Crop factor affecting FX Lenses

So... I have a d3200 and I am looking at slowly buying the odd FX lens because i know i will want to advance into an FX format body. I know that on a DX format body, the crop factor of the sensor affects the focal lengths of a lens, i.e. by say x1.5... but I am confused as to whether it affects the aperture as well? Shed some light please. Cheers.

gowge said:

So... I have a d3200 and I am looking at slowly buying the odd FX lens because i know i will want to advance into an FX format body. I know that on a DX format body, the crop factor of the sensor affects the focal lengths of a lens, i.e. by say x1.5... but I am confused as to whether it affects the aperture as well? Shed some light please. Cheers.

Click to expand...

No affects on aperture. Also, the sensor size does not affect focal length, it affects Field of View and, too a lesser extent, depth of field.

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The crop factor relates to the field of view, reducing the width and height of the subject viewed and captured by the DX sensor. Focal length of the lens remains the same, regardless of which sensor/camera to which it is attached. Since f/stop is related to focal length and not FOV, the f/stop does not change.

ahhh right!! well that is what i meant then. am i right in believing that if i had a 70-200mm with a crop factor x1.5, it would change to 105-300mm? so that is affecting the field of view.

gowge said:

ahhh right!! well that is what i meant then. am i right in believing that if i had a 70-200mm with a crop factor x1.5, it would change to 105-300mm? so that is affecting the field of view.

Click to expand...

No, the focal length would not change. The focal length never changes. Period. What changes is the field of view.

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gowge said:

ahhh right!! well that is what i meant then. am i right in believing that if i had a 70-200mm with a crop factor x1.5, it would change to 105-300mm? so that is affecting the field of view.

Click to expand...

The lens does not change in any way on DX - it is still 70-200 and f/2.8. DX does not affect aperture, or the lens at all.

All that changes is that the smaller DX sensor crops the view from the lens, to be seen as a smaller view (cropped - less width). So then the numerical idea is that DX view LOOKS LIKE the field of view that 105-300 would see if ON A FX CAMERA (standing at same spot). As compared to FX, it just LOOKS LIKE the lens was 1.5x longer than it is. But it wasn't, the DX VIEW was just cropped smaller. It is still 70-200 f/2.8.

The lens is what it is, unchanged. The DX crop factor is just a way to compare its DX VIEW to the FX VIEW, due to the smaller DX cropped sensor size. The DX view is from the same lens, just cropped smaller, and then seen enlarged back more.

Cropping ANY IMAGE later in the photo editor, reduces its area size, and then when we enlarge it back, it SIMULATES the same telephoto zoom effect we attribute to DX. The only difference is the DX image retains all of its sensor pixels, none are missing.

See FX - DX Lens Crop Factor

It doesn't change the focal length, that's a physical property of the lens that can't change. In the case of the 70-200mm, with the smaller sensor of a DX, it gives the same field of view as a 105-300 on a 35mm camera or FX Nikon digital camera, which has a sensor the same size as a 35mm film negative. it's all sort of semantics, and yet it's important to understand the difference.

Edit: I see Wayne beat me by a couple of minutes with a more complete explanation.

WayneF said:

Cropping ANY IMAGE later in the photo editor, reduces its area size, and then when we enlarge it back, it SIMULATES the same telephoto zoom effect we attribute to DX.

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And THAT is the other crucial thing to understand, in my opinion; this difference is "simulated"; it's a perception created when doing a specific comparison.

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Simply put, the focal length of a lens is the length from the sensor to a specific point in the lens. More complicated, but lets keep it simple. So, focal length of a lens does not change unless you move the lens closer or farther away from the sensor. Assuming the lens is mounted to a camera without adapter, extension, converter, etc., the focal length never changes.

Simply put, the change from FX to DX using the same lens is the size of the captured image. That's it. A DX crops compared to an FX. It does not zoom, telephoto or magnify the subject in any way.

Horoscope Fish said:

And THAT is the other crucial thing to understand, in my opinion; this difference is "simulated"; it's a perception created when doing a specific comparison.

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Yeah, I fear we don't always distinguish between analog and digital resolution. The lens creates the analog image, which the digital pixels merely try to reproduce it well.

Sports and wildlife people favor DX because it APPEARS TO magnify the view from the lens they have, by about 1.5x. Which is good, certainly can be "good enough", but it is not the same as actually using the longer lens on FX (depending on the quality of the two lenses, but FX is the way to bet, if you can afford the lenses). The sports photogs seen at the Olympic or NFL sidelines are not using DX cameras (today).

The lens has a resolution parameter, sometimes called line pairs per mm, or line pairs per screen height, etc.. But the lens image definitely does have a resolution (shows a certain amount of detail), which digital pixel sampling merely tries to reproduce the best it can. The digital pixels can never improve the lens image, actually can never quite fully reproduce the analog resolution, but can copy much of it.

Enlargement (magnification) directly reduces this original lens resolution, proportionately. Twice as large is half as sharp, numerically. We know that a printed 6x4 print looks sharper than an 8x10, which in turn looks sharper than wall mural size. Enlargement reduces resolution, and DX requires half again more enlargement than FX.

Cropping the image in the DX camera, or in the photo editor later (to same cropped size), reduces the lens image resolution the same amount (the subsequent enlargement does this).

However, DX sensor cropping retains all of the sensor pixels.. say 24 megapixels on the DX sensor.
Which in turn is reproduced better (by digital pixel sampling resolution) than if we cropped the FX 24 megapixels in the photo editor later, to be only say 10 megapixels remaining then.
24 megapixels shows the cropped and enlarged lens image better than 10 megapixels can. But neither can improve the reduced original lens image.

So... make no mistake, these pixels are only greater digital pixel sampling resolution to better reproduce the lesser cropped lens image resolution (due to the greater enlargement necessary).
But again, 1.5x is not a huge factor.

gowge said:

ahhh right!! well that is what i meant then. am i right in believing that if i had a 70-200mm with a crop factor x1.5, it would change to 105-300mm? so that is affecting the field of view.

Click to expand...

The actual focal length will not change. What will change is the field of view, due to the fact that where the lens focuses the image, there is a smaller sensor, capturing a smaller part of the focused image.

You've probably come to be conditions to think of a 50mm lens as “normal”, anything much shorter than that as “wide angle”, an anything much longer than that as “telephoto”. These characteristics, though, are not absolute to the focal length of the lens itself, but also to the size of the sensor or film that is capturing the image.

It has always been the case, even back in the stone-aged film days, that the size of the film or sensor is relevant to the field of view, and the focal length that it takes to achieve a particular view.

Imagine, for example, an 8×10 inch view camera, equipped with a 380mm lens. This would be a “normal lens” for that camera, providing a field of view that approximates a human eye. It would be equivalent to about a 50mm lens on a 35mm camera. Of course, if you mount that same lens on a 35mm camera, then it's a rather long telephoto lens. The lens hasn't changed; just the fact that there's a much smaller piece of film capturing a much smaller portion of the focused image.



So, if you mount a 70-200mm zoom on your DX camera, it doesn't “become” a 105-300mm lens. It's still 70-200mm. However, on your DX camera, it will give a field of view that is comparable to what a 105-300mm would give on an FX camera or a 35mm film camera.