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confusion between DX and FX lense Focal length

Sun P , Jun 18, 2009; 06:51 a.m.

Hi experts,

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.

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).

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

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!

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?

Please advice



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Robert Davies , Jun 18, 2009; 07:12 a.m.

Firstly the focal length of the lens remains the same for whatever format.
On DX the 50mm will have the same angle of view as a 75mm lens on FX, so the 1.5x is correct.
However, as others will probably chime in, it isn't as simple as that. :)

Matthew Brennan , Jun 18, 2009; 07:19 a.m.


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.

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.

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, when mounted to a DX body - any lens be it DX designated lenses or 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.

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.

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

Jason Richardson , Jun 18, 2009; 08:27 a.m.

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?

Matt Laur , Jun 18, 2009; 08:42 a.m.

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.

To be clear: a 70mm lens mounted on a DX body will result in the same angle of view 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.

Kent Shafer , Jun 18, 2009; 08:48 a.m.

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.

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

John Ashby , Jun 18, 2009; 08:57 a.m.

However, as others will probably chime in, it isn't as simple as that.

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.

I find it funny when cheap point and shoots talk in 35mm equivalents.

Michael R. Freeman , Jun 18, 2009; 08:59 a.m.

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.

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 exact same framing 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.

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?

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".

Ilkka Nissila , Jun 18, 2009; 09:46 a.m.

Nikon lenses have worldwide warranty so even if you purchase one in the US or UK, it will still be respected by Nikon in your country. As long as it was not gray market in the country of purchase.

William Pahnelas , Jun 18, 2009; 10:39 a.m.

this is a really good resource regarding the field of view difference between DX and FX bodies:

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