D-SLR Depth of Field: A few new tough questions

kkamin · Sep 8, 2009

I challenge someone to explain this to me. This is very confusing for me. :grumpy:

1. If I am at a set distance from a subject, with a set focal length and f-stop, and take a picture with a full frame (FF) camera and a 1.6x crop sensor camera, the depth of field should be the same right? The light entering the lens on both cameras is hitting the sensors in similar ways, but the crop sensor is only catching the middle 40% (approx.) of the projected image.

I read that if the FF shot in the above example was cropped to match the 1.6x crop shot, the depth of field would be identical. But why wouldn't it be identical before the crop? -- the FF would just have a larger field of view (FOV), more foreground, background, more everything...why would the depth of field potentially change?

2. With a crop sensor, to get the same FOV as a FF you need to either move back or shorten the focal length. Moving back will increase depth of field and...

...shortening the len's focal length to match the FOV of the FF camera will give 1.6x more DOF. Why? I know that if you keep the camera body constant but use different focal length lenses, the depth of field is the same. I just don't understand how the different sensor size change the DOF.

3. Lastly, I've read that when you reach the hyperfocal distance with a 1.6x crop sensor, you gain more than 1.6x more DOF. To me still, the image entering the lens and hitting the sensor has an inherent DOF regardless of the sensor size. I don't understand why a smaller sensor means more DOF... to me it just seems cropped--like if I took a random image into Photoshop and cropped it down to 40% of its size, it doesn't change DOF or anything.

The only things I really understand is changing distance to the subject to compensate for a smaller sensor will effect DOF, but that's the only variable in this subject that I get. help.

epp_b · Sep 9, 2009

See: Circle of Confusion. Dumb term, but basically, light forms on sensor or film as a circle that has a sharp centre and progressively blurs as it gets larger (the size of the sharp centre area is determined by the aperture).

The small image panes on point-and-shoot cameras tend to get everything in focus, even at large apertures and close focus distances, because they only see the sharp centre. Larger frames in APS-C, 35mm, medium and large format cameras break free of the sharp area and are able to "see" the progressive outward blur.

kkamin · Sep 9, 2009

epp_b said:
This depth of field calculator should pretty much answer your questions, I think.

Thanks. No I played with a few, but it just left me more confused.

Moglex · Sep 9, 2009

kkamin said:
I challenge someone to explain this to me. This is very confusing for me.

1. If I am at a set distance from a subject, with a set focal length and f-stop, and take a picture with a full frame (FF) camera and a 1.6x crop sensor camera, the depth of field should be the same right? The light entering the lens on both cameras is hitting the sensors in similar ways, but the crop sensor is only catching the middle 40% (approx.) of the projected image.

I read that if the FF shot in the above example was cropped to match the 1.6x crop shot, the depth of field would be identical. But why wouldn't it be identical before the crop? -- the FF would just have a larger field of view (FOV), more foreground, background, more everything...why would the depth of field potentially change?

2. With a crop sensor, to get the same FOV as a FF you need to either move back or shorten the focal length. Moving back will increase depth of field and...

...shortening the len's focal length to match the FOV of the FF camera will give 1.6x more DOF. Why? I know that if you keep the camera body constant but use different focal length lenses, the depth of field is the same. I just don't understand how the different sensor size change the DOF.

3. Lastly, I've read that when you reach the hyperfocal distance with a 1.6x crop sensor, you gain more than 1.6x more DOF. To me still, the image entering the lens and hitting the sensor has an inherent DOF regardless of the sensor size. I don't understand why a smaller sensor means more DOF... to me it just seems cropped--like if I took a random image into Photoshop and cropped it down to 40% of its size, it doesn't change DOF or anything.

The only things I really understand is changing distance to the subject to compensate for a smaller sensor will effect DOF, but that's the only variable in this subject that I get. help.

Always remember that total depth of field is only extremely weakly related to the focal length of the lens (although the distribution in front of and behind the focus distance does change). Apart from aperture, of course, DOF is primarily related to the distance to the subject.

The answer to your first question is, of course, that it wouldn't. You cannot possibly change the DOF by cropping.

The difference in effective DOF between crop sensor and FF (and, indeed, MF and 5x4) is based on the fact that, for any given focal length, you need to change the distance between the focal plane and the subject to get the same FOV.

The answer to (3) is that DOF is not linearly related to subject/focal plane distance.

Garbz · Sep 9, 2009

Fantastic Moglex, it couldn't have been put in a more straight forward way!

kkamin one thing to realise is that the premise in (1) never actually happens. If you crop a full frame camera you end up with an identical image to the sensor size you crop to. When people say FF has better depth of field they are taking into account the fact that you're not cropping, that your lens field of view is wider, and thus you need to get closer to your subject. Comparisons of field of view almost always use this premise as if you left the camera in the same position you end up with completely different composition, hardly a real world comparison for photography where composition should come first and foremost.

epp_b · Sep 9, 2009

Reread my post, I modified it significantly

You cannot possibly change the DOF by cropping.

That's not entirely true. Depending on the shot, you may change the apparent DOF when cropping.

Dao · Sep 9, 2009

epp_b said:
Reread my post, I modified it significantly

You cannot possibly change the DOF by cropping.

Click to expand...

That's not entirely true. Depending on the shot, you may change the apparent DOF when cropping.

Sorry for the off topic. Just noticed that your post was registered on 09/09/09 09:09 (Central Time)

Derrel · Sep 9, 2009

Here is a dPreview post that has a few links one might want to read.

Comparison shots FF vs. APS-C shallow DOF?: Canon EOS 7D / 50D - 10D Forum: Digital Photography Review

Oddly enough, one of the links is to an article I wrote for my blog, over two years ago, with a verbal example of a 6x6 120 rollfilm DOF example, a 35FF example, and an APS-C example. Each capture format has an inherent depth of field wit its standard lenses. Same angles of view, but the smaller the format, the deeper the depth of field, for the SAME angular view. Optical fact. Use it to your advantage, or not. Work with it,or fight it. It's immutable.

There is also a link to a series of photos showing FF, APS-C, and 4/3 images showing the relative DOF between those formats in outdoor shooting.

The biggest falsehood we have are people showing APS-C "samples" shot at very close range,covering TINY subject areas, like a birthday cake decoration that is 4 inches tall, or a photo of a robin or small animal in a photograph that covers an area 20 inches wide, from less than 10 feet away,using APS-C,and claiming that the format can produce shallow depth of field. YES, APS-C can produce shallow depth of field effects, but only at distances that are close and far, farshort of the hyperfocal distance. In real-world uses, at outdoor distances using wide-angle and even normal lenses, the APS-C format typically produces depth of field that measures 1.6 times greater than that of a FF digital camera producing the same picture angle, and as the focusing distance approaches the hyperfocal focusing distance, APS-C produces depth of field that measures 2.85 times greater than that produced by a FF digital camera when producing the SAME picture angle, or more.

There is a narrow, close range of distances where APS-C and FF can produce almost identical depth of field as FF digital, with the APS-C camera using an aperture roughly 1 and 1/3 stops wider than FX uses, with a shorter focal length lens. But that range is VERY limited,and as distance gets over 10 feet, the depth of field of APS-C builds very rapidly. Especially when using short focal lengths in the 18-40mm range.

This characteristic can be a plus, or a minus,depending on the situation. Bottom line though, the typical 18-55mm f/3.5~5.6 kit lens does not allow the type of selective foreground/background separation many first-time d-slr buyers would "like" to create, unless the subjects are close-ups or very small objects with very distant backgrounds.