No Crop factor on DX lenses??? - DPChallenge Forums

Pentax DA 18-55mm f/3.5-5.6 AF

07/13/2006 02:12:18 AM · #1

Ok, I was just reading about DX lenses on the Nikon site.

Not I'm not the most technical person when it comes to light and distortion etc. but from what I understand:

DX lenses are smaller than regular lenses

DX lenses project an image only the size of the sensor THEREFORE according to the diagram shown on the site there would be NO crop factor for a DX lens.

So my Nikon AF-S DX Zoom-Nikkor 18-70mm f/3.5-4.5G IF-ED would in fact be the equivalent of an 18-70mm on a 35mm camera (becaue there is no crop factor).

Cool.

ok, now tell me why I'm wrong (or how cool I am ;) )

faidoi

Pentax K10D

07/13/2006 02:17:30 AM · #2

I think it just means that it is optimized for a digital sensor and a smaller lens can be made so as not to waste the cropping in a film lens.

Nikon AF Micro-Nikkor 60mm f2.8D

07/13/2006 02:21:07 AM · #3

but the image circle projected is the size of the sensor. The reason for the multiplier is that the image circle projected is larger than the sensor (hence the sensor "crops" the image circle).

If the image circle is the same size as the sensor there would be nothing to "crop".

Edit: that's also why you can't use a DX lens on a 35mm camera - the image circle is too small so you would get vignetting.

Message edited by author 2006-07-13 02:22:36.

Vapor63

Nikon D200

07/13/2006 02:25:53 AM · #4

No. Your lens will not be "Film-Equivolent" 18-70. IT will however be an 18-70mm lens, no matter how you look at it. "Film Equivolent" simply is a standard of how lenses behave on a film sized sensor or actual film. This does not mean that when Nikon made the lens to the specifications of 18-70mm, that they then called it the "27-105mm". It simply wouldn't make sense to call a lens by what it isn't- it will act like an 18-70mm on an APS-C sensor (in Nikon's case 1.5x). The only difference is that Nikon has moved beyond the rediculous "Full-Frame" notion and is making lenses that don't cover the 35mm standard size, but rather, Nikon's standard APS-C size.

"Crop" is a silly term to describe a difference in size, and these lenses only cover what would be considered the "cropped" area. They do not compress a FF projection into a "cropped" area. I have and love the 18-70mm.

It's still a "27-105mm" Film Equivolent lens, but now you can't use it on a film or full frame camera. No loss here for me, and I appreciate the fact that Nikon is moving ahead.

Message edited by author 2006-07-13 02:28:27.

faidoi

Pentax K10D

Pentax DA 18-55mm f/3.5-5.6 AF

07/13/2006 02:29:41 AM · #5

Originally posted by Vapor63:

It's still a "27-105mm" Film Equivolent lens, but now you can't use it on a film or full frame camera. No loss here for me, and I appreciate the fact that Nikon is moving ahead.

They better because they stop making film cameras.

Nikon AF Micro-Nikkor 60mm f2.8D

07/13/2006 02:38:49 AM · #6

I think "crop" is a proper term because it's the equivalent of cropping out a rectangle from the projected image from the lens.

this is the example from the link I posted (if you haven't read the article you might want to)

On a non-DX lens it cuts it from a circle 1.5 times the size of the sensor.

On a DX lens it cuts it from a circle the size of the sensor (1x)

It DOES make a difference when thinking equating lenses to 35mm because if you have both DX and non-DX lenses you would need to multiply the non-DX lenses to figure out the equivalent size to you DX lens.

Message edited by author 2006-07-13 03:12:26.

Vapor63

Nikon D200

07/13/2006 03:36:46 AM · #7

I have my 1989, 60mm Micro f/1.8D with me right here. I also have my 18-70mm back in my room. As soon as I get off of work I will go and shoot a picture from the 60mm prime (film lens) and the 60mm setting on the 18-70mm. I'm 99% certain that they will come out identical, meaning, that while the DX lens is only for a DX format sensor, it is still not the "film equivolent" measurement- In other words, the 18-70mm is a 27-105mm Film-Equivolent lens.

If you are right, I will post a delicious cookie to you. In 1:1 Reproduction ratio from my 60mm Micro.

Canon RF 24-70mm f/2.8 L IS

07/13/2006 12:15:17 PM · #8

Bump...

Where's Bear Music when we need him? ;)

kirbic

Canon EOS R5

07/13/2006 12:20:30 PM · #9

Dan, you're correct in that cropping *is* what occurs with a 35mm lens on an APS-C cam. It doesn't follow, howvever, that the smaller image circle of the DX lenses implies no crop factor. True, there is "nothing to crop" but it's just that the image circle is already "cropped" by the optical design; it's not that it's been shrunk by the optics, and therein lies the difference.
The 18-70mm lens on a 1.5-crop DSLR will in fact give you a FoV equivalent to a 27-105mm lens on a 35mm DSLR.

fotomann_forever

07/13/2006 12:23:51 PM · #10

You still have a crop factor to deal with in "film equivalence, even with DX lenses. A 50mm lens is a 50mm lens is a 50mm lens, no matter what camera you put it on.

The difference is the "sensor" size. A 50mm lens on say a medium format camera is a wide-angle lens, compared to being a mild telephoto on a APS-C sensor.

The smaller image circle produced by the DX lens isn't an attempt to "recalibrate" 50mm, but rather just makes the lens smaller and lighter compared to a 35mm 50mm lens.

07/13/2006 12:35:50 PM · #11

I understand it's not redefining the physics of a 50mm lens - just suggesting the ratio is perhaps different because of the size of the plane projected.

OK, so say I have a pinhole camera that's 1 foot by 1 foot and I want to expose a square that's 3 inches by 3 inches - I'll get a 3 inch square of the image projected.

Now say I move the back plane back so my image projected is the size of an aircraft hanger. The image itself will be exactly the same only bigger - but the 3 inch square will capture a LOT less of that projected image.

The fact that the image wouldn't change is the constant size of the lens (like saying 50mm is 50mm no matter how you look at it). The image plane would change though and hence the amount of image the exposure area would capture would change as well.

I'm not trying to argue - just trying to understand (with cold hard facts ;) )

07/13/2006 12:42:01 PM · #12

Originally posted by Megatherian:

Bump...

Where's Bear Music when we need him? ;)

Right here.

Unfortunately, Megatherian, you're not understanding it correctly.

Let me use an example from the film world because it's easy to understand:

On a 4x5 inch film camera (a "view camera") a 90mm lens is approximately equal, in angular coverage, to a 28mm lens on a 35mm film camera. If you could somehow mount that same 90mm lens ON the 35mm camera, you can see that the 35mm film would only cover a very small portion of the image circle that the 4x5 film used, right? That's your "crop factor" right there. The exact same lens is a moderate telephoto for 35mm film and a wide angle for 4x5 film.

Going the other way, it doesn't work. Why? Because the 90mm lens that is designed for ("optimized for") 35mm film cameras throws an image circle that is barely large enough to cover the 35mm film. So if you mounted that lens on the 4x5 camera, the image you saw on the 4x5 film would be a small circle in the center of the film.

The larger your film (or your sensor), the larger must be the image circle that the lens projects. And, obviously, the larger the image circle, the greater the angular coverage.

Now, there are many advantages to optimizing a lens for a given film/sensor size. Optically, you can produce a sharper, less-distorted image by eliminating the need to have sharpness further and further from the center of the image circle. To understand why, think of how much the light rays have to fan out to cover the larger circle. There's a straight-line distance from the focal center of the lens to the center of the image circle, and there are countless "diagonals" that represent the distance the light has to travel to reach portions of the film/sensor that are NOT on center. The larger the image circle, the greater the difference between distance-to-center and distance-to-outer-edge. You can easily imagine that this is a major issue that has to be dealt with optically, so that these light rays that have to travel further are actually in focus on the same plane as the shorter rays that touch the center of the image.

This is why lens tests always show sharpness at the center and at the edges, and it is why one of the characteristics of a cheap lens is that it is much softer at the edges than a higher-quality lens of the same focal length.

So if we design a lens to fit the APS-C sensor like the Canon 20D/350xt uses (Canon calls these EF-S [for "short"] lenses), we are able to ignore the design problems that come from "reaching out" to a larger circle, and optimize the lens for the smaller image sensor. But if we were to mount this lens on a full-frame digital camera, it would have really dramatic vignetting (darkening of the corners) because the sensor actually would extend beyond the area of the image circle the lens is throwing.

Another benefit of the EF-S (or Nikon's DX) lenses is that, for a given focal length, they can be both lighter and smaller than their full-grown brethren designed for larger sensors/films. And one of the benefits of using these cropped-sensor cameras with full-frame lenses is that they use only the center of the image circle, which is always the area in which a lens is optically superior.

Nevertheless, 18mm is still 18mm, and 200mm is still 200mm, no matter how you go about expanding or contracting the actual image circle it throws; the smaller you make the image circle, the more you are "cropping" in the camera, so to speak, and we call that "crop factor".

Hope this helps.

R.

07/13/2006 12:58:49 PM · #13

Originally posted by Megatherian:

OK, so say I have a pinhole camera that's 1 foot by 1 foot and I want to expose a square that's 3 inches by 3 inches - I'll get a 3 inch square of the image projected.

Now say I move the back plane back so my image projected is the size of an aircraft hanger. The image itself will be exactly the same only bigger - but the 3 inch square will capture a LOT less of that projected image.

The fact that the image wouldn't change is the constant size of the lens (like saying 50mm is 50mm no matter how you look at it). The image plane would change though and hence the amount of image the exposure area would capture would change as well.

I'm not trying to argue - just trying to understand (with cold hard facts ;) )

Forget your pinhole camera for a minute and go back to the view camera concept, because it is very simple: On a view camera, focused at infinity, you can actually measure the distance from the optical center of the lens to the film plane, and you will find that it is the same as the focal length of the lens. So to focus a 200mm lens at infinity on a view camera, we have to extend the lens board so it is 200mm from the film plane. If we want to use a 1000mm lens, we have to rack the lens board out to 1000mm from the film plane, by physically adding "extension rails" to the normal rails of the camera.

Now, you can take whatever the heck size of film you WANT to and place it at the film plane, 8x10 or 4x5 or 35mm or whatever, and obviously the smaller the film, the more "cropped" the image is. That's your crop factor.

The pinhole camera isn't a good comparison, because with a small enough pinhole the DOF is huge, so you can effectively alter the focal length by moving your your film closer to or further from the pinhole itself; you have a zone of sharpness available to you there. But on our digital cameras, the sensor plane has a fixed relationship to the lens, so that isn't a factor.

What complicates things is that with a "simple camera" (a monorail design like a view camera) You focus by extending the physical length of the entire system until sharpness is attained. With complex optics, like the ones we use on our dSLR cameras, since the relationship fo the sensor plane to the lens mount is fixed, the focusing is done optically instead of by physically racking the whole thing out. And there are many ways to do this; just compare the compact zooms, like Canon's 70-300mm, with the larger, more conventional zooms, like the canon 70-200mm. The 70-300mm is MUCH shorter and lighter than the 70-200mm, but certain concessions are made in design to accomplish this, and the 70-200mm, consequently, gives better optical performance across the board than the 70-300mm.

R.

07/13/2006 12:59:28 PM · #14

ahh... ok.

I figured I was probably not understanding something correctly (not hard to do these days), I just like to know what exactly I'm misunderstanding (so I do understand :P )

While you were typing your post Bear I found and was reading this article on FoV and AoV and how it applies to Nikon and Olympus "optimized" lenses.

Thanks everyone for your replies, I appreciate it :)

One more question though - all the DX lenses (except the 10.5mm) have a widest apature of 4.5 - do you think that is partially due to the smaller size of the lens?

(I know I'm probably opening another can of worms here)

petrakka

07/13/2006 01:07:09 PM · #15

the 17-55 2.8 DX has a constant 2.8 aperture. I think it's just because they haven't gotten around to making many professional grade DX lenses because they are still fairly new, and it's the wide end that people are more concerned about. I think most people like the fact that their 80-200 2.8 is now a 120-300 2.8 in terms of 35mm.

but it's really still an 80-200, and I don't know why everything has to be compared to the 24x36 dimensions of that old film no one uses anymore =)

and its cheaper/easier to make fast lenses with a smaller image circle I think.

see i think alot....but i know nothing.

Message edited by author 2006-07-13 13:12:28.

fotomann_forever

07/13/2006 01:37:27 PM · #16

Originally posted by petrakka:

but it's really still an 80-200, and I don't know why everything has to be compared to the 24x36 dimensions of that old film no one uses anymore =)

Laziness of us that 'grew up" on film to learn the characteristics of "cropped" sensors, maybe?

07/13/2006 01:42:27 PM · #17

With cameras having differnt crop factors it makes life easier to have a standard reference point. Since many people grew up using film the natural reference point would be 35mm film.

If you see a great shot and it says it was shot with a Canon 50mm lens it's not so easy to figure out the Nikon or Olympus equivalent without some kind of static reference.

Message edited by author 2006-07-13 13:42:47.

Nikon AF-S Nikkor 16-35 f/4 ED VR

07/13/2006 02:14:10 PM · #18

Originally posted by Megatherian:

The Nikon and Olympus equivalents aren't the problem: 50mm is 50mm. What's key is knowing the size of the sensor as well as the focal length of the lens. I think that's probably what you meant? Gotta know what camera it was shot with, the lens alone isn't a lot of help.

But you're right about the standard reference point: so many different size sensors out there, it helps to keep a constant like 35mm film in mind for comparison.

R.

Message edited by author 2006-07-13 14:15:10.

petrakka

07/14/2006 04:42:54 PM · #19

true true I guess. I'm just not a huge fan of the idea that these '35 mm sized' sensors are the size that is best now.

but to each their own.

darnok

Nikon D850

07/14/2006 04:50:36 PM · #20

Originally posted by petrakka:

true true I guess. I'm just not a huge fan of the idea that these '35 mm sized' sensors are the size that is best now.

but to each their own.

35mm is not the best size, it's just the most common film size, that's why it's useful to know the 35mm equivalent focal length of a lens.

kirbic

Canon EOS R5

Canon RF 24-70mm f/2.8 L IS

07/14/2006 04:59:30 PM · #21

From a historical perspective, it's interesting that APS-C format cameras ( hwich is what 1.5 adn 1.6-crop DSLRs are, never made it big when first introduced (using film). Now in digital, completely different story. 35mm format DSLRs are very much a minority. Only time will tell how long the idea that 35mm is "the reference" will persist, and whether 35mm as a format will grow with respect to APS-C.
For *me*, 35mm is the best format going right now... that's certainly not true for everyone, or perhaps even for a majority.