Crop sensor and DX lenses question

giorgio · Jan 30, 2010

Hi

I have the D80 with its kit 18-135mm DX.
So far, I know that the mm's on a Crop sensor will be increased by 1.5x, so a 50mm view will look like 75mm

Now, the DX lenses, like my 18-135, when I push the zoom to 50mm as it says right there on the barrel...,
will it give me a real 50mm view(like decompensing the 1.5x factor) or it will give me the 75mm real view, even though the lense position is set on 50mm.?
I'm asking this because, well, it seems to be (DX) a lense aimed to cropped sensors so it ocurred to me that it will be like adjusted to give the mm's that it actually says.

Like...,
A 50mm DX lense will give a 50mm view on a crop sensor.
A 50mm non-DX will give 75mm view on a crop sensor

And I'm also checking this because when I set my lense to 50mm, I can see through the viewfinder pretty much almost the same angle of view as my natural view.
Shouldn't it be more of a zoomed view through the view finder because of the 1.5x factor?

I'm ofcourse checking to buy more glass, and as a way of emulating a 50mm Non DX lense view angle to see if I like how it looks, I set my DX 18-135 to 50mm, but..., is it the same view angle than the Non DX 50mm?

Should I decrease 1.5x to emulate a non-DX lense view angle?

I mean, why design a lense for a 1.5x factor sensor and not considering the real resulting mm's so it would be put on the lense specifications.?
SO THEN, when the DX lense is mounted on a FF sensor one could espect the decrement by 1.5x of the lense mm's specifications.

And therefore, the other Non-DX lenses when mounted on a crop sensor camera will actually give the increased mm view by 1.5x.

:meh:

Dao · Jan 30, 2010

The focal length of the lens is the optical property of the lens. It has nothing to do with the camera body. So a 50mm focal length is always 50mm whether the lens is mounted on a Film/FF or cropped sensor body. The optical property of the lens will not change.

The only different in cropped body is the sensor do not take the full image (i.e. 24x36mm), instead, it take part of it. Therefore, lens manufacturer started to make lens that projects an image smaller than 24x36mm and called them DX, EF-S, DII ....

So the field of view of a FF 35mm lens and DX 35mm lens should be the same in your camera. (in real life, it may off a little bit)

An if you are not coming from a FF or film world, you really do not need to worry about crop factor. That is just a reference for those people who are used to what they see with those cameras. All you need to worry is when you look through the viewfinder, is the view wide enough for you? If the lens give you the view you like/need, who cares what body it is on.

TJ K · Jan 30, 2010

No.. All lenses list the focal length by FX format. So that would mean your 18-135 really is on your D80 a 27-202mm lens.

If you're trying to get a lens that will be equivalent to 50mm on FX then you should really look into the nikon 35 1.8 lens. Awesome lens and is good for walk around because with the crop it is around 52mm.

Well you wouldn't be using DX lenses on a full frame sensor. That would be pointless. If you have FX buy FX glass. It would start to get confusing if they made glass for DX and named the focal length what it would be on FX. It's easier to leave it how it is and if you really want you can easily do the focal length * 1.5=DX FL

Imagine canon naming their lenses for the Full Frame, 1.3 crop, and 1.6 crop. You wouldn't know what you're working with. There has to be some type of standard unit to go by.
TJ

Dwig · Jan 30, 2010

giorgio said:
...I know that the mm's on a Crop sensor will be increased by 1.5x, so a 50mm view will look like 75mm

Incorrect. There is no particular angle of view associated with a particular focal length. There is no particual angle of view associated with "75mm". Your, along with most posters' on forms like this, sloppy language usage is getting you confused. What you should have said is "... so a 50mm view on a DX format camera will look like 75mm on an FX format camera."

Focal length is not a measure of field of view in and of itself and should not be confused with such. It doesn't become related to FOV until it is associated with a particular image format. Change the format and you change the relationship. Fail to state the format and you fail to communicate.

... I'm also checking this because when I set my lense to 50mm, I can see through the viewfinder pretty much almost the same angle of view as my natural view.
Shouldn't it be more of a zoomed view through the view finder because of the 1.5x factor? ...

What you are seeing has nothing directly to do with DX vs FX. The image magnification you see in an SLR VF is a factor of the relationship between the focal length of the camera lens and that of the finder eyepiece. With most current DSLRs, the lens mount and general body configuration were inherited from full frame film SLRs. This results in the VF eyepiece having a focal length of 55-60mm. This is largely due to the lens mount placement relative to the sensor and the geometry of the mirror and focusing screen placement that result from it. When the eyepiece and the camera lens have the same focal length the visual magnification is 1:1.

RyanLilly · Jan 30, 2010

TJ K said:
No.. All lenses list the focal length by FX format. So that would mean your 18-135 really is on your D80 a 27-202mm lens.

If you're trying to get a lens that will be equivalent to 50mm on FX then you should really look into the nikon 35 1.8 lens. Awesome lens and is good for walk around because with the crop it is around 52mm.

Well you wouldn't be using DX lenses on a full frame sensor. That would be pointless. If you have FX buy FX glass. It would start to get confusing if they made glass for DX and named the focal length what it would be on FX. It's easier to leave it how it is and if you really want you can easily do the focal length * 1.5=DX FL

Imagine canon naming their lenses for the Full Frame, 1.3 crop, and 1.6 crop. You wouldn't know what you're working with. There has to be some type of standard unit to go by.
TJ

Sorry but this is pretty much all wrong. Focal length is focal length, the lens does not change depending on the camera. Focal lengths on lenses are not "named" they are measured. A lens is designed at a particular focal length, and engineered to project an image circle large enough to cover the film or sensor. A 50mm lens designed for a FX sensor projects a image circle large enough to cover a area of 24x36mm, that lens will work fine on a APS-C (aka. crop body) camera but the sensor size is only ~16x 24mm(depending on manufacturer), so only the center of the projected circle is captured and, the image from crop body will have a more narrow field of view.

giorgio · Jan 31, 2010

Hi

thank you for your replies

Sorry, english is not my main language so I tried my best to explain myself

.

So, If I want to buy a Sigma 24-70mm 2.8 lense, and I want to see with my 18-135DX how the image would look with the Sigma set at, say, at 50mm...
At what mm should I put my 18-135 lense?

this is...
Sigma 24-70 at 50mm = Nikon 18-135 at ??mm

thank you again

Dwig · Jan 31, 2010

giorgio said:
...
Sigma 24-70 at 50mm = Nikon 18-135 at ??mm...

50mm, of course. If you see any difference at all it is the accuracy, or inaccuracy, of the lens' markings. Such things aren't infinitely precise and variations up to 10% are to be expected.

Focal length is focal length. As stated earlier, nothing, that is absolutely NOTHING, about the camera's format has anything to do with what focal length is.

Dao · Jan 31, 2010

50mm is 50mm because we are talking about the focal length. So Sigma 24-70mm lens at 50mm should be the same as Nikon 18-135mm at 50mm. I said should be since the actual focal length maybe off a little bit. i.e. 50mm may actually be 49.x or 50.x. (In fact some lens manufacturer sell a 18-250mm lens, and the actually the lens is 18-220mm only.)

So, 1 lb of feather should weight the same as 1 lb of steel.

Dao · Jan 31, 2010

Let me put it this way. A 150 horse power Honda engine should be the same as 150 horse power Ford engine in terms of power.

However, when you put the 150 horse power engine in a 1000lb car, the end result should be different when you put it in a 5000lb car. Assuming everything else stay the same, so there maybe a 5x different here.

So now you find an engine that is capable of output 100 to 200 horse power. And you tune it so that output at 150 horse power only and installed to the 1000lb car, will you expect the car perform about the same as the Honda engine that is 150 horse power?

giorgio · Jan 31, 2010

Perfect, thank you very much.

I do understand about the accuracy, ~more/less, I knowp.

:thumbup:

jackieclayton · Jan 31, 2010

now you got me confused!! lol... if all lenses list the focal length of a FX camera, what the heck is the difference in DX lenses?

Dao · Jan 31, 2010

jackieclayton said:
now you got me confused!! lol... if all lenses list the focal length of a FX camera, what the heck is the difference in DX lenses?

The main difference is the lens made for FF / Film 35mm cameras project a bigger image circle to the Film/Sensor plane.

In other words, the DX lens produce a smaller image that will not cover the entire film/ff sensor.

jackieclayton · Jan 31, 2010

oh ok... I understand now... my D700 has an option where I can tell it that a DX lens is mounted so it can adjust accordingly. That makes sense... thanks!