Does "Crop Factor" still apply when using a "crop lens" (a specialty lens designed specifically for crop sensor cameras) on a crop sensor camera?

[Tom-Thomas]

To set the stage for this discussion, let me first clarify my understanding of the "Crop Factor":

The focal length of a lens remains the same regardless of the sensor size. A 50mm lens is always a 50mm lens, whether it's used on a full-frame or a crop sensor camera. However, when used on a crop sensor camera, the smaller sensor captures less amount of the image circle produced by the lens than a full frame sensor would.

While this is somewhat misleading, we can think of it as if the crop sensor "crops out" a portion of the image. This leads to the concept of "Crop Factor." The Angle of View (or AoV) seen by the crop sensor is narrower compared to that of a full-frame sensor; thus, giving the impression that the image was captured with a longer focal length lens—often referred to as the "Equivalent Focal Length”.

OK. So I understand that. The confusion arises when discussing specialty "crop lenses" designed specifically for use on crop sensor cameras. There are conflicting claims about whether Crop Factor still applies when using a crop lens on a crop sensor camera.

One argument is that crop lenses are designed to produce a shrunken image circle that fit the size of the crop sensor. Since the image circle is already tailored for the crop sensor, the sensor effectively captures the same portion of the image circle as it would with a full-frame lens on a full-frame camera. In this view, the Crop Factor would no longer apply. This reasoning makes sense to me. See illustration below:

On the other hand, some argue that the Crop Factor still applies, but I haven't found a clear explanation supporting this claim. To investigate further, I checked the specifications on Nikon's website for the two types of lenses. Surprisingly (to me), according to Nikon's specs, it appears that the Crop Factor is still relevant even when using a "crop lens" on a crop sensor camera.

Here are the details:

• The 85mm FX lens (full-frame) has an AoV of 28°30’ on a full-frame camera and 18°50’ on a crop sensor camera (source: Nikon 85mm FX lens).
• The 85mm DX lens (crop sensor) also lists an AoV of 18°50’ (source: Nikon 85mm DX lens)—the same AoV as the full-frame lens when used on a crop sensor camera.

This is interesting because, if the Crop Factor didn’t apply, one would expect the AoV of the crop lens on the crop sensor camera to be the same as that of a full-frame lens on the full frame camera (i.e., 28°30’). However, both lenses show the same AoV of 18°50’ on a crop sensor camera, implying that the crop lens used on a crop sensor camera still exhibits a crop factor effect. And, if it is the case, that would mean that (at least in Nikon’s case) a crop lens does NOT shrink the image circle to fit the crop sensor. So what is so special about a crop lens?

 

[grain elevator]

Your understanding of crop factor is correct. You understanding of focal length is what leads to your confusion. Focal length is a physical property of the lens and has nothing to do with what format it is intended for, and generally, the actually physical focal lengths are given - they are not adjusted to a crop factor. There are exceptions, when it should say "35 mm/full frame equivalent" but that phrase is omitted, but I have not seen them when lenses for interchangeable lens cameras are described for sale, seems more common is zoom settings on digital compacts.
Look into pinhole photography if you want to understand focal length - focal length is simply the distance between film and pinhole and defines the angle of view for a specific film format, 7th grade trigonometry.
So yes, crop factor still applies. E.g. a 35 mm lens, be it one designed for the system with a smaller image circle, or one designed for full frame, is a "normal" lens on an APS-C sensor camera, but a slight wide angle on a full frame camera.
One consideration that might be confusing is that the 35 mm lens designed for APS-C doesn't have to be a wide angle, while the 35 mm lens designed for full frame is a wide angle. An yet, I repeat, they give the same image on the APS-C camera. This may sound confusing because, contrary to popular belief, "wide angle" or not does not designate specific focal lengths per se but an image circle that is large, by whatever standard, for the focal length. So those two lenses deliver the same angle of view when mounted on the same camera, but one has a larger image circle than the other (if you mount the 35mm designed for APS-C on a full-frame camera, you'll probably get heavy vignetting).

 

[MattKing]

"Crop factor" is simply an attempt to deal with the situation when people are trying to deal with the similarities and differences when one is switching between formats.
It means nothing by itself. It only has meaning if you include context.
Using an APS-C format frame and a 35mm format as examples.
E.g. a 30mm lens, the depth of field performance will always be the same for the same camera to subject distance - it relates to the focal length.
You really only should use the phrase this way: "The 30mm lens designed for a 35mm frame has a crop factor of 1:1.6 when used on a APS-C sensor.
In other words, the phrase is meaningless unless it is accompanied by the "when used on" companion.
Some people will use a different notation for what I've referred to as 1:1.6 - I use it to express the fact that it covers about 60% of the subject field when the smaller sensor is used.
Like many things in photography, using shorthand terms for slightly more complex ideas can often confuse people.

 

[Chan Tran]

If you use the crop factor then it should apply also to the what you call "crop lens". It's best not to use the crop factor (we have cameras in 8x10" down to Disc and yet did you hear anyone talked about crop factor back in those days?) but to know that an 85mm on a FF gives 18 30' AOV and on an APC-C gives 18 50" AOV. Except that if you use the so called "crop lens" on the FF camera you would only have 18 50" AOV with some black around your image.

 

[Tom-Thomas]

You understanding of focal length is what leads to your confusion. Focal length is a physical property of the lens and has nothing to do with what format it is intended for, and generally, the actually physical focal lengths are given.

But I am not confused about this though. I says the same thing in my post:

The focal length of a lens remains the same regardless of the sensor size. A 50mm lens is always a 50mm lens, whether it's used on a full-frame or a crop sensor camera.

That is because contrary to popular belief, wide agle or not does not designate specific focal lengths per se but an image circle that is large, by whatever standard, for the focal length.

I understand this part too. Thus, my question: If a crop lens is made so that the image circle is shrunk to fit a crop sensor, would it then imply that the AoV in the case of "a-50mm-crop-lens-on-a crop-sensor-camera" would be the same as that captured by "a- 50mm-full-frame-lens-on-a-full-frame-camera?

So, in essence, the critical point is whether or not a crop lens shrinks the image circle to fit a crop sensor as some people assert.

 

[Tom-Thomas]

"Crop factor" is simply an attempt to deal with the situation when people are trying to deal with the similarities and differences when one is switching between formats.
It means nothing by itself. It only has meaning if you include context.
Using an APS-C format frame and a 35mm format as examples.
E.g. a 30mm lens, the depth of field performance will always be the same for the same camera to subject distance - it relates to the focal length.

I understand all that and I have said so myself in my post! Has anyone really read my post? Let me repeat myself:

While this is somewhat misleading, we can think of it as if the crop sensor "crops out" a portion of the image.

The Angle of View (or AoV) seen by the crop sensor is narrower compared to that of a full-frame sensor; thus, giving the impression that the image was captured with a longer focal length lens—often referred to as the "Equivalent Focal Length”.

 

[MattKing]

I understand this part too. Thus, my question: If a crop lens is made so that the image circle is shrunk to fit a crop sensor, would it then imply that the AoV in the case of "a-50mm-crop-lens-on-a crop-sensor-camera" would be the same as that captured by "a- 50mm-full-frame-lens-on-a-full-frame-camera?

So, in essence, the critical point is whether or not a crop lens shrinks the image circle to fit a crop sensor as some people assert.

No.
A lens designed for a smaller sensor just doesn't need to cover a larger sensor.
It is a different lens - it doesn't bother with imaging outside the area of the smaller sensor.
I guess it's possible that early on the camera manufacturers might have saved some money by temporarily adapting an existing lens design by mechanically limiting the imaging to the smaller image area. But I doubt that - it costs more money to build and design lenses intended to cover larger frames.
And most importantly, it is relatively meaningless to refer to it as a "crop" lens.
It is just a lens. It is designed to accurately image film or a sensor that is any size up to a target size, but not to provide good results for anything larger - assuming of course it isn't mechanically prevented from mounting on a larger format camera.
It may very well work with larger sensors or film, if you can put up with poorer performance near the edges.
People in the large format film world regularly use lenses with all sorts of different film formats, and are happy to discuss how they perform to different standards on those different formats.
A "crop" lens is just a marketing phrase - it defines nothing more than the format of camera which is designed to be sold for.
I get regular use of a lens that was designed for the very first generation of Canon APS film and digital cameras - a cute and very handy 22-55mm zoom lens. I use it on 35mm film, because it is an EF mount. They made that lens before they invented the EF-S mount, even though the lens was designed with the smaller format in mind. The corner performance isn't perfect with 35mm film, but the utility of the lens more than makes up for it.

 

[Tom-Thomas]

Guys, I am NOT confused about 'Crop Factor". I understand that the focal length of lens does NOT change simply because you put the lens in front of sensors of different size.

 

[Tom-Thomas]

A lens designed for a smaller sensor just doesn't need to cover a larger sensor.

I GOT THAT!

Here is my question as plainly as I can put it:

There are 2 opposing claims:
(A) Crop lenses are designed so that the image circle is shrink to fit the crop sensors (see my illustration). If this is the case, then, the crop factor will not apply when a crop lens is used on a crop sensor camera.
(B) Crop lenses are NOT designed to shrink the image circle to fit the crop sensors. If this is the case, then crop factor WILL still apply when a crop lens is used on a crop sensor camera just like a full frame lens when it is used on a crop sensor camera.

 

[Tom-Thomas]

May be a better way to illustrate my question is to consider the case of using a Metabones type "speed booster" with a full frame lens on a crop sensor camera.

A speed booster's lens elements shrink the original image circle projected from a full frame lens down to a size better fit a crop sensor. It may NOT shrink it down to perfectly fit a crop sensor, but it does shrink it down. That's the whole point about a speed booster.

I have seen claim that crop lenses do a similar thing. Is this true of false? THAT is s my question.

 

[MattKing]

May be a better way to illustrate my question is to consider the case of using a Metabones type "speed booster" with a full frame lens on a crop sensor camera.

A speed booster's lens elements shrink the original image circle projected from a full frame lens down to a size better fit a crop sensor. It may NOT shrink it down to perfectly fit a crop sensor, but it does shrink it down. That's the whole point about a speed booster.

I have seen claim that crop lenses do a similar thing. Is this true of false? THAT is s my question.

No. That would change the effective focal length, and ought to require re-labelling the lens.

 

[Chan Tran]

I GOT THAT!

Here is my question as plainly as I can put it:

There are 2 opposing claims:
(A) Crop lenses are designed so that the image circle is shrink to fit the crop sensors (see my illustration). If this is the case, then, the crop factor will not apply when a crop lens is used on a crop sensor camera.
(B) Crop lenses are NOT designed to shrink the image circle to fit the crop sensors. If this is the case, then crop factor WILL still apply when a crop lens is used on a crop sensor camera just like a full frame lens when it is used on a crop sensor camera.

the crop lens has a smaller image circle but the crop factor is still applied. Look at your 2 illustration the image capture by the middle and right are identical and giving the same AOV. So if you apply the crop factor in the center image you would do that in the image on the right as well. The area outside of the sensor is irrelevant.
Crop lens doesn't do the same thing as speed booster. Speed booster changes the focal length of the lens.

 

[Tom-Thomas]

Look at your 2 illustration the image capture by the middle and right are identical and giving the same AOV.

No. They aren't. Please look at the illustration again, carefully, this time.

 

[MattKing]

No. They aren't. Please look at the illustration again, carefully, this time.

Those illustrations aren't accurate then.

 

[koraks]

owever, both lenses show the same AoV of 18°50’ on a crop sensor camera, implying that the crop lens used on a crop sensor camera still exhibits a crop factor effect.

Yes. It's still an 85mm lens. The only difference is that the DX lens on an FX camera simply won't cover the entire sensor. It'll "try" to cover the 28.30 AoV when mounted on an FX camera, but won't make it.

So what is so special about a crop lens?

Smaller & lighter because it doesn't have to throw as large an image circle as a FX lens.

Those illustrations aren't accurate then.

That's right; the illustration is wrong. Image #3 should be the same as #2, just a smaller image circle.

 

[Tom-Thomas]

Those illustrations aren't accurate then.

You are welcome (and should) point out how and why my illustration is inaccurate, instead of just throwing such accusation out. I am waiting. I am humble enough to admit I am wrong if you can point out inaccuracy of my illustration.

 

[grain elevator]

I GOT THAT!

Here is my question as plainly as I can put it:

There are 2 opposing claims:
(A) Crop lenses are designed so that the image circle is shrink to fit the crop sensors (see my illustration). If this is the case, then, the crop factor will not apply when a crop lens is used on a crop sensor camera.
(B) Crop lenses are NOT designed to shrink the image circle to fit the crop sensors. If this is the case, then crop factor WILL still apply when a crop lens is used on a crop sensor camera just like a full frame lens when it is used on a crop sensor camera.

Ok, then there is your misunderstanding: the image circle is not shrunk, it is chopped off around the edges. What is imaged will be imaged at the same magnification by all lenses of the same focal length.

 

[Tom-Thomas]

Yes. It's still an 85mm lens. The only difference is that the DX lens on an FX camera simply won't cover the entire sensor.

You are correct, of course. BUT I have not said otherwise. I have never even mention the case of putting a DX lens on a FX body. Never. Please read my post.

That's right; the illustration is wrong. Image #3 should be the same as #2, just a smaller image circle.

How could #3 be the same as #2 when the image circle in #3 is SMALLER than that of #2?

 

[koraks]

Here's what your image should have looked like.

 

[Tom-Thomas]

Ok, then there is your misunderstanding: the image circle is not shrunk, it is chopped off around the edges. What is imaged will be imaged at the same magnification by all lenses of the same focal length.

Sir, I understand that. This is what I write, please read carefully.

The focal length of a lens remains the same regardless of the sensor size. A 50mm lens is always a 50mm lens, whether it's used on a full-frame or a crop sensor camera. However, when used on a crop sensor camera, the smaller sensor captures less amount of the image circle produced by the lens than a full frame sensor would.

While this is somewhat MISLEADING, we can think of it as if the crop sensor "crops out" a portion of the image. This leads to the concept of "Crop Factor."

I am NOT talking about Crop Factor in the case of using a "standard" full frame lens on a full frame crop sensor camera. I am talking about using a crop lens (a lens designed specially for crop sensor camera) ON A CROP SENSOR CAMWERA.

 

[BrianShaw]

This thread really could use some cold water thrown on it...

or decaf coffee in the future.

 

[Tom-Thomas]

View attachment 392708
Here's what your image should have looked like.

No. You did NOT shrink the image circle in #3. You just crop out part of the image circle. The point of contention here is whether or not a crop lens, similar to a speed booster, shrinks (not crops) the image circle down to a size closer to the size of the crop sensor; therefore, allows the crop sensor to capture a large larger area of the image circle than otherwise.

 

[koraks]

You just crop out part of the image circle.

Yes, that's what a DX lens does. It just projects a smaller image circle.

The point of contention here is whether or not a crop lens, similar to a speed booster, shrinks (not crops) the image circle down to a size closer to the size of the crop sensor

A crop factor lens does not shrink the image. It just projects a smaller image circle. In a way, it really is a cropped image.

 

[Tom-Thomas]

Any Moderator out there, please just close this thread. I just don't see the point of it. My beef is NOT about people agreeing with me or not. I am here to ask a question and to learn. However, I am very frustrated because so many people don't read my post thoroughly before they reply. So, I am out of here. Thank you for your time, nonetheless.

 

[koraks]

Any Moderator out there, please just close this thread.

There's no real need.

I am here to ask a question and to learn.

Well, you've covered the first part, sort of. You seem to have gotten stuck on the latter. Don't blame it on the rest of the world, though...