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confusion between DX and FX lense Focal length


sun_p

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<p>Hi experts,</p>

<p> I have the nikon D40 and 18-55 kit lens. I was looking at buying a good portrait lens and as per expert advice I had two options, 50-150 sigma and 28-75 F2.8 Tamron. However, both these lenses are just not available here and will have to wait for someone travelling to the US or UK to get it but then the problem is if there is an issue then I am stuck.. Now, I am having a change of mind and thinking of playing it safe and going in for the 50mm 1.4AF/S and probably 105 F2.8 nikkor prime lenses so that even if something is wrong, atleast I have authorised service stations here. Tamron/Sigma are not yet here and i am getting grey models for ridiculous prices without warranty.</p>

<p>1. The reason I am choosing 50mm1.4AF/S is I can shoot in Low light and also i can stop up to shoot regulr protraits indoor under stobes. My space restriction is 18 feet by 7 feet (I am managing).</p>

<p>2. The reason I am choosing 105 2.8 is so that I can get some good face shots and probably shoulder length shots under strobes indoors and can be used outside also in</p>

<p>3. Both are nikon and robust in build without many moving parts and also I have proper repair facilitites. The other zooms etc are ridiculously expensive here, almost half the cost of a car!</p>

<p>4. My 4 issue is with focal length. I am a little confused about the lengths. The crop factor mentioned for DX bodies, is that the real FL or we need to manually calculate. E.g, the 18-55 kit lens for my d40 when I take a shot, in the Exif data I see FL as 50 when I shoot at 50. So does that mean that the snap is shot at 50 FL or do I need to multiply that by 1.5 or is the snap already cropped at 50mm! What I was trying to ask is that are all lenses expecially for DX (AF-S) ones showing the FL the correct one or are they shown for FX bodies and we need to multiply. Does it mean that my Kit DX 18*55 is actually 18*1.5 to 55*1.5?</p>

<p>Please advice</p>

<p>Thanks,<br>

Sunil</p>

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<p>Firstly the focal length of the lens remains the same for whatever format.<br>

On DX the 50mm will have the same angle of view as a 75mm lens on FX, so the 1.5x is correct.<br>

However, as others will probably chime in, it isn't as simple as that. :)</p>

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<p>Sunil,</p>

<p>All Nikon lenses indicate the same focal length be they DX lenses or FX lenses - the actual focal length of any given lens remains the same wether it be mounted on a DX body or an FX body.</p>

<p>On DX bodies there is the crop factor or multiplication factor (1.5x in Nikon's case) as you are aware. DX designated lenses are specially designed to produce a smaller circle of light through the lens which only covers the smaller foramt DX sensors but fails to 100% cover the larger format FX sensors.</p>

<p>A 50mm f/1.4 lens will produce a 50mm frame on an FX body but on a DX body the same lens will crop the frame to effectively give the equivalent of a 75mm frame on FX. So in effect, <em><strong>when mounted to a DX body</strong></em> - any lens be it DX designated lenses <strong>or</strong> other non designated DX lenses like the Ai, AiS, AF, AF-D, AF-S etc will produce an FX equivalent focal length imaged frame 1.5 x l'onger' (or tighter) than the actual lens focal length. So your current 18-55mm kit zoom is actually giving you what would be in FX equivalency a focal range of 27mm - 82mm, therefore your last line in this post is indeed correct.</p>

<p>To put it another way - I have a D700 FX body camera / you have a D40 DX body camera. If you and I were to stand side by side with a 50mm f/1.4 lens on our respective cameras, and we shoot the same subject in front of us - my camera will produce an image which is 1.5 x wider framed than your cameras image or alternatively, your camera will produce an image which is 1.5 times tighter framed than my camera.</p>

<p>I think 50mm focal length will suit you well given your stated studio dimensions.</p>

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<p>Can someone answer this, please? There are DX and FX lenses. If I have a DX body and a DX lens, say the 18-105, I actually get 18-105. But if I put a FX lens on there, like the 24-70, that is different than using a 24-70 if it were built for a DX camera, right? Putting the Nikon 24-70 on a DX body is really like putting a ~35-105 DX lens on it, right? So the 70mm on the current FX 24-70 lens would frame up exactly like the 18-105 DX lens at 105mm. Is that correct?</p>
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<p>Jason: as mentioned above, at 24-70 is a 24-70 no matter what camera you put it on. When you use a DX-format sensor, you're just recording a smaller piece of the image projected by the lens into the camera body.<br /><br />To be clear: a 70mm lens mounted on a DX body will result in the same <em>angle of view</em> as a 105mm lens mounted on an FX format body. When you put an 18-105 on a DX body, you'd need to use a 27-157 on an FX body to see the same angle of view.</p>
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<p>Jason, no. The 24-70 at 70 would frame up the same as the 18-105 at 70. The only difference between DX lenses and all the rest is that DX lenses generally won't cover a full frame (film or FX digital). See Matthew's post for more details.</p>

<p>If you only use a DX camera, you can just ignore the DX/non-DX distinction.</p>

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<blockquote>

<p>However, as others will probably chime in, it isn't as simple as that.</p>

</blockquote>

<p>Why not :). It's a crop factor not a change in focal length. So if you compare a 200mm on a DX body to a 300mm on an FX body, you won't have the same depth of field and you won't have the same background compression.</p>

<p>I find it funny when cheap point and shoots talk in 35mm equivalents.</p>

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<p>Unless you use, or have used, both formats and want to compare 35mm/FX angles of view with DX angles of view, forget about crop factors. They're meaningless and only serve to cause confusion.</p>

<p>50mm is 50mm on DX, FX, 35mm, 120, etc. The new AF-S 35mm f/1.8 DX and the "FX" AF 35mm f/2D for example, will give the <strong>exact same framing</strong> when mounted on a DX body. They'll also give the exact same framing when mounted on an FX or 35mm body, but the DX lens will not project a large enough image circle to cover the format, so there will be severe corner vignetting.</p>

 

<blockquote>

<p>Putting the Nikon 24-70 on a DX body is really like putting a ~35-105 DX lens on it, right? So the 70mm on the current FX 24-70 lens would frame up exactly like the 18-105 DX lens at 105mm. Is that correct?</p>

</blockquote>

<p>No. The FX 24-70 at 70mm will frame up exactly like the DX 18-105 lens at 70mm, not 105mm. Zoom the 18-105 from 24mm through 70mm, and that's the exact same framing you will see if you mounted the 24-70 on the same body and zoomed it through its full range. You will probably see slight differences in framing at close focus distances, but that has nothing to do with "crop factors".</p>

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<p>Okay, thanks for the clarification. Everytime I read about a FX lens on a DX, people always mention the other numbers (ex 24-70 roughly = 35-105). I just didn't understand why I should care. I'm not an FX shooter, so I guess I should disregard those statements. Thanks again.</p>
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<p>I read the link that William provided, and this is what I don't understand. Maybe I am mixing up words and saying the wrong thing. Quote from article above regarding using a FX lens on a DX camera:</p>

<blockquote>

<p>The image circle produced by the lens is larger than the sensor area, therefore the field of view is cropped and the image appears magnified. This means that the lens has an apparent focal length magnification of 1.5 times the quoted figure, the actual focal length of the lens has not changed.</p>

</blockquote>

<p>It sounds like that is saying that even though the lens is at a certain focal length, it appears as though it is not. It appears 1.5 times bigger. That's why I keep thinking that FX lens at 70 would produce the same framed image as a DX lens at 105.</p>

<p>I don't have any experience with this, so I am trying to learn. Sorry for all the questions.</p>

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<blockquote>

<p>It sounds like that is saying that even though the lens is at a certain focal length, it appears as though it is not. It appears 1.5 times bigger. That's why I keep thinking that FX lens at 70 would produce the same framed image as a DX lens at 105.</p>

 

</blockquote>

<p>In a nutshell, yes. As stated above, a 50mm lens is a 50mm lens. As long as the distance from the camera to the subject does not change, the only difference from one format (DX,FX,MF,LF, etc.) to another is one of cropping. Nothing is magnified, only the framing is different from one "sensor" size to another.<br>

Back in the film days, no one had much of a problem understanding the difference between 35mm and MF. Digital has apparently created a"problem". Therefore, digital is bad. ;-) (That was a joke!)</p>

<p>Good luck, Doug</p>

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<p>Jason: you're in the right conceptual neighborhood, you've just got it backwards.<br /><br />Take the example of a 50mm lens. A simple prime. It projects an image into an DX body that would allow you (with the camera in portrait/vertical orientation) to fill the frame with a standing human figure from about 15 feet away.<br /><br />Now, stand in exactly the same place, and use exactly the same lens, but put it on an FX body. The lens is projecting the exact same image into the body (because the lens hasn't changed), but now that projected image is falling on a bigger sensor. Some of that image was being ignored on the DX's smaller sensor, but is being recorded on the FX's larger one. So that means that your human figure no longer fills the frame, but instead has more scenery around it.<br /><br />In order for the FX camera's framing (with regard to how much of the frame the human figure fills) to look like the DX's, you'd have to walk closer to the person, to again make them fill the larger frame. But that also changes your perspective. Your other option (on the FX body) would be to use a longer lens... so that your human figure again fills the frame, but your perspective (which is a function of how close you are to the figure, and has nothing to do with lenses and cameras) remains the same.<br /><br />So in that scenario, the DX camera can fill the frame with a human from 15 feet while using a 50mm lens, but the FX camera needs a 75mm lens to make that figure land on the sensor in the same proportion.</p>
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<p>You must be thoroughly confused by now !<br>

It is all based around the diagonal of the sensor or film and a lens that has a focal length of the diagonal is deemed normal and has a 40 deg angle of view regardless of camera.<br>

An 8x10 camera is around 12" A 4x5 is 160mm 6x9 is 100 mm FX is 43 mm DX is 30 mm. All are approximate as I am to lazy to figure exactly.<br>

All see the same angle of view if you look through the camera<br>

A crop factor is just a way to to tell 35 mm film photographers what focal length they need to make the DX camera see the same as a film camera. It is about 1.5 x. So if you like 50 mm on film, get a 35 mm for DX. If you like 105 on film, get 70 for DX. </p>

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So if I use a 70mm/5.6 on DX camera, and a 105mm/5.6 on a FX camera, the only difference will be the depth of view. May I concude that the DOF on a DX camera is less, because you are nearer to the subject?
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<blockquote>

<p>Ted, I think that's <em>field of view</em> that varies, not depth of field.</p>

</blockquote>

<p>Yes.. because the focal length changes, the field of view change, just as if switching to a longer lens.<br /> <br /> But smaller sensors do provide a larger depth of field.</p>

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<blockquote>

<p>So if you compare a 200mm on a DX body to a 300mm on an FX body, you won't have the same depth of field and you won't have the same background compression.</p>

</blockquote>

<p>This is incorrect. The depth of field will be different at the same f stop, but telephoto compression depends only on field of view and has nothing directly to do with focal length. So those two examples will have the same amount of compression.</p>

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<p>Joe C,<br>

So you're saying if I use an FX camera and take an image with a 200mm lens, then crop the image so I have the same *field of view* the lens would have given me with 300mm (which is what a DX body does), I will also have the same background compression I would have had if I shot at 300mm to begin with on the FX body? That doesn't make any sense. It would follow that taking any image and cropping it changes the background compression.</p>

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<blockquote>

<p>So you're saying if I use an FX camera and take an image with a 200mm lens, then crop the image so I have the same *field of view* the lens would have given me with 300mm (which is what a DX body does), I will also have the same background compression I would have had if I shot at 300mm to begin with on the FX body? That doesn't make any sense. It would follow that taking any image and cropping it changes the background compression.</p>

</blockquote>

<p>That is correct, cropping any image will increase the telephoto compression (or reduce the wide angle expansion if the cropped image is still wide angle). The perspective distortion is because of the difference in field of view between the camera (including any form of cropping) and the final photograph <em>as seen by the person looking at it.</em> Changing the viewing distance of a photograph also changes the perspective distortion. If one gets close enough to a wide angle photograph (and can still focus one's eyes from that close, it helps if the photograph is big) then the wide angle expansion disappears.</p>

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<p>Thanks for the links, all.</p>

<p>I just want clarification on one thing. The reason I'm asking is that I want more of a good portrait lens than what I have. I have a D90 and the 17-55 DX and 18-105 DX lenses. Will the Nikon 24-70 FX lens produce the same image in the viewfinder on my D90 at 35mm that my 17-55 lens produces set at 35mm? Will the 24-70 produce the same image in my viewfinder set at 70mm as my 18-105 lens produces at 70mm? I'm taking in regards to framing and magnification, not DOF, color rendetion, etc.</p>

<p>I don't think my 17-55mm is long enough for protraits at 55mm, and I would like a larger aperture then what my 18-105 provides. Although if the answers to my above questions are "yes", then I'm not sure 70mm is enough, either on the 24-70. I tend to stay above 70mm on my 18-105 when I am taking portrait-type pictures. All the confusion on my part led me to believe that the 24-70 would reach out to ~105 like my 18-105 lens does.</p>

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<p>Jason, any Nikon F-mount lens at 35mm, regardless of whether it is a fixed 35mm DX, a fixed 35mm non-DX, a DX zoom that covers 35mm, or a non-DX zoom that covers 35mm will produce "the same image" in the viewfinder on your D90. (In reality, the quality from each lens could be a bit different in terms of sharpness, flare resistence, etc., but you shouldn't be able to tell the different looking into the viewfinder perhaps except for flare resistence.)</p>

<p>As it has already been mentioned, whether a lens is DX or not indicates the size of its image circle. Otherwise a 35mm DX and a 35mm non-DX projects the same image (in terms of magnification) on the sensor.</p>

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