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

[BradS]

What @koraks says in posts #19 and #23 above is exactly correct. The OP's misunderstanding of what is meant by image circle is the difficulty.

 

[Chan Tran]

Yes

 

[koraks]

The OP's misunderstanding of what is meant by image circle is the difficulty.

Yes, this is precisely the root cause of the problem. If one reads the OP carefully (which we should, note the several admonitions to this effect), there's this:

One argument is that crop lenses are designed to produce a shrunken image circle that fit the size of the crop sensor. [...] This reasoning makes sense to me.

(Emphasis mine)
This is where the confusion traces back to - the use of the word 'shrunken', and the apparently literal interpretation in the sense of a 'scaled down' image circle. While the reasoning might make sense when taken at face value, the problem is that the formulation of a 'shrunken image circle' is unfortunate and strictly speaking technically incorrect.

Hope this helps in any way @Tom-Thomas. This sort of thing can get confusing, especially if words are used in a slightly loose, informal manner.

 

[MattKing]

When I look again at your initial illustrative images, I realize I missed the problem initially - my apologies for that.
And if you were relying on them to form your understanding, I understand your confusion.
All of us have struggled with questions that turn out to embody incorrect assumptions in the first place - I can remember discovering one example of one of those from when I was 17 - more than 50 years ago! - and where as a result a Physics professor thanked me before the class for finding an error in a question he had asked the entire class.

 

[Chan Tran]

There's no real need.



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...

The thing about the crop factor that it's very simple but if you don't get it you simply never get it regardless

 

[koraks]

The thing about the crop factor that it's very simple but if you don't get it you simply never get it regardless

Yeah, although I think it helps to work with a large format/view camera for a while. This tends to make concepts involving image circles really come to life.

 

[grain elevator]

Yeah, although I think it helps to work with a large format/view camera for a while. This tends to make concepts involving image circles really come to life.

And designing a pinhole camera really drives home what focal length means (much simpler than people seem to think).

 

[Chan Tran]

When I look again at your initial illustrative images, I realize I missed the problem initially - my apologies for that.
And if you were relying on them to form your understanding, I understand your confusion.
All of us have struggled with questions that turn out to embody incorrect assumptions in the first place - I can remember discovering one example of one of those from when I was 17 - more than 50 years ago! - and where as a result a Physics professor thanked me before the class for finding an error in a question he had asked the entire class.

I didn't see it either that's why I said the crop sensor camera produces identical image with either the FF or crop lens. I didn't see that he resized the image circle instead of cropping the image circle which is a similar effect of using the speedbooster.

 

[grain elevator]

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

OP, if you assume that this illustration is correct (I concur that it is) just for a thought experiment, you don't have to accept it as gospel, and follow the implications, you might answer your own question.

 

[wiltw]

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.

1. A '4/3 format lens' has in image circle cuited for a 13.5x18mm frame size
2. A so-called 'crop lens' (or 'APS format lens') has an image circle suited for a 15x22mm format size
3. A '135 format lens' has an image circle suited for 24x36mm format size
4. A 'medium format lens' has an image circle suited to a 56x56mm format size (ignoring 6x7 or 6x9 or 6x12 formats, of course)

The AOV of any lens is first defined by the frame area, assuming its image circle is large enough to fill the frame!

1. 50mm FL on 4/3 format has 23 degree diagonal AOV
2. 50mm FL on 135 format has 47 degree diagonal AOV
3. 50mm FL on MF lens has 80.6 degree diagonal AOV
4. 50mm FL on 4x5 format has 117 degree diagonal AOV

Lastly, FL vs. the vertical frame dimension will frame the IDENTICAL vertical area in the scene, regardless of format size (assuming its image circle is large enough)!

• 13.5mm FL on 13.5mm frame (4/3 format) will see the identical scene height as...
• 15mm FL on 15mm frame (APS frame) will see the identical scene height as...
• 24mm FL on 24mm frame (135 ferame) will see the identical scene height as...
• 56mm FL on 56mm frame (6x6 fame) will see the identical scene height as...
• 90mm FL on 90mm frame height (4x5 frame) will see the identical scene height

And your illustration of FF vs 'crop format' seems to be correct.

 

[BradS]

....

And your illustration of FF vs 'crop format' seems to be correct.

No. It is not correct. The third image shows the smaller image circle but also shows a different magnification. That's not how it works.

 

[Tom-Thomas]

Please read my post carefully before you reply. Thanks.

(1) Guys, I have never made a claim that crop lens does or does not shrink/reduce/compress the image circle. I post a QUESTION to ask whether it is true or not. I have read that crop lenses do shrink/reduce/compress the image circle—much like a lens speed booster does. HOWEVER, nobody here has answer THAT QUESTION.

(2) Instead, the argument here have become: "Even if a crop lens does shrink/reduce/compress the image circle, the crop factor would still apply." This I don't agree.

(3) If the image circle is shrink/reduce/compress when it is projected on to the sensor, then, it follow that the sensor would now capture a larger PORTION of the image circle than otherwise. Thus, it would also follow that the AoV is now larger than before.

(4) Now, if the crop lens shrink/reduce/compress the image circle so that it cover the whole crop sensor exactly (see #2 and #3 of my illustration before)—note that I am talking about a crop lens on a crop sensor camera—then, it follow that the AoV is now the same as that of a full frame lens (of the same focal length) on a full frame camera.

(5) To illustrate my point, consider a lens speed booster. A lens speed booster increase your lens' "speed" by shrinking/reducing/compressing the image circle down; hence, allowing more light to hit the sensor.

(6) Another effect of shrinking/reducing/compressing the image circle is that the sensor now see a wider AoV. That is because the sensor now (with the Metabones) capture an larger portion of the image circle than before. See the illustration below from a Metabones White Paper.

(7) This is essentially the same as my illustration of a shrunken image circle I posted before.

(8) "Crop Factor" comes about because a crop sensor captures a smaller portion of the image circle projected from a full frame lens.

(9) We can see from the Metabones illustration that as the image circle is reduced, the area of the image circle captured by the crop sensor increase; thus, a wider AoV, and so a lowered "crop factor".

(10) So, (a) I am NOT saying that a crop lens does what a Metabones Speed Booster does. Or not. I have heard that it is the case but I don't know for sure. Instead, I am asking if anyone know from a reliable source that it does or not. And, (b) I argue that IF a crop lens does shrink the image circle down to a size that fit a crop sensor, it would follow that the normal crop factor will not apply (or having a crop factor of 1x).

 

[Tom-Thomas]

I didn't see that he resized the image circle instead of cropping the image circle

No. I did NOT crop the image circle. I DID resize it. Please look at my illustration carefully. I am posting it again below. Note that the crop sensor size is the same size in the both images, but the image circle on the right image is smaller.

instead of cropping the image circle which is a similar effect of using the speedbooster.

No. As illustrated by the Metabones white paper posted in my previous post, a Metabones doesn't crop the image circle, it shrinks it.

 

[Tom-Thomas]

No. It is not correct. The third image shows the smaller image circle but also shows a different magnification. That's not how it works.

The "magnification" is determined by the AoV the sensor is capturing, which in turn is determined by the amount of the area of the image circle is being captured by the sensor. So, yes, my illustration shows a different magnification, because, now the amount of the area of the image circle is being captured by the sensor is larger. My illustration is NOT wrong.

 

[Chan Tran]

No. I did NOT crop the image circle. I DID resize it. Please look at my illustration carefully. I am posting it again below. Note that the crop sensor size is the same size in the both images, but the image circle on the right image is smaller.

View attachment 392721


No. As illustrated by the Metabones white paper posted in my previous post, a Metabones doesn't crop the image circle, it shrinks it.

Yes you should have cropped the right image instead of shrinking it. Only the speedbooster would shrink the image the crop lens only crop the image circle. Thus you should have identical image with ff lens as crop lens.
I said I didn't see that you resized the image instead of cropping it and by resizing the image you provide the effect of the speedbooster and not a crop lens.

 

[Tom-Thomas]

Guys, reading back on your posts, many of you appear to be arguing with me about a full frame lens mounted on a crop sensor lens. But, please, I am NOT talking about that at all. I fully understand and know why crop factor applies in this situation. However, I am talking about a "CROP LENS" on a "CROP SENSOR CAMERA". Please.

 

[Tom-Thomas]

Yes you should have cropped the right image instead of shrinking it. Only the speedbooster would shrink the image the crop lens only crop the image circle. Thus you should have identical image with ff lens as crop lens.
I said I didn't see that you resized the image instead of cropping it and by resizing the image you provide the effect of the speedbooster and not a crop lens.

(A) Can you cite a reliable source that says that a crop lens does NOT shrink the image circle like a Metabones. That IS what I am psoting here for: Confirmation.
(B) Do you agree that if (I am saying IF) a crop lens does shrink the image circle down to fit the size of the crop sensor (similar to a Metabones), then, crop factor will not apply in this case? Do you agree?

 

[Chan Tran]

Guys, reading back on your posts, many of you appear to be arguing with me about a full frame lens mounted on a crop sensor lens. But, please, I am NOT talking about that at all. I fully understand and know why crop factor applies in this situation. However, I am talking about a "CROP LENS" on a "CROP SENSOR CAMERA". Please.

If you have a Nikon camera try the Nikon 35mm f/1.8 FX vs the DX version on a crop sensor Nikon camera and you see you have identical images.

 

[Chan Tran]

(A) Can you cite a reliable source that says that a crop lens does NOT shrink the image circle like a Metabones. That IS what I am psoting here for: Confirmation.
(B) Do you agree that if (I am saying IF) a crop lens does shrink the image circle down to fit the size of the crop sensor (similar to a Metabones), then, crop factor will not apply in this case? Do you agree?

If the crop lens shrinks the image then you're right but it does not because lens of whatever the coverage is (whether it's large format lens or 35mm lens or crop lens or M43 lens) of the same focal length has the same magnification at the same distance so the subject is the same size on the film plane. It doesn't shrink. If you don't believe me looking on lens magnification and no where is the size of image circle is relevant. Only the focal length.

 

[BradS]

The "magnification" is determined by the AoV the sensor is capturing, which in turn is determined by the amount of the area of the image circle is being captured by the sensor. So, yes, my illustration shows a different magnification, because, now the amount of the area of the image circle is being captured by the sensor is larger.

This is incorrect and the source of your confusion.

Magnification is independent of sensor size. If I point a 100mm lens (regardless of what format it was designed to cover) at the moon and focus the image of the moon on a a piece of cardboard, the size of the moon projected onto the cardboard does not change if I change size of the cardboard.

My illustration is NOT wrong.

I guess it depends upon what you think the illustration is illustrating. It is not illustrating the difference between two lenses with identical focal length at the same subject distance.

 

[OAPOli]

A speed booster reduces the focal length of the lens it is attached to. It is no longer the same lens hence the higher FOV or shrunk image circle.

 

[MattKing]

Did you take the illustration photos, and if so, with which lenses, and which cameras?
Unless you add some more glass to an 85mm lens, or subtract some glass from an 85mm lens, it won't change the magnification that comes from the lens.
And if you add or subtract that glass, in a way that changes the magnification, than you will have made it something different than an 85mm lens.
That is what a speed booster does - it changes the effective focal length of the two together into something shorter, and thus reduces the magnification.

 

[wiltw]

No. It is not correct. The third image shows the smaller image circle but also shows a different magnification. That's not how it works.

The illustration of Post 1 is correct...The scale of the image might not be portrayed correctly in the illustration, but the concept that is depicted is correct...the scene area fills the frame the same extent when the FL used is appropriate to the format size... e.g. 0.62*FL used when 0.62*format size, the size of an object at film plane is proportional.

• 31mm FL APS-C 'normal' lens, at 21' subject distance, sees 10' tall object filling 42% of APS-C frame height,
• 50mm FL 135 format 'normal' lens, at 21' subject distance sees, 10' tall object filling 42% of the 135 frame height.

A 100mm FL will ALWAYS portray a 10' object the IDENTICAL SIZE on the film plane, no matter what format size the 100mm lens was made to fill, when the camera position is at a fixed distance from the subject.

• 50mm FL lens for APS-C format, at subject distance of 42', will see 10' tall object fill 12mm vertically at the APS-C focal plane
• 50mm FL lens for 135 format, at subject distance of 42' will see 10' tall object fill 12mm vertically at the 135 focal plane
• 50mmFL lens for 6x6 format, at subject distance of 42' will see 10' tall object fill 12mm vertically at the 6x6 focal plane
• 50mmFL lens for 4x5 format, at subject distance of 42, will see 10' tall object fill 12mm vertically at the 4x5 film plane

 

[BradS]

The scale of the image might not be portrayed correctly in the illustration, but ....

but this is exactly the cognitive dissonance the OP is suffering. The original illustration shows the magnification changing which is exactly contrary to everything else you wrote. What you wrote is correct. The illustration in the OP is NOT correct precisely because it shows the magnification changing...which you so exhaustively show is not what happens.
We both agree. The illustration in the OP is incorrect. korak's later correction in post #19 is correct.

 

[MattKing]

The scale of the image might not be portrayed correctly in the illustration, but the concept that is depicted is correct...the scene area fills the frame the same extent when the FL used is appropriate to the format size... e.g. 0.62*FL used when 0.62*format size, the size of an object at film plane is proportional.

The problem is that it actually doesn't - see how the size of the greenish depression changes between the middle and the right illustration. That change implies a change in focal length.