Crop Body math

[Misfitlimp]

So i own a canon 50D and I know the lens i use isnt what it seems. For example if i have my lense at say 28mm it really isnt 28 because i have to multiply it by 1.6 so my question is do i add the diffrence or do I just multiply for example 28x1.6= my actaul focal length or is it 28x1.6=x and then i add x to my 28mm to get my actual focal length? sorry for the newb question and for turning TPF into math class.

 

[FrankLamont]

Your 28mm lens would have an equivalent of 44~45mm in 35mm film format/full frame.

In other words, don't worry about it, unless you're judging ultra wide, in which case get a 10-22mm for the equivalent 16-35mm.

 

[Misfitlimp]

Your 28mm lens would have an equivalent of 44~45mm in 35mm film format/full frame.

In other words, don't worry about it, unless you're judging ultra wide, in which case get a 10-22mm for the equivalent 16-35mm.

My question was how did u come to the conclusion that its 44-45? Im worring because im taking a class and on an assignment my instructor gave us he asks for certain things like to set our lense at 50mm if i go and set it to 50mm and it actually isnt 50mm I would not b fullfiling reqiuerments i have for that said assignment so you can see my predicament.

 

[camz]

So the forumla would be 1.6 x (lens focal length) = actual focal length

That's how he/she got it 28mm = 44~45mm

 

[Montana]

Multiply the focal length of the lens by 1.6. 50mm lens=80mm field of view. Its still a 50mm lens though. I think you should talk to your instructor a little. Is he wanting you to take pictures using 35mm equivalents?

 

[Misfitlimp]

Multiply the focal length of the lens by 1.6. 50mm lens=80mm field of view. Its still a 50mm lens though. I think you should talk to your instructor a little. Is he wanting you to take pictures using 35mm equivalents?

Well he knows people have crop bodys and he told everyone the whole 1.6x deal and wants us to figure it out. so im sure he wants full frame equivalants. so if i wanted 50mm i would shoot at about 35mm?

 

[Montana]

yes.

Is the class have people with both film and DSLR cameras? If it is all DSLR, I would think that the majority of the class would be shooting crop. (full frame DSLR being a tad on the expensive side for college kids) Good to see you taking a class. I wish there were some offered near me.

 

[Misfitlimp]

Thanks for the help guys

 

[KmH]

Just to be clear.

The focal length of your lens is 35mm regardless, and the crop factor produces an image that would have the same Field-Of-View (FOV) that a 50 mm lens would have on a full frame (35mm) camera.

 

[CW Jones]

Your 28mm lens would have an equivalent of 44~45mm in 35mm film format/full frame.

In other words, don't worry about it, unless you're judging ultra wide, in which case get a 10-22mm for the equivalent 16-35mm.

My question was how did u come to the conclusion that its 44-45? Im worring because im taking a class and on an assignment my instructor gave us he asks for certain things like to set our lense at 50mm if i go and set it to 50mm and it actually isnt 50mm I would not b fullfiling reqiuerments i have for that said assignment so you can see my predicament.

Ok well to start off with... the teacher will never know that it wasn't set at 50mm.... most teachers say to use the 50mm because it close to what the human eye sees.

Edit: second thought... I guess the teacher will know if its a digital class, I assumed it was 35mm for whatever reason haha

 

[choudhrysaab]

you're welcome

 

[tom beard]

I am a noob too and was confused. I found out that the FX format sensor is the same size as a standard 35mm frame, so an 18-105mm=18-105. The DX format sensor is smaller so the Field of View (FOV) changes. In the DX format an 18-105mm translates to 27-157.5mm. That is called the APS Equivalent (Advanced Photo System). I found a website that does the math for you. It is: slrgear.com/reviews/showcat.php/cat/13. They have a long list of lenses (focal lengths and brands). Just find the lens you want, click on it on it and then click on Specifications. Five lines down the APS Equivalent is listed. It will also list many other specs. I found this very helpful.

Tom Beard

 

[table1349]

Your 28mm lens would have an equivalent of 44~45mm in 35mm film format/full frame.

In other words, don't worry about it, unless you're judging ultra wide, in which case get a 10-22mm for the equivalent 16-35mm.

My question was how did u come to the conclusion that its 44-45? Im worring because im taking a class and on an assignment my instructor gave us he asks for certain things like to set our lense at 50mm if i go and set it to 50mm and it actually isnt 50mm I would not b fullfiling reqiuerments i have for that said assignment so you can see my predicament.

Ok well to start off with... the teacher will never know that it wasn't set at 50mm.... most teachers say to use the 50mm because it close to what the human eye sees.

Edit: second thought... I guess the teacher will know if its a digital class, I assumed it was 35mm for whatever reason haha

Unless the teacher has a really good eye. As stated above, focal length is focal length period. A 50mm lens is a 50mm lens no matter what format it is on. Different focal lengths have different characteristics. 50mm produces an image that is most like what the eye sees in terms of perspective.

Wider than 50mm and the perspective the lens gives tends to widen the subject matter due to the wider field of view that the lens sees. Take a 24mm lens and take a head shot portrait of a person. Take a 100mm lens and take the same head shot of that person from the proper distance to give you the exact same field of view and look at the features of the person. The 24mm will make those features look wider than the 100mm will.

When people talk about crop sensor bodies and focal length they are actually talking about the size of the field of view of view that the sensor actually sees. The lens produces the same field of view on all bodies. The sensor size determines how much of that field of view it can detect.

 

[musicaleCA]

most teachers say to use the 50mm because it close to what the human eye sees.

Incorrect. The human eye perceives a very wide field-of-view. Look straight ahead and then focus on the view around yourself; note how you can see almost a full 180º horizontally. The retina covers a whole lot of the back of the eye, save for the blind-spot where the ocular nerve connects to it. These outer edges are extremely blurry, lack detail, and are monochromatic (your brain can fool you though, if something with a continuous tone is extending from your sharpest field-of-view into this area by filling it in with approximate colour).

The fovea, on the other hand, has an extremely narrow angle of view; about 2º from centre, or a dime held at arm's length. This area is packed with colour-sensitive cones (there are three types, one each for red, green, and blue, though we have mostly green, then red, then blue photoreceptors). The fovea also has some rods, but they are vastly outnumbered by cones. This is your sharpest angle of view; a paltry 2º. You hit the blind spot on the interior side (side closest to the nose) at approximately 10º. Visual acuity has already dropped almost three-fold by this point.

See here for a graph.

Coming back to lenses and how all this shapes-up next to lenses. The first thing to keep in mind is that the eye is essentially circular; a sensor is rectangular. So while the diagonal, vertical, and horizontal angles of view are the same for the eye, they differ on a camera depending on the sensor's dimensions and size.

The fovea, at a mere 2º angle of view, is roughly equivalent to the angle of view of a very long telephoto lens. A 35mm sensor is 36mmx25mm, making the diagonal roughly 43.3mm. Then we do a quick calculation (I just know the approximate focal length off-hand; it's 600mm): angle-of-view = 2arctan(43.3/2(600)). The angle of view comes-out to around 2º, with a little rounding in there for my own sake (I don't have a scientific calculator).

So your sharpest angle of view, the fovea, is actually equivalent to the angle of view of a 600mm lens on a 35mm sensor. Doing the above calculation again, for a 50mm lens, we find that the diagonal angle of view is actually around 49º. That's sure far away from our 2º fovea. If you look at the above graph again, you'll see that our visual acuity at an angle of 49º is just so paltry as to be practically useless for anything beyond detecting fast motion.

This entire 50mm = human eye thing is a misnomer.

As for compression of field (by this I mean how relative distances from the camera lens effects how such objects are rendered on the sensor or film; wide-angle lenses produce images where what is a little closer to the lens is much larger, yadda yadda), 50mm on a 35mm sensor is pretty nice; no wide-angle-like distortion, no obvious compression. It's for that reason that I can understand how people come to see this focal length as close to what the eye sees, but that point is moot as well when one considers how the human brain interprets the information from both eyes. We compensate a lot for the wonky distortions that we see with wide-angle and telephoto focal lengths. You can see a little of this if you close an eye and look at something up close; you've removed the brain's ability to use binocular cues to interpret what you're seeing better. Though, even then the eye and brain do a great job at not messing things up (the fact that the eye is physically smaller than a camera lens, and your retina smaller than a 35mm sensor, has quite a bit to do with it). Pretty cool piece of biology, the eye.

 

[Dwig]

most teachers say to use the 50mm because it close to what the human eye sees.

Incorrect. The human eye perceives a very wide field-of-view. ...

Quite true, the human visual field is extremely wide, overall. There are other issues though. The binocular field is much narrower. Perhaps somewhat wider than the "old wive's tales of the 50mm" states, but far less than the 160-200 degree overall field.

The issue of perspective, again, is more complex and the old wive's tale is wrong again about the 50mm. The old usefull definition of a "Normal" lens is one with a focal length equal to the diagonal of the image (42mm for 35mm full frame). Such a lens gives very much the perspective that the human brain expects (normal perspective hence normal lens) if, and only if, the image is viewed at a distance roughly equal to the diagonal measure of the image or, if enlarged, the print. Shoot 35mm with a 42mm lens and view the resulting 4x6 print at 7" or an 8x12 at about 14" and you will see an image with very "normal" perspective. Change the lens or the print viewing distances and perspective changes.

The "myth of the 50" comes from 50mm being the long standing standard lens on 35mm cameras and the fact that in some uses "standard" and "normal" have very similar definitions. One gets swapped for the other due to sloppy language use and, bingo, a myth is born.

The 50mm lens was the early standard because of technical limitations. In the 1920's, when 35mm began its trek to becoming a common format, lenses needed to be about 20% longer than the image diagonal. To cover 24x36 at top quality you needed a 50mm lens at the shortest. Oscar Barnack choose 50mm for the lens on the first Leica for that reason alone. Optical designs and fabrication techniques improved over the years, but by the time shorter lenses that were true Normals became practical SLRs were driving the market and clearing the mirror became the big challenge. Even today, its noticably more difficult (read: more expensive) to make a fast (>f2) lens shorter than 50mm and clear the mirror on a full frame body and still cover the full frame image. The few 40-45mm lenses made for 35mm SLRS have been f/2.8 lenses. Many fixed lens RF cameras have used true normals in the 35-45mm range. Both Nikon and Leica marketed fast 35mm lenses as standard on their RFs in addition to 50mm lenses when such lenses became practical in the late 1950's.