question about digital lens magnification

[Luis Ortega]

On a camera with a dx size sensor like the nikon d300, do lenses get multiplied by 1.5X so that a 20mm becomes a 30mm, or does it matter if the lens is a dx lens?
If a lens is listed as a dx lens, will its focal length be as listed when used on dx sensor cameras or will it be multiplied?
If the lens still multiples the magnification, what is the purpose of calling some lenses dx lenses?

 

[flosseR]

hi, has just been discussed here: https://forums.macrumors.com/threads/936532/
and I wrote about it here in a quick and dirty way:
Crop and Full Frame (APS-C vs Full Frame)
.

 

[Gold89]

On a camera with a dx size sensor like the nikon d300, do lenses get multiplied by 1.5X so that a 20mm becomes a 30mm, or does it matter if the lens is a dx lens?
If a lens is listed as a dx lens, will its focal length be as listed when used on dx sensor cameras or will it be multiplied?
If the lens still multiples the magnification, what is the purpose of calling some lenses dx lenses?

All dx or ef-s lenses lengths are given in 35mm terms and you have to apply the crop factor to them. This makes it easier to compare focal lengths with the rest of the full frame range.

 

[Ruahrc]

The focal lengths reported on DX lenses is not already multiplied by 1.5, because the lens is showing what the actual focal length of the optics is. Same for EXIF- it will be recording the actual FL of the lens used, not the "DX version".

You're confusing focal length (a physical optical configuration) with effective field of view (what you see through the viewfinder), which is what gets multiplied by 1.5 on the DX bodies compared to FX.

The point of DX lenses is that the image circle that the lens produces only needs to cover the smaller DX sensor size, and therefore the glass elements and overall lens body does not need to be as big. The result is cheaper and lighter DX lenses.

 

[toxic]

the focal length is the focal length regardless of what camera you put a lens on.

if you have never used 35mm, the "crop factor" is of no concern to you, except for one case: you know that someone took an image with, for example, an 85mm lens on 135 format (aka 35mm/full frame/24x36mm sensor). you want to recreate it with a different format camera. you have a Nikon APS-C camera. that means you need an 85mm / 1.5 = 57mm lens to make the same image.

 

[Luis Ortega]

The point of DX lenses is that the image circle that the lens produces only needs to cover the smaller DX sensor size, and therefore the glass elements and overall lens body does not need to be as big. The result is cheaper and lighter DX lenses.

Thanks.
I guess that means that dx lenses can't be used with fx sensors or film cameras?
It seems more useful to buy non-dx lenses so that they are useful if you upgrade to a full frame sensor camera.

 

[Ruahrc]

Thanks.
I guess that means that dx lenses can't be used with fx sensors or film cameras?
It seems more useful to buy non-dx lenses so that they are useful if you upgrade to a full frame sensor camera.

For Nikon, you can use DX lenses on FX bodies, there is a "crop mode" that will reduce the captured area of the sensor to compensate. So for example the 12MP D3/700 would take a 5MP DX image I think. Obviously it's not ideal but it is a workaround while you transition glass.

Some DX lenses actually do produce an image large enough to fill the full frame- for example the 12-24 f/4 will do it above 18mm and I believe the 10-24 will do it above 15 or 16mm as well. If you keep the aperture open and limit your focus distances, the 35mm f1.8 DX also covers FX pretty good, etc. You can use these lenses in "unsupported mode" and get full coverage although I hear the corners are not as good as a real FX lens because they were not designed to be used like that.

For Canon I believe that it is not possible to use EF-S lenses on the FF bodies.

It is worthwhile to consider the DX vs. FX issue when planning gear purchases, but to only buy FX gear "just in case you might upgrade to FX several years down the road" is not always the best mode. Quality lenses hold their value very well, so if you purchased good DX lenses now, you could make back a large portion of that money later on if you sold them when upgrading to FX gear. Also due to the image circle size issue, FX lenses are going to be larger and heavier than the DX versions- if you are going lightweight with DX gear then only buying FX lenses would work against you.

An example would be the 17-55 (DX) vs the 24-70. Not only is the 24-70 larger, heavier, and more expensive, it has a less useful focal range on DX compared to the 17-55. Used prices on the 17-55 have been pretty steady for the last couple of years (as long as I have been tracking)- meaning if you bought a used 17-55 a couple of years ago and just now upgraded to FX, you could have basically sold the lens for what you paid.

 

[compuwar]

Thanks.
I guess that means that dx lenses can't be used with fx sensors or film cameras?
It seems more useful to buy non-dx lenses so that they are useful if you upgrade to a full frame sensor camera.

It depends on what you want to do.

I have no problem using DX lenses on my D3x, they produce perfectly fine 10.5MP images which can be printed out extremely large with no ill-effects. For most people, even the lower-resolution FX bodies will produce usable prints up to about 11x14. Remember that the D2h was producing full-page published images for quite some time at 4MP- so much depends on usage that it's impossible to generalize.

Finally, a lot depends on what you routinely shoot. There's absolutely no point in buying a 135mm FX lens to us on a DX camera to take portraits if you don't have the room to take the shots, and if your'e happy with a 60mm lens on a DX camera for portraits, you're not likely to be happy with the same lens on an FX body for the same purpose.

The bottom line is buy the lenses for the shots you're going to take and don't sweat it- good-condition used lenses hold their values very well, so there's almost no point in trying to hedge against some unknown future sensor that may or may not be perfectly fine shooting in DX crop mode that allows you to get the same AOV as you originally purchased the lens for versus getting a lens that's unsuited for it's purchased use once you change formats.

Paul