Questions about the 1.6X "conversion" in DSLRs

[Milton Miszputen]

Hi,


Can someone help me with the following questions about the 1.6X "conversion" when using EF lenses in a DSLR body, like the 50D?


1. Is that true that "what you see (in the viewfinder) is not what you get (in the picture)"?


2. Does the image appear "uncropped" in the viewfinderbut full in theLCD screen? If so, will we have to guess, when using the viewfinder, what will or will not appear in the final picture?


3. Does the perspective also change when the wide-angle "becomes" normal or it's just a matter of loss of the peripheral image?


4. To solve that is it necessary to have a full-frame body? If so, from which Canonmodel on we get this function?


Thanks a lot.


Yours,


Milton

[Maleko]

1. You see 95% of the image in the viewfinder


2. Not sure, I can only guess its the same 95% - someone else can clarify that, i dont use the viewfinder :P


3.? Not sure what you mean.


4. The 5D has a full frame, along with: 1DS MK1&2, the 1D has a 1.3 Crop.

[1st-Insp]

Hello Milton,


I use a Rebel XTi and just ordered a new 50D (from B&H to support Bryan's web site) because of the 1.6x factor. I shoot a lot of interior/exterior real estate and I'm getting into (children) sports photography.


What you see in the view finder (VF) is what you get in the final picture (almost). The only difference is how well designed the VF is. In the Rebel, I think I only see about 95% of the framed picture yet in the new 7D you see 100%. In other words, in my Rebel, my final picture has a little more image than what I saw in the VF.


As far as wide angle (or any lens), you simply multiply the lens by a factor of 1.6. So if you have a 35mm primary lens, you will see the full frame camera equivalent of 52mm. I use the excellent EF-S 10-22mm lens (equivalent of 16-35mm on a FF body) and get the same results (image size) as with a FF camera with a 16-35mm lens.


In my opinion, if you need a longer lens and do not have the money to buy the longer L glassor want to carry the extra weight around, get a 1.6x camera. I have a 70-200 2.8 IS and it is so long that I mostly use if for outdoor sports. My 24-70 is what is on the body most of the time. If you must have the ultimate in sharpness, and can afford the longer L's, spend the extra money on a FF body.


However, if you buy any EF-S lens, you can not use it on a FF body. All the new EF lenses can work on both a 1.6x and FF body. Someone correct me if I'm wrong with this next point. My understanding is that if you use a EF lens on a 1.6x body you are shooting in the center of the lens. In other words, the sweet spot of the lens. For example, you will see less vignetting in a EF lens on a 1.6x body vs a FF body. There are other pluses and minuses as well.


Tom

[wickerprints]

1. The oft-quoted 1.6x "crop factor" has nothing to do with viewfinder coverage. What you see in the viewfinder is almost everything you will capture in the image, save for a small margin around the edges.


2. No. The viewfinder will match (mostly) the captured image area, no matter what SLR camera you use.


3. Think of it this way. A given lens will cast a spot of light onto the focal plane, which contains the projected image. That circular spot of light is the same size for all EF lenses, but for EF-S lenses, it is smaller. In order to record the projected image, you could put a piece of light-sensitive film there, or in the case of a dSLR, you could put a digital electronic sensor. Imagine putting a rectangle inside a circle so that the two figures share a common center. If the rectangle is much smaller than the circle, you capture only that small central portion of the image. If the rectangle is made larger than the circle, then parts of the sensor will not be exposed to incoming light, leaving it black. Now if you make the rectangle so that its diagonal roughly matches the circle's diameter, then you are capturing as much as you can of the circle whilst ensuring the entire sensor is exposed to light.


How this relates to the crop factor is this: The EF lenses were made to accommodate 35mm film, so they project an image circle that is large enough to completely cover a rectangle of width 36mm and height 24mm. This is also the size of a 35mm "full-frame" digital sensor. Now, some sensors are smaller, and the ones used in some Canon bodies are called APS-C format. Just like the example I previously described, you can still use the EF lenses because all you have done is made the sensor smaller, and thus you are only capturing the smaller central portion of the available light. However, Canon has also made EF-S lenses, and these can ONLY be used with APS-C format sensors, because they project an image circle that is too small to expose a 36x24mm rectangle.


When we speak about focal length, we are talking about an intrinsic optical property of the lens, and the lens alone. Thus, a 50mm lens is always a 50mm lens, regardless of what you have used to capture the image that lens has projected. You could modify the focal length of the system by adding other optical elements or changing their position relative to the focal plane, but the fact remains that the size of the recording format is unrelated to the focal length.


However, the apparent angle of view of the captured image *does* depend on what we use to capture the image. If you use a small sensor, the angle of view is narrower than if you use a large sensor. This is what the 1.6x crop factor is for. If you use a 50mm lens on a "full-frame" body, in order to get an image with the same angle of view on an APS-C body, you would need to use a lens with focal length 50/1.6 = 31.25mm.


4. Please see my response to item 3 above. Again, you are misunderstanding the meaning of this 1.6x factor. It has nothing to do with viewfinder coverage--what you see in any SLR viewfinder will always closely match what you will capture in terms of coverage. The only thing the 1.6x factor does is tell you what focal length is needed in order to achieve the equivalent angle of view between an APS-C sensor and a 35mm sensor. If you don't own both formats, then you have no need for this so-called "crop factor.

[Ehcalum]

As was hit on eariler, what you see in the viewfinder ranges from 95-100% of the final image.


The LCD will show you the 100% image that you saw in the viewfinder.


a 1.6 or 1.3 body does not magnify a lenses range. A 300mm is still a 300mm on a crop body. What happens is the field of view is shortened, causing the appearance of a longer focal length, when in fact it's still a 300mm. In order to get true magnification, you will need a longer focal length, or step closer to the subject.





People complain about losing the wide end of a lens on a crop body, just take some steps backwards and you'll be fine.

[Daniel Browning]

Lots of good answers already, but why not one more?

1. Is that true that "what you see (in the viewfinder) is not what you get (in the picture)"?

No, it's not true. As others mentioned, there is a slight inaccuracy of the viewfinder on most cameras, but it applies equally to EF and EF-S lenses. For example, a 17mm EF lens (e.g. 17-40 f/4) will give you the same view as a 17mm EF-S lens (e.g. EF-S 17-55 f/2.8).

3. Does the perspective also change when the wide-angle "becomes" normal or it's just a matter of loss of the peripheral image?

The perspective will not change unless you change subject distance. But it's generally better and more natural to change focal length instead of subject distance. For example, say you are using the 17-40 f/4 on the 50D. At 40mm, you can take a headshot that has slightly emphasized perspective (large nose). If you upgrade to the 5D2, and continue using the same lens, and same focal length, then to get a head-and-shoulders shot, you will have to stand closer. That will cause very strong perspective (very large nose) and will probably look terrible. Instead, you will probably buy a 70-200 lens and use it at 70mm, so that you can back up to the same distance you were at when you used 40mm on the 50D. I hope that helps.

[Yves]

Well explained from "wickerprints"...with one exeption:

However, Canon has also made EF-S lenses, and these can ONLY be used with APS-C format sensors, because they project an image circle that is too small to expose a 36x24mm rectangle.

It's true, EF-S lenses ONLY can be used with APS-C format sensors. But the reason for this is, that the construction method of the EF-S lenses (lens parts closer to the focal plane) disturbs the mirror movement of full frame bodies and causes damageson the mirror. And You won't get a picture with the (broken) mirror in line with lens and focal plane...


Anyway, You can't use EF-S lenses on full frame bodies.

[wickerprints]

Yes, I am well aware of the fact that the "S" in EF-S stands for "short back focus." I am aware of the mechanical reasons why EF-S is not compatible on 35mm format bodies. I only wrote the above to avoid making my response unnecessarily complicated for the original poster and to avoid providing details that I did not consider relevant to the discussion at hand.

[Chuck Lee]

Anyway, You can't use EF-S lenses on full frame bodies.
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FYI,


EF-S lenses from Canon cannot be used but other manufactures lenses designed for APS-C sensors can. I can mount the Tamron 17-50 f2.8 DII to my 5D. The mirror does not hit it. The only problem is, it appears as if you are looking through a hole. I would rather use my EF 17-35 f2.8L on my 5D anyway....


Good instruction wickerprints!

[Milton Miszputen]

I want to thank all the members below and "The-Digital_Picture.com"for their valuable answers!


Milton, S&atilde;o Paulo, Brazil