I shoot canon products so I am mostly thinking about EF/EF-S on a rebel series camera.

Everything I can find about crop sensor lenses state that they are lighter and smaller than full frame lenses but offer no disadvantages or reduced light gathering capabilities. They do offer a slight advantage using the center of a full frame lens offering the least amount of distortion. I understand that a crop sensor lens on a crop sensor body should equate to using a full frame lens on a full frame body, but what happens when you use a full frame lens on a crop sensor body? A 300mm full frame lens on a crop sensor body will give you an angle of view at a ratio comparable to the size of the crop factor. Technical information on the difference between crop sensor lenses and full frame lenses from various manufactures from a credible source is difficult to find, so I have a few questions.

I understand the mechanical theory of moving the rear lens closer to the sensor could/would allow for the same trigonometry and ratios of light on a crop sensor as a standard lens on a full frame camera. I also understand that the light that "prints" on a crop sensor through a full frame lens is just the center.

However, I don't completely understand how light or optics work. I relate using a full frame lens on a crop body to using a lens with a larger diameter. For example, (I am assuming) if a lens with a 58mm objective lens is the standard, using a macro with a 67mm objective lens or some of the larger telephotos at 82mm allow for more light to enter allowing for a brighter photograph for a given focal length. Does a larger rear lens, allowing more light through have a similar effect? or is the light simply not usable and "wasted"

In the design of a crop sensor lens is just the rear lens that is moved rearwards (extending the overall length of the lens)? Is the mounting flange moved forward (sliding the entire assembly rearwards), thus changing the angle of view for a given focal length? (over the same focal length on a full frame lens)

So, if a bigger hose lets more water through, a bigger lens should let more light through. If you allow an abundance of light through, will the "extra" light not contacting the sensor allow for a brighter photograph, allowing for faster shutter speeds? does the extra light bouncing around inside the body run the risk of negatively effecting the photograph?

Is the amount of light throttled by the aperture, regardless of the rear lens size? Will the aperture be negated at the largest diameters using a full frame lens on a crop body?  With a FF lens on FF body or crop lens on a crop body, aperture effects the photo from through the full range.  If you have a full frame lens on a crop body is there "slack" in the aperture where low end of the aperture (large diameter) stops effecting the photo?

Speaking only of optics, given 2 lenses with the same quality of "glass", is the only difference between a FF/Crop lens on a crop body the full frame lens only uses the center of the optics, therefor reducing the risk of defects that can be more prominent in the outermost edges of the lens?

I understand that size, weight, and price reductions in crop sensor lenses as well as the optical quality in pro grade FF lenses can set the 2 apart. "Right tool for the job". If I plan on hiking or traveling with a lens I might lean towards a crop sensor lens, however if I am leaning more towards the artistic side I can look towards the higher quality of some of the full frame lenses. But for that middle of the road stuff, medium priced medium quality (where the lines between the two start to blur), what are the functional benefits of going with a crop lens and being restricted to only using it with a crop body?

You say you shoot Canon.  What do you currently have?  What are the issues with it?  What is the specific question you want answered below? 

The difference between a crop sensor body and a full-frame body is the angle of view available to the sensor. Since FF sensors are wider, they have a wider angle of view. This has no effect on the lens. It does equate to having the angle of view of a longer lens on an FF body, but all you get is the narrower view -- none of the magnification.

The advantage the lens makers can get from a cropped sensor is that since the sensor is physically smaller, the sensor can use a smaller light circle from the lens thus allowing the lens to be narrower thus using less glass.


The light coming from a full-frame lens needs to be in a circle big enough to cover the full-frame sensor. Much of this light falls outside the cropped sensor and is just wasted.


It does not work that way. Because the cameras are designed to use both FF and crop lenses, the two sets of lenses interact and connect with the camera in the same way.


In theory, all the extra light falling outside the sensor and bouncing around could negatively effect the image. However, since the lens creates a circle of light, the cropped lens still creates "wasted" light, just less waste than the FF lens. Since this is a known given, the inside of the camera is designed to absorb all the wasted light.


The amount of light is throttled by the aperture. The widest aperture is throttled by the width of the lens. The size of the rear of the lens is always bigger than the needs of the sensor.

There is no slack in the aperture of an FF lens on a crop body. The aperture governs the angle in which light focuses on the sensor. It does not control the width of the light circle generated by the lens. The aperture does govern the brightness of the light circle.


Yes, because the cropped sensor is narrower than the FF sensor, the cropped sensor uses only the middle parts of a FF lens thus eliminating any issues in the lens at its corners.


The benefits of a cropped sensor lens on a cropped sensor body (as opposed to a FF lens on the cropped sensor body) is purely size and weight. Assuming Canon is like Nikon, you can get pro-level lenses in both FF and cropped; and you can get consumer-level lenses in both FF and cropped.

You would never choose to use a cropped sensor based on the lens. You first choose to use a cropped sensor for the advantages that it has over a full-frame sensor, then you pick which lens to use on that sensor.

That is a lot of good information. This has been a long time coming for me. I got into cameras because I was traveling and wanted to share pictures. Now its more of a primary hobby and I am trying to learn a bit more about it. Most of this information is "this is what you do" but not the in depth technical "why". This has cleared up a lot for me.

Canon only makes their pro grade glass for their full frame line. Though from what I have found the new crop lenses will give old pro (L) lenses a run for their money. But, having known this, it may have lead me down the path of Nikon. I started with canon because I had several friends who shot canon, so help on the fly was always there. (Now I have a T3i, T6i, and picked up a T5 for my gfs kid.)

I guess the last question in the air is that canons EF-S (crop) lenses protrude deeper into the body (smaller rear lens closer to the sensor) I just wonder if the distance/length is added on to where a full frame lens would be. or if the whole design of the lens is shifted rearwards. I would think that moving the entire design rearwards would allow for a wider angle of view given the same diameter/length lens. (holding your hand in a ring in front of you face and then moving it slightly closer to your eye allows for more to be viewed through the same size hole.) though all bets are off if there is a bottle neck due to smaller glass inside the body of the lens.

Now, I understand that each manufacturer will have a different design, and I will probably be able to come across the answers for canon and nikon, but rokinon, tamron, sigma, etc will be more difficult to find information on. Not sure if there is a standard or not.

The way each maker go about with the physical characteristics of the lens is completely up to them as long as it remains in spec for the camera body.

However, this does create compatibility problems for lens accessories like telecoverters. Even with with same maker, not all lenses will even mount on the teleconverters. Nikon is a good example of this, where many of their consumer grade lenses sit too far back into the lens well resulting in glass touching glass with the Nikon teleconverters. In this case, teleconverters made by Kenko often will mount on those Nikon lenses due to being designed differently so that there is more space available.