Canon EF 17-40mm f/4.0 L USM equivalent focal range on 1.6x sensor body

[nimblybimbly85]

Since this lens is designed for full frame dslr, what does the focal range become on a rebel xsi?


I know the focal range will be less wide and longer, but I can't find anywhere what it is exactly.





Thank you!

[btaylor]

You just have to multiply the focal length by 1.6. This applies for both EF and EF-S lenses. So the effective focal length becomes 27.2 - 64mm and aperture remains f/4.0.


Someone correct me if I'm wrong. I'd hate to be giving out false info. I'm 99.9987234% sure I'm right though.

[neuroanatomist]

Of course, btaylor is absolutely correct. The Rebel line of cameras have a 1.6x crop factor sensor, meaning you multiply the focal length of all lenses by 1.6 to determine the angle of view provided, relative to what that focal length would produce on a full frame camera. I'll fill in the remaining0.0012766% by stating that this multiplication doesn't change the actual magnification, just the angle of view.


Read more HERE.

[btaylor]

Phew! I was a bit worried there [:P]

[Daniel Browning]

So the effective focal length becomes 27.2 - 64mm and aperture remains f/4.0.

I agree with you about the focal length: the full frame equivalent is indeed 27.2-64mm. However, I do not agree with you about f-number. If you are going to apply the crop factor to focal length, then you must also apply it to f-number.


No matter what body you put a lens on, the focal length always stays the same. 17mm is always 17mm, no matter if you use it on a 1.0X, 1.3X, 1.6X, or 7.0X body (such as the Canon XL-H1, which has a 1/3" sensor and EF lens mount). That is why I think the term "effective focal length" is very misleading -- it is much more clear to say "equivalent angle of view". For example, these are correct:

• 17mm on 1.6X has the same focal length as 17mm on full frame.
• 17mm on 1.6X has the same angle of view as 27.2mm on full frame
  

Same thing applies to f-number. No matter what body you put the lens on, the f-number is always the same. However, just as "angle of view" changes between format sizes, many factors affected by f-number change as well: total light gathered, noise, dynamic range, diffraction, depth of field, and more. Of course, some of these are affected by the sensor as well. But in the case where the sensors in both cameras have the same performance per area, such as the case of the 5D2 and 7D, then things become very simple:

• f/4 on 1.6X has the same f-number as f/4 on full frame.
• f/4 on 1.6X has the same exposure as f/4 on full frame.
• f/4 on 1.6X has the same depth of field as f/6.4 on full frame.
  
• f/4 on 1.6X has the same diffraction as f/6.4 on full frame.
• f/4 on 1.6X has the same total amount of light as f/6.4 on full frame.
• f/4 on 1.6X has the same noise level as f/6.4 on full frame.

Most people just stop at "exposure". They only consider what happens when you use the same ISO and same f-number on both cameras, in order to get the same exposure. But it's important to also consider that the larger sensor allows you to use a different exposure (i.e. higher ISO and less exposure) and still get the same (or better) results. After all, it's the results that matter.


I've done some demonstrations of this equivalence here:


http://thebrownings.name/images/2009-10-5d2-equivalence/


Here is a 1.6X image taken at 70mm f/4 ISO 640.





Compare that with the following image which is 111mm f/6.3 ISO 1600:





They have the same noise, diffraction, and depth of field, despite being at two completely different f-numbers and ISO settings. That shows that f-number scales with format size for all the important factors in photography.

[neuroanatomist]

Daniel, can you please clarify something for me?Your comparison examples refer to 'simulated APS-C' and one of your bullet points on your demo page indicates "same subject distance." But, don't the effects of crop factor on apparent aperture depend on maintaining the same subject framing (as opposed to the same magnification)? You are cropping images taken with the same sensor, but in reality to get the same overall framing on the 1.6x crop that you have with your FF sensor, you'd need to move the camera further from the subject.


Using DOFMaster's calculator to 'recreate' your simulation, 5D2 70mm f/4 at 10 ft has a DoF = 1.47 ft. 50D 111mm f/6.4 at 16 ft has a DoF = 1.5 ft. (and if I could select 112mm as a focal length, which is 70x1.6, it would be 1.47 ft.). So, to get the same DoF we need to mulitply focal length and aperture by 1.6x, but only if we also move further from the subject. Similarly, 50D 70mm f/4 at 16 ft has a DoF = 2.4 ft; same lens, same settings but put on a 5D2 body and moving forward to 10 ft subject distance to maintain framing, changing the aperture to f/6.4 gives a DoF = 2.35 ft.


In other words, as Bryan states, "...as a generalization, using a higher FOVCF DSLR will yield more DOF in your similarly cropped pictures because you will be farther from the subject."


Thanks!


--John

[Daniel Browning]

But, don't the effects of crop factor on apparent aperture depend on maintaining the same subject framing (as opposed to the same magnification)?

They do if you keep focal length the same. There are several methods to get the same subject framing. One is to keep the focal length fixed and vary subject distance with sensor size. That results in huge differences in perspective (and perspective distortion) and in many cases is not physically possible. Another method, which is the one I used for the comparison, is to keep subject distance fixed and vary focal length with sensor size. This results in the exact same subject framing no matter what the sensor size. As a further benefit, it does not cause any difference in perspective or perspective distortion.

You are cropping images taken with the same sensor, but in reality to get the same overall framing on the 1.6x crop that you have with your FF sensor, you'd need to move the camera further from the subject.

When focal length is fixed (e.g. 70mm on both), that is correct, but I was changing the focal length.

Using DOFMaster's calculator to 'recreate' your simulation, 5D2 70mm f/4 at 10 ft has a DoF = 1.47 ft. 50D 111mm f/6.4 at 16 ft has a DoF = 1.5 ft. (and if I could select 112mm as a focal length, which is 70x1.6, it would be 1.47 ft.).

Your recreation does not have the same subject framing, because you changed focal length and subject distance at the same time. If you wanted to see what happens when you change just subject distance and not focal length, here's what you would compare:

* 5D2 70mm f/6.4 at 10 ft
* 7D 70mm f/4 at 16 ft

That results in the same subject framing ("field of view"), but not the same angle of view and not the same perspective. The depth of field may not be exactly the same, either, because it depends on how close it is to the hyperfocal distance. (70mm f/4 at 16 ft is not very close, so the DOF is probably the same, but in other cases it may not be.) That's another reason why I prefer to scale focal length instead of subject distance.

In other words, as Bryan states, "...as a generalization, using a higher FOVCF DSLR will yield more DOF in your similarly cropped pictures because you will be farther from the subject."

That's correct. The same focal length and f-number combined with further distance results in deeper DOF for smaller sensors.

For example, if you have the 17-40 f/4 on the 7D and you shoot a portrait at 40mm and 5 feet, it will come out with a certain field of view and amount of distortion (big nose, small ears, etc.). If you take the same exact lens and put it on the 5D2 to shoot the same portrait, now you have to move much closer to fill the frame at 40mm. This difference in perspective results in extreme distortion -- much worse than 40mm on the 7D. That's why most photographers will use different focal lengths based on the size of their sensor. On their digicam, they'll shoot full length portraits with a 7mm lens. On 2X crop sensors, they'll use 25mm. On 1.6X, they use 31mm. On full frame, they'll use 50mm. If they used the exact same 50mm (or 7mm) on all sensors, they would get wildly different results.

In addition to perspective, there's also the fact that many times it's physically impossible to change subject distance. If you're shooting the moon with 800mm on 1.6X for example, then if you upgrade to 800mm on full frame the only option is to increase focal length, unless you have a rocket ship to take you closer to the moon. Similarly, if you're shooting a cramped room with 17mm on full frame and you switch to 17mm on a crop, you can't back up any further because you're already at the wall.

So while it's true that f-number does generally scale with sensor size when varying subject distance instead of focal length (with some DOF exceptions), I prefer to vary focal length because then all else remains equal.

Hope that helps. []

[neuroanatomist]

Thanks for taking the time to clarify, Daniel - I have a much better understanding now. []

[btaylor]

Thanks for the clarification Daniel and John. I have never been completely sure about the effects of f stop when comparing sensor size. I always understood that the DOF on a FF sensor is narrower but I guess I didn't understand the underlying reasons for that.


Cheers! Ben.

[clemmb]

I'm 99.9987234% sure I'm right though.
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You are 99.1795402116% correct.


We say the crop factor is 1.6 but it is actually 1.6132595228 therefore the 17-40 becomes 27.4250111888-64.5294380912. Thats assuming the lens is truly 17-40.


All joking aside, Daniel did an excellent job explaining as usual.


Mark