DX to FX: effect on DoF?

[photom]

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<p><br />I hope I make myself clear, but I guess that I'm asking if someone could explain to me how many stops difference it is between DX and FX with a given desired DoF?<br />Thanx!<br />Ray</p>

 

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<p>Most people estimate the iso "light gathering" benefit (if that phrase is OK) to be about 1 to 1.5 stops going from a good APS-C to FF. As to DOF, the difference really depends on what type of photos you take. For many photos, there will not be any practical difference. However, if you take a lot of portraits and love the perspective from about 12 ft. on film, then you will be disappointed that Nikon and Canon do not make a fast portrait lens that is 85/1.5(1.6) in focal length. In the DOF "battle" FF comes out ahead slightly if you want shallow DOF, because there are no special APS-C lenses that are a stop faster. If you want more DOF the practical answer is it is about a draw.</p>

[darrengold]

<p>For clarification why don't we look at the situation in a different way.<br>

If we take a picture with a 50mm lens at f5.6 from 10 feet on an FX camera we will get a picture with a specific DOF. Then we print the picture. Now we mask off the border of the image as though it had been taken on a DX camera. Same lens, same aperture and same distance - because it's the same print. Then we cut round it with a pair of scissors. The new DX print has the same DOF as the original. It must have, it's the same print. We haven't changed anything.<br>

Now, we take the same setup on a DX camera (50mm, f5.6) and move further way from the original subject matter to get an image that contains exactly the same information on the DX sensor that we had on the FX sensor. Because we are further way the DOF has become greater.<br>

This is why it is considered that FX has less DOF than DX. It is all do with the subject distance used to acquire the image size that we are looking for.<br>

Any given lens, at a given aperture, at given subject distance, has the same DOF on any format. Irrespective.<br>

It has nothing to do with the sensor at all. We just proved it with a pair of scissors.<br>

Cheers.</p>

[ilkka_nissila]

<p><em>Then we cut around it with a pair of scissors. The new DX print has the same DOF as the original.</em></p>

<p>It doesn't work like that. You must first enlarge the cropped section to match the same print size or alternatively look at the print at a closer distance so that the viewing angle is the same (as when viewing the original, uncropped print). Now you look at the size of the blur in out of focus areas and compare it to the frame size to determine if the area is acceptably in focus. You will find that the depth of field of the cropped print is shallower since less of the print is sufficiently sharp (relative to the frame size).</p>

[kelly_flanigan1]

<p>Again; </p>

<p>50mm lens can be used over many different formats; with smaller ones typically having a tighter criteria. A 50mm lens used on a tight pitched dlsr or 8mm cine camera has a tight criteria; a disposable camera for 4x6" prints has a loose criteria.</p>

<p>Same goes with a knife; THE SAME KNIFE can be considered sharp; average; or dull. It depends on what the heck you are going to use it for.</p>

<p>Unless one has some actual goals; you are going to be confused trying to get simple answers.</p>

<p>In pro work; there is an actual client; and actual goal; thus with an image the DOF *for that image* can be figured out.</p>

<p>DOF tables are a guide.</p>

<p>Even if one uses a FX format with a sensor pitch of "X" microns; a 50mm lens at F8 focused at 10 feet; the criteria can vary by what the *PURPOSE* the image is for. If the clients goals are just 600x800 pixel web images; the DOF tables are often way to tight. If you are shooting an image that will be enlarged and one places ones nose into it; the stock DOF tables are not tight enough.</p>

<p>Since amateurs typically have no hard and fast goals for their images; they get confused. It is like asking how sharp a knife has to be; but you cannot fathom if you are cutting butter; balsa wood; oak or are doing surgery; or shaving.</p>

<p>Amateurs always get confused about theses grey scale type questions; since they seek a B&W simpleton answer to a greyscale type problem.</p>

<p>Going "out of focus" is not a Black and White issue; it is a gradual one</p>

<p>A DX can be higher or lower pitch than an FX camera. One has digital slrs in the 35mm format going</p>

<p> back to just 1.3 Megapixels; thus a broad brush statement is lay; ie for the assuming types.</p>

<p>Strive to use some actual camera models and some actual goals and actual viewing distances.</p>

<p>None of this is really anything new; it was FULLY understood before any of us were born; in an era when folks assumed less; and had actual goals.</p>

<p>Without any goals it is impossible to sweat subtle DOF numbers; it is basically total BS. With some actual goals one gets a radically better feel for what works.</p>

<p>DOF requires a criteria for what is acceptable; this varies by how the image is used.</p>