Sensor Size

[Anbesol]

Something not addressed is that sensor size effects depth of field. The higher the "crop" or "correction" factor, the more depth of field you get for a given F/stop. so f/2.8 on a full-frame sensor has less depth of field than f/2.8 on an aps-c or 4/3 sensor, always (everything else being equal). If your applications need high depth of field, like in macro photography or long telephoto, a crop sensor might be preferable to a larger sensor.

So while Anbesol generalized that "We'd all love to have full frame 35mm sensors in our camera," it's simply not always true. Personally I'd loathe to give up the extra depth of field and correction factor of the 4/3 format. For me, those extras outweigh the cost of a slightly "noisier" image. What Anbesol really intended to say is that we'd all like our cameras sensor to produce images as high-quality as full frame 35mm sensors, and he'd be right about that!

Actually 'APS lens' is somewhat of a misnomer - it simply translates to smaller cuts of optics (or short back focus with canon), throw it on a full frame and you'll vignette around the edges at wide and corner problems are worsened. Essentially you cut out sweet spot in favor of smaller form factor/lighter lens with "APS designed lens". But f/2.8 dof is the same on both APS and full frame. The difference is crop factor, not optical construction, or maybe thats what you meant and I misread.

no-less, when I said 'we'd all love to have a full frame camera', i was speaking in a mystical, pipe-dream sort of way. ;)

[Sushigaijin]

The smaller the sensor the more DoF for a given aperture (at output level).

That's a factor of sensor size. The only other things that effect DoF are magnification and aperture. In the case of very wide lenses, focal distance can factor in as well - it's important to think of focal distance as different than magnification IF the lens is approaching hyperfocal distance.

F/2.8 has the same DoF for any focal length at the same magnification on the same sensor, but that changes drastically when you move to a smaller sensor. A 4/3 DSLR has roughly two stops more DoF at any given aperture (at the same magnification) than a 35mm full frame camera.

Of course, that's in relation to each other. Absolute depth of field on the same sensor is irrelevant because a 4/3 lens would never cover the sensor of a 35mm camera. This post also explains one reason why it is very hard to get short DoF on the tiny sensors in compact cameras.

[Franglais]

The smaller the sensor the more DoF for a given aperture (at output level).

That's a factor of sensor size. The only other things that effect DoF are magnification and aperture. In the case of very wide lenses, focal distance can factor in as well - it's important to think of focal distance as different than magnification IF the lens is approaching hyperfocal distance.
...

OK I see what you mean for the first statement. If you take the exact same picture at the same aperture on a camera with a big sensor and a camera with a small sensor the image from the small sensor will have more depth of field because the focal length of the lens is shorter.

I don't understand the second statement. What is "focal distance"? How does the hyperfocal distance come into it?

[Sushigaijin]

OK I see what you mean for the first statement. If you take the exact same picture at the same aperture on a camera with a big sensor and a camera with a small sensor the image from the small sensor will have more depth of field because the focal length of the lens is shorter.

I don't understand the second statement. What is "focal distance"? How does the hyperfocal distance come into it?

Focal distance in the distance from the sensor to the sharpest point of focus.

Lenses all behave similarly at the same F/stop except when nearing hyperfocal distance. So a 400mm at f/4 has the same depth of field as a 50mm at f/4, assuming the magnification is the same.

or,

As long as the magnification level doesn't bring the focal distance close to the hyperfocal distance, f/4's depth of field is always the same.

Since a 100mm and a 400mm both have very far away hyperfocal distances, they will have the same (virtually) depth of field at any given f/stop at the same subject magnification. Make a quarter the exact same size at the exact same place in focus, and the depth of field will be the same at the same aperture on the same sensor, even with lenses of different focal lengths.

Well wait, that doesn't sound right - a 24mm lens has WAY more depth of field than a 400mm at f/4.

Of course. But at the same magnification level, the 24mm is approaching or exceeding the hyperfocal distance. At f/6.3 and 24mm the hyperfocal distance is INCHES from the lens...too close for a 400mm to even focus. But if you're shooting at say, 1:3 magnification with a 24mm and a 28mm, and the object is at the same magnification, f/4 will be very similar in depth of field. You could never tell the difference without actually crunching the equation on a calculator.

The nitty gritty is that in MOST situations, focal length doesn't have anything to do with depth of field. Only magnification, focal distance, sensor size and aperture control depth of field. It's an important consideration when stepping up to a smaller sensor because you've changed one of the basic determinants permanently. I can never have the same shallow DoF as f/1.4 on a full frame - I would need an f/0.2 lens (or something like that). I do, however, get twice as much depth of field at f/2 than a full frame does.

I shoot a lot of macros, so that's a good thing.

[Loupey]

...It's an important consideration when stepping up to a smaller sensor because you've changed one of the basic determinants permanently. I can never have the same shallow DoF as f/1.4 on a full frame - I would need an f/0.2 lens (or something like that). I do, however, get twice as much depth of field at f/2 than a full frame does.

I shoot a lot of macros, so that's a good thing.

Any format smaller than full-frame (1.6x, 1.5, 2x, even PS) can still be fully replicated by a full-frame sensor as long as one knows how much cropping it will take - that is, how much smaller to make the subject magnification at the imaging plane.

With today's mega MP full-frame cameras, I bet that the level of cropping necessary to achieve the 1.6x and 1.5x (and possibly even the 2x) would still keep it on par, image quality wise, with those dedicated formats.

My "old" 5D at 12mp still produces cleaner images than my "new" 50D at 15mp. Of course the number of pixels isn't the deciding factor here.

So the real disadvantages of going as large as you can (i.e. full-frame) are the size of the equipment and price.

[OldClicker]

Any format smaller than full-frame (1.6x, 1.5, 2x, even PS) can still be fully replicated by a full-frame sensor as long as one knows how much cropping it will take - that is, how much smaller to make the subject magnification at the imaging plane.

With today's mega MP full-frame cameras, I bet that the level of cropping necessary to achieve the 1.6x and 1.5x (and possibly even the 2x) would still keep it on par, image quality wise, with those dedicated formats.

My "old" 5D at 12mp still produces cleaner images than my "new" 50D at 15mp. Of course the number of pixels isn't the deciding factor here.

So the real disadvantages of going as large as you can (i.e. full-frame) are the size of the equipment and price.

I have an xx mm prime on my 1.5x camera and stand at 10 feet and my subject fills the frame. So with the same lens at 10 feet with FF camera, if I crop to again fill the frame, I will have the same DoF? - TF

[Loupey]

I have an xx mm prime on my 1.5x camera and stand at 10 feet and my subject fills the frame. So with the same lens at 10 feet with FF camera, if I crop to again fill the frame, I will have the same DoF? - TF

Yes - the same lens shot at the same distance will produce the same DOF. How can it not? The lens doesn't care what it's attached to.

[Loupey]

OK, look - here is a two minute example I just shot with what I have at the moment at my office.

To exaggerate the DOF effect, I shot both of these at the maximum zoom (105mm) at the maximum aperture (f/4) at the closest focusing distance (the front of the lens is at the zero mark). Same ISOs, same lens.

First set - uncropped from both cameras.

Second set - cropped to the same "apparent" subject size.


EXIF data attached if you want to check.