For instance take any of the camera LENSes designed for the Canon APS-c sized sensors like those used by the 10D, 20D, 30D, 40D. If you put that lens onto a full-size sensor camera like the Canon 5D or the Canon 1Ds...
you will break the camera because the EF-s LENS is longer so will strike the sensor of a full-frame camera.
In that case, let's not actually do it, but the thought experiment still holds.
What is vignetting but the light from the image not hitting the entire sensor? If the image covered the entire sensor then there would be good light all the way to the corners. Since the image is hitting in the middle of the sensor the edges and corners are dark, hence vignetted.
Yes, that is what vignetting is. All camera lenses exhibit some degree of vignetting. Really good lenses have so little vignetting that it is not discernable across the entire sensor (or film frame). Poor lenses will have as much as 1 (or even 2) f-stops worth of exposure difference from the center to the corner of the frame. Other image quality measurments (such as chromatic abberation, corner focus, and distortion) are likewise less pronounced near the center of the image than near the edges.
However, a lens considered poor when casting an image on a 35mm frame might still cast a pretty good image on a 22mm frame. Think of it as using the "sweet spot" of the lens. This is one of the valid reasons for choosing an APS-C size sensor over a 35mm sensor. You can get good image quality out of cheaper lenses. In fact, that is the seminal idea behind the EF-S lens series. Make lenses that physically fit the same mount, but use lighter materials and less expensive designs that don't strive for image quality outside the smaller frame.
Not according to the manufacturers or the experts who have written about this in detail. See the many references I've already posted.
I'll refer you back to your chosen source
http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm. This page discusses the "crop factor", which is the portion of the focal plane that the sensor samples. A larger sensor samples a larger area and therefore has a wider "angle of view". It does not change how the lens works.
With the smaller sensor you have a 1.6x "magnification factor" which is caused by the fact that the smaller sensor is only picking up the center portion of the image created by a lens for a full-size sensor so an "L" lens set at 50mm appears to be an 80mm effective setting on the 40D.
"Appears" is a slippery word here. A 50mm lens is a 50mm lens, no matter what camera you put it on. The whole "magnification factor", or "35mm-equivalent lens" is smoke and mirrors. In essence, you have to preface each statement with "if you were using a 35mm camera, this would be 'like' using a..." A 50mm lens on an APS-C camera is "like" using a 80mm lens on a 35mm camera. A 50mm lens on 2 1/4 square format camera is "like" using a 28mm lens on a 35mm camera. A 50mm lens on a 8x10 format camera is "like" using a 6mm lens on a 35mm camera. A 50mm lens on a Canon 5D body is "like" using a 31mm lens on a Canon 40D, a 85mm lens on a 2.25"x2.25" camera, or a 450mm lens on an 8"x10" camera. These statement are all about "filling the frame", which is about the field of view - a combination of lens and film frame. It is a mistake to think the lens changes when you put it on a different body.
Everything else being equal (same f-stop, same distance to object, same focal length), the camera with the bigger sensor will have less depth of field and the camera with the smaller sensor will have more depth of field.
So to get the same size image you have to change the focal length of the "L" lens down to 50mm/1.6=31.25mm if you want the same image. And going to a wider image results in a change to the DoF, specifically you get a more DoF.
Which is it? Are you changing the focal length or not? If you change the focal length, you'll change the depth-of-field. If you don't, you won't. Depth-of-field depends on focal length, not sensor size.
So are you talking focal-length as set on the LENS or effective focal-length of the camera+LENS combination?
I am speaking of the actual focal length of the camera lens, not the so-called "effective focal-length".
Let's suppose we made test prints of our pictures, measuring 24x35mm (like 35mm contact prints). The Canon 5D shots would require no digital magnification (a 1.0 multiplier). The Canon 40D shots would require 60% (a 1.6 multiplier) enlargement. Since all we're doing is manipulating the print size, it does not affect depth-of-field. The point is that there are two kinds of magnification going on here. Optical magnification is done by the lens. Digital magnification is done to enlarge the 22mm APS-C sensor image to 35mm. Depth-of-field is related to the optical magnification but not the digital magnification. "Actual" focal length is a measurement of the optical magnification. "Effective" focal length is a comparison term that combines optical and digital magnification and is therefore more subject to confusion.
If you adjust the camera/LENS so the effective focal-length remains the same as you move the camera body/LENS around, then yes, DoF stays the same.
Are you now saying that a 31mm lens on a Canon 40D, it will produce the same depth-of-field as a 50mm lens on a 5D (both "effective" focal length 50mm)? That's the exact opposite of what you've said before and is incorrect. Depth-of-field is a function of
actual focal length, not "effective" focal length. Let's perform another thought experiment.
Take a Canon EF 200mm f2.0 L IS USM Lens (chosen simply because it has a fixed focal length and a tripod mount) and mount it on a tripod. Attach a Canon 40D body, focus on a subject, and take a picture. Now, without changing anything else, remove the 40D body and attach a 1Ds Mk III body. Take another picture using the same ISO setting and exposure (specifically f-stop).
Now, you will tell me that the first picture had an "effective" focal length of 320mm, based on the fact that the 40D has a smaller sensor, and therefore a narrower angle of view. Nevertheless, nothing about the lens changed, and nothing about the light passing through it changed. The first picture will exhibit a tighter "crop" than the second one, but they will have the exact same depth-of-field.
If you disagree, then please explain how a different sensor changed what was in focus and what was out of focus when the image on the focal plane is identical.
But if the as-set focal-length remains the same then the effective focal-length changes and your DoF changes with it.
I don't know what you mean by "as-set focal-length". Can you elaborate?
So this whole discussion really has been a potAto, potAHto conversation. We've been talking past each other.
Perhaps. We may mean a different thing by "size" (and no nearsighted blonde jokes, please). When I say any lens of the same (actual, not effective) focal length will produce the same size image, and the same depth-of-field, I mean the size of the image cast on the focal plane. I don't mean the the size proportional to the angle of veiw (a.k.a. size in the frame, or "crop factor").
You are absolutely correct, if you CHANGE the focal-length of the LENS when you change the camera/LENS then you will have a different DoF.
Whether you change the camera or not, when you change the actual focal length (or more accurately, the optical magnification), the depth-of-field changes.
My entire conversation has been around using the same focal-length on different sensors which would yield different results.
Again, this may be conflict in terminology (which is why I dislike the attempt to couch everything in 35mm equivalent terms). If you use the same
actual focal length lens on different size sensors, your photos will have different angles of view, but the same depth-of-field. If you use the same
effective focal-length (which is a different actual focal length), at the same distance, your photos will have the same angle of view, but different depth-of-field.
I want the pen in focus and everything in front of the pen and behind the pen soft so that the viewer's eyes are drawn to the pen.
That is probably the best statement that has come out of this discussion! If you would permit me, I'd like to elaborate a little, because unlike most everything else we've said, this point is truly useful to pen photography.
When you want the viewer's eye drawn to a particular subject, one useful tool is to keep that subject in sharp focus while everything else is (at least slighty) blurry. In photographic terms, you want the depth-of-field to encompass the subject, but not the foreground nor background. The goal is not minimum depth-of-field, nor maximum depth-of-field, but rather the correct depth-of-field to isolate the pen.
As Keith pointed out, depth-of-field extends farther beyond the point of focus than in front of it (because depth-of-field varies with distance). So, after choosing your camera, and arranging your photo, you want to focus on a point somewhat closer than the median distance, and pick an f-stop that will bring the depth-of-field out to just encompass the pen.
Thinking about how you want your pen presented, and composing your shot to achieve that goal makes a far greater difference to your photo than the brand and model of your equipment. Or, as an old instructor of mine used to say "what's behind the camera is more important than what's inside it".
It was fun having the discussion though.
Yep.
Regards,
Eric
