FOV Crop/Magnification on Digital SLR's

[hhorwitz]

A friend and I are uncertain of the practical implications of the FOV crop/magnification issue that exists with lower-end Digital SLR's. Can any of you technically-qualified people settle this for us?

I have a newly-purchased Digital Rebel, which is reported to have a "magnification factor" (technically that may not be the correct term, but it is apparently in common usage) of 1.6x. I understand that to mean that my 18 - 55 mm lens actually functions on this camera at a 35 mm camera-equivalent 28 - 88 mm. If that is correct, this is obviously a disadvantage in terms of wide angle lenses because it costs a lot more to buy a quality 18 mm lens than a quality 28 mm lens. But is the reverse true on the other end of the spectrum? In particular, will the Canon 70 - 200 mm f/2.8 lens that I am considering purchasing truly function on this camera in a manner equivalent to a 35 mm camera 112 - 320 mm lens? That is what I have been told, particularly in my local camera store, but my friend believes that this phenomenon is merely "digital." He says that the "magnification effect" is simply a cropping of the image at the sensor itself, so that there is no optical benefit to the magnification factor. Put another way, less of the image that the lens is capable of transmitting is recorded, so thatwhen it is enlarged to
whatever size it would have been, it appears magnified. But this magnification or larger image is less dense than if would be from a longer focal length lens.

Are there any techies who can resolve this for us? A thousand thanks if you can.

[Andy_T]

Hi Hhorwitz,

why don't you try a forum search for the search terms you mentioned?

This is one of the more widely discussed issues on this forum.

Your observation about the wide angle end is correct. However, a good wide angle lens is still about US$ 5,000 less expensive than a full frame camera.

On the tele end, what matters, of course, is the amount of pixels you get into that area corresponding to the 320 mm crop area, as they define how big you can go at enlargements. Here 6.3 MP is definitely close to the limits of Canon L glass, so IMHO you can speak of a 'real' 320 mm lens.


Regards,
Andy

[robertwgross]

All of those things you've heard bear merit. It kind of depends on which side of the argument you want to get on. To some, this x1.6 factor is "effectively" a magnification factor, while to others, the factor is simply a crop factor. It will really get into a religious war to try to resolve that any further right here. So, I just call it the x1.6 factor.

It should be sufficient to say that the x1.6 factor hurts the wide angle user, and it helps the telephoto user. I tend to fall into the latter category, so I won't complain.

---Bob Gross---

[Belmondo]

This is a thread that was kicking around a while back. You might find it helpful.

http://photography-on-the.net/forum/showthread.php?t=18880#104733

[PacAce]

hhorwitz wrote:
A friend and I are uncertain of the practical implications of the FOV crop/magnification issue that exists with lower-end Digital SLR's. Can any of you technically-qualified people settle this for us?

I have a newly-purchased Digital Rebel, which is reported to have a "magnification factor" (technically that may not be the correct term, but it is apparently in common usage) of 1.6x. I understand that to mean that my 18 - 55 mm lens actually functions on this camera at a 35 mm camera-equivalent 28 - 88 mm. If that is correct, this is obviously a disadvantage in terms of wide angle lenses because it costs a lot more to buy a quality 18 mm lens than a quality 28 mm lens. But is the reverse true on the other end of the spectrum? In particular, will the Canon 70 - 200 mm f/2.8 lens that I am considering purchasing truly function on this camera in a manner equivalent to a 35 mm camera 112 - 320 mm lens? That is what I have been told, particularly in my local camera store, but my friend believes that this phenomenon is merely "digital." He says that the "magnification effect" is simply a cropping of the image at the sensor itself, so that there is no optical benefit to the magnification factor. Put another way, less of the image that the lens is capable of transmitting is recorded, so thatwhen it is enlarged to
whatever size it would have been, it appears magnified. But this magnification or larger image is less dense than if would be from a longer focal length lens.

Are there any techies who can resolve this for us? A thousand thanks if you can.


I'm just curious...why is it that owners of non-DSLR cameras never seems to be concerned about their their lens and the high magnification/crop factor of their cameras compared to 35mm cameras? For example, the G3 has a 7.2 to 28.8mm lens which is equivalent to 35 to 140mm in the 35mm world. That's a magnification (or crop, depending on how you want to look at it) factor of a 5!!!! Compared to that, doesn't the 1.6x factor of the Digital Rebel seem negligible?

For all intents and purposes, there are no practical or technical implications to the x-factor unless one continues to insist on comparing the Digital Rebel (or any other DSLR, for that matter) with 35mm film cameras.

[hhorwitz]

I can only respond in light of my own experience. With my G2, I never had to decide what kind of lens to use so fov crop was not an issue. Now, having purchased a digital SLR so I might use interchangeable lenses, and with plans to take pictures of animals in Africa in six weeks, I am trying to make an informed buying decision on a telephoto lens. Since my sole experience with assessing such lenses is in the 35 mm realm, meaning that I believe a 300 mm lens on a 35 mm camera would probably suffice for my purposes, I have ended up getting into the fov crop issue just to figure out what will and will not be satisfactory on my new camera. I'd be happy to forget about fov crop and magnification and just pick a lens if I had the experience and knowledge to do so. What I now conclude/assume is that the 70 - 200 mm lens I was considering will still be a 70 - 200 mm lens on the Digital Rebel except that the fov will look like a 112 - 320 mm lens on a 35 mm camera, and that this is fine so long as there are sufficient pixels to give me a satisfactory picture once it is so enlarged. Did I get that right?

[RichardtheSane]

hhorwitz wrote:
What I now conclude/assume is that the 70 - 200 mm lens I was considering will still be a 70 - 200 mm lens on the Digital Rebel except that the fov will look like a 112 - 320 mm lens on a 35 mm camera, and that this is fine so long as there are sufficient pixels to give me a satisfactory picture once it is so enlarged. Did I get that right?
Exactly right.
Basically the 'crop' factor is because the CMOS sensor in your D Rebel is smaller than that of a 35mm frame which the lens is designed to use.
You will find that the 6Mp sensor ample for enlargements well over the 10x8 size, especially if you buy a nice piece of glass like a 70-200 F2.8.
I regularly get 14x20 prints done and am very satisfied with the overall quality.

[w10d]

hhorwitz wrote:
but my friend believes that this phenomenon is merely "digital."
(snip)
But this magnification or larger image is less dense than if would be from a longer focal length lens.


I think your friend is talking in these terms purely because we are talking about digital and when you do that people have a sense that its 'all done with smoke and mirrors'.

Think of the Pentax medium format range: A photographer could build a system based on the 6x7, then buy a 645 and a lens adapter, and use the 6x7 lenses on the 645. They would need to buy 645 wide angle lenses, but they got a 'free' set of long lenses, straight out of their 6x7 gear.

No one started talking about the resulting images being less dense........(?) Though I'm sure some git will have moaned about the lenses being heavy.

Just tell your friend to look on the bright side!

[PacAce]

hhorwitz wrote:
I can only respond in light of my own experience. With my G2, I never had to decide what kind of lens to use so fov crop was not an issue. Now, having purchased a digital SLR so I might use interchangeable lenses, and with plans to take pictures of animals in Africa in six weeks, I am trying to make an informed buying decision on a telephoto lens. Since my sole experience with assessing such lenses is in the 35 mm realm, meaning that I believe a 300 mm lens on a 35 mm camera would probably suffice for my purposes, I have ended up getting into the fov crop issue just to figure out what will and will not be satisfactory on my new camera. I'd be happy to forget about fov crop and magnification and just pick a lens if I had the experience and knowledge to do so. What I now conclude/assume is that the 70 - 200 mm lens I was considering will still be a 70 - 200 mm lens on the Digital Rebel except that the fov will look like a 112 - 320 mm lens on a 35 mm camera, and that this is fine so long as there are sufficient pixels to give me a satisfactory picture once it is so enlarged. Did I get that right?

Yes, except for the pixel part at the end. Your Digital Rebel had a 6.3 MP sensor. That's not going to change no matter what lens you attach to the camera. From what I've read on this forum, you should be able to get a pretty big enlargement from your Digital Rebel without much trouble unless you were thinking of a print bigger than 11x14, in which case you'll probably want to use some nifty software to artificially increase the pixel count needed for bigger enlargements.

[w10d]

PacAce wrote:
I'm just curious...why is it that owners of non-DSLR cameras never seems to be concerned about their their lens and the high magnification/crop factor of their cameras compared to 35mm cameras? For example, the G3 has a 7.2 to 28.8mm lens which is equivalent to 35 to 140mm in the 35mm world. That's a magnification (or crop, depending on how you want to look at it) factor of a 5!!!! Compared to that, doesn't the 1.6x factor of the Digital Rebel seem negligible?

For all intents and purposes, there are no practical or technical implications to the x-factor unless one continues to insist on comparing the Digital Rebel (or any other DSLR, for that matter) with 35mm film cameras.

I think you just answered your own question :-)

A G3 owner is using a G3 lens where 7.2 is wide and 28.8 is long. Same as someone with a medium format camera has a 65mm wide and a 110mm standard. The problem comes up for them if they buy a digital back that doesn't cover the 6x7 frame, and their 65mm becomes a standard.

As long as we're all familiar with 35mm focal lengths and able to buy 'full frame' cameras like the 1Ds, we're probably stuck with using these factors, just so we know what to expect when we stick a lens on the camera...

[PacAce]

w10d wrote:
PacAce wrote:
I'm just curious...why is it that owners of non-DSLR cameras never seems to be concerned about their their lens and the high magnification/crop factor of their cameras compared to 35mm cameras? For example, the G3 has a 7.2 to 28.8mm lens which is equivalent to 35 to 140mm in the 35mm world. That's a magnification (or crop, depending on how you want to look at it) factor of a 5!!!! Compared to that, doesn't the 1.6x factor of the Digital Rebel seem negligible?

For all intents and purposes, there are no practical or technical implications to the x-factor unless one continues to insist on comparing the Digital Rebel (or any other DSLR, for that matter) with 35mm film cameras.

I think you just answered your own question :-)

A G3 owner is using a G3 lens where 7.2 is wide and 28.8 is long. Same as someone with a medium format camera has a 65mm wide and a 110mm standard. The problem comes up for them if they buy a digital back that doesn't cover the 6x7 frame, and their 65mm becomes a standard.

As long as we're all familiar with 35mm focal lengths and able to buy 'full frame' cameras like the 1Ds, we're probably stuck with using these factors, just so we know what to expect when we stick a lens on the camera...

Yes, I did, didn't I? Actually, my question was more of a rhetorical one. :) I should have really started it with "Isn't it curious..." instead of "I'm just curious...".

[urycyon]

I'm a new DSLR user too, but I thought I'd throw in something that wasn't mentioned. Since you are using less of the circular image that your lens provides, the resulting image will still be 6.3 megapixels but will magnify imperfections and light dispursion in your lenses. This and the fact that many DSLR users will be enlarging a much higher percentage of their photos than they did with 35mm will make a greater need for quality lenses. Image softness becomes more of an issue, and I think that a lot of the complaints about 300D's and 10D's images being soft is due to cheap lenses.
You will get a better image reaching the film on your 35mm with the same lens than on your DSLR's digital sensor with a large focal factor.

[Belmondo]

urycyon wrote:

You will get a better image reaching the film on your 35mm with the same lens than on your DSLR's digital sensor with a large focal factor.

We had a long discussion some time ago on this subject. The contention was that focusing errors are magnified at the sensor. This isn't exactly true, but when the image is enlarged to equal that which would be captured on a full-frame sensor, it becomes true. The same would apply to defects in the lens, I suppose.

[CyberDyneSystems]

w10d wrote:
PacAce wrote:
I'm just curious...why is it that owners of non-DSLR cameras never seems to be concerned about their their lens and the high magnification/crop factor of their cameras compared to 35mm cameras? For example, the G3 has a 7.2 to 28.8mm lens which is equivalent to 35 to 140mm in the 35mm world. That's a magnification (or crop, depending on how you want to look at it) factor of a 5!!!! Compared to that, doesn't the 1.6x factor of the Digital Rebel seem negligible?

For all intents and purposes, there are no practical or technical implications to the x-factor unless one continues to insist on comparing the Digital Rebel (or any other DSLR, for that matter) with 35mm film cameras.

I think you just answered your own question :-)

A G3 owner is using a G3 lens where 7.2 is wide and 28.8 is long. Same as someone with a medium format camera has a 65mm wide and a 110mm standard. The problem comes up for them if they buy a digital back that doesn't cover the 6x7 frame, and their 65mm becomes a standard.

As long as we're all familiar with 35mm focal lengths and able to buy 'full frame' cameras like the 1Ds, we're probably stuck with using these factors, just so we know what to expect when we stick a lens on the camera...

It is all about trying to get what you want out of a given lens that was designed for a 35mm camera so the comparison has to be made in the case of the DSLR,.

...we are using the 35mm lenses.

Where as the G2 is an island,. there are no lenses to do any math on. :)

[PacAce]

CyberDyneSystems wrote:

It is all about trying to get what you want out of a given lens that was designed for a 35mm camera so the comparison has to be made in the case of the DSLR,.

...we are using the 35mm lenses.

Where as the G2 is an island,. there are no lenses to do any math on. :)


CDS, I would agree with you if the person doing the comparing is coming from a previous 35mm SLR camera experience. However, if the 10D or the Digital Rebel is their first SLR camera of any type, then there's no reason to compare the DSLR to the film SLR no matter what type of lens is being used.

As far as I'm concerned, a lens designed for a 35mm camera works even better on a 10D or a 300D since these 1.6x cameras are only using the sharper inner portion of the lens.

[Belmondo]

PacAce wrote:I would agree with you if the person doing the comparing is coming from a previous 35mm SLR camera experience. However, if the 10D or the Digital Rebel is their first SLR camera of any type, then there's no reason to compare the DSLR to the film SLR no matter what type of lens is being used.



PacAce:
That's really not an accurate statement inasmuch as the crop factor will become relevant as sensor size increases on future DSLRs. As they approach the size of the 35mm frame, they crop factor will decrease, and our lenses will effective 'shorten.' Your 16mm lens will no longer be the functional equivalent of a 26mm lens. Then, the math will be important, and you should be taking it into account as you make your lens purchases now.

[PacAce]

belmondo wrote:
PacAce wrote:I would agree with you if the person doing the comparing is coming from a previous 35mm SLR camera experience. However, if the 10D or the Digital Rebel is their first SLR camera of any type, then there's no reason to compare the DSLR to the film SLR no matter what type of lens is being used.



PacAce:
That's really not an accurate statement inasmuch as the crop factor will become relevant as sensor size increases on future DSLRs. As they approach the size of the 35mm frame, they crop factor will decrease, and our lenses will effective 'shorten.' Your 16mm lens will no longer be the functional equivalent of a 26mm lens. Then, the math will be important, and you should be taking it into account as you make your lens purchases now.



I hear you and I know where you're coming from. But the fact of the matter is is that my 16mm lens will always remain a 16mm lens no matter what type of camera I used it with, whether it be a 10D, a 1Ds or even an APS camera. Grant it, the images taken with this lens on a 10D will be different from that taken with a 1Ds, everything else being equal, but I will already know that and so if I want the exact same images coming out of each camera, I'll select the appropriate lens for each one. I guess this is where a zoom lens really comes in handy. :)

I guess the point I'm trying to make is that we shouldn't be too overly concerned about the 1.6x factor of some of the DSLR cameras. Know that it's just the way it is with some DSLR cameras.