Can 24mm cropped be equivalent to 28mm? (24 f/2.8 IS vs 28 f/2.8 IS)

misolo wrote:

dsjtecserv wrote:

misolo wrote:

If you make an exposure with the 24mm at f/2.8 and crop to the field of view of 28mm, you'll have something very similar to the image you'd have gotten with the 28mm at around f/3.3. With the 6D you'll also be down to 15MP from 20MP.

Actually, though counter-intuitively, it would be slightly the other way around: the depth of field on the cropped version would be slightly less, as if it had been taken with a slightly larger aperture (about f/2.4). The size of the blur circles recorded on the sensor itself would, of course, not be different, since both the uncropped and cropped version were made with a 24 mm lens at f/2.8. But because cropping the 24 mm image to the equivalent FOV of 28 mm makes the image smaller, the cropped version needs to be enlarged more in order to produce the standard print size for which depth of field calculations are calibrated. That enlarges the blur circles more, with the net effect of lower perceived depth of field, for any given viewing distance.

This is exactly the same as using the 24 mm lens with a smaller sensor, with a crop factor of 1.167 compared to the larger "full frame" sensor. So just as the circle of confusion of 0.030 usually used for full frame cameras needs to be adjusted to 0.019 for a 1.6 crop camera, it would need to be adjusted to 0.030 / 1.167 =~0.26 for a full frame image taken at 24 mm but cropped to 28 mm equivalent field of view. Given the same focal length, f-number and focus distance, that decrease in CoC translates into less depth of field.

It is still true that use of a smaller "cropped" sensor is associated with greater depth field, because in order to produce the same framing, either a shorter focal length or greater subject distance would be needed. Both of those offset the decrease in CoC and result in a net increase in depth of field. But this case is different -- it was stipulated that the lens, aperture and subject distance would be the same, and cropping after the fact would be used to achieve the desired framing. In that case, only the effect of CoC is "in play" and Dof is slightly reduced.

Oh the fun one can have when noodling around with DoF calculators!

Dave

Sometimes something is counter-intuitive simply because it is completely wrong...

I suggest you simply try it with your camera.

But if you really prefer online depth of field calculators try, say,

http://www.dofmaster.com/dofjs.html

with the settings:

7D, 24mm, f/2.8, distance 2m --> depth of field 0.76m

I'm sorry, you are correct; my original post was misleading and was intended to make a different point. Indeed a 24 mm f/2.8 shot cropped to 28 mm would have the field of view and depth of field similar to one shot with a 28 mm lens at f/3.3. My point was that, counter-intuitively, this is less depth of field, because the only thing that changed as a result of the cropping was the needed degree of enlargement. I shouldn't have implied that your equivalence was wrong, but rather emphasized the more surprising element of that.

So to put numbers to it:

Full frame camera, 24 mm, f/2.8, focus distance 10 ft, Circle of confusion 0.030: DoF = 11.1 ft

Full frame camera, 28 mm f/3.4, focus distance 10 ft, circle of confusion 0.030: Dof = 9.16 ft

Cropped scenario, where only CoC is adjusted to reflect greater enlargement:

Full frame camera, 24 mm, f/2.8, focus distance 10 ft, Circle of confusion 0.026: DoF = about 9.07 ft

And, as noted earlier, one observation is that when a cropped image is used (either by the sensor of after the fact) without changing focal length, f-number or focus distance, depth of field decreases.

Sorry for the confusion.

Dave