DX vs FX

[gene thornton]

<p>There must be some formula to convert the equal size seen by the sensor between DX and FX, I can not note an exact match saying 300mm FX = 450mm DX. then looking at some macro lens some list 60mm + 93mm and I noted where 85mm = 105mm. Now is there some formula to figure this out, I hardly think different manufacturers of the same focal length lens would offer a different size image on the sensor</p>

<p>Thanks</p>

[Matt Laur]

<p>The DX format crops out a piece of the FX format. It's a "1.5x" crop, and that's quite accurate in practical terms. Your 50mm lens on DX will look like a 75mm lens on FX.<br /><br />BUT: remember that the "actual" focal length of a lens can change significantly as a function of working distance. This can get especially complex with some zoom lenses used near or at their minimal focus distances.<br /><br />Remember that using a DX body doesn't change focal length. It's exactly the same as it is on FX. The difference is that you're just looking at a smaller field of view. So any use of a lens (say, at close focusing distances) that alters the effective focus length on DX will also do so on FX... because the lens doesn't care what camera you're putting it on.</p>

[lornesunley]

<p>Actually, the sensor sizes on cameras from different manufacturers are different sizes, or they might be the same. You have to check the size of each one. The field of view is what changes with different sensor sizes NOT the focal length of the lens. According to some reports Canon makes three different sensor sizes Full frame (35mm film equivalent), 1.3 and 1.6 crop factor. Nikon does FF and 1.5.</p>

[Matt Laur]

<p>I assumed that since he's using Nikon's terms ("FX" and "DX") that he's talking about Nikon bodies, which only have one crop flavor.</p>

[tony_leinster]

<p>I find that an easier way for some people to visualise this is to think of an old fashioned 35mm negative. A full frame sensor gives you all of this neg area to work with. A crop camera, effectively crops out a central portion of the neg and only allows you to work with that. The area varies from one manufacturer to another.So for instance on a Nikon a full frame is 35.9mm x 24mm. A crop camera will effectively "cut out" an area 23.1mm x 15.4mm of that and that is what you use.</p>

[dbcooper]

<p>Gene, if you want to do the math, it goes something like this:</p>

<ul>

<li>Verify the aspect ratio is the same so you're comparing apples to apples (35mm film and Nikon FX = 24mm x 36mm; or 2:3 aspect ratio [24/36; reduce to simplest terms]) Note: Nikon FX is actually 24mm x 35.9mm, but I'm rounding off here for simplification.</li>

<li>Determine the sensor dimensions (Nikon DX = 15.6mm x 23.6mm; close enough to 2:3 ratio, so far so good)</li>

<li>Take either the long or short sides and divide the larger number by the smaller one.</li>

<li>The answer is the 'crop factor' to multiply the focal length of any lens to determine it's equivalent angle of view with the 'crop sensor' camera (strictly speaking, the focal length of a lens does not change).</li>

</ul>

<p>Example:<br /> 35mm film and Nikon FX = 24mm x 36mm film or sensor dimensions; Nikon DX = 16mm x 24mm sensor dimensions<br /> 24/36 and 16/24 both are 2:3, so far so good<br /> 24 ÷ 15.6 = 1.54 (using short sides) or,<br /> 36 ÷ 24 = 1.53 (using long sides)<br /> focal length x either 1.54 or 1.53 = plenty close enough to estimate the DX equivalent angle of view; most folks just use 1.5x instead because it's simpler to figure out in your head</p>

<p>Lorne was correct in noting that Canon 'crop sensors' factors can be 1.3x or 1.6x depending on the camera model, hence there is a difference between manufacturers. That's is something aftermarket lens manufacturers (e.g. Sigma, Tamron, Tokina) must take into account, but not Canon or Nikon since neither of them manufacture lenses for the other's cameras. It's not really so much the lens designs themselves that change, but rather the mount-to-sensor distance changes significantly between camera brands, which is probably why aftermarket companies don't make lenses with interchangeable mounts like they used to for 35mm film cameras, and also why you can get adapters to mount Nikon lenses on Canon DSLRs, but not vice versa). HTH.</p>

[gene thornton]

<p>Well yes to Matt's response I am using Nikon's. And I fully understand and appreciate DB Coopers explanation. But What is confusing me was a 300mm lens used on a DX format camera will be equal to 450 mm as opposed to using the same lens on an FX camera providing only a magnification factor of 300 mm. So say the advertisements?<br>

You gotta admit it gets confusing</p>

<p> </p>

[ljwest]

<p>In day-to-day use, you really do not need to worry about whether your camera has a full frame sensor, or is a 1.3x, 1.5x or 1.6x crop, <em><strong>unless</strong></em>, you regularly use cameras that have different sensor sizes. Then, and only then, do you need to worry about it, as that full frame camera with the 50mm lens take a different picture of a subject at a given distance than the crop sensor camera with the same lens and at the same distance! In any case, your eye will tell you as soon as you bring the camera up to it, as the optics in the camera's viewfinder make sure that what you see through the viewfinder is approximately the same as what the sensor sees.</p>

<p>When buying lenses, if you only have one camera, all you need to know is that the lens you have isn't doing something for you, so you shop for a new one that will. Maybe it doesn't get wide enough, so you look for one with a shorter focal length, or maybe it doesn't have enough reach, so you look for one with a longer focal length (there are other reasons to look for new lenses, too). For example, when I was shopping for a telephoto zoom for my Canon 7D, I wasn't constantly calculating the "effective" focal length in my head for the lenses I was looking at. I had the 18-135 kit zoom, and wanted something that got longer for wildlife and sports shots, so I shopped in the 70-300 range, and didn't think once that all those lenses were "effectively" 112mm-480mm on my 1.6 crop factor camera. On the other hand, I'm not at all unhappy about the longer "effective" reach of my new lens!</p>

<p>This is really nothing new to photography. Years ago (well, even today...), if a photographer had both 35mm and medium-format cameras, they needed to be aware of the difference in focal length when moving between formats. A 50mm focal length was considered "normal" on a 35mm camera, but the same focal length would be more of a "wide angle" with medium format cameras, because more image is captured on the greater film area. But even then, it would become second-nature in a very short time.</p>

[dbcooper]

<blockquote>

<p>But What is confusing me was a 300mm lens used on a DX format camera will be equal to 450 mm as opposed to using the same lens on an FX camera providing only a magnification factor of 300 mm.</p>

</blockquote>

<p>It is not magnification, ever. It's the difference in the sensor's field of view only. You can think of it as cropping the photo from an FX camera by 2/3, then enlarging the result. When you enlarge the resulting view to match the physical dimensions of a full-view print from the FX camera, it only <em>appears</em> to have been shot using a longer focal length lens.</p>

<p>The difference is that the sensor on a DX camera is physically smaller, and 'sees' the cropped view <em>at capture</em>. If you print the resulting images from both cameras (i.e. DX and FX) to the same physical dimensions (e.g. 8 1/2" x 11"), the DX image will appear to have been shot with a longer focal length lens. If you make the prints from both cameras so that an object in the scene is the same physical size in both prints, the DX print will be smaller than the FX print; the DX print will appear to be a cropped version of the FX print. The 'crop factor' is simply a factor for estimating the equivalent angle of view between sensor formats for a given focal length lens. If an advertisement uses the word 'magnification' (or the like), it's incorrect.</p>

[dan_south]

<p>DX doesn't magnify anything. It simply crops an image out of the middle of a larger (FX) image, but it does so without the reduction in resolution that would happen if you actually did the crop.</p>