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02-07-2009, 01:31 PM   #1
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DOF and Crop Sensors

I've been playing around with my Takumars recently - sometimes I shoot from waist level on the street to avoid attention. When I do so, I focus by estimating the distance to the subject and using the DOF scale and aperture. It works somewhat well for stealth shooting (obviously I can nail focus much better if I use the viewfinder and split focus screen).

But this got me wondering - since Pentax dSLR's are crop sensors, can the depth of field scales be used accurately? I had two contradicting thoughts on this:

1. Crop sensors generally have greater DOF than a FF sensor;

2. Since crop sensors only use the center portion of the lens, the equivilant focal length is greater. So would the DOF be less than what is indicated on the scale?

An example - if I mount my Tak 35/2 on the K20D set focus as indicated below, at F/16 will the distance from approximately 4 feet to infinity appear to be in focus?

Your thoughts please.


Thanks,
Edmund

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02-07-2009, 01:58 PM   #2
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Just keep it simple. On a crop camera, everything stays the same (DOF for a given distance from the sensor plane and f/stop on the lens). The only difference with a FF camera is that on a crop camera, the picture is... cropped. So the scale on the lens is perfectly usable.
02-07-2009, 01:58 PM   #3
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Here is a good discussion of the subject:

Digital Depth of Field
02-07-2009, 02:14 PM   #4
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QuoteOriginally posted by Asahiflex Quote
Just keep it simple. On a crop camera, everything stays the same (DOF for a given distance from the sensor plane and f/stop on the lens). The only difference with a FF camera is that on a crop camera, the picture is... cropped. So the scale on the lens is perfectly usable.
Are you sure about this? I would agree with you but in practice this doesn't work.

02-07-2009, 02:16 PM   #5
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QuoteOriginally posted by Ole Quote
Here is a good discussion of the subject:

Digital Depth of Field
Ole, thanks for the link - very good information over there. This clarifies what I was beginning to suspect - that to use the scale it has to be reduced by a little over a stop. I'll paste in part of that link below:

Many prime lenses have Depth of Field scale markings. These enable an estimate of DOF to be made based on focusing distance and aperture. Below is an example. This is a 17mm lens with a DOF scale designed for use on a full frame 35mm film camera. The aperture is set to f16 and the focus is set to 0.7m. Opposite the f16 marking on the right you can see a distance of infinity is indicated and opposite the f16 marking on the left you can see a distance of something less than 0.4m is indicated. So when this lens is used at f16 and focused at 0.7m, the depth of field extends from just less than 0.4m to infinity.



Now let's look at the case when this lens is used on an EOS 10D. As the article indicates, if you use a given lens on a smaller format, depth of field is reduced and the angular coverage ("effective 35mm equivalent focal length) decreases. In the case of the 10D it's reduced by a factor of 1.6, so it gives the same angular coverage (field of view) as a 27mm lens on a full frame 35mm body. The circle of confusion value for the 10D is reduced by a factor of 1.6x and what this means in terms of DOF scales is that you need to use the markings for about 1 1/3 stops wider aperture in order to estimate the DOF. So with the lens set to f16, we need to look at the DOF scale markings about 1/2 way between f8 and f11 as shown below. In this case, if we want infinity to be at one end of the depth of field, we have to focus at 1m. This gives us a DOF extenting from just over 0.5m to infinity.



Note that the numbers quoted above are approximate. They aren't quite the same as you'd get from a detailed DOF calculation, but that's because you can't read the DOF and distance scales on a lens accurately to two decimal places, plus you don't know quite what value the manufactuer used for the circle of confusion value (it's usually between 30 and 35 microns).

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02-07-2009, 03:02 PM   #6
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edl, the conclusion you reached is correct but just be careful how you consider the effects of sensor crop factors. Asahiflex is correct in stating the crop factor of a camera sensor has no effect on the image produced by a given lens. The DOF and hyperfocal distances remain the same. What happens is the camera accepts only a portion or crop of the image which in effect magnifies that image at the sensor. It is this magnification that results in a reduced effective hyperfocal distance for a lens and my guess is your calculated example is pretty close. It's no different than when we enlarge our film images in the darkroom and crop only a portion for our prints. If you enlarge or crop an image you must allow for greater DOF when taking the shot if you want everything in the image to appear to be in focus. Only the portion of an image at the exact plane of focus at any aperture is truly sharp, even at smaller apertures. The remainder of the image within the hyperfocal range is just sharp enough to exceed the resolution capabilities of our eyes.

The hyperfocal range markings on lenses take into account a couple of things including the circle of confusion for a given aperture but again it's a relative value because lens designers can not anticipate how large our images will be displayed.

Many of us learned the use of a camera by using a 35mm film camera and we learned the several rules of thumb that apply such as setting our minimum handheld shutter speed to the reciprocal of the lens focal length. When using a crop factored DSLR we must modify those rules of thumb by the crop factor so the minimum handheld speed for a 50mm lens would be 1/90th. SR sure helps.
02-07-2009, 09:07 PM   #7
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B Grace -

Thanks for the detailed response. I also spent many years with Tri-X, HP5 and the darkroom so that's why I thought about using the scales too. I suppose with the resolutions these dSLR's are putting out, it's very important to get the focus exactly right.

Thanks again.
02-08-2009, 02:06 AM   #8
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A couple of handy things to remember regarding sensor sizes and focal lengths (assuming the same f/number in both cases):

For a given FIELD OF VIEW (which means using a shorter focal length with a cropped sensor vs. full frame), you'll get MORE depth of field with the cropped sensor.

For a given FOCAL LENGTH (which means you'll get a narrower field of view with a cropped sensor vs. full frame), you'll get LESS depth of field with the cropped sensor.

People often seem to get these mixed up, which results in lots of lively arguments...

02-08-2009, 08:19 AM   #9
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QuoteOriginally posted by Sean Nelson Quote
A couple of handy things to remember regarding sensor sizes and focal lengths (assuming the same f/number in both cases):

For a given FIELD OF VIEW (which means using a shorter focal length with a cropped sensor vs. full frame), you'll get MORE depth of field with the cropped sensor.

For a given FOCAL LENGTH (which means you'll get a narrower field of view with a cropped sensor vs. full frame), you'll get LESS depth of field with the cropped sensor.

People often seem to get these mixed up, which results in lots of lively arguments...
I really want to understand this so I'll try saying the same thing in a different way. Please excuse the redundancy but let me know if I have this right.

When I mount the same lens at the same f/stop on both a full frame and a cropped frame camera the performance of the lens including DOF is unchanged.

What actually changes is the apparent DOF in the printed images from the two cameras. When I print an 8X10 from the cropped sensor it is effectively and enlargement as apposed to an 8X10 from the full frame sensor so the enlarged image shows a natural reduction in depth of field.

So it is the print enlargement effect that "changes" the depth of field.
Not the Lens
Not the sensor

I think........

So if I have this right and all DOF scales are based on 8X10 prints at arms length, then 4X6 snapshots and 5X7 prints show greater DOF than the scale on the lens. At least in a FF sensor.

And to get silly about it, if I want greater DOF all I need to do is force people to view my photos from 3 feet away!

Thanks, joe
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02-08-2009, 10:50 AM   #10
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QuoteOriginally posted by joelovotti Quote
I really want to understand this so I'll try saying the same thing in a different way. Please excuse the redundancy but let me know if I have this right.

When I mount the same lens at the same f/stop on both a full frame and a cropped frame camera the performance of the lens including DOF is unchanged.

What actually changes is the apparent DOF in the printed images from the two cameras. When I print an 8X10 from the cropped sensor it is effectively and enlargement as apposed to an 8X10 from the full frame sensor so the enlarged image shows a natural reduction in depth of field.

So it is the print enlargement effect that "changes" the depth of field.
Not the Lens
Not the sensor

I think........

So if I have this right and all DOF scales are based on 8X10 prints at arms length, then 4X6 snapshots and 5X7 prints show greater DOF than the scale on the lens. At least in a FF sensor.

And to get silly about it, if I want greater DOF all I need to do is force people to view my photos from 3 feet away!

Thanks, joe
joe

Well, thats needlessly confusing things. It's assumed as a reference point that our output image is equal between formats in order to isolate whats happening. Moving the hardcopy around doesn't change the perspective of the content, just the persepctive of the viewer in relation to it, which is outside the scope of what were dealing with.

The hardest part to fathom is the simplicity of this, I think. Every element in this equation is passive - the only reason DoF changes is because our proximity to infinity changes.

If I'm framing something perfectly to 18x24mm (APS-C), I have to stand further back than I would with a 24x36(FF) because the crop frame is smaller by half, thus you're adding depth of field, as you're working closer to the hyperfocal distance of the lens. Thats it.
02-08-2009, 11:28 AM   #11
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QuoteOriginally posted by thePiRaTE!! Quote
The hardest part to fathom is the simplicity of this, I think. Every element in this equation is passive - the only reason DoF changes is because our proximity to infinity changes.

If I'm framing something perfectly to 18x24mm (APS-C), I have to stand further back than I would with a 24x36(FF) because the crop frame is smaller by half, thus you're adding depth of field, as you're working closer to the hyperfocal distance of the lens. Thats it.
Okay, this is the best "simple" explanation so far. Makes perfect sense to me.
02-08-2009, 05:09 PM   #12
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QuoteOriginally posted by edl Quote
B Grace -

Thanks for the detailed response. I also spent many years with Tri-X, HP5 and the darkroom so that's why I thought about using the scales too. I suppose with the resolutions these dSLR's are putting out, it's very important to get the focus exactly right.

Thanks again.
edl, by all means use the hyperfocal range markings on your lenses. I do and I'm glad my lenses have them. Your idea of crop-factoring the lens markings is one I'll use.

I've been using my crop-factored sensor DSLRs for about two years now so I'm still adjusting. I can only think this adjustment must be similar to that which photographers had to go through when changing from big film cameras to "miniature" 35mm film cameras 80 years ago or so.
02-08-2009, 06:38 PM   #13
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Arrrgh! :

As simple as possible:

Cropped sensors only have "more" DOF because people use wider lenses to maintain a field of view. It's why P&S cameras almost never have 'out of focus' backgrounds, because their lenses are 5-12mm.

To decrease DOF, you must increase the focal length, or decrease the distance to the subject.

For a given distance to a subject, those using cropped sensor cameras have to use shorter lenses, which inherently have more DOF at a similar aperture to their larger format equivalents.
02-08-2009, 07:42 PM   #14
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QuoteOriginally posted by joelovotti Quote
When I mount the same lens at the same f/stop on both a full frame and a cropped frame camera the performance of the lens including DOF is unchanged. What actually changes is the apparent DOF in the printed images from the two cameras.
Yes, you've got it!

Here's the thing: when the lens is set to focus at, say, 3 feet, the ONLY things that are PERFECTLY sharp are at EXACTLY 3 feet. In absolute terms, there is no "depth of field". Everything that's not exactly in the plane of focus is blurry to a greater or lesser extent. It's just that things further away from the 3 foot mark are blurrier than things closer to it.

So what's this "depth of field" thing all about then? Well, it has everything to do with the fact that blurry things can look sharp if they're not enlarged too much. Think of a TV set - from across the room it looks crisp and clear, but if you walk up to it and use a magnifying glass then you can see it's actually pretty unsharp.

Depth of field is all about how much you can enlarge the blurry bits before you can actually see the blurriness. The stuff that's closest to the 3 foot mark isn't very blurry and can be enlarged further while still appearing to be acceptably sharp. The DOF markings on a lens make a bunch of assumptions as to how much enlargement you're going to do to get the final image - but they're just assumptions. If you're enlarging less than those assumptions then the final image will have a greater range (ie, DOF) where items appear to be sharp (and vice versa).

So the reason that "depth of field" with exactly the same lens is smaller with an APS-C sensor (than with FF) is because you have to enlarge the image from the smaller sensor MORE in order to get a final image of the same size. This also enlarges all those blurry bits, and means that you can see the blurriness in parts of the image that were closer to the lens' actual plane of focus.
11-27-2009, 12:44 PM   #15
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QuoteOriginally posted by CSoars Quote
Arrrgh! :

As simple as possible:

Cropped sensors only have "more" DOF because people use wider lenses to maintain a field of view. It's why P&S cameras almost never have 'out of focus' backgrounds, because their lenses are 5-12mm.

To decrease DOF, you must increase the focal length, or decrease the distance to the subject. Added by arbib: And just the opposite if you want to INCREASE the DOF...IE: you will either decrease the FL, OR move farther away. At any given/set distance

For a given distance to a subject, those using cropped sensor cameras have to use shorter lenses, which inherently have more DOF at a similar aperture to their larger format equivalents.
Yes, This puts it simply...I have been taking some test images with new/used 55/1.8 S-Tak. And I saw this right away while trying to use a flash and f/16...I took a picture of my wife I knew was over 7' away. (Hyper-focal = INF - 6.8'), she was 9' away and out of focus. I had to move the close focus point to 5' for her to in focus, OR, I could back up to about 12' away.
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