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Old Jun 16, 2005, 6:30 PM   #1
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I've been getting acquinted with my new ist Ds, and have been impressed with the quality of the pics (my personal fav setting is "natural" & +1 sharpening - I can't tell the difference from raw with this, btw ..). One thing that has caught me by surprise is that apparently the DOF is reduced and not what the lens says it is. Since I use only manual focus k lenses, this is of particular concern to me. I went out and shot some test scenics, and have been noticing that while the foreground may be in focus, the infinity behind it is not! The manual mentions that there is about a one stop difference here to be observed. My personal testing has found it to be more like 2 stops, and even then, a pic focused to inf is always sharper than if set to hyperfocal (- 2 stops) on the lens barrel.

So have any of you experienced similar anomalies?? Also, why should this be? I mean the 1.5 crop factor is just a crop - the focal length is not changed. So why the difference?
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Old Jun 16, 2005, 8:53 PM   #2
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You've got more (not less) Depth of Field with your DSLR compared to a 35mm SLR for the equivalent framing, focus distance and aperture.

Remember, your subject is filling a greater percentage of the frame for any given actual focal length and focus distance, requiring you to shoot from further away (which increases Depth of Field) so your subject fills the same percentage of the frame.

This is a complaint with some users that switch from film, since they have to use larger apertures (smaller f/stop numbers) to get a Depth of Field as shallow as they had with 35mm, given the same subject framing (since you have to shoot at a 50% greater distance for your subject to occupy the same percentage of the frame).

You've also got a difference in Circle of Confusion. See this handy online DOF calculator, keeping in mind that you have to multiply the actual focal length of a lens by 1.5x to see what the 35mm equivalent angle of view would be. You use the actual focal length for DOF Calculators, but you need to be 50% further away from your subject to get the same angle of view with your DSLR model.

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




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Old Jun 16, 2005, 9:10 PM   #3
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Circle of Confusion

i'm definately!!!! there.. whew..
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Old Jun 16, 2005, 9:12 PM   #4
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just ck it with the dof preview button. it's hard to see but you will get use to it..
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Old Jun 17, 2005, 1:13 AM   #5
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robar wrote:
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just ck it with the dof preview button. it's hard to see but you will get use to it..
No kidding! it is hard to see DOF clearly even in the Ds finder! Not like in the film days ..

Thanks for the link guys, that's exactly what I was looking for. The thing is that I'm getting less DOF. Here's an example:

Pentax-A 28mm lens on Ds -> has the field of view of a 42mm on a film body.

Let's say I use f/8, and set the focus at the conveniently orange marked 10ft setting. With film, I am now expecting DOF to extend from 5ft to inf. With the Ds, I've found that the DOF does not go all the way to inf! Maybe it's getting shifted?? According to the calculator on that site, when these same parameters are entered, I get DOF extending from about 6ft to 26ft on the Ds with the same lens!

So that's very strange to me, I still don't understand this, but at least I can make some tables for reference. Kinda throws you off there when you're used to "old school" (pre AF) style of wide angle shooting ..
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Old Jun 17, 2005, 6:59 AM   #6
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Yes... I see what you mean. But, given the field of view differences, you'd need to shoot from 50% further away with the same lens for the equivalent framing.

Keep in mind that DOF Calculators are using Circle of Confusion numbers developed a long time ago, for use at specific print/viewing sizes to show the area of "Acceptable Sharpness".

So, if the lens has a 50% greater focal length on the DS (from an angle of view perspective), that's probably what is causing the issue.

Even the DOF calculator agrees with your findings (given an absolute measurement in focus distance, you're going to get less DOF with the same lens on the DS).

But, if you shoot from 50% further away from your subject with the DS (to get the same angle of view you'd have with this lens on a 35mm camera), then the DS has greater Depth of field for the same angle of view.

Unfortunately, it looks like the bottom line is that you're not going to be able to go by the Hyperfocal Distance numbers you used with 35mm anymore.

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