Nikon Lens Conversion Factor

What is the conversion factor to use when using a non-digital lens on a Nikon digital camera (I have a D70)?

Is it 1.5? For example, what does the 17-35mm non-digital lens equate to on a D70?

If the conversion factor is 1.5, it would be approx 25-52mm ... is that correct?

Also, I believe the digital lenses (DX) are already corrected for the cropped digial sensor. Is that right?

Thanks!

17mm lens is a 17mm lens is a 17mm lens. Focal lengh is a physical measurement(like the diameter or length it sticks out in front of the body) and has nothing to do with what camera you mount it on.

On your camera 17mm non-digital lens will be identical to a 17mm DX lens.

About the conversion factor thing. The conversion factor is a way of comparing the Angle of View between Cropped sensors and film (or full frame digital) cameras.

You can apply the crop factor like this: The 17-35mm lens on a D70 would act similar to a 25-52mm lens would on a full frame body. (yes it is 1.5 that you multiply by for nikon digital bodys)

This questions comes up here very often, and I made a little diagram to explain what happens:

As William says, the focal length of the lens does not change, only the field of view. The diagram above was drawn to explain 1:1 lenses, but the same principle holds to any lens. The larger sensor photographs a larger are of the image that is projected by the lens.

Thanks for the explanations.

If I understand correctly, a 16mm (non-DX) lens will act like a 24mm (16 X 1.5) lens due to the cropped sensor. However, the focal length (16mm) does not change.

In the case of a DX lens, a 16mm DX lens will act like a 16mm lens because it was designed to account for the cropped sensor.

Is this right?

Thanks again for sharing your knowledge and experience!

Almost there -- but 16mm is 16mm, DX or not. The DX lenses may have a smaller image circle, so that they "vignette" or show a falloff or even the edge of the image circle on a full sized (24 by 36 mm) sensor. This may be true for only part of the range on a zoom, as explained by Thom Galbraith in his review of the 17-55 DX AF-S zoom review, quoted briefly below:

"Light falloff is well under control throughout the range on digital bodies. On 35mm bodies, you'll see light fall off at all focal lengths and clear vignetting caused by the small image circle begins at about 26mm. In other words, you could use this lens on a full frame body from about 26-55mm in a pinch."

Also, some DX lenses may have limited clearance between the rear of the lens and the mirror/sensor/film plane stuff inside the camera -- and those would not be useable with some cameras not intended for the DX series.

All the best,

Chris

Thanks for the explanations. 

 

If I understand correctly, a 16mm (non-DX) lens will act like a 24mm (16 X 1.5) lens due to the cropped sensor.  However, the focal length (16mm) does not change.

 

In the case of a DX lens, a 16mm DX lens will act like a 16mm lens because it was designed to account for the cropped sensor. 

 

Is this right?

 

Thanks again for sharing your knowledge and experience!

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The 16mm DX lens will give the same field of view as a 16mm non-dx lens. They will both give the same field of view as a 24mm lens would give on a 35mm film camera.

While your statement was technically correct. "a 16mm DX lens will act like a 16mm lens", I think you still do not understand

With the exception of text "DX" written on the lens, on a Crop-Sensor camera a 16mm DX lens would be Identical in every way to a 16mm non-DX lens.

Just to clarify. Digital lenses - ie lenses designed for smaller digital formats such as the Nikon DX - should theoretically be designed with greater retrofocus incorporated into their design to ensure that the light strikes the sensor at as oblique an angle as possible. This is to reduce any problems such as vignetting in the corner of the frame. So whilst a lens designed for film may well have the same focal length as a full frame (35mm film of FF digital) lens, it may have somewhat different characteristics in terms of the circle it covers and the way it covers it. It probably isn't actually quite identical! This is why new designs are being produced at present.

Cropping a 35mm (film or FF digital) lens by using it on a DX camera may not produce quite as good a result as using a DX type lens although visually the image looks identical through the viewfinder. The image seen through the viewfinder on a DX camera will be the same as that seen in a 35mm (film of FF digital) when using a lens of 1.5 x focal length of the DX lens.

Just to clarify. Digital lenses - ie lenses designed for smaller digital formats such as the Nikon DX - should theoretically be designed with greater retrofocus incorporated into their design to ensure that the light strikes the sensor at as oblique an angle as possible.

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Paul, is this really the case with Nikon's DX lenses? I agree with you that it would be a good idea but is anyone actually doing this? I have heard about special lens coatings but nothing about pushing the elements out further.

Canon is taking the exact opposite approach for their crop sensor bodies. They are moving the lens closer to the sensor thus increasing the angle rather than reducing it.

Digital lenses - ie lenses designed for smaller digital formats such as the Nikon DX - should theoretically be designed with greater retrofocus incorporated into their design to ensure that the light strikes the sensor at as oblique an angle as possible.

Shouldn't the intention be for the light to strike the sensor at exactly 90 degrees (in a perfect world) with respect to the sensor plane? This would necessitate an angle as least oblique as possible...

~Matt Segal

Sorry folks, I did mean as near to 90 degrees as possible! As for whether manufacturers are actually doing this, well unless they do, wide-angles are going to remain a problem. Snag is finding out how the lenses actually work - information is scanty except for manufacturers such as Zeiss who publish loads of data.

Lenses like the 12~24 and 10~22 must be very retrofocus in their design though and I presume that the 10.5 is too. Interestingly, lenses such as Canon's 14mm and Nikon's 15mm don't seem to get very good reviews at all - even on reduced sensor cameras. I've owned 15mm Nikkors and 14mm Sigmas and found both to be adequate rather than star performers.

It will be interesting to find out if Nikon is designing the DX (made for digital) lenses with greater retrofocus.

If they are not, it would appear from reading the comments in this thread that there is no difference between Nikon's DX (digital) lenses and their non-digital lenses.

At least now I understand there is no difference in the field of view between a DX and non-DX lens.