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Lens focal lenght conversions end equivalencies
Hi. I have a Nikon DSLR and I'm used to reading how the lenses designed for 35mm cameras have an equivalent focal lenght of F*1,5 when used in an APS-C sensor. So, in my case the AF Nikkor 50mm f/1,8 is the equivalent of a 75mm lens in a 35mm camera, and so this lens provides a narrower field of vision (more zoom) in my case..

Then I thought that if a DX lens is designed for a smaller sensor like mine, a picture shot at 75mm with a DX lens would show the same field of vision as one shot with a Nikkor 50mm in the same camera. So I set up my tripod, grabbed my zoom, took a couple of shots at 75mm, swapped the lens for my 50mm, shot another one and compared the results. The 75mm shots were narrower! (more zoom) Then I set the zoom to 50mm and the result were very close this time..

I was very surprised to see this, as I was expecting the shots to be about the same. The original question for me was: would a 30mm lens designed for a DX sensor produce the same equivalent pictures (as in field of vision) than a 30mm lens designed for a 35mm camera? Because I want the field of vision of a 30mm lens for my DSLR, not a 45mm (I'm planning to buy a Sigma 30mm f/1.4).

I fail to see the flaw in my logic, so if anyone could explain this tome me I'd be very grateful. Thanks in advance and sorry for my english...

Comments (7)

Focal length is focal length, it doesn't change with cameras. If you put a standard medium format 80 mm lens one your camera, it would still be an 80 mm lens. The 80 mm lens on a medium format camera would give the equivalent FoV of a 50 mm lens on a 35 mm camera, and yuo would need 33 mm lens to get that same FoV equivalence on your APS -C sized sensor..

The 35 mm eqivalence thing is just to give some standard reference point..

If you look at the small, P&S digital camera , you will see some that have actual focal lengths on their lenses. 6mm focal length on a 1/2.5 sensor, for example, will give you the equivalence of a 36 mm lens on a 35 mm system..

Brian A...

Comment #1

Simply stated, focal length is how far behind the lens the focused image is when the subject is at infinity. This has nothing to do with image sensors or camera formats. (Lenses are complex so the actual measuring point isn't often the back of the lens.).

Get an ordinary magnifying lens and a piece of paper. You can focus an image of a lamp or something on the paper. Changing the size of the paper has no effect on how far the paper is from the lens when the image is in focus..

Lenses that are designed for a smaller sensor just illuminate a smaller area. It has nothing to do with redefining focal length...

Comment #2

Anacondo wrote:.

Hi. I have a Nikon DSLR and I'm used to reading how the lensesdesigned for 35mm cameras have an equivalent focal lenght of F*1,5when used in an APS-C sensor..

Your error is in this sentence. The differences in the lenses for FF 35mm and APS-C sensors is the field of view, not the focal length. A 30mm lens is always 30mm. One intended for a big sensor simply has to cover a wider field. That makes it bigger, heavier, and costlier..

The whole "equivalent" thingie is a farce. It implies more than just the FoV is equivalent. Take two pix...one with a FF 35mm camera...and one with a compact camera with a tiny sensor (say a 1/2.7")...make sure that the camera settings are identical (something like actual FL=50, sen=200, f:4.0, 1/1000 sec, tripod, etc)...then compare the pix...forget the noise in the pic from the compact camera...look at the DoF! They are NOT equivalent..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #3

Anacondo wrote:.

Hi. I have a Nikon DSLR and I'm used to reading how the lensesdesigned for 35mm cameras have an equivalent focal lenght of F*1,5when used in an APS-C sensor..

No, this is wrong. ANY lens, when used on a Nikon DX-format camera, will have the same field of view (FOV) as a 1.5x longer lens on a 35mm-format camera. It's the sensor size that makes the difference - whether the lens was "designed for 35mm cameras" or not is irrelevant. (As you found out by experiment.).

Focal length is not a measurement of FOV, it just relates the size of the subject to the size of it's image on the sensor:.

Image size = Focal length * Subject size / Distance.

This works (except at very short distances) regardless of sensor size and image circle size. But the image size that fills the frame, and thus the FOV, depends on the size of the frame - i.e., the sensor size..

The "35mm equivalent focal length" isn't the real focal length, it's just what you would have to use on a 35mm camera to get the same FOV..

The original question for me was: would a 30mm lens designed for aDX sensor produce the same equivalent pictures (as in field of vision)than a 30mm lens designed for a 35mm camera? Because I want the fieldof vision of a 30mm lens for my DSLR, not a 45mm (I'm planning to buya Sigma 30mm f/1.4).

If you want the same FOV as a 30mm lens on a 35mm camera, then you'll need to look for a 20mm lens for your DSLR. Since Nikon kit lenses cover 20mm, I assume you're looking for a faster lens - maybe the Sigma 20mm f/1.8? (I don't know anything about the quality of this lens, it just seems to be the fastest available at that focal length.).

Alan Martin..

Comment #4

I see. So it's all about FoV, not focal length..

So let's see if I get this straight. My 50mm lens will be 50mm everywhere, but will cover the whole sensor of a full frame camera, more than the APS-C sensor of my Nikon and will vignette on a medium format. So the reason why a 75mm DX lens on an APS-C is not the same as 50mm FF on the same camera is simply because they're different focal lenghts and that has nothing to do with sensor sizes. But, a 50mm lens designed for a 35mm camera will give me the equivalent FoV of a 75mm in APS-C, so to obtain the equivalent FoV in APS-C I would need a 33mm lens, and something like 6mm on a point-and-shoot..

The things you mention about DoF are interesting. I read somewhere that smaller sensors are able to capture more DoF than bigger ones. Is this true? Or it's just that at a given f-stop number the bigger sensor will always have less DoF?.

Thanks for your responses, guys...

Comment #5

"Then I thought that if a DX lens is designed for a smaller sensor like mine, a picture shot at 75mm with a DX lens would show the same field of vision as one shot with a Nikkor 50mm in the same camera. So I set up my tripod, grabbed my zoom, took a couple of shots at 75mm, swapped the lens for my 50mm, shot another one and compared the results. The 75mm shots were narrower! (more zoom) Then I set the zoom to 50mm and the result were very close this time.".

You are making what could a valid conclusion from an invalid starting point. this is that your 50mm lens is actually a true 50mm in focal length. also the zoom put at 75mm ends up at 75mm. the zoom control may not matched exactly to the glass and like the 50mm, the 75mm marking may not be a true 75mm..

What you could have ended up with is the 50mm actually giving you say 47mm while the zoom marked at 75mm could really be 80mm..

The above is one problem. there is another. yousaid grabbed your zoom and took a couple of shots at 75mm. but that 75mm is subject to the crop too. it is not 75mm it is 75 X 1.5 or 112.5 that you are comparing to a 50mm which is also 50 X 1.5 or 75. you are really comparing 112.5 with 75.



If you have a a nikon film camera handy put the zoom at 75mm on that take the pic and scan it in to the pc. THEN compare to the 50mm on your dx nikon...

Comment #6

Anacondo wrote:.

I see. So it's all about FoV, not focal length..

So let's see if I get this straight. My 50mm lens will be 50mmeverywhere, but will cover the whole sensor of a full frame camera,more than the APS-C sensor of my Nikon and will vignette on a mediumformat. So the reason why a 75mm DX lens on an APS-C is not the sameas 50mm FF on the same camera is simply because they're differentfocal lenghts and that has nothing to do with sensor sizes..

So far, so good..

But, a 50mm lens designed for a 35mm camera will give me the equivalentFoV of a 75mm in APS-C,.

I would say that a 50mm lens *on* a 35mm camera will give you the same FoV as a 75mm lens *on* a DX-format camera. It doesn't matter what format the lenses were designed for, unless they vignette (or physically don't fit)..

So to obtain the equivalent FoV in APS-C I would need a 33mm lens,and something like 6mm on a point-and-shoot..

To obtain the same FoV as a 50mm lens on a 35mm camera, yes..

The P&S would probably be closer to 8mm, if it uses the common 1/2.5" sensor size (7.2 mm diagonal, 6x smaller than 35mm format). The 6mm figure is a typical widest zoom setting, not a "normal" focal length..

Alan Martin..

Comment #7

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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