Lens compatibility w/ OS Rebel SXI

[coachkirk]

I have an older Rebel SXI and I want to get a 2.8 zoom lense for sports shots in low light.  Will my camera support such a lense and if so what do you recommend?

[Tronhard]

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@coachkirk wrote:

I have an older Rebel SXI and I want to get a 2.8 zoom lense for sports shots in low light.  Will my camera support such a lense and if so what do you recommend?

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HI Coachkirk:

Your Rebel SXI (otherwise known as the Canon EOS 450D) will accept both Canon EF and EF-S lenses.  Your camera has an APS-C sensor, which is smaller than the full frame sensors by a factor of 1.6, so this will have two implications:

ALL lenses are identified with focal lengths and apartures based on a their use with a Full Frame sensor. So for your crop sensor, in looiking at the performance of ANY lens (EF or EF-S), you will need to multiply by 1.6  the numbers desribing the lens performance (aperture and focal length) by that factor to get the equivalent values for your sensor.

I am assuming you will be using a zoom lens for your purposes and right now the fastest Canon telephoto zoom lens is the Canon EF 70-200mm  f2.8 IS USM L MkII lens.  Now using the factor that I described, it's Equivalent performance on an APS-C camera will actually be 112-320mm with an Equivalent* maximum aperture of f4.48.  I know of no Canon or compatible telephoto lens (fixed or zoom) that will give you an Equivalent maximum aperture of f2.8 on an APS-C sensor (it would have to have an identified aperture of f1.2).

That said, many people use the lens I identified for sports work and indoor sports and you can mitigate the loss of aperture by using a higher ISO.  Since your body is quite old, I recommend renting the lens and trying it with your body, If you are not happy with the performance, consider also hiring the Canon 80D or Canon 77D which have improved ISO performance.

Another possiblity, if you are going to consider a body upgrade, is to get a second-hand Full Frame body, such as the EOS 6D or 5D MkII or Mk III. In that scenario the numbers on the lens will be accurate as you read them, and because the sensors are larger than your APS-C, the total amount of light reaching them will be greater and should show less noise.

 For an explanation of Equivalence please read:

https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care/2

NOTE: I have altered this material after initial post to make it clear I am talking about Equivalence -  at its most simple, a way of comparing different formats (sensor sizes) on a common basis.

[jrhoffman75]

Hi Tronhard. Do you have source data for your answer?  It is my understanding that crop factor affects angle of view and depth of field relative to the indicated aperture, but not exposure. 

I ran a test with 1D X and T5i. ISO 100 Av at f/5.6 with 35mm lens on 1D and 20mm set on T5i shooting same evenly lit scene.

Shutter speed was the same.

[Tronhard]

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@jrhoffman75 wrote:

Hi Tronhard. Do you have source data for your answer?  It is my understanding that crop factor affects angle of view and depth of field relative to the indicated aperture, but not exposure. 

I ran a test with 1D X and T5i. ISO 100 Av at f/5.6 with 35mm lens on 1D and 20mm set on T5i shooting same evenly lit scene.

Shutter speed was the same.

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Hi John:  Thanks for your question and to answer it, actually I DO, but first a clarification.

The actual phusical aperture of a lens obviuosly does not change whether you put it on a full frame or crop sensor, I am talking about f-stop equivalence in my comment, and I corrected my original post to clarify this.

While with the same lens transmission performance the amount of light per sq.mm (relating to absolute f value) remains the same no matter what camera (or cell phone) sensor you use, the total light projection that is gathered from the lens is reduced in proportion to the reduction in sensor size. In practice I would say a photographer cares more about this than the per mm performance of the lens to sensor transmission, thus Equivalence becomes an issue when looking at lens specs on a crop sensor camera.  This really beomes apparent when the light becomes marginal. 

In your experiment, how much light did you project for the camea to capture?  If there is sufficient light both sensors will do fine, it is as the light is reduced (a common issue in indoor sports photography) that one sees underexposure and/or noise creeping into the images from the smaller sensor first.

If we consider Equivalence - which is what I was referring to, (it was late and forgot that critical word so I apologise), the effective field of view of a lens is reduced when used on a crop sensor. That has two implications: because it sees less of the lens' transmission, the effective focal lenght appears to be increased (it isn't actually but it appears that way, which is what one sees in the image).  That apparent increase in focal length is proportional to the crop of the sensor. 

For a non-technical explanation of this I would refer you to this article from DPREVIEW.COM

https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care

Then there is an article on the subject from Photo Life which looks at the issue in some depth

https://photographylife.com/sensor-crop-factors-and-equivalence/

If you would refer something with a more academic rigour, I would refer you to the series of lectures given by  Professor Emeritus of Computer Science at Stanford, and Principal Engineer at Google, Mark LeVoy. 

"The aperture is then specified as an F-number N, defined by the well-known formula N = f/D, where f is the focal length of the thin lens (a.k.a. the effective focal length of the original lens assembly), and D is the diameter of the diaphragm opening." - quoted from lecture notes, Prof. LeVoy.  Note the word Effective, because if we place the lens on a crop sensor, it's effective (if you will apparent) focal length increases and narrows its field of view by the crop factor of the sensor.  So that changes the equation - to lens aperture remains the same (which was never in doubt) but the effective result is this:  To balance the equiation we have to increase both sides by that amount.

Ne x crop factor = fe x crop factor/D             where Ne and Fe are Equivalent values, not the original physical ones.  The lens, its native focal length and aparture have not changed, but their effective values have because of the narrower FOV of the crop sensor and reduction in its light-gathering potential.

He has given a series of lecturse available on the internet, see:

https://sites.google.com/site/marclevoylectures/schedule

Perhaps in this case (a sports photography application) you could focus (excuse the pun) on his lecture in that series: "Why is Sports Photography So Hard?" and in particular slide 18 of that lecture.  Here is the link to the lecture: which addresses thesubject of sports photography at around 37 minutes into the lecture:  the preceding material is not relevent to this discussion.  If you wish to view ONLY his comments on sensor size then look in at th 50 minute mark.  He comments there that for sports photography the Full frame Canon 5D is too slow to read out and the APS-C sensor does nto gather enough light.  He observes that the Canon !D has a large sensor with fewer but larger pixels to gather more total light.

https://sites.google.com/site/marclevoylectures/schedule/lecture11-25apr16

I would STRONLY suggest you invest the time to view his lecture on that part, and if you are still needing further material then go back to the section of his lectures on the physics of light metering and sensors.

If that is still not sufficent, and you are OK with the maths, I would definitely encourage you to read his other lectures on the theory of light transmission, optics and sensor design.

There is also a series of interesting articles by a New Zealand photographer who references not only the impacts of sensor size but the pixel density and himself references material from sensor manufacturers.  If you are inclined to follow his links to their conclusion here is the original post:

http://www.robsphotography.co.nz/crop-factor-advantage.html

I hope I have give you sufficent material to consider, but if not please let me know!

Regards

Trevor

[kvbarkley]

"ALL lenses are identified with focal lengths and apartures based on a their use with a Full Frame sensor. So for your crop sensor, in looiking at the performance of ANY lens (EF or EF-S), you will need to multiply by 1.6  the numbers desribing the lens performance (aperture and focal length) by that factor."

You just said two contradictary things. All lenses are marked with their actual focal length. If you want to compare fields of view, you then use the crop factor to get what field of view you would get on a 35 mm ("full frame") camera. Though that is an arbitrary choice. Crop factor has no relationship to aperture, otherwise the point and shoots with 10X crop factors would need very high ISOs!

[Tronhard]

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@kvbarkley wrote:

"ALL lenses are identified with focal lengths and apartures based on a their use with a Full Frame sensor. So for your crop sensor, in looiking at the performance of ANY lens (EF or EF-S), you will need to multiply by 1.6  the numbers desribing the lens performance (aperture and focal length) by that factor."

You just said two contradictary things. All lenses are marked with their actual focal length. If you want to compare fields of view, you then use the crop factor to get what field of view you would get on a 35 mm ("full frame") camera. Though that is an arbitrary choice. Crop factor has no relationship to aperture, otherwise the point and shoots with 10X crop factors would need very high ISOs!

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I am not sure how I have said two contradictory things, but as I have said in my last response I was explaining the issue of performance in terms of EQUIVALENCE in focal length and f-sto, not the physical propteries of the lens.  It was late and I apologise for forgetting that one word.

I am not sure why there would be any debate about the identifiation of focal lenght and f-stop in terms of a FF 35mm camera.  This is a hangover from the film era when the majority of cameras were using 35mm film.  The APS-C size is based on Kodak's Advanced Photo System (APS) film of 25.1 × 16.7 mm using 135film.  As I understand it, it was decided to maintain the nomenclature of defining Focal Length and Aparture in 35mm FF terms to keep the comparison with lenses that could be used from the film era - Nikon lenses are a good example as they never changed their mounts from film to digital.

With regards to your comments about crop factor and equivalent aperture, I would refer you to my last post. I will say this. The electronics involved in sensor technologies vary along with sensor size - Prof LeVoy's team is doing some remarkable things in that regard with sensors for smartphones.

[kvbarkley]

While the DOF thing is certainly true, I have my doubts about the other. We are talking about images in focus here after all, and the same light hits its particular area of the sensor equally. If you made low res sensors with the same size pixels as larger ones, you would get the same light performance.

[Tronhard]

I have backed up my opion with quite a bit of explanatory material which i have presented and invited you to review - I still do.   It stands in its own right, is backed up by mathematics and scientific research from several sources including sensor manufacturers..  You are very welcome to your doubts and opion, but if you are going to challenge mine then perhaps it's time for you to back up yours with appropriate academic and scientific material.

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kvbarkley wrote:

While the DOF thing is certainly true, I have my doubts about the other. We are talking about images in focus here after all (I think it has to do with images projected on the sensor) and the same light hits its particular area of the sensor equall   If you made low res sensors (what do you deifine as a  LOW RES SENSOR do you mean a Crop Sensor?) with the same size pixels as larger ones, you would get the same light performance (ONLY PER PIXEL!!!!!)

The TOTAL LIGHT GATHERING CAPABILTY OF THE SENSOR IS LESS IF THE SENSOR IS SMALLER.  Think of a sensor like a photo voltaic panel.  If you have a small panel it engages with a smaller amount of light, energy, if you have a large panel it engages with a larger amount of light energy.

If I may quote from the DP review article on Equivalence:

https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care/2

"The equivalent aperture not only tells you how much depth-of-field you get, on a different system, it also tells you how much total light you'll get. And, because the more light you capture, the less noisy your image, this is key to why large sensors generally give better image quality than small ones. This, as we said at the beginning of this article, was just as true when dealing with film formats: that's why people shot medium- and large-format film.

  But surely F1.2=F1.2=F1.2?

The [absolute] F-number itself doesn't change with sensor size, just as actual focal length doesn't change with sensor size. However, the situation is essentially the same as with equivalent focal lengths: put a 50mm lens in front of a smaller-than-full-frame sensor and it's still a 50mm lens, but you get a narrower angle-of-view, because you're not capturing the full extent of the circle-of-light the lens is projecting. So we might say the lens is now 'acting as a 100mm equivalent' lens.

In a similar way, the actual F-number always tells you the intensity of the light on each square mm of the sensor - this doesn't change with sensor size. By comparison, the equivalent aperture takes into account how many square mm of sensor you've put behind the lens.

You don't take photos with individual pixels you take photos with the sensor as unit, which is why, when considering the performance of a lens on a crop sensor, one must consider its equivalence performance values.

To Sum Up:

When I responded to to the original post it was to assist a coach who wanted to know if he could take photos of his team using his old Canon 450D and he was speficied an f2.8 lens to do so.  It is reasonable to assume that, like most coaches, he wanted a 'fast' lens and the lens I recommended for him is about as fast as he will get with a decent telephoto reach, but in terms of the amount of light it will capture, it is not going to be as stamped on the box or the front of the lens.  Why? Because those specs are defined for a Full-Frame sensor - it's a convention from the 35mm film days. 

Since a smaller sensor captures less light from the lens, while the physical lens max. aperture will not change because of the crop sensor, the area it projects onto will, by a factor of the crop value of the sensor.  This, will create an Equivalence variation for both the Equivalent focal length, and the Equivalent F-stop.

To make it clear I have never suggested that the intensity per unit area would vary by crop factor, but the overall sensor light-gathering value will.

[ebiggs1]

I have heard about and read about this for a long time.  I believe it to be not true.  While the DOF of a lens may be altered by the differing AOV the f-ratio is not. I think this theory comes from guys that have nothing to do but to over think things.

As one responder said, lay a piece of paper on a table and look at it.  Draw a circle on it.  Did the light change inside the circle? No, it doesn't   Each pixel gets the same amount of light regardless of the sensor size. 

If you take the thought further to a P&S camera or to a large format camera and apply the same formula it becomes more obvious this ain't true.  A cell phone camera would have a f22 equivalence and pretty useless!

Take a 50mm f1.4 lens and put it on your cropper and view a subject. Notice the settings reported by the camera.  Now do the same only on a FF.  The settings will be the same.  Your post editor will report the same too.

I know this is probaly a useless argument as we are not likely to change each others opinion but this is the way I see.  Plus it is the way current photography works.

[kvbarkley]

35mm is just an accident of when the digital transition took place. If it would have happened during the reign of the TLR's the standard might have been different.