Calculating the true reach of a lens

[martin_jordan]

<p>So many threads I've read but can't find this where I understand.<br>

Simple question:<br>

I have a 40D. No matter what lens I put on it, what is the math equation to figure out the focal length of a lens?<br>

In other words, if I put my 70-200 IS on my 40D and crank it out all the way...what is the real magnification I'm shooting at on the 40D?<br>

thx</p>

[felix_mizioznikov]

<p>you would just need to multiply your focal length by 1.6.<br>

why would this matter to someone?</p>

<p>I'm shooting with full frame for the length of the lens is the real length.</p>

[john_macpherson]

<p>Assume 50mm is the focal length of your eye, so a 200 divided by 50 = 4, so gives 4 times magnification of the object as it appears to your eye.</p>

<p>Then factor in the 'crop' factor of the camera eg a 1DMk2 is 1.3 ................so 1.3 x 4 = 5.2 times magnification.</p>

<p>A 40D I think has a 1.6 crop factor (yes?) so ...............1.6 x 4 = 6.4 times magnification.</p>

[scott_ferris]

<p>I don't agree with John's second bit, just the first bit.</p>

<p>A 200mm lens gives an effective cropped 4x magnification. That is it. There is no extra magnification due to the crop factor, the subject is not larger on the sensor, so it does not come into the equation.</p>

<p>Your 70-200 appears to give you more magnification on your 40D than on a FF but it doesn't.The subject is the same size on the sensor. At 200mm you have 4x magnification over eye view with either camera. The scene through your 40D gives you the apparent angle of view of a 320mm lens on a FF body, but it is not, it is just a crop of what the FF could see.</p>

[tdigi]

<p>If you look at 200mm on a 5D ( full frame ) and a 40D ( 1.6 crop ) you will see the exact same magnification. The only difference is the 40D will be cropped in. This will give you more pixels in the smaller area, to achieve this on the 5D you would need to be zoomed into 320mm to get the same pixel density the 40D gives you.</p>

<p>http://jimdoty.com/Digital/fov_crop/fov_crop.html</p>

 

[martin_jordan]

<p>Thanks Tommy. That's what I was looking for.</p>

[bobatkins]

<p>Try this: <a href="http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html">http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html</a></p>

<p>It's even got diagrams as well as words and it goes intp more detail than I can here.</p>

<p>The bottom line is that the image is the same size on all sensors. It fills different fractions of the frame, but it's always the same size from the same lens.</p>

[mendel_leisk]

<p>I agree with both bits of John M.'s explanation. Cropped sensors <em>do</em> get you more magnification, for all practical purposes.</p>

[scott_ferris]

<p>Mendel, with respect, you can believe the moon is made of cheese but it isn't. If you don't believe my words look at Tommy's link or Bob's one. They explain it clearer,but the truth is crop cameras don't "give" you anything, they just give you less than a FF, a crop in fact :-)</p>

<p>By the way I use crop and full frame cameras.</p>

[tdigi]

<p>True scott but if you where to shoot something at 200mm you would have more pixels in that cropped space so there is some benefit depending on the shot. Its the wide end that is frustrating.</p>

[aral]

<p>The focal length of the lens is physical property of the lens and does not depend on anything external to the lens, like camera body or sensor size. This holds also for other properties of the lens like f ratio, angular resolution, abberations, etc.<br>

What seems to be the question is the angle of the recorded image or the extent of the captured frame. It is proportional to the size of the sensor. 40D has 1.62× smaller sensor (i.e. 22.2 mm × 14.8 mm) than Full Frame sensor (i.e. 36 mm × 24 mm), so the recorded angle is 1.62× smaller. In other words, to record equivalent image with the Full Frame sensor one should use 1.62× longer lens, i.e. 1.62×(70 to 200 mm) = 113 to 324 mm.<br>

But a 70 to 200 mm lens is always 70 to 200 mm lens, no matter what you use to record the image.</p>

 

[mendel_leisk]

<p>I understand the theory. But the <em>full</em> frame on the crop body is 1.6 times closer. That isn't cheese ;)</p>

[scott_ferris]

<p>Tommy, only useful if the density on your crop camera is higher than your ff one and even then only if those pixels actually record useful information. Take the case of my 1D, a 1.3 crop camera and the same shot taken from the same place with the same lens with a 5D MkII and then cropped down. The 5D Mk II image has more pixels in the cropped image than the crop camera did to start with. Another instance is the 50D, it has a higher pixel density than the 5D MkII but it doesn't record more information, the airy discs are too big and cover more than one pixel at generally used f stops. So in this case the 50D might have more pixels but the enlargement from a cropped 5D MkII image would be indistinguishable.</p>

[JDMvW]

<p>The only reason it matters at all is if you are shooting with the same lenses on different sensor-size cameras. there, as said, to find out the <strong>35mm-film camera equivalent</strong> for old-timers who can't disengage their brains from the salad days of their youth, you multiply the 35mm category focal length by 1.6 to get an "equivalent".</p>

<p>However, a 50mm lens is a 50mm lens, no matter what camera it is used on. I have an 80mm Biotar f/2.8 lens in a Pentacon 6 mount. On a 6cm camera, it is the normal lens, giving a field of view roughly like that seen by the human perception. On a 5D (35mm film size sensor), this lens would be a short telephoto, and very good for portrait work. On a 15x22mm sensor camera like a digital Rebel or a 50D, this same lens (still an 80mm lens) would be a regular telephoto lens. Only the <em><strong>crop</strong> </em> (part of the image thrown by the lens that is used) changes.</p>

<p>So for your camera, roughly, a<br /> "ultra-wide" = 10-17mm lens<br /> wide angle = 18-25mm<br /> "normal" lens =26-38mm<br /> portrait or "short" telephoto = 40-60mm<br /> regular telephoto = 75-100mm<br /> long telephoto = 135mm and up.</p>

<p>It's the use of a particular photographic lens that matters for a particular format, and if you don't already know what the focal lengths on 35mm film cameras were used for, there is no reason on this earth or in heaven for you to worry about conversion multipliers.</p>

[douglas lee]

<p>

 

 

<p >I understand the theory. But the <em >full</em> frame on the crop body is 1.6 times closer. That isn't cheese ;)</p>

<br />

No it isn't. That is what Bob and Tommy are trying to explain. I own both a 40D and 5D MkII. If you are fifty feet from the subject and switch to another camera, you are still 50 feet from the subject. Switching bodies does not magnify anything. The magnification of a lens is X. Putting it on a different body does not change X. Switching bodies results in a different crop. That is different than magnification.

 

 

</p>

[mendel_leisk]

<p>I full understand that putting the same lens on full frame or crop sensor, the image it casts at the sensor plane has not changed. But, the full frame of the crop sensor will deliver a "closer" picture. I understand the why's and how's of it. For the record, I've got both full frame and crop sensor cameras at hand.<br />If there's an eagle sitting in a treetop, I might take one shot with my 5D with a 200mm, then borrow my wife's 30D and swap the 200mm over for a second shot. The second shot will fill fill <em>that</em> camera's frame more, by a factor of 1.6. It will be magified by that factor, speaking practically.</p>

[William Michael]

<p>

<p> . . . or stated another way. . . <br>

<br>

Sticking the 70 - 200 on a 5D and taking a picky, and then whipping it off and taking the same iamge with a 40D . . . <br>

<br>

would be exactly the same as taking a picky with a 35mm FILM SLR and then cutting out the centre rectangle of that negative, to be the same size, as the 40d's sensor . . . <br>

<br>

the IMAGE on the negative remains the same size. <br>

<br>

If we then print a full frame crop of the 35mm negative, and then we print a full frme crop of the cut out centre bit, both to 10" x 8", for example: obviously the print from the smaller negative (sensor) will show the subjects appearing relatively bigger, in the 10 x 8 print. <br>

<br>

But the original image is the same size on the negative (or either sensor).<br>

<br>

WW<br>

</p>

<p > </p>

 

</p>

[steve_crist]

<p>Every 50mm of lens length equals approximately one power of magnification. This is based on full frame. If you have a crop body multiply that also. Example 200mm lens on full frame equals 4 power magnification (200/50=4). 200mm lens on a 1.6 crop body equals 4 power times 1.6 or 6.4 power magnification ([200/50]*1.6=6.4).</p>

[steve_crist]

<p>Every 50mm of lens length equals approximately one power of magnification. This is based on full frame. If you have a crop body multiply that also. Example 200mm lens on full frame equals 4 power magnification (200/50=4). 200mm lens on a 1.6 crop body equals 4 power times 1.6 or 6.4 power magnification ([200/50]*1.6=6.4).</p>

[douglas lee]

<p>Perhaps we all agree. The problem is the use of the term magnification. As JDM and WW point out, there is no magnification. Nothing gets made larger. A particular lens projects a particular image circle. Changing the "screen" size does not change the magnification.</p>

[jphotog]

<p>You must be shooting really, really small people to fit them on the sensor if you have a 6.4 power magnification. The 1.6 crop factor has nothing to do with magnification. It is useful to calculate a full frame (35mm film) equivalent to understand how a lens will affect your photography.<br>

I do not understand what you mean when you say a 50mm has one power of magnification? A 50mm lens is called a 'normal lens' because the relation between objects appear similar to how we see it with our eyes. A telephoto lens makes object close appear further away and object at a distance closer (compressing the depth) and a wide angle lens does the opposite.</p>

 

[scott_ferris]

<p>Jonas,</p>

<p>Traditionally one power of magnification represents the scene to the same scale as your eye, if you have a 50mm lens on your ff camera and put it to your eye the objects will appear as they did before you held it up, not the same field of view just the object size, it is the same as your eye so is 1x, 0x would be with your eyes closed :-)</p>

[jphotog]

<p>Scott, I believe that is not correct.</p>

[scott_ferris]

<p>Jonas, unless the laws of optics changed in the last 30 years then it is. A telephoto lens is a greatly misused term, it actually meant a lens that is shorter than its focal length, but it has come to mean anything with a FOV narrower than an 85mm lens and below on 35mm cameras (longer focal length).</p>

<p><em>"A telephoto lens makes object close appear further away and object at a distance closer (compressing the depth) and a wide angle does the opposite" </em> The first part of this doesn't make sense though, a telephoto, long focal length lens, makes all objects appear closer and apparently compresses the perspective.</p>

[jphotog]

<p>Ok, I expressed myself poorly. My statement assume that you ´zoom´ with your feet to cover about the same subject.<br>

I do not have a clue about the definition of a telephoto lens so I give you right there.<br>

At least in my camera, the objects appear smaller in the viewfinder than before I put the camera to my eye using a 50mm lens. However, the perspective <br />(relation between objects) is similar to the human eye. Maybe we mean the same?</p>