Lens Sizes / Uses / Meanings

[Caroline_7]

I currently have the EF-S 18-55mm and EF 75-300mm lens for my Canon Rebel T3i. I was graciously gifted a 40mm lens for Christmas with the option to exchange it for another lens. One would want to think that the 40mm is covered within the 18-55mm lens but I am completely unsure how this works so any assistance with suggestions, size meanings and/or what these are greatly used for would be greatly appreciated! Thank you in advance!

[Skirball · Accepted solution]

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@Caroline_7 wrote:

I currently have the EF-S 18-55mm and EF 75-300mm lens for my Canon Rebel T3i. I was graciously gifted a 40mm lens for Christmas with the option to exchange it for another lens. One would want to think that the 40mm is covered within the 18-55mm lens but I am completely unsure how this works so any assistance with suggestions, size meanings and/or what these are greatly used for would be greatly appreciated! Thank you in advance!

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Yes, the 18-55mm lens can capture the same field of view as the 40mm lens.  The difference is that the 40mm can do so at a wider aperture (allows more light in, and allows a smaller depth of field for that 'blurry background' look).  It will also produce sharper images than the 18-55.  How much sharper depends on how you use it.

Prime lenses (no zoom) aren't for everybody.  You have to use your feet instead of zooming in and out.  Also, they (usually) don't have image stabilization,which your 18-55 has.  The tradeoff is that they can produce extremely sharp images, and allow you to shoot in dim light situations. 

I happen to love them, so perhaps I'm not the best person to comment.  But one of the main benefits of prime lenses is a really wide aperture.  The 40mm "pancake" lens actually isn't all that wide at f/2.8.  It's better than your zoom, but not as wide as something like the 50mm 1.8 "nifty fifty".  The main benefit of the pancake lens is just that, it's extrememly thin, for people who want a small lens.  Personally I would recommend the 50mm lens over the 40mm, since it has a wider aperture.  But I've never used the 40mm - there are a lot of people who like it.

Either way, I do recommend trying a prime lens.  As I said, they're not for everyone.  But you won't know until you try.  And what better way to try than from a gift.

[ScottyP]

Exactly. Don't be fooled by the seemingly redundant focal length that a 50mm or 40mm prime is not a great addition. The f/2.8 40mm will shoot in a fraction of the light that your zoom lens can, and the f/1.8 will shoot in less than half of what the 40 would need. Also sharper. Also giving you the professional looking blurred background with sharp subject popping out from it.

[RCOONa]

Good advice above, but keep in mind that the 50mm is actually 80mm on your t3i. Depending on circumstances, it may or may not make any difference when compared to the 40 mm prime, but at least one should be aware.

[Skirball]

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@RCOONa wrote:
Good advice above, but keep in mind that the 50mm is actually 80mm on your t3i. Depending on circumstances, it may or may not make any difference when compared to the 40 mm prime, but at least one should be aware.

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No it's not 80mm. It would produce something similar to what you would see if you put a 85mm lens on a full frame camera, but I hardly see what that has to do with the conversation other than confuse it.

The 50mm is 50mm, that never changes.  The only time conversions have any use is if you want to change camera formats (crop to FF or vice versa) and you want to know what focal length you should use to mimic the other.  If you only shoot one format then don't even bother paying attention to it.

The 50mm will produce the same image as the 18-55 would if set at 50mm.  Which would be slightly narrower than the 40mm or the 18-55 at 40mm.

[TCampbell]

One of these days I'm going to post a video or picture to help clear this up.

Take a pencil (or any straight dowel, stick, chopstick, wooden bar-b-que squewer, etc.)   Measure 44mm from the end of that 'stick" and draw a mark.  

Remote the from the camera and use a rubber band to strap the "stick" to the side of your lens so that the 44mm length sticks out beyond the BACK of the lens (using that mark you drew as the reference point).

Take a sheet of plain white paper and draw two rectangles on it.  The larger rectangle should measure 36mm x 24mm.  The smaller rectangle should be drawn just inside the first rectangle (centered) and it's dimensions should be 22mm x 15mm.

Hold that sheet of plain white paper behind the lens so that the stick is just touching the paper.  This means your piece of paper is now exactly 44mm from the back of your lens (which is precisely the distance to your camera sensor had that lens been attached to the camera.

Point the lens at anything (preferable the paper and lens should be in shadow but it'll be easier to see the image if your subject is brightly lit.

The image you see in the SMALLER rectangle is what the lens produces when attached to any camera with an APS-C size sensor (any Rebel body as well as any mid-range body such as a 60D or 70D as well as any 7D model.)

The image you see in the LARGER rectangle is what the lens produces when attached to any camera with a full-frame sensor (the 6D, any 5D, and the 1D X. 

The image you see is not merely an analogous to how the "crop factor" works... it is PRECISELY how the crop factor works.  The lens really is projecting the entire round image you see on that sheet of paper into the camera body (btw, this image will look "dim" and that's because inside your camera... it actually IS dim.)  The rectangles are actually drawn to the same dimensions as your camera sensor (depending on if you have a "full frame" model vs. an APS-C "crop frame" model.)

The point I hope the experiment will show is that it really is the same lens and the same image.  The difference is just the size of the sensor.  More image spills off the sides when you have an APS-C sensor than woudl spill off the sides when you have a full-frame sensor.  But that also implies that the angle of view is slightly narrower AND if you planned to print an arbitrary size (e.g. pick a 4x6" print) then you'd need to enlarge the APS-C image a little more than the full-frame image to fill the size of the print.

It turns out that these two 4x6" prints wont look the same (because the angle of view is different).

HOWEVER... if you took an 80mm lens and attached that to the "full frame" camera body, but left the 50mm lens on the APS-C body, then the resulting images would have approximately the same angle of view.

50mm is 50mm... the difference is how much the projected image got cropped due to the different sensor sizes.

[RCOONa]

Mea Culpa, Skirball, you are right, of course. 50 is a 50 is a 50, It would just " produce a picture similar to an 80 mm on a full frame camera" as you say. I stand corrected.

[ScottyP]

People always talk about the DOF being shallower on FF.

Is that only if you adjust the distance to the subject to create the equivalenf framing for both lenses in the comparison? (You increase the distance to subject on the crop to get equivalent framing, and longer distance = thicker DOF?)

[Skirball]

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@ScottyP wrote:
People always talk about the DOF being shallower on FF.

Is that only if you adjust the distance to the subject to create the equivalenf framing for both lenses in the comparison? (You increase the distance to subject on the crop to get equivalent framing, and longer distance = thicker DOF?)

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Not sure if you're referring to my comment or not.  I wasn't referring to the increased bokeh (presumably, thinner DoF) with respect to FF; I was comparing a f/1.8 50mm versus the f/4.5 or whatever the kit lens would be at 50mm.

[ScottyP]

Hi Skirball. No, I was following up on Tim's comment explaining crop v. FF.