QUESTION ABOUT DOF

Sirs,
If we take a picture from an APS-C with Tamron 17-50 f/2.8 and another shot taken by the same camera but with Canon EF 17-40mm f/4 by using the same focal length, would the dept of field (DOF) of the two pictures be the same ?

Please explain. Thank you so much.
Langin

If you're using the same aperture from the same distance, yes.

Langin wrote in post #15920542
Please explain.

For normal photography, DoF is determined by 4 factors: the focal length of the lens, the f ratio (or 'aperture'), the distance to the subject in sharpest focus, and a factor called the "circle of confusion", which is most easily understood by considering it to depend on the size of the recording format used.

Since you are changing none of these parameters (assuming the distance and aperture are held constant, as mentioned above), there is no reason for the DoF to change.

We've had this before. If you view both images at the same size then the APS-C version will show a smaller DOF because it is effectively a blow-up of the wider image captured by the FF sensor. Xarqi's circles of confusion are therefore magnified more, making visible the ones that were only just small enough not to be visible in the FF image, and therefore look unsharp, in the APS-C version.

Of course, the two images would not look the same because you kept focal length equal, resulting in a different field of view for each sensor size.

EDIT: Dammit, you said "same camera". MY comments all refer to the use of different cameras to match the image circles of the two lenses. With the same camera, as xarqi says, you haven't changed anything. Apologies for any confusion, circular or otherwise.

Nobody mentioned changing cameras. Both pictures are taken on aps-c.

Madweasel wrote in post #15920698
We've had this before. If you view both images at the same size then the APS-C version will show a smaller DOF because it is effectively a blow-up of the wider image captured by the FF sensor. Xarqi's circles of confusion are therefore magnified more, making visible the ones that were only just small enough not to be visible in the FF image, and therefore look unsharp, in the APS-C version.

Of course, the two images would not look the same because you kept focal length equal, resulting in a different field of view for each sensor size.

... But the OP didn't mention anything about changing cameras

Madweasel, he only changed lenses, not camera. Just different lens. Althought he didnt specify if its same f-stop, focal length, and distance.

IanE wrote in post #15920700
Nobody mentioned changing cameras. Both pictures are taken on aps-c.

DunnoWhen wrote in post #15920701
... But the OP didn't mention anything about changing cameras

musashi wrote in post #15920702
Madweasel, he only changed lenses, not camera. Just different lens. Althought he didnt specify if its same f-stop, focal length, and distance.

Thanks all, I caught my mistake and edited my post a second after posting, when I scrolled back up. Apologies.

Langin wrote in post #15920542
Sirs,
If we take a picture from an APS-C with Tamron 17-50 f/2.8 and another shot taken by the same camera but with Canon EF 17-40mm f/4 by using the same focal length, would the dept of field (DOF) of the two pictures be the same ?

Please explain. Thank you so much.
Langin

As long as you are using the same aperture, the DoF is the same. Just because you are using a lens which fits a FF doesn't make it work any differently from the lens designed for a crop sensor. Focal length, aperture, and distance from subject are the pertinent factors.

I was always taught (and I teach) that the 4 factors are: 1 - Aperture (the one everyone figures out.) 2 - working distance. Closer = less DOF. Reason macro is such a difficult thing for many. And astrophotography does not care about DOF. 3 - MM of the lens. Teles have less DOF, wides more. 4 - Size of sensor (negative). Larger = less DOF, smaller more. Pocket P&S cameras typically have a whole lot of DOF. The reason Ansel Adams shot at f/90. Gene

The only reasons the size of the sensor comes into play is because you would either A. Have to move to get the same framing which changes the distance to subject and the math or B. You would crop the full frame image to get the same framing as the APS-C and the perception of the depth of field could change with that.

It's not quite the same relationship as you'd have between a 35mm film SLR and a 4x5 view camera. There are other things involved in the view camera and how Ansel Adams did some of his work. As well as the fact that the smallest aperture on a view camera is f/64. But that's a bit OT and a whole other discussion.

As far as the OP's question if the math doesn't change the depth of field doesn't change.

Just to be clear, if you use the same focal length but use the Tamron at 2.8 and the Canon at 4 then the Tamron will have a narrower depth of field all else being equal.

Sorry, but all other factors aside, size of sensor (negative) does matter.

gasrocks wrote in post #15921939
Sorry, but all other factors aside, size of sensor (negative) does matter.

Not if all shots are made with the same camera.

gasrocks wrote in post #15921939
Sorry, but all other factors aside, size of sensor (negative) does matter.

but if you took a picture at 50mm, f2 and 6 ft away on a crop, and on a FF camera...would the larger sensor size really give you shallower DOF...

no...it'd actually give you more DOF...