around to say that one of the advantages of small-sensor digicam is the dramatic increase in DOF.

To the best of the understanding of a mathematically-challenged liberal arts type like me, a camera body does not really have a DOF, it's the focal length of the lenses together with the apertures you use that dictates your DOF. It so happens that a small-sensor digicam requires lenses with focal length numbers like the 7 to 24 mm of the new Sony V1. Put a 7-24 zoom on your D60 and you'll get great DOF too.  

In practical terms, the field-of-view multiplier AKA focal-length multiplier of your D60 is 1.6x. This means you will generally be using lenses that are 1.6 times wider than you would use to frame the same shot with a 35mm camera. Therefore, your DOF will be proportionately greater.

Again, this is not because - for example - a 35mm lens on your D60 suddenly acquires more DOF than it has on a EOS 30; but because you would use a 35mm lens on the D60 where you would have used a 56mm lens on the EOS 30.

If you're in the market for greater DOF, then the D60 (not to mention APS) gives you a leg up. If you switch to a format larger than 35mm, such as 120 film, you take a corresponding hit in DOF.
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rswinford
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« Reply #2 on: February 25, 2003, 08:39:56 PM »
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Ok another naive/learning photography question.  Up until recently I have been an owner of a point-and-shoot digital (nikon coolpix) and switched to the d60 in December of 2002.  I have been reading Understanding Exposure by Bryan Peterson and have reached an impass.

I currently have a Canon 70-200 f2.8 lens, why doesn't this lens have a depth-of-field scale? That may be a null question, as it just doesnt, but does the depth-of-field preview button compensate?

Thank you in advance.
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rswinford
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« Reply #3 on: February 26, 2003, 08:51:26 PM »
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Thank you for that helpful reply.  Taken from one of the links...........

"This is one of the unspoken drawbacks of low-end digital cameras. Only expensive SLRs like the Nikon D1x, Canon 1D and their ilk have chips close to the size of a 35mm frame, and therefore offer enough DOF to allow creative control over out-of-focus backgrounds"

Where do you think the D60 fits?

Thank you.
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