Depth of Field Calculator Question

WRX_Jeff

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I took a trip out west last summer to visit a few of the amazing national parks (Glacier, Yellowstone, Grand Teton, Badlands, Mt. Rainier and Devils Tower). I'm relatively new to photography, but have a decent general understanding of shooting techniques, including manual shooting.

The vast majority of my shots were landscapes. For the most part, I was pretty happy with a fair amount of my shots. There were however, some shots where I seemed to have trouble keeping the foreground and background in focus (foreground usually in focus - background a bit soft) even though I was shooting F11-F18.

I did some research on hyperfocal distance and downloaded a DOF calculator. I have a couple questions on how to use it.

- When it asks for 'Focal Length', am I entering the setting I've made on the lens for the particular shot? i.e. I'm using a 10-20mm lens and I'm at 20mm, so 20mm is the focal length.

- This particular calculator asks for 'Subject Distance'. What counts as the subject if I'm shooting a mountain range that's potentially miles away?

Thanks in advance!
 

480sparky

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- When it asks for 'Focal Length', am I entering the setting I've made on the lens for the particular shot? i.e. I'm using a 10-20mm lens and I'm at 20mm, so 20mm is the focal length.

That's correct.

- This particular calculator asks for 'Subject Distance'. What counts as the subject if I'm shooting a mountain range that's potentially miles away?

Not much. With such a wide-angle lens, you can easily set your focus to just less than infinity, shoot at f/8, and never bother with focus.
 

Gavjenks

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"Focal length" is not necessarily actually what you have on your lens. It is instead the visual angle that will be in your final print. These are different if you crop at all. For example, if I shoot an image with a 50mm prime lens, and then crop it down halfway, my "focal length" for purposes of DOF is 100mm, because I have the same image I would have had with a 100mm lens. Essentially, when you crop, you are magnifying the blur and making it easier to see, narrowing the range of what will appear in focus in the final print.

"Subject" is just the distance of your focus, which is always a perfectly flat geometric plane. The fact that you can see things in focus in three dimensions / outside of that perfectly flat plane is simply a result of the limits of resolution of film/sensors/eyeballs not being able to detect what is in fact slight blurriness. The distinction between miles away and "infinity" is usually irrelevant. After a couple hundred meters, it usually becomes a difference of only a millimeter or so on your focusing ring and acts the same way.




Be careful with hyperfocal distance, too. I suggest always focusing a bit further away than what a calculator would tell you is hyperfocal distance if you can possibly get away with it. If you focus exactly at hyperfocalt, what it means is that infinity is just baaaaaaaaaaaaarely on this side of the edge of what's considered "in focus." Which is fine sometimes, but gives you NO latitude to do anything weird like make unusually large prints, and you can't crop AT ALL if you shoot at hyperfocal, or infinity will rapidly enter the out of focus range, since your effective focal length is increasing, and DOF is getting smaller when you crop.

If you need hyperfocal to get the shot you want, though, then you need hyperfocal. It's a tool like any other, appropriate sometimes (low light, mostly. Or if the DOF you need is pushing the limits of diffraction), inappropriate other times.
 

KmH

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Many lenses have an aberration known as 'field curvature' making the plane of focus less than "always a perfectly flat geometric plane".
 
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WRX_Jeff

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I really appreciate all of the helpful replies!

I suppose my question now is, for landscape photography, do I really need to worry about hyperfocal distance or will I get decent results simply by ensuring I use a small aperture (F8 or higher) and focus 2/3 of the way out?

Thanks!
 

Derrel

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I really appreciate all of the helpful replies!

I suppose my question now is, for landscape photography, do I really need to worry about hyperfocal distance or will I get decent results simply by ensuring I use a small aperture (F8 or higher) and focus 2/3 of the way out?

Thanks!

When you make large prints, or view images on a large monitor, like a 32 inch CInema Display, or on a big screen TV, you can usually see focus issues. Some lenses exhibit focus shift when stopped down to shooting aperture. Environmental haze can cut sharpness over long distances, especially during spring, summer, and fall, when there can be huge amounts of pollen, smoke, dust, or agricultural airborne particulate matter, so "sometimes" objects at longer distance will not be clear, so it does little to focus with the intent of rendering the farthest zones in critically sharp focus.

As far as hyperfocal distance: it actually works if a print is seen at the so-called "appropriate viewing distance", and in small prints, like under 5x7 inches or so. As prints are made larger, or the image printed big and seen from close distances, or examined critically at 100% pixels on a good, big monitor, the hyperfocal distance focusing idea often leaves a less than pleasing taste for the critical observer. If the foreground is the most-important and most-interesting part of a shot, then focus to favor the foreground.

One of the BEST things you can do is to do some focus bracketing! Move the focus through a short range of different distances! MOST, as in doggone near all low-end lenses and AF lenses, have VERY sketchy, skimpy focusing distances, and even worse DOF scales, plus DOF scales on many lenses are for FX frames, not DX, so the scales and DOF markings, if any exist, are dubious in most cases. In landscapes, sometimes a really nifty foreground feature looks a LOT BETTER, sharper, clearer, and more interesting if the focus is cheated closer to the camera. If it's a vista type shot, often focusing on Infinty, or short of Infinity, will actually look better than any hyperfocal focusing shot, at least when seen big, or cropped in on, etc.

TRY some focus bracketing, and look carefully, and see how much even a subtle re-focusing can be on some scenes. The FACT IS: ONLY ONE DISTANCE is critically in-focus, and everything else is only "acceptably sharp". As you move into longer focal lengths and lenses that magnify things a fair amount, this truth becomes much more visible, and easily so. With short lenses and small format sensors, like 18-55 on APS-C, this is not quite so apparent, since much of the image is small and significant detail is pretty small, not huge and easily-seen. TRY some focus bracketing! See what altering the point of sharpest focus can actually do. MANY times, the fore- and mid-ground areas are what we wish to be critically sharp, with the background left to a more "suggested" rendering that is ever-so-slightly less-sharp than the closer objects. At other times, the exact opposite is true. There is often a BEST focus distance; not always, but many times there **will be** a truly optimal focus placement.
 

Braineack

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Helen B

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"Focal length" is not necessarily actually what you have on your lens. It is instead the visual angle that will be in your final print. These are different if you crop at all. For example, if I shoot an image with a 50mm prime lens, and then crop it down halfway, my "focal length" for purposes of DOF is 100mm, because I have the same image I would have had with a 100mm lens. Essentially, when you crop, you are magnifying the blur and making it easier to see, narrowing the range of what will appear in focus in the final print.

Think about this. If you crop in printing isn't it more like using a smaller sensor than switching lenses? In the example you give it would be wrong to increase the focal length but right to decrease the format size (and implicitly the MACC - maximum acceptable circle of confusion) used in the calculator. You are suggesting that if you use a crop sensor you should enter the equivalent focal length, not the actual focal length.

Using a smaller format in the calculator (and keeping everything else the same) is a way of raising the sharpness criterion for large prints in the sort of circumstances Derrel refers to.


Technical: the two changes might take the result in the right direction, but DoF is (roughly) inversely proportional to the square of the focal length but proportional to the MACC diameter. Work it out from there. Simple.
 
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Braineack

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a smaller, less MP sensor, yes.
 

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