2

edit: ^^ the answer to that question talks about the lens being the same and but not how or why this is the case, and does not answer the question about resultant image,

First of all, to let you know what I understand:

  • I am aware that focal length is focal length, and that doesn't change* (that's usually the first line of every answer I've read),
  • the difference between full frame and smaller sensors and the effect that the same lens will have on different bodies (with different frame sizes),
  • the difference between focal length, crop factor and field of view angle,
  • what some are calling "effective" focal length as compared to FOV, cropping and multiplication factor.

(* see second question)

I know this question sounds the same as Shooting 50mm EF vs EF-S and Is an EF 50mm f/1.4 the same as 50mm with an EF-S lens on a Canon 550D? (and a whole heap of others **) but they all seem to go off in slightly different tangents, namely what I will see on different bodies (I don't care about this, I have only one body type), they also cover things such as the effect of (say) an EF-S lens on a FF body (I'm not doing that either).

Put very specifically, here is my question broken into (now) four parts:

  1. Does mounting an EF vs an EF-S lens produce the same results on a camera body that has an APS-C frame size? (my thinking is NO because of the design of the lens and "short back focus" of the EF-S, and the fact that the EF lens produces a picture for a FF sensor and the fact it is further away—when directly comparing 50mm EF and 50mm EF-S—do they have the same field of view, which then leads onto the next question),

  2. (this is probably where my confusion lies, but) aren't the EF-S range of lenses physically closer to the sensor whereby the focal length IS actually shorter?

  3. If EF-S lenses (as a whole unit) are shorter and have a shorter back focus plane, wouldn't that make the FOV wider (ergo not producing the same image)?

edit:

  1. is it the case that Canon have labelled their EF-S 50mm lens as 50mm to the focal plane AS IF the focal plane was a FF 35mm plate at 50mm? Or is the focal distance 50mm to the actual smaller frame? (I think my confusion here is that a 50mm EF lens sits further forward than a 50mm EF-S lens, which to me seems to indicate that one of them isn't 50mm :)

(** others):

  • The question Is an EF 50mm f/1.4 the same as 50mm with an EF-S lens on a Canon 550D? asks exactly the same thing you are asking. It does not refer to different camera bodies, but asks whether there is any difference between a 50mm EF lens and a 50mm EF-S lens on an APS-C body. – Arkanon Jan 30 '15 at 19:33
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    Your entire question is based upon a false assumption: that the registration distance for Canon's FF cameras is longer than the registration distance for Canon's APS-C cameras. It is not. In both cases the registration distance is 44mm. The APS-C cameras allow lens elements to protrude back past the flange for certain wide angle lenses, but at 50mm this is not the case for any EF-S zoom lens of which I am aware. – Michael C Jan 30 '15 at 23:55
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    Please Clarify what is meant by same result. Do you mean same Field of View? Or are you referring to other optical qualities? Even the three 50mm EF lenses in Canon's current lineup have different optical qualities and all three are "full frame" EF lenses. – Michael C Jan 31 '15 at 0:40
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    @MichaelClark Thanks for the registration distance remark. Knew there was something I forgot in my answer. – inkista Jan 31 '15 at 1:01
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    Despite the edit, I closed this. The question is now 4, which isn't a good option on the site and the answers have lost context as a result. The first posting is a duplicate, I think that's fairly clear. Having said that, you could ask your new question(s) separate from this presuming that they haven't been asked before. – John Cavan Feb 1 '15 at 18:17
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Does mounting an EF vs an EF-S lens produce the same results on a camera body that has an APS-C frame size?

Yes. They will produce the same results, assuming all the same settings, and similar optical performance between the two lenses, and 100% accuracy in the actual focal length of the lens as reported (manufacturers have been known to fudge the number). Theoretically, an EF 50/1.8 and an EF-S 50/1.8 with identical optical performance and identical aperture settings would produce identical images with identical exposure settings and depth of field when mounted on the same body.

... aren't the EF-S range of lenses physically closer to the sensor whereby the focal length IS actually shorter?

No. You're assuming that registration distance (i.e., how far the lens is held from the image plane) affects the focal length. It doesn't. Focal length is an internal measurement and physical property of the lens itself. Changing how far away from the image plane it sits or the size of the image plane behind it doesn't affect the lens's focal length at all.

In addition, as Michael Clark points out in a comment, the registration distance for EF-S and EF lenses is identical: 44mm. That the "S" in EF-S stands for "short", and the back element can protrude farther than with EF lenses doesn't mean that the lens overall is held closer to the image plane for crop bodies, but that because the sensor is smaller, the mirror is also smaller, which means as it swings up, there's a larger clearance at the back end of the lens, so EF-S rear elements can stick farther out than the rear elements of EF lenses can on a full-frame body without hitting the mirror.

... If EF-S lenses (as a whole unit) are shorter and have a shorter back focus plane, wouldn't that make the FOV wider (ergo not producing the same image)?

Think about moving a projector closer to the screen. Does the image get bigger or smaller? Sharper or fuzzier? Because the sensor area that has to be covered by the lens's projected image circle is smaller and the lens can therefore produce a smaller, sharper image with less optical correction.

  • Thanks, that does sound the best answer I've seen so far, maybe I am misunderstanding the actual definition of focal distance. If you have a look at most answers, especially the answer @Håkon suggests, it says: that a lens' focal length is defined as the distance from the film plane needed when the lens is focused at infinity to cast point light sources as a single point on the film plan (and many others say something very similar). – Madivad Jan 30 '15 at 19:48
  • For a single objective lens (i.e., one piece of glass). Maybe this will clear it up? What is focal length when there is also flange focal distance? – inkista Jan 30 '15 at 20:04
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    Thanks again, the more reference material the better, but I'm obviously missing something pretty simple here, because all those answers appear (to me) to be saying from the lens to the image sensor (the second answer has a graphic I think I understand, but obviously don't). Analysing the accepted answer, am I to read it (going from back to front) it is from the sensor to the "nodal point of the objective lens" which (I have no idea what it means, but I suspect) means a "mean" or "calculated" point inside the lens, and not necessarily the back of the lens??? – Madivad Jan 30 '15 at 20:12
  • @Madivad. Yes. The calculated "optical center" of the lens. Given the way you can design multiple element lenses, this point can even be outside the physical lens. I.e., whether the lens is EF-S or EF, (or rangefinder, or medium format, or micro four-thirds...) if it's designated as 50mm, the optical center of the lens was designed to be 50mm away from the image plane. I've changed "includes" to "affects" in my answer. :) – inkista Jan 30 '15 at 20:25
4

The answer is yes.

50mm focal length is 50mm focal length, no matter what the mount is.

-1

I took the opportunity to simply go for it and do it.

I placed the camera (40D, being APS-C) in the same position, that is, the body is the same distance from the subject in both shots. (I'm on a slow network, so I upload only the embedded previews from the raw files)

The first one, with an EF-S zoom lens dialed to "50" The first one, with an EF-S zoom lens dialed to "50"

The second one with a 50mm EF lens The second one with a 50mm EF lens

I assume that the labelling on the EF-S lens is showing "FF equivalent focal length", because they are not supposed to be attached to a FF camera. But I don't know.

Does mounting an EF vs an EF-S lens produce the same results on a camera body that has an APS-C frame size?

Nope, not the same result.

  • There is nowhere near a 1.6x difference in subject size between these two photos. There is no 50mm mark on any Canon EF-S 18-55mm lens I have seen. If you are using any other EF-S lens, then the discrepancy is likely caused by an inaccurate mark on the zoom barrel. What focal length does the EXIF info give for the photo taken with the EF-S lens? – Michael C Jan 31 '15 at 0:01
  • Both images show 50mm as focal length in the image properties. The image was taken with an EF-S 17-85mm. – null Jan 31 '15 at 0:23
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    I assume that the labelling on the EF-S lens is showing "FF equivalent focal length", because they are not supposed to be attached to a FF camera. But I don't know. Your assumption is incorrect. If that were the case, then the second photo should be only the center 42% (area) of the first photo. I suspect camera position is the culprit. The center of the second image is aimed at a point quite a bit lower than the first. Changing angles will affect several factors, including distortion at the edges as well as focus breathing caused by pointing at the same objects that are now at a... – Michael C Jan 31 '15 at 0:33
  • ...longer distance caused by a change in the direction of the center optical axis of the lens. – Michael C Jan 31 '15 at 0:33
  • @MichaelClark It's a camera on a shoe box, not an optical bench. ;) If I can find my quick release plate, I will try this again tomorrow. – null Jan 31 '15 at 0:51
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In my opinion they don't produce the same result. Here is the comparison. Imagine you using a macro tube. One optical element is moved away from the sensor to create magnification.

What occurs here is you loose quality. Light is a dispersing medium over distance if ever so slight. You lose that slight to dispersion on the distance. Also your angle is different too. If you plot the light on paper you will see what I am talking about. The magnification factor is equivalency.

Also if you look at the actual aperture opening one may even be smaller then the other. Has to be. One sits closer while the other sits at a distance. I believe the one that sits closer produces the better quality optically.

But the best test is to take it into photoshop and blow up side by side and do a pixel to pixel comparison. The other thing you looking at is the fact that at 18 mega pixels you may not even notice a difference. But if you go to higher mega pixel you may have pixel resolving where you see the difference.

And I am just talking about APC that is what I use but when doing macro work I use tubes to move the lens away from the sensor and magnify. I thing this is what AF versus AF-s does and due to this the 50mm is equivalent not exact measurement but measurement from the front lens element to the sensor. So due to this one lens may have a wider diameter and also may have a wider aperture due to the physics needed to accomplish the same result.

Now what does this mean. It means that one lens may have more pleasing bokhe. Also bokhe has to do with taste of the person looking too. Hope this answers some of your questions regarding this. But the most important point is are you at a high enough resolution to even resolve the quality drop. Also do you need that extra perhaps 5 or 10 degree difference in field of view.

  • Your entire answer is based upon the same false assumption that the question is based on: that the registration distance for Canon's FF cameras is longer than the registration distance for Canon's APS-C cameras. It is not. In both cases the registration distance is 44mm. The APS-C cameras allow lens elements to protrude back past the flange for certain wide angle lenses, but at 50mm this is not the case for any EF-S zoom lens of which I am aware. – Michael C Jan 30 '15 at 23:54
  • Did you say that lens elements protrude past a certain place. Is that what i was referring to in my answer? Glass being close in efs versus ef and thus better quality also glass having to be larger to accomplish the same physics being farther from sensor. And if a glass is closer less light dispersion less quality loss. is an art. – Robert Fried Jan 31 '15 at 0:06
  • But light coming in at a wider angle being bent more and that has to have an affect on quality too as far as fringing or what ever happens to information carried by light when it is bent more to be collected in a point or pixel. When you loose light you loose information thus quality of information it carries to the sensor. Thus optical performance. Things become washed out or hollowed. Ghosted. You be the judge. Optics – Robert Fried Jan 31 '15 at 0:06
  • But when set at 50mm I know of no Canon EF-S lens with elements that protrude back past the flange. Do you? This is only the case with some EF-S lenses when zoomed to much shorter focal lengths. – Michael C Jan 31 '15 at 0:36
  • I don't think the question is about optical quality. It is about field of view. Even the three EF 50mm lenses (f/1.2L, f/1.4, f/1.8) in Canon's current lineup have differing optical qualities, and they are all full frame EF lenses. – Michael C Jan 31 '15 at 0:42

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