Small Sensor ?

[michael_wilson9]

Just purchased my first DSLR - Canon40D

 

I understand that it has a smaller sensor.

 

Does this increase the telephoto on an 50mm f/1.4 prime to a little more than

75mm?

 

I understand that canon is making S- lenses.

 

So if I were to purchase a 17-50mm lens would that also increase my wide angle

to a 26mm lens on the short end?

[Hector Javkin]

This is a point of confusion for many photographers, and has been asked on photo.net many, many times. The focal length doesn't change. The field of view of at 17mm -- what the sensor captures -- is equivalent to what a 35mm or a full-size sensor would capture with a 26mm lens.

 

It might help to think about it as if you were capturing on a 35mm negative (36x24mm) and trimming it so that you only get the central portion.

[tommyinca]

50mm will have a field of view of a 80mm. 17-50/2.8 will have a field of view closer to a 28-80/2.8. The 40D field of view multiplier is 1.6.

[gdanmitchell]

On your 1.6x crop sensor camera a given lens provides an angle of view the same as a

lens 1.6x as long used on a 35mm film SLR.

 

Essentially, while your 50mm lens is still a 50mm lens, on the new camera it works more

or less the same as an 80mm lens would have in the old days.

 

To get the equivalent angle of view that a 50mm lens provided on a 35mm SLR you would

want to put a lens with a focal length of about 31mm on your 40D.

 

Dan

[colinsouthern]

Hi Michael,

 

With academics & semantics placed temporarily aside, in terms of focal length, you can safely think of the 40D as haveing a 1.6x focal length multiplier.

 

As I say, strictly speaking it isn't true, but that the net effect at the end of the day is the same.

 

D of F is different - it's shallower on a Full Frame camera compared to a crop-factor camera with the same lens.

[mark u]

"D of F is different - it's shallower on a Full Frame camera compared to a crop-factor camera with the same lens."

 

A frequently encountered misconception that is not correct. With the same lens (focal length) at the same aperture depth of field is shallower on a crop camera than on a full frame camera. With the same angle of view (different focal length) at the same aperture, depth of field is shallower on a full frame camera than on a crop camera.

 

http://www.photo.net/learn/optics/dofdigital/

[robin_sibson1]

A mathematician writes ... the article by Bob Atkins to which Mark U provides a link is, as one would expect, both comprehensive and accurate. But perhaps relatively simple way of summarising part of what it says will aid to achieving the deeper understanding offered by the article. So let's ask the following question: how do you get the SAME result on a final print of a given size from cameras of different formats? SAME does mean that out-of focus parts of the scene are out of focus to the same extent, but it ignores effects to do with quality of bokeh and of sensor resolution and performance at different ISO settings. We also have to enter a caveat: this breaks down for macro or close-up work but is a plenty good enough approximation when the focal lengths involved are small compared to the focsed distance.

 

The answer is as follows.

 

1. Have the two cameras in the same position (so one after another is good) with the lens axis and camera orientation aligned. That's obvious - otherwise you are taking a different photograph.

 

2. Use lenses whose focal lengths are in the ratio of the linear dimensions of the sensors - we are assuming the sensors are the same shape, and the final print will be made from the whole of what the sensor captures. That's obvious too - otherwise your shots will have different angles of view.

 

3. Focus to the same distance - also obvious, otherwise there's no chance of the same things being sharp.

 

4. Now the tricky bit. On the smaller format, you are using a shorter focal length, which, other things being equal, will increase depth of field. But other things aren't equal. You will demand a smaller circle of confusion on the smaller sensor in order to get the final prints to match, since the sensor image has to be magnified more as between the physical dimensions of the sensor and those of the print. This is going to work in the opposite direction, by reducing depth of field. But the two effects DO NOT balance at constant f-number, they DO balance at constant aperture diameter. So to get exactly the same result on the final print, you have to open up the f-stop for the smaller format, in the same ratio as the ratio of sensor sizes.

 

5. You can then choose how to compensate for that by adjusting either or both of ISO and exposure time.

 

6. Er ... that's it.

[gdanmitchell]

<p>Michael, I should have included the simple math for you in my post. Sorry about that.

 

<p>If you are thinking in terms of your old 35mm lenses and want to know "how the lenses will behave" on your new crop camera, just <i>multiply the focal length of the old

lens by 1.6</i>. Example:

 

<blockquote>That old 50mm lens multiplied by 1.6 will "act like" and 80mm lens</blockquote>

 

<p>It works the other way, too. If you liked a particular focal length on your 35mm film SLR you can <i>divide the focal length by 1.6</i>. Example:

 

<blockquote>If you want the angle of view of your old 50mm lens on the new camera divide the focal length by 1.6 to get 31.25mm.</blockquote.</blockquote>

 

<p>I sort of like a term I heard someone else use for this: <i>effective focal length</i>. If you live in a crop sensor world with lenses used on full frame bodies, you might

describe your 50mm lens as having an <i>effective focal length of 80mm</i>. (No need to point out the semantic, etc. problems with this - I concede at the outset that it is a

flawed and imperfect notion. :-)

 

<p>I'm not going to wade into the DOF issue. Let's just say that it is partially true, more complicated than some people want to think, depends on enlargement factor, and not

worth losing sleep over.

 

<p>But do keep in mind that your use of the smaller apertures (e.g. f/16) may be limited on your crop body because <i>diffraction</i> can start to compromise sharpness at

apertures smaller than about f/8. (There have been more than a few posts around here by folks who got a new crop sensor DSLR and wanted to know why their f/16 shots were

never as sharp as they remembered getting on film...)

 

<p>Dan

[William Michael]

Considering another recent post of mine, I shall do what Kesuke Miyagi taught me, and sidestep the DoF portion and focus on this part of the question, which has not yet been addressed:

 

>>> I understand that canon is making S- lenses. <<< (HJ)

 

I think you might be referring to the range of EF-S lenses.

 

These are designed to work with APS-C sensor cameras, like the 40D.

 

The sensor in the 40D is smaller than the sensors in 135 format cameras, (some call these `Full Frame`) which the 5D is, for example. These Full Frame sensors are (about) the same size as the negative on 35mm film SLRs.

 

Also, the range of EF-S lenses cannot fit onto, nor work effectively on the APS-H format digital cameras, such as the 1 series of Digitals having a x1.3 FoV factor.

 

And, although they were designed for the APS-C format size, the EF-S lenses DO NOT work on ALL APS-C bodies.

 

In summary the EF-S lenses fit the 300D an all subsequent models in that series; and the 20D and all subsequent models in that series.

 

As one example, they do NOT work on a 10D.

 

[Note my term `do not work` does not take into account some people`s passion to dismantle the lens and remove a portion and file off a small lug, which indeed can be done.]

 

WW

[William Michael]

>>> A mathematician writes ... <<<

 

Ah! That (new) piece of the Venn Diagram makes absolute sense :)

 

WW