Understanding the focal length 1.5 conversion rate from lenses on an FX sensor to a DX sensor?

[brodie_thomson]

<p>I'm pretty sure i've misunderstood something in here, so please do correct me if i'm wrong. In my interest to purchase a Nikkor AF-S 70-300mm VR f/4.5-5.6 lens for my Nikon D60 DLSR <strong>(which has a DX format sensor)</strong>, i've been told in forums as well as reading it in reviews that the 70-300mm focal length is <em>effectively </em>equal to a focal length of 105-450mm <strong>when put on a camera with a DX sensor - </strong>which is worked out to be a 1.5 rate of conversion I suppose? I roughly understand the whole logic of this because of the crop factor: DX sensors being smaller then FX sensors. But what I don't understand is if this conversion applies to all lenses on DX sensors? OR does it have something to do with what i often hear; the AF-S 70-300mm VR being endorsed as a lens that is still compatible if someone upgrades to a FX camera (full frame).<br>

Thanks</p>

[brodie_thomson]

<p>I'm starting to think it may be the other way around. The focal length of a lens on a full frame (FX) sensor being 1.5 times what it is on a DX sensor. Hopefully someone can clear this up</p>

[rich_b1]

<p>Other than the actual focal length of the lens, the "equivalent" field of view of the lens is not lens dependent, but is sensor-size dependent. The difference with "digital only" lenses is that they have a smaller circle of light since they only need to accomodate the smaller sensor. Full-frame lenses have a larger circle of light, only part of which is used by the APS-C sensor, but the full circle is used by the full frame. Sort of like shining a pen light or a full size flash light at a mirror. The regular flash light will have a much wider beam than the small pen light.<br>

In relationship to the sensor, let's say you shine a flashlight on a hand mirror and a wall mirror. The actual size of the flashlight beam is exactly the same, however, it may cover 90% of the hand mirror, while only covering 25% of the wall mirror. This is the same principle as on an APS-C sensor (Hand mirror) vs. a full-frame sensor (wall mirror). The image size is the same, but the field of view is what differs.<br>

A very good explanation is here: http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html</p>

[brodie_thomson]

<p>Thanks for your response Rich, that clears up the theory. I'm still a bit unclear on how this applies practically with the lenses I'm talking about. Does this conversion apply to all lenses on DX sensors? OR does it have something to do with what i often hear; the AF-S 70-300mm VR being endorsed as a lens that is still compatible if someone upgrades to a FX camera (full frame). Because I imagine an 18-55mm lens would not be equal to 27-82.5mm. Is the 70-300mm being reffered to as being equal to 105-400mm on a DX sensor <strong>just</strong> because it is a lens that is marketed for full frame sensors also?</p>

[NetR]

<p>Hi Brodie. Focal length is a factor of optics - it is a constant for a specific optical design, but the angle of view will vary depending on the format you use. For example, a 50mm lens is a medium telephoto on a D60, normal on an F5, wide on a Pentax 645 and ultrawide on a 4x5 inch camera. The 70-300 AFS VR lens is designed to cover the full 35mm film/FX sensor - it isn't just "marketed" as such. So on your D60 it will give you image magnification the same as if you were using a 105-450 lens on a 35mm film/FX sensor camera. ANY lens will give you a 1.5x effective crop factor like that. The DX lenses are designed to only cover the APS-C sensor and most will not cover the FX format at any focal length, but FX lenses will always cover the smaller sensors, at the cost of extra weight to carry around. Hope this helps, Ross</p>

[ian_tindale]

<p >The focal length obviously stays the same as what it says on the lens itself — that’s the focal length of the lens no matter what you do with that lens or what you attach it to. However, the DX sensor is smaller than the full frame of a 135 film, so it’s like taking a smaller crop into the centre of the picture.</p>

<p >With 135 film, if you take a scene with a standard 50mm lens, and crop it down to concentrate on whatever the subject is in the middle, losing the extraneous nonsense around the outside of the scene, that’s pretty much the same as if you took the 50mm lens off and put a 75mm lens on instead, to isolate the subject from its surroundings so you don’t have to crop the picture.</p>

<p >DX is like that — it’s only going to cover the bit in the middle of what would have been a 135 equivalent frame, so it’s as if you’re using a tighter lens. Of course, the bit in the middle is what DX considers the whole frame, so you get your whole frame, but with a tighter crop.</p>

[brodie_thomson]

<p>Thanks Ross, that explains everything<br>

And thanks Ian. I had never thought of it like that, makes sense though</p>

[frank_skomial]

<p>"on your D60 it will give you image magnification the same as if you were using a 105-450 lens on a 35mm film/FX sensor camera. " - ?</p>

<p>- actually the lens has the same magnification on FX and on DX camera. That is the lens does not develop any magic if installed on DX. Image size of your objects will be the same on both sensors. </p>

<p>That is, the lens will not bring your subject any closer on DX camera, than on FX camera. You could just see the smaller cropped area stretched over entire viewfinder in camera, or may be not so - subject of the camera viewer optics. E.g. on D70 with smaller "channel vision" viewfinder you will just see central portion unmagnified.</p>

<p>The magification will be your own doing when the smaller picture of narrower angle of view from DX camera will get magnified by the equivalent 1.5 value for display on a computer screeen, or for printing on an equivalent paper size. </p>

<p>You will just see 1.5 times smaller central part of the picture area of the FX camera, enlarged for you, after the picture was taken (for print, or on computer screen viewing), or during viewing on camera viewfinders that have some optical magnification built-in the camera viewer just for better viewing, that does not affect the picture on the sensor.</p>

[frank_skomial]

<p>The central portion of the DX cropped image on FX camera will be the same physical size on DX image. </p>

<p> However, due to possible larger pixel density in D60 camera, the picture size counted in pixels will be larger on D60 than on e.g. D700 FX cropped picture size camera, and about the same size on a D3X FX size camera. You will be getting magnification caused by denser pixels on DX but you will be trading pixel size/quality for pixel quantity, but that may not matter in good lighting conditions?</p>

[dbcooper]

<p>1) Pixel density or pixel quality has nothing to do with this issue whatsoever. It's strictly about the sensor's physical size and the size of a given lens' projected image circle.<br /> 2) A lens' marked focal length doesn't change depending on the camera it's mounted to. Ever.<br /> 3) DX sensors are physically smaller than FX sensors (about 16x24mm, or 2/3 the size of an FX sensor or a 35mm film frame, which are 24x36mm).<br /> 4) You can use FX and 35mm film lenses on DX cameras freely (assuming mount and electronic compatibility), but there may be problems using DX lenses on FX cameras, and it might not be possible without severe vignetting, depending on the camera. It is not possible to use DX lenses on any 35mm film cameras without severe vignetting.</p>

<blockquote>

<p>But what I don't understand is if this conversion applies to all lenses on DX sensors?</p>

</blockquote>

<p>It indeed applies to <em>all </em> lenses mounted on DX cameras, due to the smaller sensor size. A DX sensor effectively crops the image, because a DX sensor is physically smaller than an FX sensor or a 35mm film frame.</p>

<p>DX lenses are designed to <em>just</em> cover the smaller DX sensor's corners with their image circles, while 35mm film/FX lenses are designed to cover an area that's <em>at least </em> 24 x 36mm with their image circles (24 x 36mm being a 35mm film frame or FX sensor size). Film/FX lenses' image circles will cover the smaller DX sensors with lots of room to spare.</p>

<blockquote>

<p>OR does it have something to do with what i often hear; the AF-S 70-300mm VR being endorsed as a lens that is still compatible if someone upgrades to a FX camera (full frame).</p>

</blockquote>

<p>If a DX lens is used on an FX camera, it will vignette badly because DX lenses' image circles will not cover an FX sensor, unless the FX camera can emulate a DX sensor electronically (my D700 can do this).</p>

<p>FX lenses and 35mm film lenses behave normally on an FX camera, and can be used freely, assuming the mount and electronics are compatible (some older film lenses can damage newer cameras if mounted).</p>

<p>Conveniently, most FX cameras will emulate a DX sensor, and one can use DX lenses with them. In that case, the 'crop factor' applies, and the megapixel count drops, because only the central 2/3 of the FX sensor is being used.</p>

<p>I can use my 80-400 VR zoom (designed for film cameras) on either my D700 (FX) or D200 (DX) with no internal adjustment in either camera. Due to the DX sensor's smaller size in the D200, it's the equivalent of a 120-600 zoom on that camera. On my D700, it performs at its marked focal length(s) of 80-400mm.</p>

<p>For a visual and more in-depth description, see http://www.kenrockwell.com/tech/crop-factor.htm</p>

<p> </p>

[Alan Marcus]

<p>To understand crop factor (magnification factor) one must appreciate what is “normal” – “wide-angle” – “telephoto”. These are conventions based on the working focal length of the camera lens relative to the format dimensions of the film or digital chip inside the camera. First I must prove to you what “normal” is. Stand before a glass window and gaze at a landscape. With wax pencil, outline on the glass, a generalization of the shapes you see. This drawing, as to perspective, is the human experience. This “normal” view is replicated by the camera when the mounted lens has a focal length that is about equal to the diagonal measure of the film/chip.<br>

For the 8x10 view camera this is 325mm. For the 5x7 camera this is 220mm. For the 4x5 camera this is 160mm. For the 8 exposure 120/620 camera this is 100mm. For the 120/620 12 exposure this will be 80mm. For the 35mm full frame this will be 43mm. For the APS-C digital this will be 30mm.<br>

All have in common an angle of view of 53º said to duplicate the human experience. Thus over the years, it has been the convention to mount a camera with a lens equal to the diagonal measure of the format.<br>

The full frame using 35mm wide film was the brain child of Ernst Leitz and his Leica camera of 1913. The format established by his chief engineer Mr. Oskar Barnack, was 24mm by 36mm. the diagonal measure is 43.27mm. Because it was an odd size, the Leica was outfitted with a 50mm as “normal”. Such tiny departures are of no consequence.<br>

By 1996 Kodak was coating professional movie film with a transparent magnetic overcoat. It was great for movie sound tracks. In that year Kodak, Fuji, Minolta, Nikon and Canon marketed a new format. The Advance Photo System (APS) was based on this magnetic overcoat. The idea was a hybrid film/digital system. Format size was 16mm by 25mm on a film 24mm wide; the diagonal measure is 30mm. This idea failed but the format lingers in the compact frame size digitals.<br>

Note 43.27 ÷ 28.84 = 1.5 the infamous focal length multiplier or magnification factor or crop factor. It simply means the full frame 35mm format is 1.5 times larger than the APS or conversely the APS is 66% of the size of a 35mm full frame.<br>

Now a wide-angle is 70% of normal or shorter. Thus for the 35mm full frame, wide-angle is 43 x 0.70 = 30mm or shorter. For the APS it is 30 x 0.70 = 20mm or shorter.<br>

Now for telephoto, it is 200% of normal or longer. Thus for the 35mm full frame, telephoto is 43 x 2 = 85mm or longer. For the APS it is 30 x 2 = 60mm or longer.<br>

These generalities hold for all format sizes regardless.<br>

Sidebar: Portrait is 2.5 times normal or longer. For the full frame 35mm it is 105mm. For the APS it is 75mm or longer.<br>

If you own lenses for the full frame 35mm format, and you mount them on an APS size camera, only the central part of the image will be seen, the edges will be baffled off by the camera’s internal mask. The image will appear to be cropped or magnified. In actuality the lens focal length is not changed. This is like setting up a slide projector in an auditorium and getting everything adjusted to fill the screen. Suddenly a professor comes and says the screen you are using is his, he takes it away. In the closet is a smaller but useable screen. You quickly set it up only to discover that the smaller screen crops top and bottom and both sides. Image size is the same but the smaller screen seems to have increased the apparent magnification of the set-up.<br>

Nobody said this stuff is easy.</p>