35 mm Equivalance ...meaning

[niranjan]

I was looking for a lens (nikon) for my DSLR (DX format) and i found this lens to fulfill my needs. but the website mentions one LINE which i wish to understand...

18-200mm, equivalent to 27-300mm in 35mm format

What does that means and how.. (do they want to suggest that same lens can be used on FX format cameras..? )

 

[molested_cow]

Doesn't make sense. Is it's 18mm on FX, then it makes sense to be 27mm equivalent on DX, but it's implying the opposite.

It's kinda weird how it's phrased, because usually, it's the other way around. Like.... 70~300mm lens equivalent to 105~450mm on DX or something like that.

 

[niranjan]

Nikon India Private Limited

 

[eshane]

With FX format, it's 18-200. DX format it's 27-300. DX is croped 1.5 so its 18x1.5=27, 200x1.5=300.

 

[dericcainphoto]

The size of your sensor is smaller than 35mm therefore making the field of view more like a 27-300mm.

 

[Helen B]

It means that it is an 18-200 mm lens (no matter what format camera it is mounted on) and the field of view on a DX camera is equivalent to the field of view of a 27-300 mm lens on a 35 mm or FX camera. It doesn't mean that it will work as a 27-300 mm lens if mounted on an FX camera - it will still be an 18-200 mm lens. Because it is a DX lens, if you mount it on an FX camera it will not form an image that covers the entire sensor area. Though it can be very confusing, this method of quoting a 35 mm equivalent focal length allows a quick comparison of the fields of view of different lenses on different formats. Unfortunately it is confusing unless you understand that it is intended to apply only to field of view comparisons.

If you search this forum you should find many previous debates on the wisdom or otherwise of using 35 mm equivalent.

Best,
Helen

 

[bennielou]

I think they are talking full frame?

 

[480sparky]

What they are 'comparing' it to is the equivalent focal length of 35mm film cameras.

Back in the day before digital (yes, little ones, there was such a time... some of us were there), 35mm film was all the same size. Color negatives, slides and B&W films were all the same dimension. So basically, a Canon 20mm lens had pretty much the same angle of view as a Nikon 20mm lens and a Pentax 20mm lens, as well as a Minolta, Leica, Yashica, etc. 20mm lenses.

Today, however, there are all sorts of different sensor sizes, so the focal length of a lens is no longer a measuring stick by which one can use to figure out field of view. The dimensions of of the sensor is a factor in calculating the field of view. What the manufacturers are doing is using the 'crop factor' multiplier to give the buying public a reference point by which to compare apples to apples again.

For instance, Nikons DX crop factor is 1.5. So a 50-100mm lens would have the same field of view as a 75-150mm lens on a film camera. Canon's crop factor is 1.6, so the same 50-100 lens in front of it's sensor would produce the equivalent of a 80-160mm lens on a film camera. Although the focal length of the 50-100 lens itself does not change, the field of view does when placed on different cameras.

 

[fortinw]

That's just tech jargon you really need not be concerned with. You're shooting with a DSLR, so unless your viewfinder shows more than 100% of the view through your lens (which it doesn't), what you see is pretty much what you'll get. Don't worry about the "crop factor" and just go have some fun shooting!

 

[480sparky]

It is 'tech jargon' you DO need to concern yourself with.... if you want to make an intelligent lens purchase.

 

[Big Mike]

What they are 'comparing' it to is the equivalent focal length of 35mm film cameras.

It would be better to say that they are comparing it to the equivalent field of view of a 35mm film camera. Or at least, that's what they should be doing.

 

[Infinite_Day]

I'm confused - it's a DX lens. Wouldn't the equivalent field of view for 35mm be shorter? At 18mm the image the lens projects wouldn't even cover a 35 mm sensor would it? My brain may not be working correctly this morning, though.

 

[Big Mike]

I'm confused - it's a DX lens. Wouldn't the equivalent field of view for 35mm be shorter? At 18mm the image the lens projects wouldn't even cover a 35 mm sensor would it? My brain may not be working correctly this morning, though.

You have it backwards. With a DX sensor (not lens) the field of view is smaller/narrower...which is equivalent to a longer focal length.

And because it's a DX lens, that does not change the FOV. 18mm is the same on any lens, the difference in FOV comes when you use a different camera.

 

[480sparky]

Think of it this way:Take two photos with your camera. Let's say one is taken at 50mm, the other at 100mm.

Now, load both images into any graphics software. Crop to the center 50% of the 50mm shot. In other words, remove the outer 50% of the frame. Now it should look the same as the 100mm shot.

Even though they were taken with different focal lengths, the cropped 50mm shot now has the same field of view as the 100mm.


Now imagine two cameras, one with a full frame sensor and one with a cropped sensor (ie, Nikon's FX & DX formats). The cropped (DX) sensor is physically smaller than the full frame (FX), so it 'sees' a narrower field of view than the full-frame sensor with the same lens. In effect, it's the same as cropping the 50mm shot in post.

However, lens's focal lengths are not determined by sensor size.... it's all a function of the optics. So a 50mm FX lens has the same field of view as a DX lens. The difference is the size of the projected image produced by the lens. The FX lens will need to create an image large enough to cover the FX (full-frame) sensor, while the DX lens (cropped) only needs to create an image large enough to cover the smaller sensor. Both lenses have the same field of view optically, but the DX sensor will only see the center 50% of it.

 

[Big Mike]

However, lens's focal lengths are not determined by sensor size.... it's all a function of the optics. So a 50mm FX lens has the same field of view as a DX lens. The difference is the size of the projected image produced by the lens. The FX lens will need to create an image large enough to cover the FX (full-frame) sensor, while the DX lens (cropped) only needs to create an image large enough to cover the smaller sensor. Both lenses have the same field of view optically, but the DX sensor will only see the center 50% of it.

Actually, I think you are incorrect on that last point. The FX lens will probably have a wider field of view, because it needs a wider view to create a larger image circle for film/FX sensors.
The DX lens, only has an image circle large enough to cover the DX sensor. And because it has a smaller image circle, it's actual FOV is narrower.