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Occasional Contributor
Posts: 13
Registered: ‎10-21-2014

Canon Lense basics

I have not bought a lense for my Canon EOS XT in probably ten years and since I have picked up the camera once again I am interested in acquiring a lense for it.  Please tell me how to tell which lenses will fit my old canon EOS XT.

Respected Contributor
Posts: 1,861
Registered: ‎12-02-2012

Re: Canon Lense basics

Hi,

You can use Canon EF lenses and EF -s lenses. The only Canon lenses made in the last 20+ years you can't use would be the few mirrorless lenses, designated EF-m.

You can also use lenses made for Canon cameras such as Sigma, Tamron, etc...

Shooting a "crop" sensor camera like your Rebel, you have the ability to use essentially all the lenses made for modern Canon cameras, other than the mirrorless ones. You really only need to be careful if you are shooting full frame, since they can't use the EF-s. canon lenses, nor the 3rd party crop-only lenses, which have different designations in the 3rd party brands.
Scott

Canon 5d mk 4, Canon 6D, EF 70-200mm L f/2.8 IS mk2; EF 16-35 f/2.8 L mk. III; Sigma 35mm f/1.4 "Art" EF 100mm f/2.8L Macro; EF 85mm f/1.8; EF 1.4x extender mk. 3; EF 24-105 f/4 L; EF-S 17-55mm f/2.8 IS; 3x Phottix Mitros+ speedlites

Why do so many people say "FER-tographer"? Do they take "fertographs"?
Occasional Contributor
Posts: 13
Registered: ‎10-21-2014

Re: Canon Lense basics

Assuming that I know absolutely nothing about photography and lenses, please translate the following into non-tech speak:

 

Canon - EF 50mm f/1.8 II Standard Lense Gaussian optics, 52mm filter size, 1.5' minimum foucs distance

 

 

Highlighted
Valued Contributor
Posts: 425
Registered: ‎01-19-2014

Re: Canon Lense basics

[ Edited ]

Back in the day when a SLR was a camera that shot 35mm wide film, sold in a small metal canister with the end of the roll sticking out like a tongue, there was a "standard" for cameras that used lenses that focused on this 24 x 36mm image area and all was good. Then came the age when a camera with film was not desirable because a competing format, called digital, came to be. The new format offered benefits of "on-the-spot" image review and computer-based photo "developement". And so things where changed.

 

But the new digital format proved challenging because the camera had always been an instrument which had aided the imaging of a photo medium, or film. Though the new camera would also image the scene, it too would be the "film" and with a sensor where there once was film, capture the image. And that too was good.

 

Yet, the imaging of the sensor proved challenging because the size of the sensor made a big difference. If the sensor was still desired to image the film area of 24x36 it must be made large and this was a big challenge for the scientists. Eventually, such a sensor was made and became known as the "full frame" sensor. Full because it was sized make it image as the old 35mm film had imaged. And so the, the 50m lens attached to this camera, with its 35mm sensor, gave the same "normal" perspective as had been. And people where happy.

 

So it was and the digital camera existed with lenses that could be used with camera's designed to accept their fitting but whose performance (specifically their angle of acceptance) was determined by the size of the sensor which received the focused light.

 

The world of photography had changed forever but in this new world of photography the manufacturers continued to name the lenses with the "terms" that where from the age of 35mm film. In that age, the 50mm lens was generally considered a "normal" lens because it approximated the viewing angle that a human eye could see. A big "iris" combined with a short "back" focus gave big advantanges for collecting light where little exists. The comparision of the size of the opening and the "back" distance is described as a function using the initial "f". F1.0 is a wide open lens which loses nothing to the "human" eye. As a extraordinarily well evolved species, as far as light is concerned, a lens of f/1.0 is extremely rare. But a lens of f/1.8 is very "wide" open when used to its fullest potential. You, the photographer, can choose to use this capabililty but like most things in life, that wide opening comes with compromise. The bottom line, however, is that the greater the front opening was when compared to the distance to the back, the more light is was said to gather.

 

The 50mm Canon lens you mention , has a "normal" perspective for a "35mm" camera. However, your camera contains a sensor which is smaller than this "35mm" area . So instead of receiving the full image, it receives that portion of the image that the sensor covers. In this case, it is the center-most area with a size smaller than the 35mm standard. (Remember, it was the manufacturers that decided to continue to use terms that came from the age of 35mm film.)

 

If you want to imagine the size of the sensor in your Rebel XT, you must first image two frames. The larger is the 35mm frame which has outside dimensions of 36 (w) x 24 (h) in millimeters (24.5mm = 1 inch). The second box is located immeditately inside the larger box and it has a size of 22.2 (w) x 14.8 (h) in mm. This second box is known as APS-C for Canon. (APS-C = Advanced Photo System - Compact). Both have a similar shape because both 35mm and APS-C formats have the same relationship of width to height.

 

A "normal" lens is only normal when using the language that was created during the time of 35mm film photography. When you capture a smaller "frame", as described in the previous paragraph, the angle of view changes. Like an eye that has closed slightly to "focus" on a subject in the center, the smaller APS-C sensor captures a narrower field of vision and the difference between the narrower field and the 35mm standard can be said as a multiplier, or factor. The Canon APS-C factor is X1.6. 

 

Instead of seeing the same range of "normal" vision that a 50mm lens sees when focusing its light in a image area 36mm x 24mm in size the 50mm lens gets most, but not all, of that image. It gets the center part which shows an area with a size that is equivalent to what a 80mm lens would see on that 35mm camera. (Really, it was the camera manufacturers that insisted that we continue to describe everything in terms of the 35mm film cameras!!!)

 

When you capture the center of something, you leave the outsides behind. If you look at this new image at the same size as you looked at the image before it was changed, it will look like everything got bigger. That is why a 50mm lens is described as being "equivalent" to a 80mm lens (50 x 1.6 = 80) when mounted to a Canon camera containing as APS-C sensor..

 

The lens you mention will photograph a scene to capture the equivalent of a 80mm lens, making objects larger than normal, and it will do it with the maximum "eye" opening that is a ratio of f/1.8. For capturing images in low light (a dimly lit room) it is best to use a camera lens that has a maximum opening that is described with a number less than 3 (2.8 to be exact). Your lens can do that and objects will appear larger than "normal" when captured with the combination of your APS-C sensor and lens.

 

 

Good luck.

Honored Contributor
Posts: 5,708
Registered: ‎11-13-2012

Re: Canon Lense basics

Capture.JPG

John Hoffman
Conway, NH

1D X, Rebel T5i, Many lenses, Pixma PRO-100, MX472, LRCC Classic
Esteemed Contributor
Posts: 4,951
Registered: ‎06-25-2014

Re: Canon Lense basics

[ Edited ]

@cale_kat wrote:

... 

The world of photography had changed forever but in this new world of photography the manufacturers continued to name the lenses with the "terms" that where from the age of 35mm film. In that age, the 50mm lens was generally considered a "normal" lens because it approximated the viewing angle that a human eye could see. A big "iris" combined with a short "back" focus gave big advantanges for collecting light where little exists. The comparision of the size of the opening and the "back" distance is described as a function using the initial "f". F1.0 is a wide open lens which loses nothing to the "human" eye. ...

 


I have to quibble with that last point. The "f" value (as in "f/1.0" or "f/5.6", etc.) is the ratio of the lens's focal length ("f") to the diameter of its maximum aperture (expressed in the same units, e.g. millimeters). It has nothing directly to do with the human eye.

 

Having said that, I guess I should point out that the commonly accepted notation may be a little confusing. "f/n" is not to be read as "f over n". The (dimensionless) number n is itself the ratio (of the focal length to the aperture).

Bob
Philadelphia, Pennsylvania USA
VIP
Posts: 11,096
Registered: ‎12-07-2012

Re: Canon Lense basics

Not feeling like starting the "crop factor" argument again but normal is considered to be the diagonal of the film or in this case the sensor.  A Full Frame DSLR is approx 43 mm and your Rebel is approx. 30mm.  So on the FF a 50mm  lens looks close to normal as to what the eye sees.  On a Rebel a 35mm lens looks as to have the same "normal" view.

 

Most of this is just numers, nothing more.  They are for you to refference or compare lenses.  You can tell if a lens is longer than 35mm it will act like a telephoto and if it is less than 35mm it will seem more wide angle. In this case the 50mm, you questioned, will seem like a very mild telephoto. Rest asured, your camera will take a pictrure that looks exactly like what you see in your view finder.

EOS 1DX and 1D Mk IV, even less and less other stuff.
Esteemed Contributor
Posts: 4,951
Registered: ‎06-25-2014

Re: Canon Lense basics


@ebiggs1 wrote:

Not feeling like starting the "crop factor" argument again but normal is considered to be the diagonal of the film or in this case the sensor.  A Full Frame DSLR is approx 43 mm and your Rebel is approx. 30mm.  So on the FF a 50mm  lens looks close to normal as to what the eye sees.  On a Rebel a 35mm lens looks as to have the same "normal" view.

 

Most of this is just numers, nothing more.  They are for you to refference or compare lenses.  You can tell if a lens is longer than 35mm it will act like a telephoto and if it is less than 35mm it will seem more wide angle. In this case the 50mm, you questioned, will seem like a very mild telephoto. Rest asured, your camera will take a pictrure that looks exactly like what you see in your view finder.


Well... almost exactly. Most Canon viewfinders show a little less than the sensor actually sees. It appears that you have to go to a 7D (crop) or 5D3 (FF) to get "approximately 100%" coverage. Even the 6D gives you only 97%, according to the published specs.

 

Note that there's a price to be paid for that accuracy. Those top-notch cameras give you no leeway to be sloppy with your framing. What you see is what you get.

Bob
Philadelphia, Pennsylvania USA
Valued Contributor
Posts: 425
Registered: ‎01-19-2014

Re: Canon Lense basics

Amusing how my post generates nothing but critique. Warm bunch.
VIP
Posts: 11,096
Registered: ‎12-07-2012

Re: Canon Lense basics

Bob from Boston.

 

I should have said, more or less, what you see!  Smiley Embarassed  But absolutely more than any 1.6x crop factor which was my intention.

 

Now my turn ....

 

"It appears that you have to go to a 7D (crop) or 5D3 (FF) to get "approximately 100%" coverage."

 

All my 1 series are 100% coverage.  Smiley Happy

EOS 1DX and 1D Mk IV, even less and less other stuff.
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